Notes on the Troubleshooting and Repair of Television Sets

Version 3.30 (17-Nov-09)

Copyright © 1994-2013
Samuel M. Goldwasser
--- All Rights Reserved ---

For contact info, please see the
Sci.Electronics.Repair FAQ Email Links Page.


Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:
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Table of Contents



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    Preface

    Author and Copyright

    Author: Samuel M. Goldwasser

    For contact info, please see the Sci.Electronics.Repair FAQ Email Links Page.

    Copyright © 1994-2013
    All Rights Reserved

    Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:

    1. This notice is included in its entirety at the beginning.
    2. There is no charge except to cover the costs of copying.

    DISCLAIMER

    Working inside a CRT-based television set, or computer or video monitor can be lethal from line-connected and high voltage power supplies as well as CRT implosion. Read and follow ALL of the safety guidelines found in Safety Guidelines for High Voltage and/or Line Powered Equipment and the section: SAFETY, below. If in doubt about your abilities or experience, leave repair and internal adjustments to a professional.

    We will not be responsible for damage to equipment, your ego, county wide power outages, spontaneously generated mini (or larger) black holes, planetary disruptions, or personal injury or worse that may result from the use of this material.



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    Introduction

    Television at the crossroads

    Television in substantially its present form has been with us for nearly 50 years. It is a tribute to the National Television Standards Committee (NTSC) that the color television standards agreed upon in the early 1950s have performed remarkably well making quite efficient use of valuable radio spectrum space and the psychovisual characteristics of the human eye-brain system. However, HDTV (High Definition TV) will supplant and ultimately replace the current standards. We will all come to expect its superior resolution, freedom from noise and ghosting, and pure CD sound. Yet, the perceived quality of TV broadcasts and cable will never likely be the major issue with most consumers. Content will continue to be the biggest problem.

    As of June 2009, all analog broadcasting in the USA has been discontinued by law, except for some low power local transmissions. This (so the justification goes) frees up a large amount of electromagnetic spectrum for other (more lucrative!) applications, since the Digital TV (DTV) channels occupy less bandwidth. Low cost DTV converter boxes enabled existing TVs to receive the digital signals so there was no need to buy new digital TVs, though this did make for one darn good excuse to upgrade to a 60" flat screen HDTV! :)

    There is plenty of information available elsewhere with regard to the pros and cons of the DTV conversion, the trials and tribulations of people using antennas (as opposed to cable or optical fiber), and the ultimate benefits, real and perceived. Suffice it to say that when digital reception is good, the picture and sound are very very good and there is essentially no comparison with the analog system it replaced. This is true even when a converter box is used with an analog TV in good condition and standard definition digital is better than the best analog. HDTV with multi-channel sound is simply exquisite.

    However, unlike analog TV, poor reception doesn't result in snow or ghosts, but rather the picture (and sound) totally drops out or (in the case of the video) freezes or pixilates. For over-the-air reception, the type of antenna and its orientation becomes much more critical. And even in areas close to the transmitter, local terrain and obstructions in the line-of-sight like hills and buildings may result in problems.

    Television receiver fundamentals

    The basic color television receiver must perform the same functions today as 40 years ago. (Since B/W is a subset of the color standard, most references in this document will be for color except as noted). A studio video monitor includes all of the functions of a television receiver except the tuner and IF (which rarely fail except for bad connections or perhaps lightning strikes to the antenna or cable connection). Therefore most of the repair information in this document is applicable to both TVs and studio monitors. Modern computer monitors share many similarities with TVs but the multisync and high scan rate deflection circuitry and more sophisticated power supplies complicates their servicing.

    As of this writing, the majority of TVs are still based on the Cathode Ray Tube (CRT) as the display device. Tiny pocket sets, camcorder viewfinders, and the like have started using LCD (Liquid Crystal Display) panels but these are still inferior to the CRT for real time video. There has always been talk of 'the picture on the wall' display and these are now appearing as large screen plasma panel displys but their cost is still high compared to even projection TVs using CRTs. The reason is simple economics - it is really hard to beat the simplicity of the shadow mask CRT. Of course, prices will drop as the technology matures.

    Projection - large screen - TVs, on the other hand, are able to take advantage of a novel development in integrated micromachining - the Texas Instruments Inc. Digital Micromirror Device (DMD), now called DLP for "Digital Light Processing". This is basically an integrated circuit with a tiltable micromirror for each pixel fabricated on top of a static memory - RAM - cell. This technology would permit nearly any size projection display to be produced and would therefore be applicable to HDTV. Since it is a reflective device, the light source can be as bright as needed. This technology is already appearing in commercial high performance projectors and is competing for use in totally digital movie theaters to replace the film projector and has begun appearing in high-end consumer projection TV sets - yet.

    (From: Kurk MacKay (kurk_mackay@telus.net).)

    "DMD TVs have been on the marketplace for about a year now. The DMD is more commonly referred to as DLP (Digital Light Processing) in the marketplace. From what I've heard Samsung has had a DLP TV on the consumer marketplace as of last year in the US and this year here in Canada. My boss was looking at buying one so we went around to view them. The picture quality looks to be between the LCD projection and Plasma. I believe the current Samsung uses a color wheel but they are working on a three color independent system.

    For more info or if you want to buy one, see DLP TV Showcase."

    As noted, the plasma panel flat screen display has been around for several years in high-end TVs, typically in the 42 inch diagonal range. However, they are very expensive ($5,000 to $15,000 as of Winter, 2003), and their life expectancy may be limited due to the gradual degradation of the active pixel cells - which occurs faster than for a CRT. The physical resolution is also still low enough that visible discrete pixels may be objectionable to some viewers. However, there is little doubt that this or a similar technology will eventually replace the direct view CRT and 3-tube projection TVs in the mid to large screen sizes in the not too distant future.

    The remainder of this document concentrates on CRT based analog TVs since these still dominate the market and realistically, these are the only type where there is a good chance of repair without access to specialized test equipment and parts. I wouldn't recommend any sort of attempt at repair of flat screen TVs or monitors - no matter what the size - beyond checking for bad connections, dead power supplies, or other obvious problems. The chance of success is vanishingly small and it's very likely that even with great care, damage could occur to the panels or circuitry.

    TV repair

    Unlike VCRs or CD players where any disasters are likely to only affect your pocketbook, TVs can be dangerous. Read, understand, and follow the set of safety guidelines provided later in this section whenever working on TVs, monitors, or other similar high voltage equipment.

    If you do go inside, beware: line voltage (on large caps) and high voltage (on CRT) for long after the plug is pulled. There is the added danger of CRT implosion for carelessly dropped tools and often sharp sheetmetal shields which can injure if you should have a reflex reaction upon touching something you should not touch. In inside of a TV or monitor is no place for the careless or naive.

    Having said that, a basic knowledge of how a TV set works and what can go wrong can be of great value even if you do not attempt the repair yourself. It will enable you to intelligently deal with the service technician. You will be more likely to be able to recognize if you are being taken for a ride by a dishonest or just plain incompetent repair center. For example, a faulty picture tube CANNOT be the cause of a color television only displaying shows in black-and-white. The majority of consumers probably do not know even this simple fact. Such a problem is usually due to a bad capacitor or other 10 cent part.

    This document will provide you with the knowledge to deal with a large percentage of the problems you are likely to encounter with your TVs. It will enable you to diagnose problems and in many cases, correct them as well. With minor exceptions, specific manufacturers and models will not be covered as there are so many variations that such a treatment would require a huge and very detailed text. Rather, the most common problems will be addressed and enough basic principles of operation will be provided to enable you to narrow the problem down and likely determine a course of action for repair. In many cases, you will be able to do what is required for a fraction of the cost that would be charged by a repair center.

    Should you still not be able to find a solution, you will have learned a great deal and be able to ask appropriate questions and supply relevant information if you decide to post to sci.electronics.repair. It will also be easier to do further research using a repair text such as the ones listed at the end of this document. In any case, you will have the satisfaction of knowing you did as much as you could before taking it in for professional repair. With your new-found knowledge, you will have the upper hand and will not easily be snowed by a dishonest or incompetent technician.

    Repair or replace

    If you need to send or take the TV to a service center, the repair could easily exceed half the cost of a new TV. Service centers may charge up to $50 or more for providing an initial estimate of repair costs but this will usually be credited toward the total cost of the repair (of course, they may just jack this up to compensate for their bench time).

    TV prices have been dropping almost as fast as PC prices. Therefore, paying such prices for repair just may not make sense. Except for picture tube problems, most TV faults can be corrected without expensive parts, however. Keeping a 5 year old TV alive may be well worthwhile as basic TV performance and important features have not changed in a long time.

    If you can do the repairs yourself, the equation changes dramatically as your parts costs will be 1/2 to 1/4 of what a professional will charge and of course your time is free. The educational aspects may also be appealing. You will learn a lot in the process. Thus, it may make sense to repair that old clunker for your game room or beach house. (I would suggest the kid's room but most TV watching just rots the brain anyhow so a broken TV may be more worthwhile educationally than one that works.)



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    TV Receivers 101

    Subsystems of a television set

    A TV set includes the following functional blocks:

    1. Low voltage power supply (some may also be part of (2).) Most of the lower voltages used in the TV may be derived from the horizontal deflection circuits. Sometimes, there is a separate switching power supply but this would be the exception. Rectifier/filter capacitor/regulator from AC line provides the B+ to the switching power supply or horizontal deflection system. Degauss operates off of the line whenever power is turned on (after having been off for a few minutes) to demagnetize the CRT.

    2. Horizontal deflection. These circuits provide the waveforms needed to sweep the electron beam in the CRT across and back some 15,734 times per second (for NTSC). The horizontal sync pulse from the sync separator locks the horizontal deflection to the video signal.

    3. Vertical deflection. These circuits provide the waveforms needed to sweep the electron beam in the CRT from top to bottom and back 60 times per second (for NTSC). The vertical sync pulse from the sync separator locks the vertical deflection to the video signal.

    4. CRT high voltage (also part of (2).) A modern color CRT requires up to 30 kV for a crisp bright picture. Rather than having a totally separate power supply, nearly every TV on the planet derives the HV (as well as many other voltages) from the horizontal deflection using a special transformer called a 'flyback' or 'Line OutPut Transformer (LOPT) for those of you on the other side of the lake.

    5. Tuner, IF, AGC, video and audio demodulators. Input is the antenna or cable signal and output are baseband video and audio signals. There is usually someplace inside the TV where line level video and audio are present but it may not be accessible from the outside of the cabinet unless you paid for the more expensive model with the A/V option. Very often, the tuner is a shielded metal box positioned on the bottom right (as viewed from the front) separate from the main circuit board. Sometimes it is on the main circuit board. The IF section may be in either place.

      On older or cheap TVs with a knob tuner, this is usually mounted to the front panel by itself. There are usually separate boxes for the VHF and UHF tuners.

    6. Chroma demodulator. Input is the baseband video signal. Outputs are the individual signals for the red, green, and blue video to the CRT.

    7. Video drivers (RGB). These are almost always located on a little circuit board plugged directly onto the neck of the CRT. They boost the output of the chroma demodulator to the hundred volts or so needed to drive the cathodes of the CRT.

    8. Sync separator. Input is baseband video. Output is horizontal and vertical sync pulses to control the deflection circuits.

    9. Audio amplifier/output. The line level audio is amplified to drive a set of speakers. If this is a stereo TV, then these circuits must also perform the stereo demultiplexing.

    10. System control. Most modern TVs actually use a microcontroller - a fixed program microcomputer to perform all user interface and control functions from the front panel and remote control. These are becoming increasingly sophisticated. However, they do not fail often. Older TVs use a bunch of knobs and switches and these are prone to wear and dirt.

    Most problems occur in the horizontal deflection and power supply sections. These run at relatively high power levels and some components run hot. The high voltage section is prone to breakdown and arcing as a result of hairline cracks, humidity, dirt, etc.

    The tuner components are usually quite reliable unless the antenna experiences a lightning strike. However, it seems that even after 20+ years of solid state TVs, manufacturers still cannot reliably solder the tuner connectors and shields so that bad solder connections in these areas are common even in new sets.

    Why projection TVs are not just normal TVs in big boxes

    In order to achieve the necessary brightness with a large display format, three separate monochrome CRTs are used with optics to combine the three images properly at the screen. This creates an entire set of additional problems in design.

    (From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).)

    The average projection TV has about twice as many parts as its direct-view counterpart. Some of the extra parts are essential for projection because CRT projection tubes require dynamic convergence. The other extra parts have to do with the fact that a more expensive TV also should have some extra features, like Dolby ProLogic sound, a satellite tuner and such.

    Generally, the electronics are based on a standard chassis that is also used for direct-view CRT television. Even the deflection circuits require minor adaptations at most. The high-voltage circuit is different because the EHT, focus and G2 voltages must be distributed over 3 CRTs. So this requires a special high-voltage part, which also includes an EHT capacitor and bleeder.

    There will be 3 CRT panels with video amplifiers. Because of the extremely high brightness, projection tubes will burn the phosphor screen immediately in fault conditions so a protection circuit is essential.

    And last but certainly not least, there is the dynamic convergence panel. The heart is a waveform generator IC, often of a Japanese brand but nowadays there's also a digital variant by Philips. The old-fashioned way requires many many potentiometers to program the waveforms. Then there's 5 or 6 convergence amplifiers and a corresponding extra power supply. And usually this is where the single deflection circuits are distributed to the 3 CRTs. At the same time the deflection currents are sensed for the protection circuits.

    Designing a PTV from a DVTV requires several man-years of work. In the factory, a special corner is devoted to the assembly. There you'll find specially educated people and the speed of the assembly line is a lot lower than usual. It requires many more adjustments, e.g. 3 optical and 3 electrical focus adjustments and then convergence.

    For more information on TV technology

    The books listed in the section: Suggested references include additional information on the theory and implementation of the technology of television standards and TV receivers.

    Philips/Magnavox used to have a very nice on-line introduction to a variety of consumer electronics technologies. Although their site has disappeared - and even people who work for them have no clue - I have now recovered several of the articles including those on TVs, VCRs, camcorders, satellite reception, and connections. See the Introductory Consumer Electronics Technology Series. These as well as most or all of the other articles, as well a glossary and much more, can be also be accessed via the Internet Archive Wayback Machine. Copy and paste the following URL into the search box:

    The earliest (Nov 09, 1996) archive seems to be the most complete.

    Also see:

    On-line tech-tips databases

    A number of organizations have compiled databases covering thousands of common problems with VCRs, TVs, computer monitors, and other electronic equipment. Most charge for their information but a few, accessible via the Internet, are either free or have a very minimal monthly or per-case fee. In other cases, a limited but still useful subset of the for-fee database is freely available.

    A tech-tips database is a collection of problems and solutions accumulated by the organization providing the information or other sources based on actual repair experiences and case histories. Since the identical failures often occur at some point in a large percentage of a given model or product line, checking out a tech-tips database may quickly identify your problem and solution.

    In that case, you can greatly simplify your troubleshooting or at least confirm a diagnosis before ordering parts. My only reservation with respect to tech-tips databases in general - this has nothing to do with any one in particular - is that symptoms can sometimes be deceiving and a solution that works in one instance may not apply to your specific problem. Therefore, an understanding of the hows and whys of the equipment along with some good old fashioned testing is highly desirable to minimize the risk of replacing parts that turn out not to be bad.

    The other disadvantage - at least from one point of view - is that you do not learn much by just following a procedure developed by others. There is no explanation of how the original diagnosis was determined or what may have caused the failure in the first place. Nor is there likely to be any list of other components that may have been affected by overstress and may fail in the future. Replacing Q701 and C725 may get your equipment going again but this will not help you to repair a different model in the future.

    Please see the document: On-Line Tech-Tips Databases for the most up to date compilation of these resources for TVs, VCRs, computer monitors, and other consumer electronic equipment.

    Additional TV technology and repair information

    See Sam's Neat, Nifty, and Handy Bookmarks under "Television" and "Manuals/Schematics/Repair Guides" for additional links.



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    CRT Basics

    Note: Most of the information on TV and monitor CRT construction, operation, interference and other problems. has been moved to the document: TV and Monitor CRT (Picture Tube) Information. The following is just a brief introduction with instructions on degaussing.

    Color CRTs - shadow masks and aperture grills

    All color CRTs utilize a shadow mask or aperture grill a fraction of an inch (1/2" typical) behind the phosphor screen to direct the electron beams for the red, green, and blue video signals to the proper phosphor dots. Since the electron beams for the R, G, and B phosphors originate from slightly different positions (individual electron guns for each) and thus arrive at slightly different angles, only the proper phosphors are excited when the purity is properly adjusted and the necessary magnetic field free region is maintained inside the CRT. Note that purity determines that the correct video signal excites the proper color while convergence determines the geometric alignment of the 3 colors. Both are affected by magnetic fields. Bad purity results in mottled or incorrect colors. Bad convergence results in color fringing at edges of characters or graphics.

    The shadow mask consists of a thin steel or InVar (a ferrous alloy) with a fine array of holes - one for each trio of phosphor dots - positioned about 1/2 inch behind the surface of the phosphor screen. With most CRTs, the phosphors are arranged in triangular formations called triads with each of the color dots at the apex of the triangle. With many TVs and some monitors, they are arranged as vertical slots with the phosphors for the 3 colors next to one another.

    An aperture grille, used exclusively in Sony Trinitrons (and now their clones as well), replaces the shadow mask with an array of finely tensioned vertical wires. Along with other characteristics of the aperture grille approach, this permits a somewhat higher possible brightness to be achieved and is more immune to other problems like line induced moire and purity changes due to local heating causing distortion of the shadow mask.

    However, there are some disadvantages of the aperture grille design:

    Apparently, there is no known way around the need to keep the fine wires from vibrating or changing position due to mechanical shock in high resolution tubes and thus all Trinitron monitors require 1, 2, or 3 stabilizing wires (depending on tube size) across the screen which can be see as very fine lines on bright images. Some people find these wires to be objectionable and for some critical applications, they may be unacceptable (e.g., medical diagnosis).

    Degaussing (demagnetizing) a CRT

    Degaussing may be required if there are color purity problems with the display. On rare occasions, there may be geometric distortion caused by magnetic fields as well without color problems. The CRT can get magnetized:

    Degaussing should be the first thing attempted whenever color purity problems are detected. As noted below, first try the internal degauss circuits of the TV or monitor by power cycling a few times (on for a minute, off for at least 20 minutes, on for a minute, etc.) If this does not help or does not completely cure the problem, then you can try manually degaussing.

    Note: Some monitors have a degauss button, and monitors and TVs that are microprocessor controlled may degauss automatically upon power-on (but may require pulling the plug to do a hard reset) regardless of the amount of off time. However, repeated use of these 'features' in rapid succession may result in overheating of the degauss coil or other components. The 20 minutes off/1 minute on precedure is guaranteed to be safe. (Some others may degauss upon power-on as long as the previous degauss was not done within some predetermined amount of time - they keep track with an internal timer.)

    On portable TVs, degauss may only function when they are run on AC, not the internal battery. If color purity problems are present, plugging the TV into an AC outlet may be needed to enable it to degauss itself.

    Commercial CRT Degaussers are available from parts distributors like MCM Electronics and consist of a hundred or so turns of magnet wire in a 6-12 inch coil. They include a line cord and momentary switch. You flip on the switch, and bring the coil to within several inches of the screen face. Then you slowly draw the center of the coil toward one edge of the screen and trace the perimeter of the screen face. Then return to the original position of the coil being flat against the center of the screen. Next, slowly decrease the field to zero by backing straight up across the room as you hold the coil. When you are farther than 5 feet away you can release the line switch.

    The key word here is ** slow **. Go too fast and you will freeze the instantaneous intensity of the 50/60 Hz AC magnetic field variation into the ferrous components of the CRT and may make the problem worse.

    WARNING: Don't attempt to degauss inside or in the back of the set (near the CRT neck. This can demagnetize the relatively weak purity and convergence magnets which may turn a simple repair into a feature length extravaganza!

    It looks really cool to do this while the CRT is powered. The kids will love the color effects (but then lock your degaussing coil safely away so they don't try it on every TV and monitor in the house!).

    Bulk tape erasers, tape head degaussers, open frame transformers, and the "butt-end" of a weller soldering gun can be used as CRT demagnetizers but it just takes a little longer. (Be careful not to scratch the screen face with anything sharp. For the Weller, the tip needs to be in place to get enough magnetic field.) It is imperative to have the CRT running when using these whimpier approaches, so that you can see where there are still impurities. Never release the power switch until you're 4 or 5 feet away from the screen or you'll have to start over.

    I've never known of anything being damaged by excess manual degaussing as long as you don't attempt to degauss *inside* or the back of the set - it is possible to demagnetize geometry correction, purity, and static converence magnets in the process! However, I would recommend keeping really powerful bulk tape erasers-turned-degaussers a couple of inches from the CRT.

    Another alternative which has been known to work is to place another similar size monitor face-to-face with the suspect monitor (take care not to bump or scratch the screens!) and activate degauss function on the working monitor. While not ideal, this may be enough to also degauss the broken one.

    If an AC degaussing coil or substitute is unavailable, I have even done degaussed with a permanent magnet but this is not recommended since it is more likely to make the problem worse than better. However, if the display is unusable as is, then using a small magnet can do no harm. (Don't use a 20 pound speaker or magnetron magnet as you may rip the shadow mask right out of the CRT - well at least distort it beyond repair. What I have in mind is something about as powerful as a refrigerator magnet.)

    Keep degaussing fields away from magnetic media. It is a good idea to avoid degaussing in a room with floppies or back-up tapes. When removing media from a room remember to check desk drawers and manuals for stray floppies, too.

    It is unlikely that you could actually affect magnetic media but better safe than sorry. Of the devices mentioned above, only a bulk eraser or strong permanent magnet are likely to have any effect - and then only when at extremely close range (direct contact with media container).

    All color CRTs include a built-in degaussing coil wrapped around the perimeter of the CRT face. These are activated each time the CRT is powered up cold by a 3 terminal thermister device or other control circuitry. This is why it is often suggested that color purity problems may go away "in a few days". It isn't a matter of time; it's the number of cold power ups that causes it. It takes about 15 minutes of the power being off for each cool down cycle. These built-in coils with thermal control are never as effective as external coils.

    Note that while the monochrome CRTs used in B/W and projection TVs and mono monitors don't have anything inside to get magnetized, the chassis or other cabinet parts of the equipment may still need degaussing. While this isn't likely from normal use or even after being moved or reoriented, a powerful magnet (like that from a large speaker) could leave iron, steel, or other ferrous parts with enough residual magnetism to cause a noticeable problem.

    See the document: TV and Monitor CRT (Picture Tube) Information for some additional discussion of degaussing tools, techniques, treatments for severe magnetization from lightning strikes, and cautions.



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    TV Placement And Preventive Maintenance

    General TV placement considerations

    Proper care of a TV does not require much. Following the recommendations below will assure long life and minimize repairs:

    Preventive maintenance

    Preventive maintenance for a TV is pretty simple - just keep the case clean and free of obstructions. Clean the screen with a soft cloth just dampened with water and at most, mild detergent. DO NOT use anything so wet that liquid may seep inside of the set around the edge of the picture tube - you could end up with a very expensive repair bill when the liquid shorts out the main circuit board lurking just below. If the set has a protective flat glass faceplate, there is usually an easy way (on newer sets with this type of protection) of removing it to get at the inner face of the CRT. Clean both the CRT and the protective glass with a soft damp cloth and dry thoroughly. If you have not cleaned the screen for quite a while, you will be amazed at the amount of black grime that collects due to the static buildup from the high voltage CRT supply.

    In really dusty situations, periodically vacuuming inside the case and the use of contact cleaner for the controls might be a good idea but realistically, you will not do this so don't worry about it.

    For LCD TVs, LCD computer monitors, and laptop displays, the cleaning is particularly critical. The front surface of these facing the viewer is generally not made of glass like those in CRT displays, but rather a plastic layer or film. Thus, any cleaning method that uses harsh chemicals can permanently damage the screen, with or without an anti-reflection coating. Some glass cleaners, acetone (nail polish remover), and other strong solvents can attack the plastic very quickly. By the time you realize there is damage, it may be too late. And, of course, NEVER use anything even mildly abrasive.

    A damp cloth with soap or detergent and water is safe, as is generally a damp clost with a solution of 70 percent isopropyl (rubbing) alcohol diluted in the ratio 1:1 with water.

    However, it is essential to avoid allowing any liguid to seep inside along the edges as this can short out the circuitry, especially the high voltage back-light driver,which often located behind the trim at the bottom, and possibly ruin the display entirely, or at least requiring a major repair.

    Warning about using a TV as a computer or video game display

    "I remember a while back (about 10 years) most home computers used to hook up to televisions. I seem to remember them having some effect on the TV though. I think they made the TV go blurry after a while. I was just wondering what these computers used to do to the televisions to mess them up like that. I thought a TV signal was a TV signal."
    The problem was screen burn. Since computers of that era were mostly text and video games tended to use fixed patterns for scenery, patterns tended to be burned into the phosphor such that they were noticeably darker and less sensitive in those areas. This was exacerbated by the tendency to run those devices at very high brightness levels.

    Modern computers and video games should not be nearly as much of a risk since the displays are so much more varied and dynamic. Nevertheless, setting the brightness at a moderate level would be prudent.

    However, projection sets with their much higher intensity CRTs may still be susceptible to screen burn and the manufacturer will likely NOT cover the cost of repairs. There is probably a disclaimer to this effect in the warranty.



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    TV Troubleshooting

    SAFETY

    TVs and computer or video monitors are among the more dangerous of consumer electronic equipment when it comes to servicing. (Microwave ovens are probably the most hazardous due to high voltage at high power.)

    There are two areas which have particularly nasty electrical dangers: the non-isolated line power supply and the CRT high voltage.

    Major parts of nearly all modern TVs and many computer monitors are directly connected to the AC line - there is no power transformer to provide the essential barrier for safety and to minimize the risk of equipment damage. In the majority of designs, the live parts of the TV or monitor are limited to the AC input and line filter, degauss circuit, bridge rectifier and main filter capacitor(s), low voltage (B+) regulator (if any), horizontal output transistor and primary side of the flyback (LOPT) transformer, and parts of the startup circuit and standby power supply. The flyback generates most of the other voltages used in the unit and provides an isolation barrier so that the signal circuits are not line connected and safer.

    Since a bridge rectifier is generally used in the power supply, both directions of the polarized plug result in dangerous conditions and an isolation transformer really should be used - to protect you, your test equipment, and the TV, from serious damage. Some TVs do not have any isolation barrier whatsoever - the entire chassis is live. These are particularly nasty.

    The high voltage to the CRT, while 200 times greater than the line input, is not nearly as dangerous for several reasons. First, it is present in a very limited area of the TV or monitor - from the output of the flyback to the CRT anode via the fat HV wire and suction cup connector. If you don't need to remove the mainboard or replace the flyback or CRT, then leave it alone and it should not bite. Furthermore, while the shock from the HV can be quite painful due to the capacitance of the CRT envelope, it is not nearly as likely to be lethal since the current available from the line connected power supply is much greater.

    Safety guidelines

    These guidelines are to protect you from potentially deadly electrical shock hazards as well as the equipment from accidental damage.

    Note that the danger to you is not only in your body providing a conducting path, particularly through your heart. Any involuntary muscle contractions caused by a shock, while perhaps harmless in themselves, may cause collateral damage - there are many sharp edges inside this type of equipment as well as other electrically live parts you may contact accidentally.

    The purpose of this set of guidelines is not to frighten you but rather to make you aware of the appropriate precautions. Repair of TVs, monitors, microwave ovens, and other consumer and industrial equipment can be both rewarding and economical. Just be sure that it is also safe!

    Warning about disconnecting CRT neck board

    Some manufacturers warn against powering a TV or monitor CRT without the CRT neck board connected. Apparently, without something - anything - to drain the charge resulting from the current flow due to residual gas ions inside the CRT, the shortest path may be through the glass neck of the tube to the yoke or from the pins outside the CRT to whatever is nearby. There aren't many ions in a modern CRT but I suppose a few here, a few there, and eventually they add up to enough to cause a major disaster at least on some CRTs.

    This is probably not a problem on small CRTs but for large ones with high high voltages and high deflection angles where the glass of the neck is very thin to allow for maximum deflection sensitivity, the potential does exist for arcing through the glass to the yoke to occur, destroying the CRT.

    There is really no way to know which models will self destruct but it should be possible to avoid such a disaster by providing a temporary return path to the DAG ground of the CRT (NOT SIGNAL GROUND!!) via the focus or G2 pins preferably through a high value high voltage rated resistor just in case one of these is shorted.

    This probably applies mostly to large direct-view TVs since they use high deflection angle CRTs but it won't hurt to take appropriate precautions with video and computer monitors as well.

    Troubleshooting tips

    Many problems have simple solutions. Don't immediately assume that your problem is some combination of esoteric complex convoluted failures. For a TV, it may just be a bad connection or blown fuse. Remember that the problems with the most catastrophic impact on operation like a dead TV usually have the simplest solutions. The kind of problems we would like to avoid at all costs are the ones that are intermittent or difficult to reproduce: the occasional interference or a TV that refuses to play 'StarTrek Voyager'.

    If you get stuck, sleep on it. Sometimes, just letting the problem bounce around in your head will lead to a different more successful approach or solution. Don't work when you are really tired - it is both dangerous (especially with respect to TVs) and mostly non-productive (or possibly destructive).

    Whenever working on precision equipment, make copious notes and diagrams. You will be eternally grateful when the time comes to reassemble the unit. Most connectors are keyed against incorrect insertion or interchange of cables, but not always. Apparently identical screws may be of differing lengths or have slightly different thread types. Little parts may fit in more than one place or orientation. Etc. Etc.

    Pill bottles, film canisters, and plastic ice cube trays come in handy for sorting and storing screws and other small parts after disassembly. This is particularly true if you have repairs on multiple pieces of equipment under way simultaneously.

    Select a work area which is wide open, well lighted, and where dropped parts can be located - not on a deep pile shag rug. The best location will also be relatively dust free and allow you to suspend your troubleshooting to eat or sleep or think without having to pile everything into a cardboard box for storage.

    Another consideration is ESD - Electro-Static Discharge. Some components (like ICs) in a TV are vulnerable to ESD. There is no need to go overboard but taking reasonable precautions such as getting into the habit of touching a **safe** ground point first.

    WARNING: even with an isolation transformer, a live chassis should **not** be considered a safe ground point. When the set is unplugged, the tuner shield or other signal ground points should be safe and effective.

    A basic set of precision hand tools will be all you need to disassemble a TV and perform most adjustments. These do not need to be really expensive but poor quality tools are worse than useless and can cause damage. Needed tools include a selection of Philips and straight blade screwdrivers, socket drivers, needlenose pliers, wire cutters, tweezers, and dental picks. For adjustments, a miniature (1/16" blade) screwdriver with a non-metallic tip is desirable both to prevent the presence of metal from altering the electrical properties of the circuit and to minimize the possibility of shorting something from accidental contact with the circuitry. A set of plastic alignment tools will be useful for making adjustments to coils and RF transformers.

    A low power (e.g., 25 W) fine tip soldering iron and fine rosin core solder will be needed if you should need to disconnect any soldered wires (on purpose or by accident) or replace soldered components. A higher power iron or small soldering gun will be needed for dealing with larger components.

    CAUTION: You can easily turn a simple repair (e.g., bad solder connections) into an expensive mess if you use inappropriate soldering equipment and/or lack the soldering skills to go along with it. If in doubt, find someone else to do the soldering or at least practice, practice, practice, soldering and desoldering on a junk circuit board first! See the document: Troubleshooting and Repair of Consumer Electronic Equipment for additional info on soldering and rework techniques.

    For thermal or warmup problems, a can of 'cold spray' or 'circuit chiller' (they are the same) and a heat gun or blow dryer come in handy to identify components whose characteristics may be drifting with temperature. Using the extension tube of the spray can or making a cardboard nozzle for the heat gun can provide very precise control of which components you are affecting.

    For info on useful chemicals, adhesives, and lubricants, see "Repair Briefs, an Introduction" as well as other documents available at this site.

    Test equipment

    Don't start with the electronic test equipment, start with some analytical thinking. Your powers of observation (and a little experience) will make a good start. Your built in senses and that stuff between your ears represents the most important test equipment you have.

    However, some test equipment will be needed:

    Incredibly Handy widgets

    These are the little gadgets and homemade testers that are useful for many repair situations. Here are just a few of the most basic:

    CAUTION: Keep any large transformer of this type well away from your monitor or TV. The magnetic field it produces may cause the picture to wiggle or the colors to become messed up - and you to think there is an additional problem!

    Safe discharging of capacitors in TVs and video monitors

    It is essential - for your safety and to prevent damage to the device under test as well as your test equipment - that large or high voltage capacitors be fully discharged before measurements are made, soldering is attempted, or the circuitry is touched in any way. Some of the large filter capacitors commonly found in line operated equipment store a potentially lethal charge.

    This doesn't mean that every one of the 250 capacitors in your TV need to be discharged every time you power off and want to make a measurement. However, the large main filter capacitors and other capacitors in the power supplies should be checked and discharged if any significant voltage is found after powering off (or before any testing - the CRT capacitance in a TV or video monitor, for example, can retain a dangerous or at least painful charge for days or longer!)

    The technique I recommend is to use a high wattage resistor of about 5 to 50 ohms/V of the working voltage of the capacitor. This isn't critical - a bit more or less will be fine but will affect the time it takes to fully discharge the capacitor. The use of a current limiting resistor will prevent the arc-welding associated with screwdriver discharge but will have a short enough time constant so that the capacitor will drop to a low voltage in at most a few seconds (dependent of course on the RC time constant and its original voltage).

    Then check with a voltmeter to be double sure. Better yet, monitor while discharging (not needed for the CRT - discharge is nearly instantaneous even with multi-M ohm resistor).

    Obviously, make sure that you are well insulated!

    Again, always double check with a reliable voltmeter!

    Reasons to use a resistor and not a screwdriver to discharge capacitors:

    1. It will not destroy screwdrivers and capacitor terminals.

    2. It will not damage the capacitor (due to the current pulse).

    3. It will reduce your spouse's stress level in not having to hear those scary snaps and crackles.

    Additional information on discharging CRTs

    You may hear that it is only safe to discharge from the Ultor to the Dag. So, what the @#$% are they talking about? :-).

    (From: Asimov (mike.ross@juxta.mnet.pubnix.ten).)

    'Dag' is short for Aquadag. It is a type of paint made of a graphite pigment which is conductive. It is painted onto the inside and outside of picture tubes to form the 2 plates of a high voltage filter capacitor using the glass in between as dielectric. This capacitor is between .005uF and .01uF in value. This seems like very little capacity but it can store a substantial charge with 25,000 volts applied.

    The outside "Dag" is always connected to the circuit chassis ground via a series of springs, clips, and wires around the picture tube. The high voltage or "Ultor" terminal must be discharged to chassis ground before working on the circuit especially with older TV's which didn't use a voltage divider to derive the focus potential or newer TV's with a defective open divider.

    CAUTION: The Dag coating/springs/clips/etc. may not be the same as signal ground on the mainboard. Discharging to that instead could result in all sorts of expensive blown components. Discharging between the CRT anode cap and Dag should be low risk though it is best to use a HV probe or properly rated high value resistor.

    For more details, see the document: TV and Monitor CRT (Picture Tube) Information.

    Removing the CRT HV connector

    WARNING: Make sure the CRT has been discharged FIRST!

    The rubber part is usually not glued down so it can be lifted rather easily. However, there may be some silicone type grease between the rubber boot (that looks like a suction cup) and the CRT glass to seal out dust.

    A metal clip with a spring keeping it spread out attaches inside the button.

    While there are a variety of types of clips actually used, pushing the connector to one side and/or squeezing it in the appropriate direction (peel up one side of the rubber to inspect) while gently lifting up should free it. Probably :-).

    The clip (when removed) and CRT button look sort of like this:

    
                          ||======= HV Cable
                          /\
                  Clip   |  |
             (Removed)  _|  |_
                                  (No DAG coating in vicinity of HV connector)
           ____________.-    -.___________
      CRT  ____________|______|___________ Glass
                     Metal Button
    

    Replacement is done in reverse order!

    This isn't rocket science and excessive force should not be needed! :-)

    Safe troubleshooting techniques for line powered TVs

    TVs are particularly dangerous with respect to troubleshooting due to the fact that a substantial portion of their circuitry - sometimes all of it - is directly line connected. Even if your are working in a totally unrelated area like the sound circuits, awareness of the general design and location of the line-connected circuits can prove to be a life saver.

    These designs may take several forms:

    1. Separate switchmode power supply (SMPS): In this case, only the primary side of the power supply is line connected. The remainder of the TV is usually isolated from the line by the high frequency transformer and feedback device (transformer or optoisolator) of the switchmode power supply.

    2. On-board SMPS: A portion of the circuitry on the mainboard is directly line-connected. In the best case, this is restricted to the area around the power cord connections and well marked on both top and bottom but don't count on it. Again, the rest of the TV may be isolated but avoiding hazardous areas is more difficult especially in cramped quarters.

    3. Flyback derived power supply: A non-isolated linear (usually) power supply provides B+ to the horizontal deflection (and startup circuit). All other system power is derived from secondary windings on the flyback transformer. Similar comments to (2) above apply.

    (1) to (3) may be found in TVs with A/V inputs and outputs.

    1. Full hot chassis: A bridge rectifier/filter capacitor/linear regulator provides some voltages including B+. The flyback secondaries provide the remaining voltages. All share a common return which is at the intersection of two of the diodes of the bridge rectifier. There is no isolation.

      This type of design will usually not be found in a TV where there are external connections (other than the RF antenna/cable connector which can be capacitively isolated and you may actually get an AC reading or even sparks between the RF shield and an earth ground due to this capacitance.)

      WARNING: Never attempt to add A/V inputs or outputs to such a TV as the signals and shields will be electrically live.

      However, some TVs with A/V inputs/outputs actually had a live chassis and used an isolated means of coupling the signals from/to the external jacks:

      (From: Bill Coffel (bc@datamix.com).)

      The late 70's and early 80's Sony CVM 1250/1750/2150 (12"/17"/21") monitors (TV) have a HOT chassis. In fact they are kV-1201/1701/2101 TV sets in larger cabinet With a 3 prong plug!!!! The inputs and outputs are isolated via opto couplers and transformers on an additional circuit board (about 6" by 8" and powered by a small transformer) the connector panel is the only part grounded via the third prong.

      If someone thinks its not a live chassis they are in for quite a shock.

    Always use an isolation transformer, whatever kind of design is used in the equipment you are troubleshooting. There are very few situations in which an isolation transformer will hurt. If you use it automatically, you will never have a chance to screw up.

    Identify the appropriate ground point (return) for your multimeter or scope. These should be marked in the Sams' Photofact or service manual. There may be several such returns such as: non-isolated, signal, and CRT. Selecting the wrong one - even momentarily connecting to it - can ruin your whole day.

    If you are not using an isolation transformer (a no-no), connecting your scope to the wrong ground point can result in (1) blown fuses and/or blown parts, and a very dangerous situation and (2) readings that don't make sense generally with distorted power line frequency signals of high amplitude.

  • Use the non-isolated ground (A) (with your isolation transformer on the TV *only* for measurements of voltage on the line-connected power supply.

  • Use the signal ground (B) for all measurements of tuner, IF, video, and sound circuits.

    Whenever you get a reading or waveform that is grossly wrong, confirm that you are using the proper ground point! Note that failures of fusable resistors in the *return* of the HOT or power supply chopper or elsewhere can also result in points that should be near ground floating at unexpected voltage levels.

    The general arrangement of components for a typical TV using a linear B+ supply with isolated auxiliary supplies for the signal circuits is shown below including the (linear) line-connected power supply, horizontal deflection output (drive, horizontal output transistor, flyback), and a typical Aux power supply output.

    
        Line fuse   Main bridge                            Part of flyback
              _      rectifier    +----------+  B+           transformer
        H o--_ --+------|>|---+---|          |-----------------+            Aux 1
                 |            |   | Filter,  |                  ):: +--|>|--+--o
                 |  +---|>|---+   | REG, etc.|                  )::(       _|_
      115 VAC    |  |             |          |                  )::(       ---
                 +--|---|<|---+   +----------+             +---+ ::(        |
                    |         |         |     H-drive      |        +-------+ B
     +-> N o---------+---|<|---+---------+   transformer  |/ C             __|__
     |                                A _|_       || +----|    Horizontal   -_-
     +-> G - Power line earth ground   ///        ||(     |\ E Output      Signal
            via building wiring         ^        ||(       |  Transistor  ground
                                        |        || +------+  (HOT)
                                        '               A _|_
                                 Non-isolated return --> /// 
                                  (connected points)
    
    

    For this power supply, what if?:

    1. You connect your scope ground clip to the non-isolated ground (A) and you are *not* using an isolation transformer?

      Answer: you blow the line fuse and/or melt your scope probe ground lead. Other parts may be damaged as well. In effect, you have just shorted across the bottom diode of the bridge.

    2. You attempt to monitor a video signal with your scope ground connected to the non-isolated ground (A)?

      Answer: you see only a highly distorted power line waveform of roughly 100 V p-p In effect, you are measuring across one of the diodes of the bridge rectifier, stray capacitance, etc.

    The series light bulb trick

    When powering up a TV (or any other modern electronic devices with expensive power semiconductors) that has had work done on any power circuits, it is desirable to minimize the chance of blowing your newly installed parts should there still be a fault. There are two ways of doing this: use of a Variac to bring up the AC line voltage gradually and the use of a series load to limit current to power semiconductors.

    Actually using a series load - a light bulb is just a readily available cheap load - is better than a Variac (well both might be better still) since it will limit current to (hopefully) non-destructive levels.

    What you want to do is limit current to the critical parts - usually the horizontal output transistor (HOT). Most of the time you will get away with putting it in series with the AC line. However, sometimes, putting a light bulb directly in the B+ circuit will be needed to provide adequate protection. In that location, it will limit the current to the HOT from the main filter capacitors of line connected power supplies. This may also be required with some switchmode power supplies as they can still supply bursts of full (or excessive) current even if there is a light bulb in series with the AC line.

    Actually, an actual power resistor is probably better as its resistance is constant as opposed to a light bulb which will vary by 1:10 from cold to hot. The light bulb, however, provides a nice visual indication of the current drawn by the circuit under test. For example:

    Note: for a TV or monitor, it may be necessary (and desirable) to unplug the degauss coil as this represents a heavy initial load which may prevent the unit from starting up with the light bulb in the circuit.

    The following are suggested starting wattages:

    A 50/100/150 W (or similar) 3-way bulb in an appropriate socket comes in handy for this but mark the switch so that you know which setting is which!

    Depending on the power rating of the equipment, these wattages may need to be increased. However, start low. If the bulb lights at full brightness, you know there is still a major fault. If it flickers or the TV (or other device) does not quite come fully up, then it should be safe to go to a larger bulb. Resist the temptation to immediately remove the series light bulb totally from the circuit at this point - I have been screwed by doing this. Try a larger one first. The behavior should improve. If it does not, there is still a fault present.

    Note that some TVs and monitors simply will not power up at all with any kind of series load - at least not with one small enough (in terms of wattage) to provide any real protection. The microcontroller apparently senses the drop in voltage and shuts the unit down or continuously cycles power. Fortunately, these seem to be the exceptions.

    Getting inside a TV

    You will void the warranty - at least in principle. There are usually no warranty seals on a TV so unless you cause visible damage or mangle the screws, it is unlikely that this would be detected. You need to decide. A TV still under warranty should probably be returned for warranty service for any covered problems except those with the most obvious and easy solutions. Another advantage of using warranty service is that should your problem actually be covered by a design change, this will be performed free of charge. And, you cannot generally fix a problem which is due to poor design!

    Getting into a TV is usually quite simple requiring the removal of anywhere from 4 to 16 Philips or 1/4" hex head screws - most around the rear edge of the cabinet or underneath, a couple perhaps in the middle. Disconnect the antenna and/or antenna or cable wiring first as it may stay with catch on the rear cover you are detaching. Reconnect whatever is needed for testing after the cover is removed.

    As you pull the cover straight back (usually) and off, make sure that no other wires are still attached. Often, the main circuit board rests on the bottom of the cover in some slots. Go slow as this circuit board may try to come along with the back. Once the back is off, you may need to prop the circuit board up with a block of wood to prevent stress damage and contact with the work surface.

    Most TVs can still be positioned stably on any of three sides (left, right, bottom) even without the rear cover. However, some require the cover for mechanical strength or to not easily fall over. Be careful- larger TVs, in particular, are quite heavy and bulky. Get someone to help and take precautions if yours is one of the unstable variety. If need be, the set can usually safely be positioned on the CRT face if it is supported by foam or a folded blanket.

    Reassemble in reverse order. Getting the circuit board to slide smoothly into its slots may take a couple of attempts but otherwise there should be no surprises.

    Specific considerations before poking around inside a TV or monitor

    Both electrical and mechanical dangers lurk:

    Dusting out the inside of a TV

    The first thing you will notice when you remove the cover is how super dusty everything is. Complements to the maid. You never dreamed there was that much dust, dirt, and grime, in the entire house!

    Use a soft brush (like a new paintbrush) and a vacuum cleaner to carefully remove the built up dust. Blowing off the dust will likely not hurt the TV unless it gets redeposited inside various controls or switches but will be bad for your lungs - and will spread it all over the room. Don't turn anything - many critical adjustments masquerade as screws that just beg to be tightened. Resist the impulse for being neat and tidy until you know exactly what you are doing. Be especially careful around the components on the neck of the CRT - picture tube - as some of these are easily shifted in position and control the most dreaded of adjustments - for color purity and convergence. In particular, there will be a series of adjustable ring magnets. It is a good idea to mark their position in any case with some white paint, 'white out', or a Magic Marker so that if they do get moved - or you move them deliberately, you will know where you started.

    Troubleshooting a TV with the mainboard disconnected

    There are times when it is desirable to remove the chassis or mainboard and work on it in a convenient location without having to worry about the equipment which will simulate the critical functions but this is rarely an option for the doit-yourselfer.

    My approach is usually to do as much work as possible without removing the main board and not attempt to power it up when disconnected since there are too many unknowns. Professionals will plug the chassis into a piece of equipment which will simulate the critical functions.

    Note that if you have a failure of the power supply - blown fuse, startup, etc., then it should be fine to disconnect the CRT since these problems are usually totally unrelated. Tests should be valid.

    However, if you really want to do live testing with the main board removed, here are some considerations. There are usually several connections to the CRT and cabinet:

    If you do disconnect everything, make sure to label any connectors whose location or orientation may be ambiguous. Most of the time, these will only fit one way but not always.



  • Back to TV Repair FAQ Table of Contents.

    TV Adjustments

    These include both controls accessible to the user (and often not understood) as well as internal adjustments that may need to be touched up due to the aging of components or following a repair.

    User picture adjustment

    For general viewing, subdued lighting but not total darkness is probably best. However, for most dramatic impact, a darkened environment may be preferred. Make the following adjustments under the expected viewing conditions.

    Tune to a strong channel or play a good quality tape.

    Turn the brightness, contrast, and color controls all the way down. Center the tint control (NTSC, may not be present on PAL sets).

    Increase the brightness until a raster is just visible in the darkest (shadow) areas of the picture and then back off until it **just** disappears.

    Increase the contrast until the desired intensity of highlights is obtained.

    Since brightness and contrast are not always independent, go back and forth until you get the best picture.

    Initially adjust the color control for pastel shades rather than highly saturated color. Set the tint control for best flesh tones. Then, increase the color control to obtain the desired degree of color saturation.

    Internal adjustments

    All of the service adjustments are accomplished either using controls inside the set (though some may be accessible by holes in the rear of the cabinet). These are usually pots on the mainboard and CRT neck boards, or in newer TVs, mostly via a service menu accessed from the remote or by using a manufacturer specific computer interface.

    A Web site with some information on the general objectives of video and color setup procedures for both direct view and projection TVs is Tru-line Video Technologies.

    Focus adjustment

    On a decent TV, you should be able to make out the individual scanning lines. If they are fuzzy, especially in bright areas, then focus may need to be adjusted.

    The focus pot is usually located on the flyback transformer or on an auxiliary panel nearby. Where there are two adjustment knobs on the flyback transformer, the top one is generally for focus and the bottom one is for G2.

    The focus wire usually comes from the flyback or the general area or from a terminal on a voltage the multiplier module (if used). It is usually a wire by itself going to the little board on the neck of the CRT.

    Let the set warm up for at least half an hour. Display a good quality signal. Turn the user color control all the way down and the brightness and contrast controls all the way up. This will be the worst case. Adjust the focus control for best overall sharpness - you may not be able to get it perfect everywhere - center as well as corners. If best focus is at one end of the focus pot's range and still not good enough, there may be a problem in the focus divider, focus pot, or some related component.

    Adjustment of the internal SCREEN and color controls

    The screen should be adjusted with a white pattern (snow from the tuner should do or turn the user COLOR control all the way down to get a black and white picture). Put the set in Service mode (horizontal line) if it has such a switch in the back or inside. If not, just use the raster in a darkened room. Adjust screen for a dim white line (raster). If the line is not white at its dimmest point, you will need to adjust the drive and cutoff controls for R, G, & B.

    Alternatively, you can use the following procedure:

    Turn R, G, and B screen (or background) controls down. Now turn color control fully counterclockwise -- off. Now turn up red screen until the screen just shows a red hue. Now turn red gun down until red tint just goes away. Now do the same with the green and blue screen controls. Now adjust the two DRIVE controls for the best black and white picture. That`s all there is to it. I don`t like to work with just a thin "SETUP" line. Cartoons seem to be the best thing to have on while doing the above procedure. You can also use just plain snow (no program) if you prefer. If you can obtain a good b@w pic. when you`re done, the tube is good and the set if most likely functioning properly. Be patient and go slow while watching the large mirror that you are using during this procedure. (LEE)

    Optimal procedure for setting brightness/background and screen adjustments

    For slight tweaks, the following is not necessary. However, if someone turned all the internal controls or if you are making significant changes that affect G2 (screen), then following the procedure below is desirable to achieve best performance and maximize life of the CRT.

    The typical user controls - brightness and contrast can, of course, be set arbitrarily, depending on video content and ambient lighting conditions.

    Set the user brightness and contrast controls in the middle for the following adjustments and let the set warm up for 20 minutes or so.

    (From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).)

    Now the screen control, that's another matter. It sets the voltage on the second grid of the electron guns, typically between +500 and +1000 V. You will want to use a well-isolated screwdriver for that if it is a naked potentiometer. In the old days there used to be 3 separate potentiometers for 3 G2s, now there is generally only one.

    Its purpose is to set the cutoff voltage for the guns, i.e. the voltage between K and G1 at which the beam is just off. The higher you set the VG2, the higher VK - VG1 must be to cut off the beam.

    If you set VG2 too low then your picture will be dark. You can compensate for that with the brightness control, which in effect will lower the VKs. A disadvantage is that you will not get optimum sharpness and peak brightness from your picture tube.

    If you set VG2 too high then your picture will be bright. You can compensate for that with the brightness control, which in effect will raise the VKs. You might even get retrace lines which can usually not be made to disappear with the brightness control. Another disadvantage is that you will not get optimum LIFETIME from your picture tube. With a too high cutoff voltage the cathode (electron emitting surface) will wear out too soon.

    You will need to see the picture tube specifications (or possibly the Sams' Photofact or service manual for the set --- sam) in order to find the correct setting for the cutoff voltage. This is measured as VK - VG1 (for each channel RGB) and is typically 130-160 V max. There will be spread between the 3 channels, typically the highest of the 3 measured values will be set against the upper limit.

    The usual adjustment procedure is as follows:

    Color balance adjustment

    Color balance needs adjustment if the highlights and/or shadows of a black and white picture (turn the color control all the way down) are not a perfectly neutral gray.

    Note: Some TV designs (Zenith uses this in a few models) automatically balance CRT cathode drive by sensing emission from the red, green, and blue guns using a gray scale reference pulse outside the viewable picture. If this is the case with your set, there may be no user OR service adjustments :-(. A color balance problem in this case means either a failure of this circuitry or a CRT where the emission from the 3 cathodes is so unbalanced (usually due to one being much much weaker than the others) that compensation is not possible.

    To adjust the color balance: Turn the color control all the way down so that you get what should be a B/W picture. Set the user brightness and contrast controls about mid-range. The tint control should not matter (if it does at this point, you have other chroma problems or an 'autocolor' switch is on limiting the range of some controls).

    Adjust the sub-brightness controls (may be called color screen, background, or the like) so that the dark areas of the picture are just visible and neutral gray. Then, adjust the color gain controls until the brightest areas are neutral white but not so bright that there is 'color bleeding' in the highlights.

    This should get you close. If something is still shifting after warmup and get some cold-spray or even a little blower and try to locate the component that is drifting. Most likely a transistor or capacitor.

    More on 'Calibrating' TV color

    If you don't know how to access the color and tint controls on your TV and are happy with green flesh tones and cartoon colors, you can skip this section. Most people can adjust their color and tint controls resulting in a reasonable hue and saturation. However, here are some ways of getting closer to perfection.

    (From: Chris Johnson (wjohnson@palmnet.net).)

    If you have access to a DVD player, get the 'Video Essentials' DVD and follow the directions.

    If you have access to a LaserDisc player, get 'A Video Standard' and follow the directions.

    But, here are a few quick pointers:

    Back off the color control all the way on a program. Set the color balance for a true grey appearance without any bias toward red, green, or blue. (Or any other color.)

    Max brightness should not be high enough that vertical lines (like the needle pulse on the test LD or DVD) don't bend.

    On the test pattern with the multicolored bars, the large bars should have the following colors: white (actually grey, but fairly light), yellow, cyan, green, purple, red, blue.

    The purpose of the narrow bars just below the large bars is to match up the tint balance. You do this with a blue filter, or killing the R and G guns. The narrow bars will be exactly the same intensity as the large bars above them, if the color and tint balance is correct.

    Here's a quick trick if you're lucky enough to have a set with separate gun killer switches for R, G, and B guns: When properly set, the narrow bars will be the same intensity as the large bars above them, if you only turn one gun on at a time. First do red, then green, then blue. Go through the process once and you'll never forget what it looks like.

    Last point: The pluge bar (in the lower right section of the color bar screen) should NOT be visible, being ten percent below black level. If you can see it, back off the brightness.

    Remember:

    Most people set the color too high. News programs should NOT be as colorful as the movie 'Jingle All The Way'. (Which, by the way, is a real test of your TV's abilities.) Most people's faces should not be red.

    You know you have it all set right when black clothing on people on TV has texture and depth to it, and also when purple and blue are two distinct colors. Some TV sets don't do that very well as shipped.

    Horizontal position, size, and linearity adjustment

    Horizontal position may be set via a switch or jumper, a pot, or (mostly in B/W TVs) a set of rings on the CRT neck.

    Horizontal size should be set so that there is about 10-15 percent overscan left and right. This will allow ample margin for power line voltage fluctuations, component aging, and the reduction in raster size that may occur with some VCR special effects (fast play) modes.

    Many sets no longer have any horizontal size adjustments and depend on accurate regulation of the voltage to the horizontal output stage to control horizontal size. There may be a B+ adjustment to perform first.

    On those that do, the adjustment may either be done by setting the B+ voltage, by a pot, or a width coil in series with the horizontal deflection coils.

    Modern sets do not generally have any linearity control but you may find this on older models. You will need to go back and forth between size and linearity as these adjustments are usually not independent.

    Some of the newest sets control all these parameters via settings in non-volatile memory and use service menus accessed via the remote control for nearly all setup adjustments.

    Vertical position, size, and linearity adjustment

    Vertical position may be set via a switch or jumper, a pot, or (mostly in B/W TVs) a set of rings on the CRT neck.

    Vertical size should be set so that there is about 10-15 percent overscan top and bottom. This will allow ample margin for power line voltage fluctuations, component aging, and the reduction in raster size that may occur with some VCR special effects (fast play) modes.

    Some sets no longer have any vertical size adjustments and depend on the accurate regulation of the voltage to the vertical output stage to control vertical size.

    On those that do, the adjustment is usually a pot in the vertical output circuitry. If your set has a linearity control, you will need to adjust this in conjunction with the size control as these are usually not independent.

    Some of the newest sets control all these parameters via settings in non-volatile memory and use service menus accessed via the remote control for nearly all setup.

    Pincushion adjustments

    There may be two controls - amplitude and phase. Pincushion amplitude as its name implies, controls the size of the correction. Pincushion phase affects where on the sides it is applied. Don't expect perfection.

    If the controls have no effect, there is probably a fault in the pincushion correction circuitry.

    It is best to make these adjustments with a crosshatch or dot test pattern

    Geometry adjustment

    This refers to imperfections in the shape of the picture not handled by the pincushion and size adjustments. These types of defects include trapezoidal or keystone shaped raster and jogs or wiggles around the periphery of the raster. Unfortunately, one way these are handled at the factory is to glue little magnets to strategic locations on the CRT and/or rotate little magnets mounted on the yoke frame. Unless you really cannot live with the way it is (assuming there isn't something actually broken), leave these alone! You can end up with worse problems. In any case, carefully mark the position AND orientation of every magnet so that if this happens, you can get back to where you started. If the magnets are on little swivels, some experimenting with them one at a time may result in some improvement. Of course, it is best to obtain a service manual and follow its instructions.

    Why is the convergence on my set bad near the edges

    Very simple - nothing is quite perfect. Perfect convergence is not even necessarily possible in theory with the set of adjustments available on a typical TV. It is all a matter of compromises. Consider what you are trying to do: get three electron beams which originate from different electron guns to meet at a single point within a fraction of a mm everywhere on the screen. This while the beams are scanning at an effective writing rate of 20,000 mph across the face of a 27" CRT in a variable magnetic environment manufactured at a price you can afford without a second mortgage!

    CRT purity and convergence problems

    Purity assures that each of the beams for the 3 primary colors - red, green, and blue - strikes only the proper phosphor dots for that color. A totally red scene will appear pure red and so forth. Symptoms of poor purity are blotches of discoloration on the screen. Objects will change shades of color when the move from one part of the screen to another.

    Convergence refers to the control of the instantaneous positions of the red, green, and blue spots as they scan across the face of the CRT so that they are as nearly coincident as possible. Symptoms of poor convergence are colored borders on solid objects or visible separate R, G, and B images of fine lines or images,

    Note: It is probably best to face the set East-West (front-to-back) when performing any purity and convergence adjustments. Since you probably do not know what orientation will eventually be used, this is the best compromise as the earth's magnetic field will be aligned mostly across the CRT. This will minimize the possible rotation of the picture when the unit is moved to its final position but there may be a position shift. Neither of these is that significant so it probably doesn't really matter that much unless you are super fussy. Of course, if you know the final orientation of the TV in your entertainment center - and you don't expect to be redecorating, use that instead. Or, plan to do the final tilt and position adjustments after the set is in position - but this will probably require access to the inside!

    First, make sure no sources of strong magnetic fields are in the vicinity of the TV - loudspeakers, refrigerator magnets, MRI scanners, etc. A nearby lightning strike or EMP from a nuclear explosion can also affect purity.

    Cycle power a couple of times to degauss the CRT (1 minute on, 20 minutes off) - see the section: Degaussing (demagnetizing) a CRT. If the built in degaussing circuits have no effect, use an external manual degaussing coil.

    Assuming this doesn't help, you will need to set the internal purity and/or convergence adjustments on the CRT. Modern CRTs usually use a combination of a series of magnetized moveable rings on the neck, and yoke position and orientation to set purity and convergence.

    First, mark the positions of all adjustments - use white paint, 'White out', or a Magic Marker on the ring magnets on the neck of the CRT, the position and tilt of the deflection yoke, and any other controls that you may touch deliberately or by accident.

    However, if your set is still of the type with a drawer or panel of knobs for these adjustments, don't even think about doing anything without a service manual and follow it to the letter unless the functions of all the knobs is clearly marked (some manufacturers actually do a pretty good job of this).

    Note: some CRTs do not have any adjustable rings for purity (and static convergence). Either an internal structure in the neck of the CRT or an external 'permalloy' sleeve is permanently magnetized at the factory and there is not way of tweaking it in the field. However, it may be possible to use a normal set of magnet rings in addition to or in place of it to correct for purity or convergence problems due to loss of magnetism due to age or someone waving a 10 pound magnet near the CRT neck!

    CRT purity adjustment

    Purity on modern CRTs is usually set by a combination of a set of ring magnets just behind the deflection yoke on the neck of the CRT and the position of the yoke fore-aft. As always, mark the starting position of all the rings and make sure you are adjusting the correct set if rings!

    Use the following purity adjustment procedure as a general guide only. Depending on the particular model TV, the following purity adjustment procedure may substitute green for red depending on the arrangement of the guns in the CRT. This description is based on the Sams' Photofact for the RCA CTC111C chassis which uses a slot-mask CRT. The procedures for dot-mask and Trinitron (aperture grille) CRTs will vary slightly. See you service manual!

    Obtain a white raster (sometimes there is a test point that can be grounded to force this). Then, turn down the bias controls for blue and green so that you have a pure red raster. Let the set warm up for a minimum of 15 minutes.

    Loosen the deflection yoke clamp and move the yoke as far back as it will go,

    Adjust the purity magnets to center the red vertical raster on the screen.

    Move the yoke forward until you have the best overall red purity.

    Now, move the yoke forward until you have the best overall red purity. Tighten the clamp securely and reinstall the rubber wedges (if you set has these) to stabilize the yoke position. Reset the video adjustments you touched to get a red raster.

    CRT convergence adjustment

    In the good old days when TVs were TVs (and not just a picture tube with a little circuit board attached) there were literally drawers full of knobs for setting convergence. One could spend hours and still end up with a less than satisfactory picture. As the technology progressed, the number of electronic adjustments went down drastically so that today there are very few if any.

    Unless you want a lot of frustration, I would recommend not messing with convergence. You could end up a lot worse. I have no idea what is used for convergence on your set but convergence adjustments are never quite independent of one another. You could find an adjustment that fixes the problem you think you have only to discover some other area of the screen is totally screwed. In addition, there are adjustments for geometry and purity and maybe others that you may accidentally move without even knowing it until you have buttoned up the set.

    Warning: Accurately mark the original positions - sometimes you will change something that will not have an obvious effect but will be noticeable later on. So it is extremely important to be able to get back to where you started. If only red/green vertical lines are offset, then it is likely that only a single ring needs to be moved - and by just a hair. But, you may accidentally move something else!

    If you really cannot live with it, make sure you mark everything very carefully so you can get back to your current state. A service manual is essential!

    Convergence is set using a white crosshatch or dot test pattern. If you do not have a test pattern generator, any static scene (from a camcorder or previously recorded tape, for example) with a lot of fine detail will suffice. Turn the color control all the way down so you have a B/W picture.

    Static convergence sets the beams to be coincident in the exact center of the screen. This is done using a set of ring magnets behind the purity magnets on the CRT neck.

    From the Sams' for the RCA CTC111C: "adjust the center set of magnets to converge blue to green at the center of the screen. Adjust the rear set of magnets to converge red to green at the center of the screen." Your set may have a slightly different procedure.

    Dynamic convergence adjusts for coincidence at the edges and corners.

    On old tube, hybrid, and early solid state TVs, dynamic convergence was accomplished with electronic adjustments of which there may have been a dozen or more that were not independent. With modern sets, all convergence is done with magnet rings on the neck of the CRT, magnets glued to the CRT, and by tilting the deflection yoke. The clamp in conjunction with rubber wedges or set screws assures that the yoke remains in position.

    From the Sams' for the RCA CTC111C: "Loosen the screws at the 6 o'clock and 10 o'clock positions to permit the yoke to be tilted vertically. Rock yoke up and down to converge the right and left sides of the screen. Tighten screw at 6 o'clock and loosen screw at 3 o'clock to permit the yoke to be tilted horizontally. Rock yoke from side to side to converge the top and bottom of the screen. Tighten screws at 3 o'clock and 10 o'clock."

    Many sets simply use the main clamp which locks the yoke to the neck of the CRT in conjunction with rubber wedges between the yoke and the funnel of the CRT to stabilize the yoke position position.

    Refer to your service manual. (Is this beginning to sound repetitious?)

    For additional comments on convergence adjustments, see the sections: "Tony's notes on setting convergence on delta gun CRTs" and "Saga and General setup for large CRT TVs".

    Tilted picture

    You have just noticed that the picture on your fancy (or cheap) TV is not quite horizontal - not aligned with the front bezel. Note that often there is some keystoning as well where the top and bottom or left and right edges of the picture are not quite parallel - which you may never have noticed until now. Since this may not be correctable, adjusting tilt may represent a compromise at best between top/bottom or left/right alignment of the picture edges. You may never sleep again knowing that your TV picture is not perfect! BTW, I can sympathize with your unhappiness. Nothing is more annoying than a just noticeable imperfection such as this. However, since TVs always overscan, the only time you will really notice a minor tilt without going out of your way to look for it is if there is text or graphics near the edge of the screen.

    There are several possible causes for a tilted picture:

    1. Set orientation. The horizontal component of the earth's magnetic field affects this slgithly. Therefore, if you rotate the TV you may be able to correct the tilt. Of course, it will probably want to face the wall!

      Other external magnetic fields can sometimes cause a rotation without any other obvious effects - have you changed the TV's location? Did an MRI scanner move in next door?

    2. Need for degaussing. Most of the time, magnetization of the CRT will result in color problems which will be far more obvious than a slight rotation. However, internal or external shields or other metal parts in the set could become magnetized resulting a tilt. More extensive treatment than provided by the built-in degaussing coil may be needed. Even, the normal manual degaussing procedure may not be enough to get close enough to all the affected parts.

    3. You just became aware of it but nothing has changed. Don't dismiss this offhand. It is amazing how much we ignore unless it is brought to our attention. Are you a perfectionist?

    4. There is an external tilt control which may be misadjusted. Newer Sony monitors have this (don't know about TVs) - a most wonderful addition. Too bad about the stabilizing wires on Trinitron CRTs. A digital control may have lost its memory accidentally. The circuitry could have a problem.

    5. There is an internal tilt control that is misadjusted or not functioning. The existance of such a control is becoming more common.

    6. The deflection yoke on the CRT has gotten rotated or was not oriented correctly at the time of the set's manufacture. Sometimes, the entire yoke is glued in place in addition to being clamped adding another complication.

      If the TV was recently bumped or handled roughly, the yoke may have been knocked out of position. But in most cases, the amount of abuse required to do this with the yoke firmly clamped and/or glued would have totally destroyed the set in the process.

      There is a risk (in addition to the risk of frying yourself on the various voltages present inside an operating TV) of messing up the convergence or purity when fiddling with the yoke or anything around it since the yoke position on the neck of the tube and its tilt may affect purity and convergence. Tape any rubber wedges under the yoke securely in place as these will maintain the proper position and tilt of the yoke while you are messing with it. (Don't assume the existing tape will hold - the adhesive is probably dry and brittle).

    7. The CRT may have rotated slightly with respect to the front bezel. Irrespective of the cause of the tilt, sometimes it is possible to loosen the 4 (typical) CRT mounting screws and correct the tilt by slightly rotating the CRT. This may be easier than rotating the yoke. Just make sure to take proper safety precautions when reaching inside!

    B/W TV size, position, and geometry adjustments

    These tend to be a lot simpler and less critical than for color monitors or TV sets.

    On a B/W TV you will probably see some of the following adjustments:

    1. Position - a pair of rings with tabs on the neck of the CRT. There may be electronic position adjustements as well though this is not that common on small TVs.

    2. Width and height (possibly linearity as well) controls. There may be some interaction between size and linearity - a crosshatch test pattern is best for this. Vertical adjustments are almost always pots while horizontal (if they exist) may be pots and/or coils. Size will normally be set for 5-10% overscan to account for line voltage fluctuations and component drift. Confirm aspect ratio with test pattern which includes square boxes.

    3. Geometry - some little magnets either on swivels around the yoke or glued to the CRT. If these shifted, the the edges may have gotten messed up - wiggles, dips, concave or convex shapes. There may be a doxen or more each mostly affecting a region around the edge of the raster. However, they will not be totally independent.

    Check at extremes of brightness/contrast as there may be some slight changes in size and position due to imperfect HV regulation.

    There may be others as well but without a service manual, there is no way of knowing for sure. Sams' often has folders for B/W TVs.

    Just mark everything carefully before changing - then you will be able to get back where you started.



  • Back to TV Repair FAQ Table of Contents.

    Low Voltage Power Supply Problems

    Low voltage power supply fundamentals

    TVs require a variety of voltages (at various power levels) to function. The function of the low voltage power supply is to take the AC line input of either 115 VAC 60 Hz (220 VAC 50 Hz or other AC power in Europe and elsewhere) and produce some of these DC voltages. In all cases, the power to the horizontal output transistor of the horizontal deflection system is obtained directly from the low voltage power supply. In some cases, a variety of other DC voltages are derived directly from the AC line by rectification, filtering, and regulation. In other designs, however, most of the low voltages are derived from secondary windings on the flyback (LOPT) transformer of the horizontal deflection system. In still other designs, there is a separate switchmode power supply that provides some or all of these voltages. There are also various (and sometimes convoluted) combinations of any or all of the above.

    There will always be:

    1. A power switch, relay, or triac to enable main power.

    2. A set of rectifiers - usually in a bridge configuration - to turn the AC into DC. Small ceramic capacitors are normally placed across the diodes to reduce RF interference.

    3. One or more large filter capacitors to smooth the unregulated DC. In the U.S., this is most often a voltage around 150-160 V DC. In countries with 220 VAC power, it will typically be around 300-320 V DC.

    4. A discrete, hybrid, or IC regulator to provide stable DC to the horizontal deflection system. Sometimes feedback from a secondary output of the flyback or even the high voltage is used. This regulator may be either a linear or switching type. In some cases, there is no regulator.

    5. Zero or more voltage dividers and/or regulators to produce additional voltages directly from the line power. This relatively rare except for startup circuits. These voltages will not be isolated from the line.

    6. A degauss control circuit usually including a thermistor or Posistor (a combination of a heater disk and Positive Temperature Coefficient (PTC) thermistor in a single package). When power is turned on, a relatively high AC current is applied to the degauss coil wrapped around the periphery of the CRT. The PTC thermister heats up, increases in resistance, and smoothly decreases the current to nearly zero over a couple of seconds.

    7. A startup circuit for booting the horizontal deflection if various voltages to run the TV are derived from the flyback. This may be an IC or discrete multivibrator or something else running off a non-isolated voltage or the standby power supply.

    8. A standby power supply for the microcontroller and remote sensor. Usually, this is a separate low voltage power supply using a small power transformer for line isolation. However, some sets use other (probably cheaper) approaches. See below.

    Always use an isolation transformer when working on a TV but this is especially important - for your safety - when dealing with the non-isolated line operated power supply. Read and follow the information in the section: Safety guidelines.

    Standby power supplies

    Where the TV has a remote control (which most do nowadays), there needs to be some source of voltage(s) for the remote receiver, microcontroller, and other circuitry that watch for the 'power on' commend. These sets are never totally off.

    The standby supply may consist of:

    Typical TV power supply front end

    The partial schematic below is similar to those found in the majority of TVs sold in countries with 110 to 120 VAC power. Many parts are not shown including the power switch or relay, RFI bypass capacitors across the rectifier diodes, and RFI line filter.

    
                                             Bypass resistor
         Line fuse   Main bridge  Fusable    +----/\/\-----+
               _      rectifier   resistor   |   +-----+   |
         H o--_ --+------|>|---+---/\/\--+---+---| REG |---+---+---o B+
                  |            |         |       +-----+       |
                  |  +---|>|---+     C1 _|_ Main    |         _|_ Regulator
       115 VAC    |  |           400 uF --- filter  |         --- output
                  +--|---|<|---+  200 V  |  cap     |          |  capacitor
                     |         |         |          |          |
     +-> N o---------+---|<|---+---------+----------+----------+---o Non-isolated
     |                                                                 return
     +-> G - Power line earth ground via building wiring
    
    

  • The line fuse is typically 2 to 4 A, usually a normal fast blow type. Even so, it may not blow as a result of faults down the line - the fusable resistor or regulator may fail first.

  • The main bridge rectifier is often composed of 4 discrete diodes (similar to 1N400Xs) but may also be a single unit. Failures - usually shorted diodes - are common.

  • The main filter capacitor can range in size from 200 to 800 uF or more at 200 to 250 V. THIS CAN BE LETHAL! A typical TV may continue to work at normal line voltage without any noticeable degradation in performance (hum bars, hum in sound, or shutdown) even if this capacitor is reduced in value by 75%. Its uF value is therefore not critical.

  • The regulator is often an IC or hybrid module. Failures resulting in no or reduced output, or no regulation are common.

  • The regulator output capacitor is needed for the B+ regulator to function properly. If this capacitor is reduced in value or develops a high ESR, regulation may fail resulting in instability, oscillation, or excessive B+ and shutdown.

  • The regulator bypass resistor reduces the amount of current control needed of the regulator. Caution: even if the regulator has been pulled, the B+ line will have substantial voltage as a result of this resistor.

    Totally dead set

    This can be as simple as a bad outlet (including blown fuse or tripped circuit breaker due to some other fault), switched outlet and the switch is off, or bad cordset.

    Beyond these basic causes, troubleshooting will be needed inside the set to determine what is defective. Also see the section: Intermittently dead set - bad cordset.

    Intermittently dead set - bad cordset

    There are two problems which are common with the line cord on appliances. Don't overlook these really simple things when troubleshooting your vacuum cleaner - or fancy electronic equipment! If wiggling the cord has an effect, then the following are likely causes:

    It is important to deal with these symptoms as soon as possible as erratic power cycling can lead to much more serious and expensive problems down the road.

    Power button on set is flakey

    If the on/off (or other button) on the set itself behaves erratically but the remote control works fine, then it could be a dirty button or cable or other connections to the switch PCB, particularly if the buttons on the set itself are rarely used. There could possibly be a bad pullup resistor or something of that sort - but is it worth the effort to locate?

    Why not just continue to use the remote? There is no reason to suspect that it will develop similar symptoms. However, there is some risk that if the button is dirty, you may find the TV coming on at random times in the middle of the night (of course!).

    I think I have an older Sylvania that does that sort of thing - don't really know as I never use the power button on the set!

    If power is controlled by a hard switch - a pull or click knob, or mechanical push-push switch and this has become erratic due to worn contacts, replacements are available but often only directly from the original manufacturer to physically fit and (where applicable) have the volume or other controls built in. As an alternative, consider mounting a small toggle switch on the side of the cabinet to substitute for the broken switch. This will almost certainly be easier and cheaper - and quite possibly, more reliable.

    TV blows fuse

    A blown fuse is a very common type of fault due to poor design very often triggered by power surges due to outages or lightning storms. However, the most likely parts to short are easily tested, usually in-circuit, with an ohmmeter and then easily removed to confirm.

    Note that it *may be* useful to replace a fuse the *first* time it blows (though it would be better to do some basic checks for shorted components first as there is a small chance that having a fuse blow the second time could result in additional damage which would further complicate the troubleshooting process). However, if the new one blows, there is a real problem and the only use in feeding the TV fuses will be to keep the fuse manufacturer in business!

    Sometimes, a fuse will just die of old age or be zapped by a power surge that caused no damage to the rest of the TV. However, it must be an EXACT replacement (including slo-blow if that is what was there originally). Else, there could be safety issues (e.g., fire hazard or equipment damage from too large a current rating) or you could be chasing a non-existent problem (e.g., if the new fuse is not slo-blow and is blown by the degauss circuit inrush current but nothing is actually wrong).

    If the fuse really blows absolutely instantly with no indication that the circuits are functioning (no high pitched horizontal deflection whine (if your dog hides under the couch whenever the TV is turned on, deflection is probably working).) then this points to a short somewhere quite near the AC power input. The most common places would be:

    You should be able to eliminate these one by one.

    Unplug the degauss coil as this will show up as a low resistance.

    First, measure across the input to the main power rectifiers - it should not be that low. A reading of only a few ohms may mean a shorted rectifier or two or a shorted Posistor.

  • Test the rectifiers individually or remove and retest the resistance.

  • Some sets use a Posistor for degauss control. This is a little cubical (about 1/2" x 3/4" x 1") component with 3 legs. It includes a line operated heater disk (which often shorts out) and a PTC thermister to control current to the degauss coil. Remove the posistor and try power. If the monitor now works, obtain a replacement but in the meantime you just won't have the automatic degauss.

    If these test good, use an ohmmeter with the set unplugged to measure the horizontal output transistor. Even better to remove it and measure it.

    If any readings are under 5 ohms, the transistor is bad. The parts sources listed at the end of this document will have suitable replacements.

    If the HOT tests bad, try powering the set first with your light bulb and if it just flashes once when the capacitor is charging, then put a fuse in and try it. The fuse should not blow with the transistor removed.

    Of course, not much else will work either.

    If it tests good, power the set without the transistor and see what happens. If the fuse does not blow, then with the good transistor (assuming it is not failing under load), it would mean that there is some problem with the driving circuits possibly or with the feedback from the voltages derived from the horizontal not regulating properly.

    Look inside the TV and see if you can locate any other large power transistors in metal (TO3) cans or plastic (TOP3) cases. There may be a separate transistor that does the low voltage regulation or a separate regulator IC. Some TVs have a switchmode power supply that runs off a different transistor than the HOT. There is a chance that one of these may be bad. If it is a simple transistor, the same ohmmeter check should be performed.

    If none of this proves fruitful, it may be time to try to locate a schematic.

    A blown fuse is a very common type of fault due to poor design very often triggered by power surges due to outages or lightning storms. However, the most likely parts to short are easily tested, usually in-circuit, with an ohmmeter and then easily removed to confirm.

    If you find the problem and repair it yourself, the cost is likely to be under $25.

    Fuse blows or TV blows up when sync is disrupted

    This is a problem which is not going to be easy to identify. One possibility is a drive problem. The messed up sync resulting from swtiching channels, or changing input connections might be resulting in an excessively long scan time for just one scan line. However, this may be enough to cause a current spike in the horizontal output circuit or an excessive voltage spike on the collector of the horizontal output transistor. Normally, the HOT current ramps up during scan. During flyback, the current is turned off. This current is normally limited and the voltage spike on the collector of the HOT is also limited by the snubber capacitors to a safe value. If scan time is too long, current continues to increase. At some point, the flyback core saturates and current goes way up. In addition, the voltage spike will be much higher - perhaps destructively so.

    Troubleshooting these sorts of problems is going to be tough. However, a likely area to investigate would be:

  • Drive circuitry for the HOT including the coupling components.

  • The chip that generates takes the sync input and generates the horizontal drive signal.

  • A bad low voltage regulator might permit the B+ to rise to excessive levels during black scenes (i.e., video mute during channel changing).

    Internal fuse blew during lightning storm (or elephant hit power pole)

    Power surges or nearby lightning strikes can destroy electronic equipment. However, most of the time, damage is minimal or at least easily repaired. With a direct hit, you may not recognize what is left of it!

    Ideally, electronic equipment should be unplugged (both AC line and phone line!) during electrical storms if possible. Modern TVs, VCRs, microwave ovens, and even stereo equipment is particularly susceptible to lightning and surge damage because some parts of the circuitry are always alive and therefore have a connection to the AC line. Telephones, modems, and faxes are directly connected to the phone lines. Better designs include filtering and surge suppression components built in. With a near-miss, the only thing that may happen is for the internal fuse to blow or for the microcontroller to go bonkers and just require power cycling. There is no possible protection against a direct strike. However, devices with power switches that totally break the line connection are more robust since it takes much more voltage to jump the gap in the switch than to fry electronic parts. Monitors and TVs may also have their CRTs magnetized due to the electromagnetic fields associated with a lightning strike - similar but on a smaller scale to the EMP of a nuclear detonation.

    Was the TV operating or on standby at the time? If it was switched off using an actual power switch (not a logic pushbutton or the remote control), then either a component in front of the switch has blown, the surge was enough to jump the gap between the switch contacts, or it was just a coincidence (yeh, right).

    If the TV was operating or on standby or has no actual power switch, then a number of parts could be fried.

    TVs usually have their own internal surge protection devices like MOVs (Metal Oxide Varistors) after the fuse. So it is possible that all that is wrong is that the line fuse has blown. Remove the cover (unplug it first!) and start at the line cord. If you find a blown fuse, remove it and measure across the in-board side of fuse holder and the other (should be the neutral) side of the line. The ohmmeter reading should be fairly high - well certainly not less than 100 ohms - in at least one direction. You may need to unplug the degaussing coil to get a reasonable reading as its resistance may be 25 or 30 ohms. If the reading is really low, there are other problems. If the resistance checks out, replace the fuse and try powering the TV. There will be 3 possibilities:

    1. It will work fine, problem solved.

    2. It will immediately blow the fuse. This means there is at least one component shorted - possibilities include an MOV, line rectifiers, main filter cap, regulator transistor, horizontal output transistor, etc. You will need to check with your ohmmeter for shorted semiconductors. Remove any that are suspect and see of the fuse now survives (use the series light bulb to cut your losses - see the section: The series light bulb trick.

    3. It will not work properly or appear dead. This could mean there are open fusable resistors other defective parts in the power supply or elsewhere. In this case further testing will be required and at some point you may need the schematic.

    If the reading is very low or the fuse blows again, see the section: TV blows fuse.

    Fuse replaced but TV clicks with power-on but no other action

    The click probably means that the power relay is working, though there could be bad contacts.

    Since the fuse doesn't blow now (you did replace it with one of the same ratings, right?), you need to check for:

    If any of these are bad, they will need to be replaced with flameproof resistors of the same ratings (though you can substitute an ordinary resistor for testing purposes). Before applying power, check: Rectifier diodes, horizontal output transistor, regulator pass or chopper transistor (if present), and main filter capacitor for shorts.

    An initial test with an ohmmeter can be done while in-circuit. The resistance across each diode and the collector to emitter of the transistors should be relatively high - a few hundred ohms at lest - in at least one direction (in-circuit). If there is a question, unsolder one side of each diode and check - should be in the Megohms or higher in one direction. Removed from the circuit, the collector-emitter resistance should be very high in one direction at least. Depending on the type, the base-emitter resistance may be high in one direction or around 50 ohms. If any reading on a semiconductor device is under 10 ohms - then the device most likely bad. Assuming that you do not have a schematic, you should be able to locate the rectifiers near where the line cord is connected and trace the circuit. The transistors will be either in a TO3 large metal can or a TOP3 plastic package - on heat sinks. The filter capacitor should eventually measure high in one direction (it will take a while to charge from your ohmmeter). It could still be failing at full voltage, however.

    If you find one bad part, still check everything else as more than one part may fail and just replacing one may cause it to fail again.

    Assuming everything here checks out, clip a voltmeter set on its 500 V scale or higher across the horizontal output transistor and turn the power on. Warning - never measure this point if the horizontal deflection is operating. it is ok now since the set is dead. If the voltage here is 100-150, then there is a problem in the drive to the horizontal output circuit. If it is low or 0, then there are still problems in the power supply or with the winding on the flyback transformer.

    Other possible problems: bad hybrid voltage regulator, bad startup circuit, bad standby power supply (dried up filter capacitor, etc.) bad relay contacts as mentioned above. However, these probably would not have blown the fuse in the first place so are less likely.

    Power-on tick-tick-tick or click-click-click but no other action

    A variety of power supply or startup problems can result in this or similar behavior. Possibilities include:

    One possible test would be to vary the line voltage and observe the set's behavior. It may work fine at one extreme (usually low) or the other. This might give clues as to what is wrong.

    Also see the section: Dead TV with periodic tweet-tweet, flub-flub, or low-low voltage.

    No picture or raster and no sound

    The screen is blank with no raster at all. There are indications that the channel numbers are changing in the display. This indicates that some of the low voltages are present but these may be derived from the standby supply.

    Assuming there is no deflection and no HV, you either have a low voltage power supply problem, bad startup circuit, or bad horizontal output transistor (HOT) or other bad parts in the horizontal deflection.

    Check for bad fuses.

    (If you have HV as indicated by static electricity on the front of the screen and you hear the high pitched whine of the horizontal deflection when it is turned on, then the following does not apply).

    1. Use an ohmmeter to test the HOT for shorts. If it is bad, look for open fusable resistors or other fuses you did not catch.

    2. Assuming it is good, measure the voltage on the collector-emitter of the HOT (this is safe if there is no deflection). You should see the B+ - probably between 100 and 150 V.

    3. If there is no voltage, you have a low voltage power supply problem and/or you have not found all the bad/open parts.

    4. If there is voltage and no deflection (no high pitched whine and no HV), you probably have a startup problem - all TVs need some kind of circuit to kick start the horizontal deflection until the auxiliary power outputs of the flyback are available. Some Zeniths use a simple multivibrator for this - a couple of transistors. Others power the horizontal osc. IC from a special line-derived voltage. The multivibrator type are sometimes designed to fail if someone keeps turning the set on and off (like kids playing) since the power rating is inadequate.

      Test the transistors if it is that type with an ohmmeter. If one is shorted, you have a problem. The usual way a TV service person would test for startup problems is to inject a signal to the base of the HOT of about 15.75 kHz. If the TV then starts and runs once this signal is removed, the diagnosis is confirmed. This is risky - you can blow things up if not careful (including yourself). See the section: Bypassing the Startup Circuit for details.

    If you hear the high pitched whine of the deflection and/or feel some static on the scree, confirm that the horizontal deflection and high voltage are working by adjusting the SCREEN control (probably on the flyback). If you can get a raster then your problem is probably in the video or chroma circuits, not the deflection or high voltage.

    Reduced width picture and/or hum bars in picture and/or hum in sound

    The most likely cause is a dried up main filter capacitor. Once the effective capacitance drops low enough, 120 Hz (or 100 Hz in countries with 50 Hz power) ripple will make its way into the regulated DC supply (assuming full wave rectification).

    Another likely cause of similar symptoms is a defective low voltage regulator allowing excessive ripple. The regulator IC could be bad or filter capacitor following the IC could be dried up.

    Either of these faults may cause:

    1. A pair of wiggles and/or hum bars in the picture which will float up the screen. For NTSC where the power line is 60 Hz but the frame rate is 59.94 Hz, it will take about 8 seconds for each bar to pass a given point on the screen. (On some sets, a half wave recitifier is used resulting in a single wiggle or hum bar).

    2. Hum in the sound. This may or may not be noticeable with the volume turned down.

    3. Possible regulation problems resulting in HV or total shutdown or power cycling on and off.

    The best approach to testing the capacitors is to clip a good capacitor of approximately the same uF rating and at least the same voltage rating across the suspect capacitor (with the power off). A capacitor meter can also be used but the capacitor may need to be removed from the circuit.

    Once the capacitors have been confirmed to be good, voltage measurements on the regulator should be able to narrow down the problem to a bad IC or other component.

    Excessive B+ from fixed regulator like STR30123/STR30130/STR30135

    These are fixed regulators that do fail but the problem may be elsewhere.

    If the B+ goes to high, the X-ray protection circuitry may kick in and shut down the horizontal deflection.

    If there is little or no load (horizontal deflection not running at all), all bets are off as well - the resistor that is likely across input-output will dominate and boost the voltage above the proper output for the regulator chip. Use a Variac to bring up the voltage to the TV. If the deflection does not start up at any voltage even with the B+ ramping up past its normal value, the problem is probably in the horizontal deflection/startup circuitry, not the regulator.

    Some of these may go out of regulation if the output electrolytics are dried up. There might a a 10 uF 200 V or so electrolytic across the output to ground. Test it or substitute a known good one of about the same uF rating and at least equal voltage rating. If you can get the TV to work at reduced voltage using a Variac (but possibly with hum bars in the picture and hum in the audio), check the output capacitor.

    Otherwise, it could be the regulator or one of its biasing components (sets current to B input - the voltage at this input should be close to the output voltage value). Also check to be sure the input voltage is solid - main filter capacitor is not dried up.

    TV power cycling on and off

    The power light may be flashing or if you are runing with a series light bulb it may be cycling on and off continuously. There may be a chirping or clicking sound from inside the set. (Note: using too small a series light bulb load during testing for the size of the TV may also result in this condition.)

    If there is a low voltage regulator or separate switching supply, it could be cycling on and off if the horizontal output, flyback, or one of its secondary loads were defective.

    Does this TV have a separate low voltage regulator and/or switching power supply or is it all part of the flyback circuit? For the following, I assume it is all in one (most common).

    Some simple things to try first:

    Verify that the main filter capacitor is doing its job. Excessive ripple on the rectified line voltage bus can cause various forms of shutdown behavior. An easy test is to jumper across the capacitor with one of at least equal voltage rating and similar capacitance (make connections with power off!).

    Use a Variac, if possible, to bring up the input voltage slowly and see if the TV works at any point without shutting down. If it does, this could be an indication of X-ray protection circuit kicking in, though this will usually latch and keep the set shut off if excessive HV were detected.

    Dead TV with periodic tweet-tweet, flub-flub, or low-low voltage

    A TV which appears to be dead except for a once a second or so tweet or flub usually indicates an overload fault in the power supply or a short in one of its load circuits. In some cases, the low voltage (including B+) will just be reduced to a fraction of their normal value as a result of an overload on one of the outputs - usually the main B+.

    This may be caused by a shorted rectifier in the power supply, flyback, or even the yoke, but check the the loads first. Wait a few minutes for the filter caps to discharge (but stay away from the CRT HV connector as it may retain a dangerous and painful charge for a long time), use an ohmmeter across the various diodes in the power supply. Using an ohmmeter on the rectifier diodes, the resistance in at least one direction should be greater than 100 ohms. If it is much less (like 0 or 5 ohms), then the diode is probably bad. Unsolder and check again - it should test infinite (greater than 1M ohms) in one direction.

    Summary of possible causes:

    Bypassing the Startup Circuit

    Where the TV is dead and a startup problem is suspected, a TV service person would test for startup problems by injecting a signal into the base of the HOT of about 15.75 kHz. If the TV then starts and runs once this signal is removed, the diagnosis is confirmed. This is risky - you can blow things up if not careful (including yourself). A 555 timer based circuit will work fine as a signal source for this.

    WARNING: be careful if you do this. The HOT circuit may be line-connected and it is possible to destroy the HOT and related components if this is not done properly. I once managed to kill not only the HOT but the chopper transistor as well while working in this area. An expensive lesson.

    You can reduce the risk somewhat (to the TV at least) by using a series light bulb load and/or running on reduced line voltage. The most important thing to avoid is putting in an excessively long drive pulse which will result in the flyback transformer saturating, huge amounts of current, and likely a dead HOT and possibly other parts if there is nothing to limit the current. For NTSC/PAL, it is fairly safe to assume that a 50 percent duty cycle 15 to 16 kHz drive signal will not result in fireworks as long as there aren't other problems (like a shorted flyback/LOPT).

    If after a second or so, the TV fires up (not literally!) and stays happy until it is turned off, a startup problem is almost certain. It could be the standby supply (if used) or a dedicated startup circuit that has failed.

    But, don't push your luck - if the TV starts after a second or so of your drive signal but doesn't continue to run when it is removed, don't be tempted to leave your circuit connected - it could still be stressing other parts. Find out why the normal horizontal drive is not being generated - possibly a power supply or horizontal oscillator problem.

    If nothing happens, either startup is not the problem or there are other components that have failed preventing the HOT drive signal from having any effect.

    Shorted Components

    A failure of the horizontal output transistor or power supply switchmode transistor will blow a fuse or fusable resistor.

    Look for blown fuses and test for open fusable resistors in the power circuits. If you find one, then test the HOT and/or switchmode transistor for shorts.

    Other possibilities: rectifier diodes or main filter capacitor.

    While you are at it, check for bad connections - prod the circuit board with an insulated stick when the problem reoccurs - as these can cause parts to fail.

    Startup problems - nothing happens, click, or tick-tick-tick sound

    TVs and monitors usually incorporate some kind of startup circuit to provide drive to the horizontal output transistor (HOT) until the flyback power supply is running. Yes, TVs and monitors boot just like computers.

    There are two typical kinds of symptoms: power on click but nothing else happens or a tick-tick-tick sound indicating cycling of the low voltage (line regulator) but lack of startup horizontal drive.

    Check the voltage on the horizontal output transistor (HOT). If no voltage is present, there may be a blown fuse or open fusable resistor - and probably a shorted HOT.

    However, if the voltage is normal (or high) - usually 100-150 V, then there is likely a problem with the startup circuit not providing initial base drive to the HOT.

    The startup circuits may take several forms:

    1. Discrete multivibrator or other simple transistor circuit to provide base drive to the HOT.

    2. IC which is part of deflection chain powered off of a voltage divider or transformer.

    3. Other type of circuit which operates off of the line which provides some kind of drive to the HOT.

    The startup circuit may operate off of the standby power supply or voltage derived from non-isolated input. Be careful - of course, use an isolation transformer whenever working on TVs and especially for power supply problems.

    Note that one common way of verifying that this is a startup problem is to inject a 15 kHz signal directly into the HOT base or driver circuit (just for a second or two). If the TV then starts up and continues to run, you know that it is a startup problem. However, this is a risky procedure for both you and the TV. See the section: Bypassing the Startup Circuit.

    I have also seen startup circuits that were designed to fail. Turning the TV on and off multiple times would exceed the power ratings of the components in the startup circuit. Some Zenith models have this 'feature'.

    When this situation exists, it could be that the circuit is not providing the proper drive or that due to some other circuit condition, the drive is not always sufficient to get the secondary supplies going to the point that the normal circuits take over.

    I would still check for bad connections - prod the circuit board with an insulated stick when the problem reoccurs.

    TV turns off after warming up

    If you can turn it back on with the s momentary key or power button:

    When it shuts off, do you need to push the power button once or twice to get it back on? Also, does anything else about the picture or sound change as it warms up?

    1. If once, then the controller is shutting the TV down either as a result of a (thermally induced) fault in the controller or it sensing some other problem. Monitoring the voltage on the relay coil (assuming these is one) could help determine what is happening. The controller thinks it is in charge.

    2. If twice, then the power supply is shutting down as the controller still thinks it is on and you are resetting it. A couple of possibilities here would be low voltage or high voltage regulation error (excessive high voltage is sensed and causes shutdown to prevent dangerous X-ray emission). A partially dried up main filter capacitor could also cause a shutdown but there might be other symptoms like hum bars in the picture just before this happened. Clipping a good capacitor across the suspect (with power off!) would confirm or eliminate this possibility.

    If it uses a pull-knob (or other hard on/off switch), then this may be like pulling the plug and would reset any abnormal condition.

    TV doesn't power up immediately

    The TV may do nothing, cycle on and off for a while, power up and then shutdown in an endless cycle - or at least for a while. Then it comes on and operates normally until it is turned off.

    A couple of possibilities:

    1. The main filter capacitor or other filter capacitors in the low voltage power supply is dried up and this can cause all kinds of regulation problems.

    2. The power supply regulator is defective (or marginal) allowing excessive voltage on its output and then the X-ray protection circuitry shuts you down.

    If you can get access to a Variac, it would be worth bringing up the input voltage slowly and seeing if there is some point at which it would stay on.

    If there is, then if the picture has serious hum bars in it the main filter cap could be bad. If more or less a decent picture with minor hum bars then it could be the regulator.

    Old TV requires warmup period

    So, what else is new? In the old days, a TV was expected to take a few minutes (at least) to warm up. We are all spoiled today. Of course, you usually maintained a full time technician or engineer to fiddle with the convergence adjustments!

    A TV (from around 1983) needs at least 5 min. to warm up (lighting up the screen and making sound if I give it a cold start. Once warmed up, you can it off and on again from the front panel and it will work immediately. Another thing this TV has a sub-power switch in the rear.

    1983 sounds a bit late, but sets in the late '70 during the transition from tubes to all solid state chassis often had the 'sub-power' switch providing some power to the filaments of the CRT and other tubes - usually in the deflection and high voltage circuits since these would take a while to heat up and stabilize. The idea was to leave this switch on all the time (except when going on vacation - it was sometimes labeled 'vacation') so that you would have nearly instant warm up. Supposedly, this led to an increased risk of fire as well (see the section: About instant-on TVs).

    If it is a totally solid state chassis, then there is some component - probably a capacitor in the power supply since it affects both picture and sound - that is drifting with temperature and needs to be located with cold spray or a heat gun.

    TV shuts down with bright picture or when brightness is turned up

    This is probably a protection circuit kicking in especially if turning power off or pulling the plug is required to restore operation.

    The detection circuit could be in the power supply or horizontal deflection output circuit. It may be defective or the current may be too high for some other reason. A couple of tests can be performed to confirm that it is due to beam current:

    Relays in the Power Circuitry of TVs

    Where power on or power off is erratic or only seems to work part way (e.g., the picture goes off but not the sound), it may just be a sticking or dirty relay. Of course, general on/off problems could also be relay related but could also be a lot of other things.

    For erratic on/off problems, gently tapping on the relay when the problem occurs will confirm that the relay is at fault - if the set then switches on or off properly, it's almost certainly the relay and replacing it will fix the problem. But double check its solder connections to make sure it isn't a simple bad connection to the relay or in its vicinity.

    So what exactly is the purpose of such a power relay in a TV set? Why doesn't the power switch on the TV just apply power directly instead of through a relay?

    The usual reason for a relay instead of a knob switch is to permit a remote control to turn power on and off. If your TV does not have a remote, then it is simply the same chassis minus 24 cents worth of circuitry to do the remote function. Isn't marketing wonderful?

    The only unknown is the coil voltage. It is probably somewhere in the 6 to 12 volt range. You should be able to measure this on the coil terminals in operation. It will be a DC coil.

    However, the relay controls the 125 VAC (or 220 VAC) which you should treat with respect - it is a lot more dangerous than the 25kV+ on the CRT!

    Almost certainly, the relay will have 4 connections - 2 for power and 2 for the coil. If it is not marked then, it should be pretty easy to locate the power connection. One end will go to stuff near the AC line and the other end will go to the rectifier or maybe a fusable resistor or something like that. These will likely be beefier than the coil connections which will go between a transistor and GND or some low voltage, or maybe directly into a big microcontroller chip.

    However, a few sets use a latching relay - separate coils (sharing a common connection) to 'set' and 'reset' the relay - like a flip flop. In such a case, the controller knows which state the TV is in (on or off) and sens the appropriate pulse to the relay to change state. And, there will be NO voltage on the coils of a latching relay except when turning the set on or off.

    Of course, the best thing would be to get the schematic. Some big public libraries carry the Sams' photofact series for TVs and VCRs. If not, take 10 minutes and trace it. You should be able to get far enough to determine the relay connections.

    Once you are sure of the AC connections - measure across them while it is off and also while it is on. While off, you should get 110 to 125 VAC. While on and working - 0. While on and not working either 110 to 125 VAC if the relay is not pulling in or 0 if it is and the problem is elsewhere. We can deal with the latter case if needed later on. Note the even if the relay contacts are not working, the problem could still be in the control circuitry not providing the correct coil voltage/current, though not likely.

    It may be expensive and/or difficult to obtain an exact replacement, but these are pretty vanilla flavored as relays go. Any good electronics distributor should be able to supply a suitable electrical replacement though you may need to be creative in mounting it.

    What is a posistor?

    A posistor is a combination of a PTC (positive temperature coefficient) resistor and another resistor-element to heat it up and keep it hot. Sometimes, these will go by the name posister or thermistor. The heater is a disk shaped resistor across the power line and the themister is a disk shaped device in series with the degauss coil. They are in clamped together to be in close contact thermally. You can pry off the lid and see for yourself.

    The most common failure mode is for the part to short across the line.

    Its function is to control degauss, so the only thing you lose when you remove one of these is the degauss function on power-on. When you turn the TV or monitor on, the PTC resistor is cold and low resistance. When heated, it becomes very high resistance and turns off the degauss coil but gradually - the current ramps down to zero rather than being abruptly cut off..

    Computer Component Source stocks a wide variety, I believe but it may be cheaper to go direct to the manufacturer if they will sell you one.

    Flameproof Resistors

    Flameproof Resistor or Fusable Resistor are often designated by the symbol 'FR'. They are basically the same. The designation "Flameproof" means that if they fail due to excessive current, there will be no chance of, well, them going up in flames. :) They will also have a power rating and thus can act as a protective device, though a specific circuit may not depend on a precise fuse rating, rather that the resistor will open with massively excessive current.

    You may see these in the switchmode power supplies used in TVs and monitors. They will look like power resistors but will be colored blue or gray, or may be rectangular ceramic blocks. They should only be replaced with flameproof resistors with identical ratings. They serve a very important safety function.

    These usually serve as fuses in addition to any other fuses that may be present (and in addition to their function as a resistor, though this isn't always needed). Since your FR has blown, you probably have shorted semiconductors that will need to be replaced as well. I would check all the transistors and diodes in the power supply with an ohmmeter. You may find that the main switch mode transistor has decided to turn into a blob of solder - dead short. Check everything out even if you find one bad part - many components can fail or cause other components to fail if you don't locate them all. Check resistors as well, even if they look ok.

    Since they function as fuses, flameproof resistors should not be replaced with higher wattage types unless specifically allowed by the manufacturer. These would not blow at the same level of overload possibly resulting in damage to other parts of the circuitry and increasing the risk of fire.

    Then, with a load on the output of the power supply use a Variac to bring up the voltage slowly and observe what happens. At 50 VAC or less, the switcher should kick in and produce some output though correct regulation may not occur until 80 VAC or more. The outputs voltages may even be greater than spec'd with a small load before regulation is correct.

    Width and height change with warmup

    Since both width and height are affected, this points to something common like the low voltage power supply. If there are any indications of hum bars, first check the main filter capacitor(s) or substitute a known good one. There might even be other symptoms like faint retrace lines on at least part of the screen.

    Start by monitoring the B+ to the flyback (feeding the HOT) to see if this drifts at all. If it does, then there is probably a low voltage regulator problem - bad capacitor, resistor, or chip. Use freeze spray to narrow it down. If this is solid, then there could be a high voltage drift but this would be somewhat unusual without other symptoms (like arcing) since the HV is nearly always tracks the low voltage supply.

    Problems with SCR based regulators

    Here are typical symptoms:

    "Sharp TV has a short blast of high voltage and sound then shuts down. All components in regulator area test good. I have two of these sets." Is there a good sharp tech out there thats seen this problem?"

    (From: Mr. Caldwell (jcaldwel@iquest.net).)

    There is a bulletin from Sharp on troubleshooting *any* SCR regulated TV, this can easily be adapted to RCA, GE, Emerson and Panasonic sets that have similar circuits given a little thought but the technician.

    You are going to need to figure part of this out as I no longer have the schematics available. All this will do is allow you to rule out either the regulator or the horizontal section.

    Don't plug this in until you've read the whole list.

    Figure out how to bypass the turn on circuit from the microprocessor (unless it's a manual one). This is usually just jumpering the relay but sometimes Sharp puts a horizontal Vcc turn on transistor that also must be jumpered.

    Next jumper across the SCR anode to cathode.

    Now using an *variable isolation transformer* turn the voltage on it down and plug the set in. Bring the voltage up slowly, if you can bring the AC up so that the DC on the jumper across the SCR is within the regulated voltage you should have a picture and this rules out the horizontal section as the culprit.

    If the set shuts down prior to getting the DC up enough then you've got problems in the horizontal section. Either you have something wrong with the high voltage transformer or the tuning caps or there is a problem with the x-ray protect pick off voltage to the deflection IC.

    If it's the horizontal section you can set the AC at approx. 25v and look at the waveforms in the horizontal output section for defects like ringing.

    I've never gotten a good troubleshooting technique down for the regulator since it's an active circuit the waveforms and voltages are not stable when it's failed. A good diode, transistor and capacitor checker will help.

    It would help to get the service manual for that set, the training manual for that chassis and the bulletin dealing with troubleshooting SCR regulators.

    Also the training manual should have a good explanation of how this regulator works. In a nutshell the regulator is a switched mode circuit that uses a winding from the high voltage transformer to turn off the SCR. The regulator is always turned off at the same time by a pulse from the high voltage transformer. Regulation is achieved by controlling when the SCR is allowed to turn on.

    TV shuts down with dark picture or when changing channels

    This may happen at any time or possibly after being on for awhile in which something heats up and drifts out of spec.

    The low voltage regulator may be letting the voltage rise excessively. Then, a dark picture or video muting during a channel change triggers the X-ray or power supply overvoltage protection.

    Monitor the output of the low voltage power supply B+ to see if it is stable as the brightness/scene changes.



  • Back to TV Repair FAQ Table of Contents.

    Deflection Problems

    Deflection fundamentals

    Note: the following is just a brief introduction. For more detailed deflection system theory of operation and sample circuits, see the document: TV and Monitor Deflection Systems.

    The electron beams in the CRT need to be scanned horizontally and vertically in a very precise manner to produce a raster - and a picture.

    For NTSC and PAL, the horizontal scan rates are 15,734 and 15,625 Hz respectively.

    For NTSC and PAL, the vertical scan rates are 60 and 50 Hz (approximately) respectively.

    The deflection yoke includes sets of coils for horizontal and vertical scanning oriented at 90 degrees with respect to each other. Additional coils are needed to correct for pincushion and other geometric defects.

    The deflection circuits must be synchronized and phase locked to the incoming video signal.

    Therefore, we have the following functions:

    1. Sync separator to obtain horizontal and vertical synchronization pulses.

    2. Horizontal oscillator which locks to horizontal sync pulses.

    3. Horizontal drive followed by horizontal output which feeds deflection yoke (and flyback for HV and other voltages), Yoke requires a sawtooth current waveform for linear horizontal deflection. Horizontal output in all but the smaller TVs is a large discrete power transistor, most often an NPN bipolar type.

    4. Vertical oscillator which locks to vertical sync pulses. Yoke requires sawtooth waveform for linear vertical deflection.

    5. Vertical drive/output which feeds vertical deflection yoke. Newer TVs use ICs for vertical drive and output.

    6. Various additional deflection signals to correct for the imperfections in the geometry of large angle deflection CRTs. These may be fed into the normal deflection coils and/or there may be separate coils mounted on the neck of the CRT.

    About the vertical scan rate

    Some people believe that the TV scan rate is locked to the local power line. TVs never ever used the line frequency for vertical rate. The vertical rate is not even equal to line frequency, actually 59.94 Hz (NTSC). It was set originally to 60 Hz to minimize the visibility of interference between the deflection and power transformer. When NTSC added color, it changed to 59.94 Hz for highly technical reasons. And, TVs no longer have power transformers.

    See Symptoms of Some Common Deflection Problems when referring to the specific descriptions below.

    Picture squeezed in then died

    You were watching 'Knight Rider' reruns and all of a sudden, the picture "squeezed in" slowly from the right hand side. It "squeezed in" about 2 inches or so when the entire picture went dead - has remained like this since. Sound is fine, but no activity at all from the tube. Has it died? How much time, effort, and expense to fix?

    No, it's not dead, at least it certainly is not the picture tube.

    Your set probably didn't like Knight Rider - at least that episode!

    Seriously, how old is the set? Is it a totally solid state chassis or are there tubes in the deflection circuits?

    Is there any indication of light on the screen? Any indication of the 15735 Hz horizontal running at all? (You would normally hear the high pitch sound).

    Newer TVs almost always derive voltages for the sound circuits from the horizontal deflection but older hybrids may run the sound off of its own power.

    In any case, there is a problem in the horizontal deflection and you probably have no high voltage as well assuming no light on the screen.

    The fact that it squeezed in first indicates that a partial short or other fault may have developed in the horizontal deflection circuits - possibly the deflection yoke or flyback transformer. It could also have been a bad connection letting loose. Once it failed completely, the horizontal output transistor may have bought the farm or blown a fuse.

    TV non-linearity

    Most modern TVs are nearly perfect with respect to linearity. There are never any user adjustments and there may not even be any internal adjustments. See the sections: Horizontal position, size, and linearity adjustment and/or Vertical position, size, and linearity adjustment.

    A sudden change in linearity or a TV that requires a warmup period before linearity becomes acceptable may have a bad component - probably a capacitor in the horizontal deflection circuits. For the latter, try some cold spray or a heatgun to see if you can locate the bad part.

    Horizontal deflection shutting down

    Confirm that the horizontal deflection is shutting down (along with the high voltage since it is derived from horizontal deflection: listen for the high pitched deflection whine, test for static on the screen, see if the CRT filaments are lit, turn up the brightness and/or screen control to see if you can get a raster) and then why:

    1. Power is failing to the horizontal output transistor - this could be due to a low voltage power supply problem, bad connection, etc.

    2. Base drive to the horizontal output transistor is failing - could be a fault in the horizontal oscillator or bad connection.

    3. Problem with the flyback transformer or its secondary loads (flyback may provide other power voltages).

    4. X-ray protection is activating - either due to excess HV or due to a fault in the X-ray protection circuitry.

    If the problem comes and goes erratically it sounds like a bad connection, especially if whacking has an effect. If it comes and goes periodically, then a component could be heating up and failing, then cooling, etc.

    TV will not sync

    There are a wide variety of causes for a TV that will not display a stable or properly configured picture. Among the symptoms are:

    Horizontal lock lost

    A TV which loses horizontal lock when changing channels, momentarily losing the signal, or switching inputs may have a horizontal oscillator that is way out of adjustment or has drifted in frequency due to aging components.

    Note that the characteristics of this are distinctly different than for total loss of sync. In the latter case, the picture will drift sideways and/or up and down while with an off frequency oscillator, the torn up picture will try at least to remain stationary.

    This could be a capacitor or other similar part. Or, the oscillator frequency may just need to be tweaked (particularly with older sets). There may be an internal horizontal frequency adjustment - either a pot or a coil - which may need a slight tweak. If a coil, use a plastic alignment tool, not metal to avoid cracking the fragile core.

    A schematic will be useful to locate the adjustment if any or to identify possible defective parts. Try a large public library for the Sams' Photofact for this set.

    Insufficient width (without hum bars)

    If there are hum bars or wiggles in the picture and/or hum in the sound, see the section: Reduced width picture and/or hum bars in picture and/or hum in sound.

    If both width and height are affected, the cause is likely something common: low, low voltage power supply voltages or excessive high voltage (resulting in a 'stiffer' beam).

    (From: Jerry G. (jerryg@total.net).)

    Lack of width is usually caused by defective power supply, low horizontal drive to the yoke and flyback, defective circuits in the pincushioning amplifier section, excessive high-voltage caused by defective voltage regulation, and or excessive loading on the secondary side of the flyback.

    Vertical lock lost

    This indicates a picture that is correct but rolling vertically.

    If the picture is rolling down the screen the frequency of the vertical oscillator is incorrect - too high - and this may be the problem. Generally, the free run frequency of the vertical oscillator should be a little below the video rate (of around 50 or 60 Hz depending on where you live).

    If it is rolling continuously without jumping, then there is a loss of sync from the sync separator or faulty components in the vertical oscillator causing it to totally ignore the sync pulses.

    If it is rolling up rapidly and not quite able to remain locked, the free run frequency may be too low or there could be a fault in the sync circuits resulting in an inadequate vertical pull-in range.

    On older sets, there was actually a vertical hold (and possibly even a separate vertical frequency) control. On anything made in the last decade, this is unlikely. There may be Vertical Frequency and Vertical Pull-in Range adjustments (and others) accessible via the service menu. However, if any of these ever change, it indicates a possible problem with the EEPROM losing its memory as component drift is unlikely.

    As with everything else, bad connections are possible as well. You will need a schematic and possibly setup info to go beyond this.

    Vertical squashed

    This is a vertical deflection problem - possibly a bad capacitor, bad connection, flyback/pumpup diode, or other component. None of these should be very expensive (in a relative sort of way).

    If the symptoms change - particularly if they become less severe - as the set warms up, a dried up electrolytic capacitor is most likely. If they get worse, it could be a bad semiconductor. Freeze spray or a heat gun may be useful in identifying the defective component.

    It is often easiest to substitute a good capacitor for each electrolytic in the vertical output circuit. Look for bad connections (particularly to the deflection yoke), then consider replacing the vertical output IC or transistor(s).

    A defective deflection yoke is also possible or in rare cases, a bad yoke damping resistor (e.g., 500 ohms, may be mounted on the yoke assembly itself).

    Where the entire top half or botton half of the picture is squashed into into the center (i.g., only half the picture shows), a missing power supply voltage, defective vertical output IC, or a component associated with it is likely bad. A bad connection or blown fusable resistor may be the cause of a missing power supply voltage.

    The following are NOT possible: CRT, flyback (except possibly where it's the source for a missing voltage but this is more likely just a bad solder connection at a flyback pin), tuner (except for the famous RCA/GE/Proscan or Sony models where the controller is at fault - see the sections on these specific brands). I am just trying to think of really expensive parts that cannot possibly be at fault :-).

    Note that some movies or laser karaoke discs are recorded in 'letterbox' format which at first glance looks like a squashed vertical problem. However, the picture aspect ratio will be correct and turning up the brightness will reveal a perfectly normal raster above and below the picture.

    Part of picture cut off

    The following applies if the part of the picture is missing but not otherwise squashed or distorted. For example, 85% is missing but the portion still visible is normal size.

    Wow! That's an interesting one, more so than the typical run-of-the-mill "my TV just up and died on me". Or, "my pet orangutan just put a hole in the CRT, what should I do"?

    Since the size of the picture fragment is correct but 85% is missing, my first thought would be to check waveforms going into the vertical output stage. The supply voltage is probably correct since that often determines the size. It almost sounds like the waveform rather than being mostly on (active video) and off for the short blanking period is somehow only on during the last part of the active video thus giving you just the bottom of the picture. If there is a vertical output IC, it may be defective or the blanking input to it may be corrupted. The problem may be as far back as the sync separator. Then again who knows, maybe wait for the schematics.

    CAUTION: To prevent damage to the CRT phosphors, immediately turn down the brightness so the line is just barely visible. If the user controls do not have enough range, you will have to locate and adjust the master brightness or screen/G2 pots.

    Since you have high voltage, the horizontal deflection circuits are almost certainly working (unless there is a separate high voltage power supply - almost unheard of in modern TVs and very uncommon in all but the most expensive monitors).

    Check for bad solder connections between the main board and the deflection yoke. Could also be a bad horizontal coil in the yoke, linearity coil, etc. There is not that much to go bad based on these symptoms assuming the high voltage and the horizontal deflection use the same flyback. It is almost certainly not an IC or transistor that is bad.

    Single Horizontal Line

    CAUTION: To prevent damage to the CRT phosphors, immediately turn down the brightness so the line is just barely visible. If the user controls do not have enough range, you will have to locate and adjust the master brightness or screen/G2 pots.

    A single horizontal line means that you have lost vertical deflection. High voltage is most likely fine since there is something on the screen.

    This could be due to:

    1. Dirty service switch contacts. There is often a small switch located inside on the main board or perhaps accessible from the back. This is used during setup to set the color background levels. (On some sets, this is located on the CRT neck board and may be a jumper plug or other means of selecting service mode - not an actual switch.)

      When moved to the 'service' position, it kills vertical deflection and video to the CRT. If the switch somehow changed position or got dirty or corroded contacts, you will have this symptom. Flip the switch back and forth a couple of times. If there is some change, then replace, clean, resolder, or even bypass it as appropriate.

    2. Bad connection to deflection yoke or other parts in vertical output circuit. Bad connections are common in TVs and monitors. Check around the pins of large components like transformers, power transistors and resistors, or connectors for hairline cracks in the solder. Reseat internal connectors. Check particularly around the connector to the deflection yoke on the CRT.

    3. Bad vertical deflection IC or transistor. You will probably need the service manual for this and the following. However, if the vertical deflection is done with an IC, the ECG Semiconductor Master Substitution guide may have its pinout which may be enough to test it with a scope.

    4. Other bad parts in vertical deflection circuit though there are not that many parts that would kill the deflection entirely.

    5. Loss of power to vertical deflection circuits. Check for blown fusable resistors/fuses and bad connections.

    6. Loss of vertical oscillator or vertical drive signals.

    The most likely possibilities are in the deflection output stage or bad connections to the yoke.

    Keystone shaped picture

    This means that the size of the picture is not constant from top to bottom (width changes) or left to right (height changes). Note that some slight amount of keystoning is probably just within the manufacturing tolerance of the deflection yoke and factory setup (geometry magnet placement, if any). On a TV, this is only noticeable with scenes having straight edges (e.g., video games) in relationship to the CRT bezel.

    However, a sudden increase (and it will usually be rather substantial in a TV) may indicate a problem with the deflection yoke.

    An open or short in a winding (or any associated components mounted on the yoke assembly) will result in the beam being deflected less strongly on the side where that winding is located. Typical TV yokes have fewer individual windings in parallel than high scan rate monitors so the effects of one such fault are more dramatic. See the section: Deflection yoke testing.

    If the set has been dropped off a 20 story building, the yoke may have shifted its position on the neck, of the CRT resulting in all sorts of geometry and convergence problems (at the very least).

    Loss of Horizontal Sync (also applies to vertical) after Warmup

    The problem lies either in the horizontal oscillator or in the sync system. If it really is a problem with sync pulses not reaching the oscillator, the picture will move around horizontally and can be brought to hold momentarily with the hold control. If the picture breaks up into strips, there is a problem in the horizontal oscillator. Rotate the hold control: if the frequency is too far off, the picture will not settle into place at any adjustment of the hold control. Look around the horizontal oscillator circuit: all of the oscillator parts will be right there, or check on the horizontal oscillator module. Another horizontal problem can occur if the set is an RCA made from around 1972-1980: these sets are designed to slip very far off sync if the high voltage is too high, to protect against radiation. Turning up the brightness will decrease the number of bars if this system is in question, as the high voltage is decreasing. In this case, check around the high-voltage regulation system on the deflection systems board. I've had 2 1970's RCA's with this problem.

    Intermittent jumping or jittering of picture or other random behavior

    This has all the classic symptoms of a loose connection internal to the TV or monitor - probably where the deflection yoke plugs into the main PCB or at the base of the flyback transformer. TVs and monitors are notorious for both poor quality soldering and bad connections near high wattage components which just develop over time from temperature cycling. The problem may happen any time or more when cold or hot.

    The following is not very scientific, but it works: Have you tried whacking the TV when this happened and did it have any effect? If yes, this would be further confirmation of loose connections.

    What you need to do is examine the solder connections on the PCBs in the monitor, particularly in the area of the deflection circuits and power supply. Look for hairline cracks between the solder and the component pins - mostly the fat pins of transformers, connectors, and high wattage resistors. Any that are found will need to be reflowed with a medium wattage (like 40W) or temperature controlled soldering iron.

    It could also be a component momentarily breaking down in the power supply or deflection circuits.

    One other possibility is that there is arcing or corona as a result of humid weather. This could trigger the power supply to shut down perhaps with a squeak, but there would probably be additional symptoms including possibly partial loss of brightness or focus before it shut down. You may also hear a sizzling sound accompanied by noise or snow in the picture, static in the sounds, and/or a smell of ozone.

    Horizontal output transistors keep blowing (or excessively hot)

    Unfortunately, these sorts of problems are often difficult to definitively diagnose and repair and will often involve expensive component swapping.

    You have just replaced an obviously blown (shorted) horizontal output transistor (HOT) and an hour (or a minute) later the same symptoms appear. Or, you notice that the new HOT is hotter than expected:

    Would the next logical step be a new flyback (LOPT)? Not necessarily.

    If the set performed normally until it died, there are other possible causes. However, it could be the flyback failing under load or when it warms up. I would expect some warning though - like the picture shrinks for a few seconds before the poof.

    Other possible causes:

    1. Improper drive to horizontal output transistor (HOT). A weak drive might cause the HOT to turn on or (more likely) shut off too slowly (greatly increasing heat dissipation. Check driver and HOT base circuit components. Dried up capacitors, open resistors or chokes, bad connections, or a driver transformer with shorted windings or broken or loose core can all affect drive waveforms.

    2. Excessive voltage on HOT collector - check LV regulator (and line voltage if this is a field repair), if any.

    3. Defective safety capacitors or damper diode around HOT. (Though this usually results in instant destruction with little heating).

    4. New transistor not mounted properly to heat sink - probably needs mica washer and heat sink compound.

    5. Replacement transistor not correct or inferior cross reference. Sometimes, the horizontal deflection is designed based on the quirks of a particular transistor. Substitutes may not work reliably.

    6. CRT shorting internally. If this happens only once in two weeks, it may be diffuclt to track down :-(.

    The HOT should not run hot if properly mounted to the heat sink (using heatsink compound). It should not be too hot to touch (CAREFUL - don't touch with power on - it is at over a hundred volts with nasty multihundred volt spikes and line connected - discharge power supply filter caps first after unplugging). If it is scorching hot after a few minutes, then you need to check the other possibilities.

    It is also possible that a defective flyback - perhaps one shorted turn - would not cause an immediate failure and only affect the picture slightly. This would be unusual, however. See the section: Testing of flyback (LOPT) transformers.

    Note that running the set with a series light bulb may allow the HOT to survive long enough for you to gather some of the information needed to identify the bad component.

    Horizontal output transistors blowing at random intervals

    The HOT may last a few months or years but then blow again.

    These are among the hardest problems to locate. It could even be some peculiar combination of user cockpit error - customer abuse - that you will never identify. Yes, this should not happen with a properly designed TV though newer horizontal processor chips are quite smart about preventing HOT killing signals from reaching the horizontal driver.

    However, a combination of channel changing, loss of sync when switching video sources, and frequent power cycles, could test the TV in ways never dreamed of by the designers. It may take only one scan line that is too long to blow the HOT.

    On the other hand, the cause may be along the lines of those listed in the section: Horizontal output transistors keep blowing (or excessively hot) and just not as obvious - blowing in a few days or weeks instead of a few seconds but in this case, the HOT will likely be running very hot even after only a few minutes.

    Another possible cause for random failures of the HOT are bad solder connections in the vicinity of the flyback and HOT (very common due to the large hot high power components) as well as the horizontal driver and even possibly the sync and horizontal oscillator circuits, power supply, or elsewhere.

    Vertical foldover

    The picture is squashed vertically and a part of it may be flipped over and distorted.

    This usually indicates a fault in the vertical output circuit. If it uses an IC for this, then the chip could be bad. It could also be a bad capacitor or other component in this circuit. It is probably caused by a fault in the flyback portion of the vertical deflection circuit - a charge pump that generates a high voltage spike to return the beam to the top of the screen.

    Test components in the vertical output stage or substitute for good ones.

    Comments on vertical problems

    (From: Bert Christensen (bert.christensen@rose.com).)

    As a general rule, vertical faults can be divided into two types: ones that cause geometric distortion (a circle will not be round) and those that simply black out a portion of the screen. The former are faults in the vertical oscillator, drive, or output stages. The latter are blanking faults. Blanking faults are almost always caused by electrolytic capacitors changing value and thereby changing the timing of the pulses which blank the screen during vertical retrace. In other words, the pulses are turning off the video signals at the wrong time.

    The most common true vertical fault is geometric distortion and a foldover of white lines at the top of the screen. This is almost always caused by the electrolytic capacitor on or near the collector of the vertical output transistor or part of the IC which has the supply voltage (B+) on it. In the old tube days, the general rule was that bottom distortion was in the cathode of the output tube and distortion at the top was caused by a fault in the drive circuit.

    Jagged or uneven vertical sweep

    (From: Matthias Meerwein (Matthias.Meerwein@rt.bosch.de).)

    I recently fixed two CRT display devices that both developed a very similar problem: The vertical deflection was severely "jagged" with uneven line spacing and partial vertical foldover. One patient was a nameless el-cheapo 28-inch TV (1988 made), the other one a 14 inch ADI SVGA monitor (1991 vintage).

    My first suspicions were bad contacts on the PCB or yoke connectors or isolation / connectivity problems inside the yoke. However, as the picture didn't change with warmup or tapping, those causes could be ruled out. Examining the vertical deflection waveform with the scope showed the problem being a parasitic high frequency oscillation around the vertical output IC. On the TV, the oscillation extended over the entire scan period, while the monitor exhibited the problem only near the vertical current zero cross.

    In both cases I found the capacitor of the RC damping network on the amp output to be at fault. Replacing it fixed the problem in both sets. This is not the well-known dried-up-electrolytic problem described in the FAQ. The culprits were mylar caps (.1 and .47 uF) looking completely unsuspicious. They were probably a bit underrated voltage-wise (40 volts) so I replaced them with 100 volts rated ones. The 2.2 ohms resistor in series with the cap was fine in both cases.

    Excessive width/pincushioning problems

    This would mean that the left and right sides of the picture are 'bowed' and the screen looks something like the diagram below (or the opposite - barrel distortion).

    However, the obvious symptoms may just be excess width as the curved sides may be cut off by the CRT bezel.

     ============================================
     \                                          /
      \                                        /
       \                                      /
        \                                    /
         \                                  /
          \                               /
           |                              |
           |                              |
           |                              |
          /                                \
         /                                  \
        /                                    \
       /                                      \
      /                                        \
     /                                          \
    ==============================================
    

    In particular, this sounds like a pincushion problem - to correct for pincushion, a signal from the vertical deflection that looks something like a rectified sinewave is used to modify width based on vertical position. There is usually a control to adjust the magnitude of this signal and also often, its phase. It would seem that this circuit has ceased to function.

    If you have the schematics, check them for 'pincushion' adjustments and check signals and voltages. If not, try to find the 'pincushion' magnitude and phase adjustments and look for bad parts or bad connections in in the general area. Even if there are no adjustment pots, there may still be pincushion correction circuitry.

    If the internal controls have absolutely no effect, then the circuit is faulty. With modern digital setup adjustments, then it is even tougher to diagnose since these control a D/A somewhere linked via a microprocessor.

    Pincushion adjustment adds a signal to the horizontal deflection to compensate for the geometry of the CRT/deflection yoke. If you have knobs, then tracing the circuitry may be possible. With luck, you have a bad part that can be identified with an ohmmeter - shorted or open. For example, if the pincushion correction driver transistor is shorted, it will have no effect and the picture will be too wide and distorted as shown above.

    However, without a schematic even this will be difficult. If the adjustments are digital this is especially difficult to diagnose since you don't even have any idea of where the circuitry would be located.

    Faulty capacitors in the horizontal deflection power supplies often cause a similar set of symptoms.

    Deflection yoke testing

    A faulty deflection yoke can affect the geometry (size and shape) of the raster, result in insufficient high voltage and/or other auxiliary power problems, and blow various components in the low voltage power supply or elsewhere.

    The deflection yoke consists of the horizontal coils and vertical coils (wound on a ferrite core), and mounting structure. Little magnets or rubber/ferrite strips may be glued in strategic locations. DO NOT disturb them! In rare instances, there may be additional coils or other components mounted on the same assembly. The following deals only with the actual deflection coils themselves - the other components (if any) can be tested in a similar manner.

    Where the test procedure below requires removal of the yoke, see the section: Removing and replacing the deflection yoke first.

    Deflection yoke repair

    So you found a big black charred area in/on one of the yoke windings. What can be done? Is it possible to repair it? What about using it for testing to confirm that there are no other problems before ordering a new yoke?

    If the damage is minor - only a few wires are involved, it may be possible to separate them from each other and the rest of the winding, thoroughly clean the area, and then insulate the wires with high temperature varnish. Then, check the resistances of each of the parallel/interleaved windings to make sure that you caught all the damage.

    Simple plastic electrical tape can probably be used for as insulation for testing purposes - it has worked for me - but would not likely survive very long as a permanent repair due to the possible high temperatures involved. A new yoke will almost certainly be needed.

    Testing of flyback (LOPT) transformers

    How and why do flyback transformers fail?

    Flybacks fail in several ways:

    1. Overheating leading to cracks in the plastic and external arcing. These can often be fixed by cleaning and coating with multiple layers of high voltage sealer, corona dope, or even plastic electrical tape (as a temporary repair in a pinch).

    2. Cracked or otherwise damaged core will effect the flyback characteristics to the point where it may not work correctly or even blow the horizontal output transistor.

    3. Internal shorts in the FOCUS/SCREEN divider network, if present. One sign of this may be arcover of the FOCUS or SCREEN sparkgaps on the PCB on the neck of the CRT.

    4. Internal short circuits in the windings.

    5. Open windings.

    More than one of these may apply in any given case.

    First, perform a careful visual inspection with power off. Look for cracks, bulging or melted plastic, and discoloration, Look for bad solder connections at the pins of the flyback as well. If the TV or monitor can be powered safely, check for arcing or corona around the flyback and in its vicinity,

    Next, perform ohmmeter tests for obvious short circuits between windings, much reduced winding resistances, and open windings.

    For the low voltage windings, service manuals may provide the expected DC resistance (Sams' PhotoFact, for example). Sometimes, this will change enough to be detected - if you have an ohmmeter with a low enough scale. These are usually a fraction of an ohm. It is difficult or impossible to measure the DC resistance of the HV winding since the rectifiers are usually built in. The value is not published either.

    Caution: make sure you have the TV or monitor unplugged and confirm that the main filter capacitor is discharged before touching anything! If you are going to remove or touch the CRT HV, focus, or screen wires, discharge the HV first using a well insulated high value resistor (e.g., several M ohms, 5 W) to the CRT ground strap (NOT signal ground. See the section: Safe discharging of capacitors in TVs and video monitors.

    Partially short circuited windings (perhaps, just a couple of turns) and sometimes shorts in the focus/screen divider will drastically lower the Q and increase the load the flyback puts on its driving source with no outputs connected. Commercial flyback testers measure the Q by monitoring the decay time of a resonant circuit formed by a capacitor and a winding on the flyback under test after it is excited by a pulse waveform. It is possible to easily construct testers that perform a well. See the companion document Testing of Flyback (LOPT) Transformers for further information.

    Horizontal or vertical flipped picture

    The picture is flipped left-to-right or is upside-down or both. This cannot happen as a result of a failure. For a CRT-based TV or monitor, it almost certainly means that the wires to the horizontal or vertical deflection yoke have been swapped to enable the picture to appear correct when viewed via a mirror (horizontal only) or if the unit were mounted base-up to a ceiling (both). The remedy is simply to swap the two wires to the relevant deflection yoke(s). There may even be obvious splices to guide you. There is usually a connector with 4 relatively fat wires that go to the deflection yoke on the CRT neck (NOT the PCB attached to the tube base). If you don't have a schematic, trace these on the main PCB back to their origin. The horizontal will originate somewhere in the vicinity of the flyback transformer. It may be possible to disengage the wires from the connector shell and swap them there. If not, cut, splice, and solder. Adjustment of the appropriate centering controls may be needed.

    For flat panel displays, it is even more unlikely this would happen as a result of a hardware failure. Most likely, there is a mode setting in the one of the setup menus for the TV or monitor itself. It could also be in the receiver for the TV, or the driver or application software of the PC. If the source is a video projector, the menu setting is likely there, to select between front and rear projection (horizontal) or table or ceiling mount (both). So, don't bother to open up the flat panel TV or monitor. The problem is not there! :)



  • Back to TV Repair FAQ Table of Contents.

    High Voltage Power Supply Problems

    HV power supply fundamentals

    Most, if not all, TVs derive the high voltage for the CRT second anode, focus, and (sometimes) screen (G2) from the horizontal deflection system. This technique was developed quite early in the history of commercial TV and has stuck for a very simple reason - it is very cost effective. A side effect is that if the horizontal deflection fails and threatens to burn a (vertical) line into the CRT phosphors, the high voltage dies as well.

    Most TV high voltage supplies operate as follows:

    1. Horizontal output transistor (HOT) turns on during scan. Current increases linearly in primary of flyback transformer since it appears as an inductor. Magnetic field also increases linearly. Note: flyback is constructed with air gap in core. This makes it behave more like an inductor as far as the primary drive is concerned.

    2. HOT shuts off at end of scan. Current decreases rapidly. Magnetic field collapses inductively coupling to secondary and generates HV pulse. Inductance and capacitance of flyback, snubber capacitors, and parasitic capacitance of circuitry and yoke form a resonant circuit. Ideally, voltage waveform across HOT during flyback (retrace) period will be a single half cycle and is clamped by damper diode across HOT to prevent undershoot.

    3. Secondary of flyback is either a single large HV winding with HV rectifiers built in (most often) or an intermediate voltage winding and a voltage multiplier built in or a separate unit (see the section: What is a tripler?. The output will be DC HV pulses.

    4. The capacitance of the CRT envelope provides the needed filtering to adequately smooth the HV pulses into a DC voltage.

    5. A high resistance voltage divider provides the several kV focus voltage and sometimes the several hundred volt screen (G2) voltage as well. Often, the adjustments for these voltages are built into the flyback. Sometimes they are mounted separately. The focus and screen are generally the top and bottom knobs, respectively.

    What is a tripler?

    In some TVs, the flyback transformer only generates about 6-10 kV AC which is then boosted by a diode-capacitor ladder to the 18-30 kV needed for modern color CRTs. The unit that does this is commonly called a tripler since it multiplies the flyback output by about 3 times. Some TVs use a quadrupler instead. However, many TVs generate the required HV directly with a winding with the required number of turns inside the flyback transformer.

    Triplers use a diode-capacitor ladder to multiply the 6-10 kV AC to 18-30 kV DC. Many triplers are separate units, roughly cubical, and are not repairable. Some triplers are built in to the flyback - it is probably cheaper to manufacture the HV diodes and capacitors than to wind a direct high voltage secondary on the flyback core. In either case, failure requires replacement of the entire unit.

    For external multipliers, the terminals are typically marked:

    Symptoms of tripler failure are: lack of high voltage or insufficient high voltage, arcing at focus protection spark gap, incorrect focus voltage, other arcing, overload of HOT and/or flyback, or focus adjustment affecting brightness (screen) setting or vice-versa. Where there is overloading, if you disconnect the tripler and everything else comes back to life (obviously, there will be no HV or picture), then it is very likely bad.

    High voltage shutdown due to X-ray protection circuits

    A TV that runs for a while or starts to come on but then shuts down may have a problem with the X-ray protection circuitry correctly or incorrectly determining that the high voltage (HV) is too great (risking excessive X-ray emission) and shutting everything down.

    A side effect of activation of this circuitry is that resetting may require pulling the plug or turning off the real (hard) power switch.

    Was there anything else unusual about the picture lately that would indicate an actual problem with the HV? If this is the case, then there may be some problem with the HV regulation. If not, the shutdown circuit may be overly sensitive or one of its components may be defective - a bad connection of leaky cap (or zener).

    If the horizontal frequency is not correct (probably low) due to a faulty horizontal oscillator or sync circuit or bad horizontal hold control (should one exist!), HV may increase and trigger shutdown. Of course, the picture won't be worth much either!

    One symptom of excessive HV (but not required) is an overly bright picture of reduced size.

    The HV shutdown circuit usually monitors a winding off of the flyback for voltage exceeding some reference and then sets a flip flop shutting the horizontal drive off.

    On some Sony models, a HV resistive divider performs this function and these do fail - quite often. The big red Hstat block is a common cause of immediate or delayed shutdown on certain Sony monitors and TVs. See the section: Sony TVs/monitors and Hstat.

    Excessive low voltage supply may trigger high voltage shutdown

    (From: Ray Chandos (rchandos@ivc.edu).)

    Modern television receivers and video monitors are all equipped with a safety circuit to shut down the high voltage feeding the anode of the picture tube if that high voltage becomes excessive. (This is to prevent dangerous x-rays emitted when electrons with too much energy strike the metal shadow mask just inside the TV screen.) Unfortunately, high voltage shutdown problems can be very difficult to diagnose because, once shutdown has occurred, the horizontal pulses used to generate the high voltage are turned off, and with them the high voltage itself.

    In many cases I have encountered, the high voltage is not excessive, but the shutdown circuit itself has failed and falsely triggers. A common cause of this is failure of the circuitry that samples the high voltage and feeds a portion back to the input of the shutdown circuit. Typically, a tap from the flyback transformer feeds a diode and a filter capacitor to produce a sample DC voltage proportional to the high voltage. As the high voltage increases, so does this sample. It is usually further reduced by a voltage divider, then sent through a series zener diode to the "horizontal shutdown" input of a video processor chip, so that, if the divided down voltage exceeds the rating of the zener diode, the latter will conduct and trigger the shutdown input, which then latches off the horizontal pulses. Now if the bottom resistor in the voltage divider opens, or increases above its nominal value (common for high value carbon resistors), the sampled voltage will increase, possibly enough to falsely trip the shutdown input. Check it with an ohmmeter.

    Incidentally, if you don't have a schematic, you can still attempt to diagnose and repair your shutdown problem. Start with the video processor IC, a huge chip that controls most of the TV functions. Get the pinout from this web site, the ECG semiconductor replacement guide, or data sheet archives on the Internet. Find the horizontal output and horizontal shutdown pins, and attach oscilloscope probes to verify that you have a shutdown problem. If you do, you will see horizontal pulses for a brief instant on power up, but suddenly disappearing as the shutdown input voltage goes up and turns them off. (This is a latching circuit, so the shutdown voltage will normally stay high until the power is turned off.)

    Now trace the shutdown signal voltage back through the voltage divider, the filter capacitor, and the diode to the flyback winding. Test out all these parts as you go.

    If the shutdown circuitry all seems OK, it may be doing its intended job of detecting and disabling excessively high voltage. Too much high voltage often results when the lower voltage DC supply feeding the high voltage supply circuitry somehow gets too high. This voltage, often around +160 VDC, nowadays comes from the TV's main regulated power supply and is applied to one end of the flyback transformer primary. The other end connects to the collector of the horizontal output (a large, high voltage power transistor on a heat sink), the emitter of which connects to ground. Horizontal drive pulses originating in the video processor circuit drive the base terminal of this transistor, switching it on when the pulses are high and thus supplying current to the flyback transformer primary. The secondary winding, having many more turns, steps up the +160 Volts applied to the primary to 25 - 45 kV, which is rectified, filtered, and applied to the anode of the picture tube. Now if the +160 Volts increases, say to +200 Volts, due to some malfunction in the main power supply regulation, then the secondary voltage will also increase by the same percentage, and trip the high voltage shutdown circuit.

    Fortunately, although the high voltage quickly vanishes after shutdown, preventing you from measuring it, the low voltage usually stays on. You can measure it (carefully) from the collector of the horizontal output transistor to ground. Of course, if you lack a schematic, you won't know if this voltage is correct or not, so again trace it back from the flyback transformer primary to the main TV power supply. There you may find a label printed on the printed circuit board telling you the normal voltage. You can also get a clue by looking at the voltage rating of any filter capacitor connected from this voltage line to ground. For example, if the filter capacitor is rated at 200 V and you are measuring 220 V, you know you have a problem. Sometimes the voltage will come from a linear voltage regulator IC whose pinout and output voltage you can look up from the chip number. These linear regulators can short from input to output, raising the output voltage and leading to the shutdown problem.

    If the low voltage comes instead from a switching regulated supply and you can't readily determine the normal output voltage, check for a bad filter capacitor on the feedback winding. Most such power supplies put out several regulated voltages, derived from separate windings on the switching supply transformer, then rectified and filtered, for use in various places in the set. Regulation of these voltages is accomplished by sampling the output from a dedicated feedback winding, and then cranking up the transistor switch if that voltage is too low, or cutting back the transistor switch if the voltage is too high. The idea is that, since all of the output voltages come from the same transformer, with the output voltage of each determined by the number of turns on its winding of the transformer, if one voltage (from the feedback winding)is correct, then they all will be correct. Now if the filter capacitor on the feedback winding opens, lowering the sensed DC voltage from that winding, what will the voltage regulator circuit do? Not realizing that the reduced feedback is due to a bad filter capacitor, it simply cranks up the transistor switch to get the voltage back up where it belongs. But that raises all the other output voltages as well, making them higher than they should be, including the one powering the high voltage supply! And that will trip the shutdown circuit.

    When replacing filter capacitors, be sure to use good ones rated for 105 (not 85) degrees C, and able to withstand the high frequency pulses they are getting hammered by in these circuits.

    Low or no high voltage

    Most of these problems are due to faults in the horizontal deflection system - shorted HOT, shorted windings or HV rectifiers in the flyback, defective tripler, or other bad parts on the primary side of the flyback.

    However, if you discover an inch layer of filth inside the TV, the HV could simply be shorting out - clean it first.

    In most cases, these sorts of faults will put an excessive load on the horizontal output circuits so there may be excessive heating of the HOT or other components. You may hear an audible arcing or sizzling sound from internal shorts in the flyback or tripler. Either of these may bet hot, crack, bulge, or exhibit visible damage if left on with the fault present.

    Most modern TVs do not regulate HV directly but rather set it via control of the low voltage power supply to the HOT (B+), by snubber capacitors across the HOT, and the turns ratio of the flyback. The HV is directly related to the B+ so if this is low, the HV will be low as well. Faulty snubber capacitors will generally do the opposite - increase the HV and the X-ray protection circuits may kick in. However, low HV is also a possibility. The only way the turns ratio of the flyback can change is from a short which will manifest its presence in other ways as well - excessive heating and load on the horizontal output circuits.

    While a shorted second anode connection to the CRT is theoretically possible, this is quite unlikely (except, as noted, due to dirt).

    Excessive high voltage

    Any significant increase in HV should cause the X-ray protection circuits to kick in and either shut down the set or modify the deflection in such a way as to render it harmless.

    Symptoms include arcing/sparking of HV, smaller than normal picture, and under certain scenarios, possible excessive brightness.

    Causes of the HV being too high are:

    1. Excess B+ voltage to the HOT. The likely cause is to a low voltage regulator failure.

    2. Open snubber capacitors across the HOT. These are under a lot of stress and are located near hot components so failure is possible.

    3. Incorrect excessively long scan drive to HOT caused by failure of horizontal oscillator/sync circuits. However, other things like the HOT will probably blow up first. The picture will definitely be messed up.

    4. Failure of HV regulator (tube sets and a few solid state sets - actual HV regulators are relatively uncommon today.) This may result in an underscanned (smaller than normal) picture.

    Snaps, crackles, and other HV breakdown

    Various problems can result in occasional or sustained sparking or arcing sounds from inside the monitor. Note that a static electricity buildup is common on the front of the screen. It is harmless and there iss nothing you can do about it anyhow.

    The following may result in occasional or sustained sounds not commonly associated with a properly working TV or monitor. There may or may not be flashes or blanking of the screen at the same time as the audible noise. See the same-named sections that follow for details.

    Arcing, sparking, or corona from CRT HV anode (red wire/suction cup)

    Symptoms could include a sizzling corona or more likely, an occasional or rapid series of sharp snaps - possibly quite loud and quite visible - from the anode connection (at the suction cup) on the CRT to the grounded coating on the outside of the CRT or a chassis ground point (or any other conductor nearby). Corona is a high resistance leakage through the air without total breakdown. The snapping is caused by the sudden and nearly complete discharge of the CRT anode capacitance through a low resistance ionized path similar to lightning.

    There are two likely causes:

    1. Dirt, dust, grime, around and under the suction cup on the CRT are providing a discharge path. This may be more severe in humid weather. Safely discharge the HV and then remove and thoroughly clean the HV suction cup and the area under it and on the CRT for several inches around the HV connection. Make sure there are no loose wires or other possible places for the HV to discharge to in the vicinity.

    2. The high voltage has gone through the roof. Usually, the X-ray protection circuitry should kick in but it can fail. If cleaning does not help, this is a likely possibility. See the sections: "High voltage shutdown due to X-ray protection circuits" and "Excessive high voltage".

    Arcing from flyback or vicinity

    Arcing may be visible or audible and result in readily detectable levels of ozone. Note that very slight traces of ozone may not indicate anything significant but if the TV smells like an office copier, there is probably some discharge taking place.

    WARNING: It is possible for arcing to develop as a result of excessive high voltage. Symptoms might be a smaller than normal excessively bright picture but this may not be able to be confirmed until the flyback is repaired or replaced. See the section: Excessive high voltage.

    Where repair seems possible, first, clean the areas around the arc thoroughly and then try several layers of plastic electrical tape. If the TV works normally for say, an hour, then there is probably nothing else wrong and you can try for a proper sealing job or hope that tape holds out (put a few more layers on - each is good for about 8-10 kV theoretically).

    However, replacement of the flyback really is the best alternative to minimize risk of future problems. This is the only option where there could be a potential issue of liability should subsequent failure result in a fire.

    Once I had a TV where the main problem was a cracked flyback arcing but this took out one of the fusable resistors for the power supply to the *vertical* output so the symptoms included a single horizontal line. Don't ask me to explain - replacing that resistor and the flyback (the flyback tested good, but this was for someone else) fixed the TV.

    In another case, a pinhole developed in the flyback casing probably due to poor plastic molding at the time of manufacture. This resulted in a most spectacular case of sparking to a nearby bracket. A few layers of electrical tape was all that was needed to affect a permanent repair.

    Dave's complete procedure for repair of an arcing flyback

    (From: Dave Moore (penguin@datastar.net).

    First I clean the afflicted area with Electromotive spray from Autozone. It's for cleaning alternators. On Z-line I remove the focus control and wash with the alternator cleaner and a tooth brush until all dirt and carbon deposits are removed. Then I take an xacto knife and carve out the carbonized hole where the arcing broke through. Then take your soldering iron and close the hole by melting adjacent plastic into it. (clean any solder off your iron with solder-wick first). Then cut some plastic off of some other part off the flyback where it wont be needed and use this to plastic weld (with your iron) a hump of a patch into and over the arc hole. Smooth and seal with iron. Next apply as thick a layer of silicone rubber as you can and let dry overnight.

    Arcing at spark gaps and gas discharge tubes on CRT neck board or elsewhere

    These are protective devices intended to breakdown and divert excessive voltage away from the CRT (usually).

    This is rarely due to a defective sparkgap or gas discharge tube but rather is a safety mechanism like a fuse designed to protect the internal electrodes of the CRT if the focus or screen voltage should become excessive. The sparkgap breaks down first and prevents internal arcing in the CRT. These sparkgaps may be built into the CRT socket as well.

    Arcing at a sparkgap or a glowing or flashing discharge tube may be accompanied by total loss of picture or bad focus, brightness or focus fluctuations, or any of a number of similar symptoms. A common cause is a breakdown inside the focus divider (usually part of the flyback or tripler) but could also be due to excessive uncontrolled high voltage due to a failure of the B+ regulator or HOT snubber capacitor, or (ironically) even a short inside the CRT.

    Therefore, like a fuse, don't just replace or disable these devices, locate and correct underlying problem. The CRT makes an expensive fuse!

    Arcing due to bad connections to or disconnected CRT return

    The Aquadag coating on the outside of the CRT is the negative plate of the HV filter capacitor. If this is not solidly connected to the HV return, you will have your 25 kV+ trying to go where it should not be. There should be a wire solidly attached to the CRT neck board or chassis. Without this, voltage will build up until it is able to take some other path - possibly resulting in damage to sensitive solid state components in the process. Therefore, is is important to rectify the situation.

    Warning: If you find this disconnected, don't just attach it anywhere. You may instantly kill ICs or other solid state components. It must be connected to the proper return point on the CRT neck board or chassis.

    Flashovers inside the CRT

    Due to sharp edges on the electron gun electrodes, impurities, and other manufacturing defects, there can be occasional arcing internal to the CRT. Properly designed HV, deflection, and power supply circuits can deal with these without failing but not all monitors are designed well.

    There is nothing you can do about flashovers assuming your HV is not excessive (see the section: Excessive high voltage. If these persist and/or become more frequent, a new CRT or new TV will be needed.

    Ozone smell and/or smoke from TV

    Smoking is just as bad for TVs as for people and usually more quickly terminal.

    White acrid smoke may indicate a failed electrolytic capacitor in the power supply probably in conjunction with a shorted rectifier. Needless to say, pull the plug at once.

    A visual inspection should be able to easily confirm the bad capacitor as it will probably be bulging and have condensed residue nearby. Check the rectifier diodes or bridge rectifier with an ohmmeter. Resistance across any pair of leads should be more than a few ohms in at least one direction. Remove from the circuit to confirm. Both the faulty diode(s) and capacitor should be replaced (though the capacitor may work well enough to test with new diode(s).

    If a visual inspection fails to identify the smoking part, you can probably plug the set in for a few seconds until the source of the smoke is obvious but be prepared to pull the plug in a real hurry.

    If the smell/smoke is coming from the flyback, then it has probably gone belly up. You may be able to see a crack or bulge in the case. While the flyback will definitely need to be replaced, it is likely that nothing else is wrong. However, it might be prudent to use a Variac when performing initial testing with the replacement just in case there is a secondary short circuit or excess HV problem.

    X-ray and other EM emission from my TV or monitor?

    X-ray radiation is produced when a high velocity electron beam strikes a target containing heavy metals. In a modern TVs and monitors, this can only take place at the shadow mask/aperture grille and phosphor screen of the CRT.

    For X-rays, the amount of radiation (if any) will be proportional to brightness. The energy (determined by the CRT high voltage, called kVP in the medical imaging field) is not affected. This is one reason many monitors and TVs are designed with brightness limiting circuits.

    In any case, there will be virtually no X-ray emissions from the front of the CRT as the glass is greater than an inch thick and probably contains some lead for added shielding. Also see the section: Should I be worried about X-ray exposure while servicing a TV or monitor?.

    Electromagnetic radiation (EM) is produced mostly from the deflection yoke and to a lesser extent from some of the other magnetic components like transformers and inductors. Depending on monitor design (some are specifically designed to reduce this), EM emissions can vary quite a bit. Frequencies range from the 50/60 Hz of the power line or vertical scan rate to several hundred kHz in the AM broadcast band. The intensity and spectral distribution will vary depending on horizontal and vertical scan rate.

    A totally black screen will reduce X-ray emission to zero. It will not affect EM emissions significantly as most of this comes from the magnetic parts, particularly the deflection yoke.

    There is no measurable microwave, IR, or UV radiation.

    I refuse to get into the discussion of what, if any, health problems result from low level EM emissions. There is simply not enough data.

    Should I be worried about X-ray exposure while servicing a TV or monitor?

    The only source of X-rays in a modern TV or monitor is from the CRT. X-rays are generated when a high velocity electron beam strikes a heavy metal target. For anything you are likely to encounter, this can only happen in a vacuum - thus inside the CRT. The higher the voltage, the greater the velocity and potential danger. Really old TVs (prior to around 1975) may still have HV rectifier and regulator tubes - other sources of X-rays. However, modern TVs and monitors implement these functions with solid state components.

    The thick front CRT faceplate protects users adequately but there may be some emission from the thinner sides. At 25-30 kV (quite low as X-ray energies go) X-rays will be stopped by almost any metal so what you have to worry about is where there are no shields.

    Other than lowering the brightness (or high voltage!), there isn't anything you can do to reduce X-ray emission from the front of the monitor. Any sort of add-on screen (grounded or otherwise) unless it is made of thick leaded glass, will have no significant effect on X-rays. If you are still concerned, sit farther away.

    However, realistically, there is very little danger. I would not worry about exposure unless you plan to be sitting for hours on the sides, behind, or under the TV or monitor - with a picture (there will be none if the screen is black).

    It is interesting that even those 1.5" Watchman and .5" camcorder viewfinder CRTs have X-ray warning labels even though the high voltage used with these isn't anywhere near high enough to be of any concern!

    More on radiation from TVs and monitors

    (From: Jerry Greenberg (jerryg50@hotmail.com).)

    Your standard TV set or monitor should not exceed about 0.2 mR/Hr of radiation from a distance of 5 cm from any part of the cabinet. Most TV monitor equipment is less than half of this amount.

    The CRT has a coating on the inner wall of its glass envelope, and also there is a metal shadow mask or aperture grill in the front. There is also a metal shroud around its parameter.

    The type of emission from the CRT is known as soft X-Ray emission. This is because it is low power, and is in the lower X-Ray region.

    The X-Ray emission is strongest at the rear of the TV set because there is some opened area where the electron gun is located. But, this is very weak as well. The radiation from a TV or monitor is not being focused to one point, and is also below the threshold level of being dangerous.

    The long term effect of the total radiation from normal operating TV equipment is not fully known. However, the effect of X-Ray radiation is accumulative over time if there are no breaks in between the exposures. As for standard focused X-Rays like the ones used in a medical or security facility, these and most of their effects are well known.

    As for normal working TV equipment, when used normally, the total radiation is less that what you would get when walking on the street. There are many satellites beaming down signals, radio and TV broadcast stations, communications systems, and then cell phones.

    The X-Ray radiation in a TV set is emitted from the effect of the High Voltage drive generating the electron beam. If the High Voltage exceeds the designed safety limit for the CRT, then there is concern that the X-Ray radiation may have some effect on anyone that is in close proximity to the CRT. The amount of by which the high voltage exceeds the design specfifications will determine the total X-Ray emission. Since this emission is not focused into a fine area, its immediate danger is also greatly reduced.

    All TV sets by law must have in their design some type of protection to shut the TV down if there is excessive High Voltage, excessive High Voltage current drive, and a number of other safety criterias.

    There is also the concern about electromagnetic radiation. In fact all radio frequencies are based on electromagnetic radiation (EMR).

    There was a great concern about the low frequency EMR. This would come from the power supply, deflection amplifier stages, and then from the deflection yoke and flyback transformer. There different types of EMR from TV sets.

    Concerning TV's and monitors, this radiation worry comes up from time to time. If a woman is pregnant it would be wiser for her to not expose the unborn baby by working close to a terminal or monitor. This nonexposure is a good policy to make sure that everyone is safe rather than suffer any type of damage or health risks.

    As for a safety concern for a mother to be, or a small baby, they can be in front of a TV set but at least 5 to 7 feet away. From this distance there should not be any danger at all.

    The above is from my personal observations and is very general. I have also read various publications over the years that pertain to this subject.

    I have a personal concern about the radiation from TV sets and monitors because I do an extensive amount of service on these. I am also doing a lot of picture tube changes in monitor equipment. I am then exposed for a few hours because I must do the purity and convergence setups of these sets. I have some days where I work 10 to 12 hours doing TV and monitor service work.

    If you want a TV monitor that will put out near zero X-Ray radiation, and very low electromagnetic radiation, then go for one of the new LCD flatscreen monitors.

    Flyback shot by 4 year old with water pistol

    Your 4 year old son shot the Sony in the flyback transformer. Smoke and sparks everywhere. Great aim!

    Who says these FAQs cannot be funny?

    Needless to say, unplug the set immediately. Inspect around the target area for obviously blown or damaged components. Test fuses and fusable resistors. Repair burnt solder connections and circuit board traces. Once the set is entirely dried out, power it up - preferably through a series light bulb and/or Variac until you are sure nothing else will let loose. Look, listen, and smell for any unusual behavior. If it now works, then consider yourself lucky. If not, there may be damage to transistors, ICs, or other components.

    (From: Richard Symonds (edison@nelson.planet.org.nz).)

    We're seeing another 'hazard' these days, people cleaning their television screens with window cleaner - no problem in the days of separate chassis but with the entire circuit board jammed under the tube on most TVs these days just a few drips and its all over. Some have just corroded the switch banks (had one recently just got into the A/V switch - when you walked around the room the set changed to A/V and back by itself!) but a few have got around the microprocessors and surface mount components and resulted in complete write-offs. I suppose the damage is the opposite of electroplating as the microprocessors have constant voltage to them. Never mind, they'll be a good source of parts for future use.

    Blooming or breathing problems

    There are several symptoms that are basically similar:

    In both these cases, if this just started after some work was done to the TV, the brightness limiter and/or video drive may simply be set so high that the TV cannot supply enough current to the high voltage. If the brightness is acceptable with these turned down slightly and still have acceptable brightness, then there may be nothing wrong.

    Erratic focus or screen (G2) voltage and/or controls on flyback

    Symptoms may include fluctuating focus or brightness. In extreme cases, the result may be a too bright or dark picture or other behavior caused by breakdown in the Focus/Screen(G2) divider network.

    Usually, this will require flyback replacement to repair reliably. Sometimes, the section with the controls can be snapped apart and cleaned but this is not common.

    First, just try rotating the screen (G2) control back and forth a few times. This may clean up the contacts and eliminate the erratic behavior. Possibly, positioning it a bit to one side of the original location will help. Then, use the individual or other master background/bias adjustments to compensate for the improper brightness.

    If pressing in on the erratic control helps to stabilize the setting, you might try adjusting it to the optimal position and then put a dab of hot-melt glue (or Superglue if you can manage not to stick your fingers together) on the shaft to hold it with a little more contact force.

    If none of this helps, here is a 'well it's going in the dumpster anyhow' procedure to try:

    After discharging the CRT (so you don't get zapped) drill a tiny hole in the plastic cover near the bad control. Be careful you don't damage anything inside - you just want access to the contacts of the controls. Use a hand drill with, say, a 1/16" bit. Don't drill more than about 1/8" deep which should enter the airspace. Then spray some contact cleaner through the hole and work the controls. Wait sufficient time (say, 24 hours) for everything to dry COMPLETELY and see if behavior changes (or it works at all).

    This is a 'you have got to be kidding' type of repair so no guarantees :-).

    If by some miracle it does work, fill the hole with a drop of RTV or just put a couple of layers of electrical tape over it.

    Focus/Screen divider bypass surgery

    This is kludge number 41256 but may be the difference between a bit more life and the dumpster.

    If the previous extreme measures don't help, then it may be possible to simply substitute a good divider network externally.

    Note that if there is evidence of internal breakdown in the divider of the original flyback (hissing, cracks, overheating, bulging case, etc.), this will not work unless you can disconnect it from its HV connection.

    There are two issues:

    1. Is this a stable situation? Even if you provide an external substitute, the parts inside the flyback may continue to deteriorate eventually resulting in other more total failure of the flyback or worse.

    2. If you provide an external focus/screen divider, it must be done is such a manner (including proper mounting and super insulation) such that it cannot be called into question should there be a fire where the monitor is even the slightest bit suspect.

    Various size external focus/screen divider networks can be purchased but whether this is truly a cost effective solution is not obvious.

    (From: Larry Sabo (sabo@storm.ca).)

    I just ordered a 'bleeder resistor' from Data Display Ltd (Canadian sub of CCS) to use as a cure for flybacks with flaky focus/screen pots. It contains focus and screen pots, and costs Cdn$ 16.99, which is a lot less than a complete flyback, that's for sure. I expect it will be compatible with quite a wide range of flybacks.

    I have used bleeder resistor assemblies from duff flybacks a couple of times with good success. You connect the HV lead into the HV cap of the original flyback, ground all pins of the sub flyback, and use the focus and screen leads from the sub bleeder assembly in place of the originals.

    Looks like hell but works fine. Mounting (and securing) the substitute is a challenge given the limited space available. I only use this approach on what would otherwise be uneconomical to repair, and always advise the owner or customer of the cobbling job. It also enables you to verify whether it is the flyback that needs replacement, versus the CRT.

    Decaying or erratic focus or screen (G2) voltages

    The following applies to both CRT focus voltage (which should be a few kV) and screen or G2 voltage (which should be several hundred V).

    "The screen voltage will come up to normal after sitting over night, 400 V or so. After approximately 5 minutes or slightly longer, I hear a slight arcing. From that point on, the screen voltage will wander anywhere from 75 V up to maybe 150 V. Adjustment of the screen control on the flyback has only a small effect and is not permanent. Removing the CRT pcb results in the screen voltage returning to normal."

    This is very likely a short between electrodes inside the CRT unless there is something on the neck board that is breaking down as a result of some connection to the CRT. The flyback should largely not know the difference with the socket plugged into the CRT. However, on rare occasions, there is contamination within the 'plastic alignment base' on the end of the CRT neck. (It is possible to *carefully* remove the plastic piece and clean the CRT glass/pins. Reinstall the plastic piece if it is still intact or leave it off - just take care in replacing the CRT neck board.)

    One possibility is that glue used to hold components down on some circuit boards has deteriorated and turned conductive. Check for tan to brown stuff shorting traces on the CRT neck board. If this is present on the focus or screen traces or wires, it may just be your problem. Scrape off all of the old glue and then clean thoroughly. Repair any damaged traces.

    What happens to the HV? A HV breakdown possibly inside the CRT would result in all the voltages being dragged down.

    What happens to the picture?

    If you connect a charged HV capacitor (guessing a couple hundred volts, a couple microfarads) between G2 and G1 or focus, you **will** know if tapping the neck results in a momentary short! I cannot predict whether this will be a temporary cure or permanent killer. See the section: Rescuing a shorted CRT.

    Here is another thing to try: put a 100 M ohm or so resistor between SCREEN and the CRT socket. This should not affect the behavior much until the failure occurs. Then, check the voltage on both sides with a high impedance voltmeter (1000 M). If the CRT is arcing, it will be much lower on the CRT side and will probably fluctuate. You can play similar games with focus voltage.

    Disconnecting flyback wire(s) from CRT driver board

    In some cases, there may be one or more separate wires running to directly to the CRT socket. These are typically for focus which has a relatively high voltage so better insulation is needed but there may be no obvious means of removal should flyback replacement be needed.

    One alternative is simply to cut the wire(s) in a location that is well away from any place to short out, solder, and then do a most excellent job of insulating the splice. If there is more than one wire, make sure to label them first if they aren't color coded.

    However, you may find that the cap on the CRT socket snaps off using a thin knife blade or screwdriver. The wire may be soldered or just pressed in place in such a way that pulling it out is difficult or impossible without removing the cover. If there is more than one wire, label them before removal unless the locations are clearly marked. Sometimes the color is stamped on the plastic but there may just be a designation like "A" and "B".

    (From: Raymond Carlsen (rrcc@u.washington.edu).)

    The last one I worked on puzzled me for a few moments. See if you can see a space between the little cup (where the wire enters the socket) and the socket itself. Pry up on the cap with a knife and it should pop right off. The wire is soldered to a pin under it. Don't apply heat for very long... you may melt the socket.

    Focus or screen voltage drifts after warmup only when CRT is connected

    "I have a 3-5 yr old TV that loses screen voltage. I believe that the problem is specific to the CRT or the flyback, either one is a guess I'd rather be sure of prior to ordering a part.

    The screen voltage will come up to normal after sitting over night, 400 V or so. After approximately 5 minutes or slightly longer, I hear a slight arcing. From that point on, the screen voltage will wander anywhere from 75 V up to maybe 150 V. Adjustment of the screen control on the flyback has only a small effect and is not permanent. Removing the CRT pcb results in the screen voltage returning to normal.

    I cannot find the source of the arcing, as it happens quickly and I have always been on the other side of the set when it happens. I have replaced the CRT socket, thinking the spark gap was arcing. I have checked the CRT for G1 and HK shorts on a sencore CRT checker, it checks good, but I am aware that since it is an intermittent problem, that the checker probably will not catch it."

    This is very likely a short between electrodes inside the CRT unless there is something on the neck board that is breaking down as a result of some connection to the CRT. The flyback should largely not know the difference with the socket plugged into the CRT. However, on rare occasions, there is contamination within the 'plastic alignment base' on the end of the CRT neck. (It is possible to *carefully* remove the plastic piece and clean the CRT glass/pins. Resinstall the plastic piece if it is still intact or leave it off - just take care in replacing the CRT neck board.)

    One possibility is that glue used to hold components down on some circuit boards has deteriorated and turned conductive. Check for tan to brown stuff shorting traces on the CRT neck board. If this is present on the focus or screen traces or wires, it may just be your problem. Scrape off all of the old glue and then clean thoroughly. Repair any damaged traces.

    What happens to the HV? A HV breakdown possibly inside the CRT would result in all the voltages being dragged down.

    What happens to the picture?

    If you connect a charged HV capacitor (guessing a couple hundred volts, a couple microfarads) between G2 and G1 or focus, you **will** know if tapping the neck results in a momentary short! I cannot predict whether this will be a temporary cure or permanent killer.

    Here is another thing to try: put a 100 M ohm or so resistor between SCREEN (or FOCUS) and the CRT socket. This should not affect the behavior much until the failure occurs. Then, check the voltage on both sides with a high impedance voltmeter (>1000 M). If the CRT is arcing, it will be much lower on the CRT side.



  • Back to TV Repair FAQ Table of Contents.

    Raster, Color, and Video Problems

    No color - black and white picture

    This means absolutely no color - equivalent to a black and white picture. Not even a hint of color.

    First, confirm that the source is actually in color - try another channel or input device.

    Next, check the settings of the color control - it may have accidentally been turned down. If your TV has some kind of automatic picture mode, try turning if off and adjusting the color control. Try adjusting fine tuning if you have such a control and the problem is with a broadcast or cable transmission.

    At this point with a confirmed color signal source, there is a problem with the chroma circuitry.

    Note that to the average person, the obvious question becomes: is my color picture tube bad? The answer is a definitive NO. It is virtually impossible for a defective CRT to cause a total loss of color. A defective CRT can cause a lack of a primary color - R, G, or, B or a short between two colors which will mess up the color but is not likely to result in a black and white picture.

    Some possibilities in no particular order:

    1. Weak signal or defect in tuner/IF causing loss of signal strength.

    2. Coler killer set too high (internal control) if it has one.

    3. Defective part around the chroma chip/circuit. Faulty color oscillator.

    4. Bad connections in area of chroma chip/circuit.

    5. Defective chroma chip (don't suspect this first just because it is probably very expensive).

    A service manual or Sams', DMM, and scope will help greatly in attempting to troubleshoot this unless it is an obvious bad connection. Try prodding the main board around the chroma chip with an insulated tool to see if you can make the color come and go.

    I had one set where a $.02 resistor decided to open up causing just this problem - perfect BW picture, no color. Another had a coil with a broken wire.

    Saturated color but almost no brightness

    This means you have lost the luminance input to the chroma decoder or final video chip. A failure of the brightness limiter may result in similar symptoms.

    A few common causes are:

    With a scope and schematic (or even just a pinout for the chip), you should be able to trace the luminance signal to see where it is getting lost.

    This is also *not* a picture tube problem. :-)

    (From: John Mehrtens (sarge@drag-net.com).)

    I have had several TVs and monitors where the delay line in the luminance circuitry has failed. Usually it's made out of glass, and inherently is fragile. Sometimes whacking the monitor would make it come back, leading to the thought of connectoritis or a cold solder joint -- where in fact it was the delay line (long rectangular unit with two to four leads). Replacing the delay line was the solution, but to check it first it'd be a good idea to look for 'in' and 'out' on the line and short the pins. The picture may be shifted, colours may not line up, but it'll tell you that it's the delay line if the picture comes back at all. It's better than looking at a saturated picture with no luminance! :)

    Brightness control has no effect

    The following assumes that the picture is fine but the brightness is fixed - probably at too high a level. However, there could be several interrelated problems if a common supply voltage were missing, for example.

    If it is a knob, then it should be varying the control grid (G1) voltages relative to the cathodes (K) of the CRT. This is not likely to be a very complex circuit. If you do not have a schematic, I would start by tracing from the control, check continuity and solder connections. Check the control itself for proper operation with an ohmmeter. A power supply going to one side of the control (negative probably) may be missing. Tbe control grid voltage will end up on the little board on the neck of the CRT - check there as well for bad solder connections or open resistors.

    If brightness is a digital control, then you will need a schematic unless there is an obvious bad connection.

    One color is too weak or too strong

    If the problem is slight and/or has gradually gotten worse, this may just require an adjustment of the color brightness/background/bias and/or color gain/drive controls inside the TV. See the section: Color balance adjustment.

    Note that if it is possible to obtain a good black and white picture with the user color control set to its minimum, then this is not likely a problem with one of the primary color channels (red, green, or blue) but with the chroma decoding circuitry. Or, perhaps, you are just watching MTV!

    Even if it appears as though there is an excess, this may actually be a reduction in one of the primary colors. For example, a magenta tinge is represents a reduction in the strength of the green signal.

    A color that that is now suddenly brighter or darker than normal resulting in incorrect color balance or a tint in the background could be due to a number of causes:

    Psychodelic color

    The means colors that are not normal and that adjustment of the user controls is not able to correct it so that all colors of the picture are properly displayed at the same time. For example, you are unable to get any yellows or blues in scenes that should have these colors..

    Make sure the user color and tint controls have not been accidentally turned while cleaning or purposedly misadjusted by small (or large) kids.

    Perform the user setup described in the section: User picture adjustment.

    Confirm that the source is not a weird color video - try another channel or a tape.

    Verify that this is not a missing color problem - one of the primary R, G, or B, has disappeared. If so, refer to the section: Intermittent or missing colors.

    Once these have been eliminated, you are left with the following possibilities:

    1. Defective part around the chroma chip/circuit. Misadjusted color oscillator.

    2. Bad connections or short circuit in area of chroma chip/circuit.

    3. Defective chroma chip (don't suspect this first just because it is probably very expensive).

    4. Bad degauss circuit resulting in lack of degauss or abrupt termination of degauss current rather than smooth tail off. The CRT is not being properly demagnetized and color purity is totally messed up.

    5. Bad CRT - the shadow mask has been damaged and it is impossible to properly adjust purity across the screen.

    A service manual or Sams', DMM, and scope will help greatly in attempting to troubleshoot this unless it is an obvious bad connection. For (1)-(3), try prodding the main board around the chroma chip with an insulated tool to see if you can restore normal color. For (4) try manually degaussing (see the section: Degaussing (demagnetizing) a CRT. If this clears up the colors until at least when it is power cycled, then a degauss problem is likely.

    Something as simple as a bad resistor or inductor can be the cause - don't immediately suspect the most expensive and difficult to replace part.

    No picture/dark picture/erratic picture

    If everything else works - there is sound and high voltage (static on the screen) - but there is no picture (including no on-screen menus), this limits the likely area of problems is the video output circuitry or CRT filament connections. Check to see if the CRT filaments are glowing. If they are not glowing, check for bad soldering on the CRT neck board and around the flyback transformer.

    Where the picture is erratic - coming and going entirely or changing brightness suddenly, with power off, remove the picture tube socket (carefully!) and clean the pins with fine sandpaper and use contact cleaner on the socket. This source of bad connections can result in a variety of erratic symptoms. Check for bad solder connections on the CRT neck board.

    TV and monitor manufacturing quality and cold solder joints

    Any intermittent problems with monitors that cause random sudden changes in the picture brightness, color, size, or position are often a result of bad connections. Strategically placed bad connections can also cause parts to blow. For example, a bad connection to the SCR anode in a phase controlled power supply can result in all the current passing through the startup resistor, blowing it as well as other components. I had a TV like this - the real problem was a bad solder joint at a pin on the flyback. Thus, erratic problems, especially where they are power or deflection related, should not be ignored!

    Bad solder joints are very common in TVs and monitors due both to poor quality manufacturing as well as to deterioration of the solder bond after numerous thermal cycles and components running at high temperature. Without knowing anything about the circuitry, it is usually possible to cure these problems by locating all bad solder connections and cleaning and reseating internal connectors. The term 'cold solder joint' strictly refers to a solder connection that was either not heated enough during manufacturing, was cooled too quickly, or where part pins were moved before the solder had a chance to solidify. A similar situation can develop over time with thermal cycling where parts are not properly fastened and are essentially being held in by the solder alone. Both situations are most common with the pins of large components like transformers, power transistors and power resistors, and large connectors. The pins of the components have a large thermal mass and may not get hot enough during manufacturing. Also, they are relatively massive and may flex the connection due to vibration or thermal expansion and contraction.

    To locate cold solder joints, use a strong light and magnifier and examine the pins of large components for hairline cracks in the solder around the pin. Gently wiggle the component if possible (with the power off). Any detectable movement at the joint indicates a problem. With the power on, gently prod the circuit board and suspect components with an insulated tool to see if the problem can be effected.

    When in doubt, resolder any suspicious connections. Some monitors may use double sided circuit boards which do not have plated through holes. In these cases, solder both top and bottom to be sure that the connections are solid. Use a large enough soldering iron to assure that your solder connection is solid. Put a bit of new solder with flux on every connection you touch up even if there was plenty of solder there before.

    Why can't TV manufacturers learn to solder properly?

    I can think of several potential reasons - all solvable but at higher manufacturing cost.

    1. Mass of large component leads (like shields) does not get adequately heated during manufacture leading to latent cold solder joints. While they may look ok, the solder never actually 'wetted' the heavy pins and therefore did not form a good mechanical or electrical bond.

    2. Thermal cycles and differential thermal coefficients of circuit boards, traces, and solder. While it is not easy to do anything about the material properties, using plated through-holes or a similar mechanical via would greatly increase the surface area of the joint and prevent the formation of cracks.

    3. Vibration. This is also directly related to the single sided circuit boards without plated through-holes to strengthen the joints.

    4. Lack of adquate mechanical support (single sided circuit boards without plated through-holes (vias).

    I believe that the single most significantimprovement would come about by using plated trhough-holes but this would add to the cost and apparently the consumer is not willing to pay more for better quality and reliability! Some designs have used rivlets - mechanical vias instead of plated ones. While this is good in principle, the execution has often been flawed where cold solder joints resulted between the rivlets and the circuit board traces due to lack of adequate process control.

    The Sony and RCA/GE tuner shield problem is interesting because this could have been solved years ago at essentially no additional cost as other manufacturers - and their own repair procedures - have proven.

    Intermittent or missing colors

    This is a catch-all for some of the most common TV and monitor problems.

    Note that due to the additive color scheme used in all emissive color displays like CRT or flat panel TV sets and video monitors, a single missing primary color (red, green, or blue) will result in the following appearance (for a white screen):

        Missing Color       Appearance
      ------------------------------------------------
           Red              Cyan (blue-green)
           Green            Magenta (reddish-purple)
           Blue             Yellow
    

    Which color is affected may be even more obvious if the set has a color on-screen display for which you recall the proper colors.

    To narrow down the problem:

    Most of the causes of intermittent colors boil down to bad connections of one form or another. For totally dead colors - not intermittent - bad components are also a possibility.

    Some commentary on monitor and TV whacking

    Anytime that intermittent symptoms are experienced, I recommend gently whacking the patient to determine if mechanical shock or vibration affects the behavior. Here are a couple of responses to this suggestion.

    (The following is from Marc Gelfond (71363.1700@CompuServe.COM).)

    I just love the bit about "whacking it". It brings to mind an episode from the old Andy Griffith show, where a new fangled piece of electronics gear, was broght into Emmets repair shop. After many long hours of fruitless troubleshooting, out of frustration Emmet gave the thing a whack, and sure enough it fixed the problem.

    As we say in the Telephony business, it "CCWT" or Came Clear While Testing. Another saying is that it "CCBFM" Came Clear By F------ Magic!!

    (To which Gavin Adams (gaa@hopi.com) comments):

    In the video industry we had a saying concerning malfunctioning gear:

    "If it's broke, hit it with a hammer"
    "If that doesn't fix it, paint it and sell it"

    My DEC 16" monitor is case in point. Evey once in a while it would lose sync, and smacking it would bring it back (sometimes a few smacks). Recently it gave up the ghost completely, and after the local DEC office gave me a quote of $900 to fix it (Bermuda), I ordered a new Viewsonic 17" for the same price.

    I ripped the guts out of the DEC beast, painted it with a marble finish, put plants in it, and sold it! :>

    Retrace lines in picture

    During the time the electron beam is returning from right to left at the end of a line and bottom to top (over the course of multiple lines), it is supposed to be result in no visible light on the screen. However, a number of faults can result in visible retrace lines.

    The appearance will likely be a general reduction in contrast from the visible horizontal retrace on every scan line and two dozen or so diagonal lines lines (lower left to upper right) resulting from the vertical retrace.

    The retrace lines may be either white or gray (possibly with a slight color tint due to unequal settings of the color adjustments) or a primary color - red, green, or blue. Anything in between is also possible but less likely.

    White/gray retrace lines

    Where all colors are involved - the lines are essentially white or gray (or with a slight tint due to slight unequal settings of the color adjustments), look for something common like an incorrectly adjusted screen (G2) or master brightness/background/bias control or a problem in one of these circuits, a defective power supply or a problem in the blanking circuitry:

    Red, green, or blue retrace lines

    Where only one color is showing, suspect an incorrectly adjusted individual background/bias control or bad part on the CRT neck board for that color.

    There is a slight possibility that a bad CRT may result in visible retrace lines. To eliminate this possibility:

    See the section: Bad CRT causing retrace lines.

    Bad CRT causing retrace lines

    (From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).)

    The TV which I bought last started developing retrace lines after a month or so of use. I took it back to the lab for warranty (special deal) and had it examined by the real experts. They found that even with the filament supply disconnected and VG2 at 0V the screen would still light up. They could even see that the electrons weren't even coming from the cathode. That was with only the picture tube in a test rig. So in this case the obvious conclusion had to be that the tube was bad, and it was replaced (32" 16:9 SF, very $$). It had something to do with processing problems during manufacturing of the electron guns.

    So even if this was a rare case, it *can* happen that retrace lines are due to a bad picture tube. It's more usual to suspect the VG2 (screen voltage) or a defect somewhere in the RGB video path.

    Red, green, or blue full on - fog over picture

    This could be a heater-cathode (H-K) short in the CRT, a failure of a component in the chroma circuits or video output (driver board), or bad connections there or elsewhere.

    Don't panic - heater-cathode shorts in CRTs can often be worked around.

    Note: before proceeding, it is a good idea to make sure that the screen is degaussed - else you could be attempting to track down problems with the wrong color!

    Some simple tests can confirm or rule out other possibilities.

    Here is the procedure in more detail (example for red full on):

    (From: J. K. Emerine (jkemerine@aol.com).)

    To identify if the fault is in the CRT or a control problem try this (WITH SET OFF):

    On the CRT board, lift the output end of the green cathode final resistor. Do the same with the offending red cathode's resistor. Use short insulated jumpers to 'swap' drive signals - drive the red cathode with the green drive and the green cathode with red drive. (Note that if this problem only occurs after a warmup period, color at turn on will be - well - wierd, but it is just a test.)

    Shorts in a CRT

    Occasionally, small conductive flakes or whiskers present since the day of manufacture manage to make their way into a location where they short out adjacent elements in the CRT electron guns. Symptoms may be intermittent or only show up when the TV or monitor is cold or warm or in-between. Some possible locations are listed below:

    Except for the high voltage to other places, the short may actually be located in the CRT *socket* or even on the CRT neck board, probably in the spark gap(s) for the problem pins. Remove the socket and test between the suspect pins on the CRT itself. If the CRT itself is fine, the spark gaps should be inspected and cleaned/repaired and/or components replaced. At this point, the cause may still be present - a short inside the flyback for example resulting in excessive voltage on one or more pins.

    Assuming this is not the case, replacing the CRT may be the best solution but there are a variety of 'techniques' that can often be used to salvage a TV that would otherwise end up in the dump since replacing a CRT is rarely cost effective:

    1. Isolation - this will usually work for H-K shorts as long as only one gun is involved.

    2. Blowing out the short with a capacitor - depending on what is causing the short, this may be successful but will require some experimentation.

    3. Placing the CRT (TV or monitor) face down on a soft blanket and *gently* tapping the neck to dislodge the contamination. Depending on the location of the short, one side or the other might be better as well. Sometimes, this can be done in-place while watching the picture.

    A combination of (2) and (3) may be required for intermittent shorts which don't appear until under power. See the sections below for additional details. However, for shorts involving the focus and high voltage elements, even a sharp edge can result in arcing even if there is no actual short. There is no remedy for these types of faults.

    Providing isolation for a CRT H-K short

    This procedure will substitute a winding of your own for the one that is built in to the flyback to isolate the shorted filament from the ground or voltage reference. Note that if you have a schematic and can determine where to disconnect the ground or voltage reference connection to the filament winding, try this instead.

    The flyback is the thing with the fat red wire coming out of it (and perhaps a couple of others going to the CRT board or it is near this component if your set has a separate tripler) and may have a couple of controls for focus and screen. It should have some exposed parts with a ferrite core about 1/2-3/4" diameter.

    The filament of the CRT is the internal heater for each gun - it is what glows orange when the set is on. What has happened is that a part of the fine wire of the bad color's filament (assuming this is indeed your problem) has shorted to the cathode - the part that actually emits the electrons. Normally, the heater circuit is grounded or tied to a reference voltage so when it shorts to the cathode, the cathode voltage level is pulled to ground or this reference.

    You will need some well insulated wire, fairly thick (say #18-22). Find a spot on the flyback where you can stick this around the core. Wrap two turns around the core and solder to the CRT filament pins after cutting the connections to the original filament source (scribe the traces on the board to break them). Make sure you do not accidentally disconnect anything else.

    This winding should cause the filaments to glow about the same brightness as before but now isolated from ground. If they are too dim, put another turn on the flyback to boost the voltage as this will result in low emission, blooming, and possible damage to the cathodes after awhile. (Don't go overboard as you may blow the filament totally if you put too many turns on the core - you then toss the TV.)

    Route the wires so that there is no chance of them getting near the high voltage or any sharp metal edges etc. Your picture quality may be a tad lower than it was before because of the added stray capacitance of the filament wiring being attached to the the (formerly bad) video signal, but hey, something is better than nothing.

    If you are not inclined to build your own isolation transformers, kits are available.

    Rescuing a shorted CRT

    If the short is filament-cathode (H-K), you don't want to use the following approach since you may blow out the filament in the process. If this is the case, you may be able to float the filament and live with the short (see the section on: "Red, green, or blue full on - fog over picture".

    Shorts in the CRT that are between directly accessible electrodes can be dealt with in a more direct way than for H-K shorts. At this point you have nothing to loose. A shorted CRT is not real useful.

    If the short is between two directly accessible electrodes like cathode-grid, then as a last resort, you might try zapping it with a charged capacitor. Start with a relatively small capacitor - say a few uF at a couple hundred volts. Check to see if the short is blown after each zap - few may be needed. Increase the capacitance if you fell lucky but have had little success with the small capacitor.

    If the fault is intermittent, you will, of course, need to catch the CRT with the socket disconnected and the short still present. Try some gentle tapping if necessary. If you do this with the charged capacitor across the suspect electrode, you **will** know when the short occurs!

    Also see the section: High voltage to focus short.

    Picture tube replacement

    It is possible to replace the picture tube. However, this is likely to be both expensive and possibly time consuming with respect to adjustments like purity and convergence. When replacing:

    "The 25VCXP22 picture tube of my RCA Accutouch XL-100 CCU-942 TV start fading. Its 100% transistorized, everything still works perfectly after about 20 years service. But:

    (From: Chris Jardine (cjardine@wctc.net).)

    What you have here is genericly referred to as the 25V as opposed to the 25A picture tube. While there are minor differences with respect to the letters after the V for the most part they are interchangeable. When I worked my way through engineering college I worked at a TV repair shop and my job was mostly changing picture tubes. Yeah, we did enough of them to keep a tech busy 4 to 5 hours a day changing them and I got pretty good and could change, color balance, convergence, etc. the tube in about 45 minutes. We for the most part used 3 major tubes, 1) 25A, 2) 25V, and 3) 21FJ (a little nostalgic for those who remember this one). This was back when your TV would have been fairly new (1981 to 82). These are available from many different sources - RCA, Channel Master, Wisconsin Tube, etc. The price would vary depending on the quality of the tube. I remember that we could get a 25A for about $35 at the time due to our volume - one truck per month. The most expensive I've seen them has been just over $200. This is quite a range and there are now many other types of tubes including in-line, trinitron, etc.

    I hope this helps and thanks for the trip down memory lane!

    (From: Chris Jardine (cjardine@wctc.net).)

    The important thing here is that the tube begins with 25V. If it does it should work in your set. The only thing you have to know is whether the tube has 'ears' attached permanently. The 25V comes both with and without these mounting ears permanently attached. I know that you can still get one of these from any of a number of suppliers. I know that Channel Master and RCA (Thomson, whatever!) still make them available as well as any of a number of local CRT rebuilders.

    High voltage to focus short

    Symptoms would be (with the unit powered and high voltage present):

    If the CRT is gassy or up to air, forget it - it might make a decent fish tank :-). In this case, there would be visible arcing INSIDE the CRT probably not confined to a single location.

    However, if there is just a metal whisker between the F and HV, that might be able to be cleared by careful tapping or a charged capacitor. You may even be able to see it if you were to remove the yoke - the gap is pretty large, about 1-2 mm - the last gap between electrodes before the start of the internal (Dag) coating.

    See the section: Rescuing a shorted CRT.

    Note that other damage may have been done as

    Other components including the flyback, HOT, and parts on the CRT neck board and beyond, may have been damaged as a result of the short. Zapping the CRT may be just the beginning of what is required to repair it all.

    Dark picture

    A TV or monitor with a picture that is too dark may have a fault or the CRT may just be near the end of its useful life.

    First, confirm that your video source - computer, camera, etc. - is producing a proper signal.

    Is the brightness at all erratic? Does whacking the monitor have any effect? If so, then you may have bad connections on the CRT driver card or elsewhere. If the brightness tends to fade in and out over a 10 to 20 second period, a bad filament connection is likely. Check for the normal orange glow of the filaments in the neck of the CRT. There should be 3 orange glows. If they are excessively reddish, very dim, or fade in and out, you have located a problem. See the section: Picture fades in and out.

    Common causes of brightness problems:

    1. Dirty CRT faceplate or safety glass. Don't laugh. It sounds obvious, but have you tried cleaning the screen with suitable screen cleaner? It is amazing how dirty screens can get after a few years - especially around smokers!

      Wipe gently with a slightly dampened cloth - not soaking or you may end up with real problems when the water drips down inside and hits the electronics! On TVs with a separate protective faceplate, clean both the front and rear surfaces of this plate as well as the CRT itself.

    2. Old CRT. The brightness of the CRT deteriorates with on-time. It does not matter much how bright your run your TV. An indication of a weak CRT would be that turning up the SCREEN (G2) or master brightness control only results in a not terribly bright gray raster before the retrace lines show up. There may be indications of poor focus and silvery highlights as well. A CRT brightener may help. See the section: Brightening an old CRT.

    3. Bad component in filament circuit or bad connection reducing filament voltage. This should be easy to check - there are only a few parts involved. If it is erratic, bad connections are likely.

    4. Brightness control faulty - bad pot, bad connections, or problem with its power supply. Depending on specific problem, control may or may not have any effect. If digitally adjusted, there could be a problem with the logic or control chip. If the button or menu item has no effect at all, then a logic or control problem is likely.

    5. Improperly set SCREEN (G2) voltage (usually on flyback) or faulty divider network. See the section: Adjustment of the internal SCREEN and color controls.

    6. Improperly set video bias (background) levels or fault in video drive circuitry. See the sections starting with: "Optimal procedure for setting brightness/background and screen adjustments".

    7. Fault in video amplifiers. With all three color affected equally, this would most likely be a power supply problem. A video amplifier problem is likely if turning up the SCREEN (G2) or master brightenss control results in a very bright raster before the retrace lines appear. Cheack signals out of the video/chroma(IC.

    8. Fault in beam or brightness limiter. Many TVs and monitors measure the beam current (possibly indirectly) and limit the maximum to a safe value. The purpose of this may be to protect the CRT phosphors, and/or to assure that the power supply does not go out of regulation, and/or to limit X-ray emission. If this circuit screws up, a dark picture may result. Checking the signals and voltages at the CRT socket should determine if this is the problem.

    9. High voltage is low. However, this would likely result in other symptoms as well with focus, size, and geometry.

    Brightening an old CRT

    If performing adjustments of the internal background and/or screen controls still results in a dark picture even after a long warmup period, the CRT may simply be near the end of its useful life. In the old days of TVs with short lived CRTs, the CRT brightener was a common item (sold in every corner drugstore, it seemed!).

    You can try a similar approach. Caution: this may shorten the life of the CRT - possibly quite dramatically (like it will blow in a couple of seconds or minutes). However, if the monitor or TV is otherwise destined for the scrap heap, it is worth a try.

    The approach is simple: you are going to increase the voltage to the filaments of the electron guns making them run hotter. Hopefully, just hotter enough to increase the brightness without blowing them out.

    Voltage for the CRT filament is usually obtained from a couple of turns on the flyback transformer. It is usually easy to add an extra turn or two which will increase the voltage and thus the current making the filaments run hotter. This will also shorten the CRT life - perhaps rather drastically. However, if the TV or monitor was headed for the dumpster anyhow, you have nothing to lose.

    Picture tube brightener

    (From: Kevin Carney (carneyke@mhv.net).)

    Try a CRT brightener from MCM Electronics about $20. It boosts the filament voltage a volt or two. I have used them before and they help. You can also try running a power supply on the filament with the monitor OFF. Set the supply at the filament voltage and slowly bring the voltage up. If the filament is 6.3 volt bring it up gradually to 10 -12 volts for about a half hour. This will brighten it up some. Be careful because too much voltage can open the filament !

    Before doing this did you check the screen voltage setting and the RGB settings for drive and background ?

    There are also commercial CRT rejuvenators that supposedly zap the cathodes of the electron guns. A TV repair shop may be able to provide this service, though it is, at best, a short term fix.

    More drastic measures to brighten CRT

    (From: LEE (leep@mailhub.scf.lmsc.lockheed.com).)

    As a start, I crank the brightness control all of the way up. I then turn the color control all of the way up. I let the set run with a bright screen for around 15 min. This procedure cleans up the cathode surfaces so that they can emit more electrons. Now turn the controls back to normal and see if any improvement took place. If not, Wrap 2 or 3 turns of around 18 gauge insulated wire around the flyback and add this extra power in series with existing filament leads from flyback. You can experiment with the number of turns etc. to get brighter filaments. do not run the filaments white - just a brightened yellow. This will probably turn out to be around 8-9v in most cases. I had to do this on two different Sanyo replacement flybacks as they had low filament voltage from the factory. (flakey replacement parts). I`ve been running one of these Sanyos for around 4 years now with a nice bright picture (13")

    Left portion of screen is dark or faded

    "I've got an old TV where the left 1/3 of the screen is 'faded'. It is especially noticable when a dark picture is showing (like a night time scene)."

    This is normally caused by a bad filter capacitor on the power supply line (typically 200 V) that feeds the RGB output transistors. It is usually a scan derived voltage off of the flyback. Look for an electrolytic capacitor of around 4.7 to 10 uF, 160 to 250 V fed from a rectifier diode on this supply.

    Color balance changes across screen from left to right

    The characteristics are that a solid white screen will tend to be blue tinted on one side and red tinted on the other. This is usually a subtle effect and may be unavoidable with some designs.

    There are several possibilities:

    1. Purity - this means the beams are landing on the wrong phosphor dots. This is what would be affected by moving from one location to another or even rotating the TV on its base without degaussing. If the problem just appeared, degaussing may be needed.

      What do you have near the TV or monitor? Loudspeakers or other devices which generate magnetic fields can easily cause all sorts of color purity problems. Relocate the offending device(s) or the TV or monitor and then degauss it.

      See the section: Degaussing (demagnetizing) a CRT.

      If the problem still persists, purity adjustment may be needed. However, this isn't likely to have changed so look for other causes before tackling these adjustments.

    2. Unequal electron gun to shadowmask/screen distance - the electron beams for the red and blue video travel slightly different distances on the left and right sides of the screen so their intensity (due to focus not being optimal and other factors) in each case may differ slightly affecting color balance.

    3. Doming - This would only happen in very bright areas and causes the shadow mask to expand and distort. (Doming should not be a problem with Trinitron CRTs which use tensioned wires in their aperture grill.) This would also not really affect left-right color balance in particular.

    I don't really know how much of a problem (2) is in practice or whether some manufacturers compensate for it.

    Bleeding highlights

    On very bright areas of the picture, one or more colors may bleed to the right resulting in a trail of those colors. The difference between this problem and the section: Trailing lines in one or more colors is that in this case, only highlights are affected.

    One cause of this is that the color gain, contrast, or intensity controls (whatever they are called on your set) are set too high. See the section on: "Color balance adjustment". Check the settings of any brightness limiter controls as well.

    Trailing lines in one or more colors

    Assuming this is not a form of ghosting resulting from poor reception conditions, then it could be any of the following:

    One simple test would be to swap two of the color outputs to the CRT pins. If the behavior moves with the swap (i.e., from red to blue), then it is likely an electronic problem. If it is still the same colors, it is probably the CRT.

    Brightness changes from left-to-right across screen

    Slight variations in brightness across the face of the CRT are not unusual. In fact, if you used a photometer to actually measure the brightness, you might be amazed at the actual variance even with the best monitor or TV - you just don't notice it. However, a major variation - usually a decay from left to right but could be the other way indicate a component failure. Of course, make sure the face of the screen is clean!

    Picture fades in and out

    If the picture faded away on the order of 10-20 seconds (and if it comes back, also comes up to full brightness in same time frame - possibly with the persuasion of some careful whacking) AND with NO other significant changes such as size, focus, etc., then take a look in the back of the tube for the filament to be lit - the orange glow near the CRT socket. If there is none, then you probably have a bad solder connection on the circuit board on the neck of the CRT. Look for fine cracks around pins on that board. Try prodding it with an insulating stick to see if the picture comes back. Resolder if necessary. Dirty or corroded CRT pins/socket contacts can also do this - remove, inspect, clean, and replace the neck board. It is probably not a bad CRT as the filaments are usually wired in parallel and all would not likely go bad at the same time.

    However, if only a single color fades in and out, then a bad connection inside the CRT is a distinct possibility - look for only one of the filament's glow to be coming and going. This is probably not worth fixing.

    If the picture faded away with other symptoms, then there is probably a fault in the video amplifier/output one of its power supplies - still probably a loose connection if you are able to get it back by whacking.

    Occasional brightness flashes

    These may last only a fraction of a scan line or much much longer.

    This could mean an intermittent fault in a variety of places including the video circuitry and SCREEN power supply:

    If you get it from all sources, then tuner/IF is ruled out.

    Suppose you just have no signal to a direct video input. What do you get? If you still get flashes, it should be real easy to monitor either the video outputs or SCREEN supply (with a HV divider on your scope) for noise. Then trace back to power or noise source.

    Excessive brightness and/or washed out picture

    There are a number of possibilities including incorrect screen (G2) or bias (G1) voltages, or a problem in the video or blanking circuitry. Any of these could be the result of bad connections as well. A short in the CRT can also result in these symptoms.

    First, check for bad connections/cold solder joints by gently prodding with an insulating stick. Check voltages and bias levels.

    Bad focus (fuzzy picture)

    Focus voltage on the CRT is usually in the range of 2-8 kV DC and should be controllable over a fairly wide range by the focus pot - usually located on the flyback or a little panel in its vicinity:

    Also see the sections: "Focus adjustment" and "Focus drifts with warmup".

    The focus wire usually comes from the flyback or if the general area or from a terminal on a voltage multiplier module in some cases. It is usually a wire by itself going to the little board on the neck of the CRT.

    If a sparkgap (a little 2 terminal device with a 1/8" gap in the middle) is arcing with power on, then the resistive divider has shorted inside the flyback, focus board, or HV multiplier - whatever you TV has - and the this unit will need to be replaced. Ditto if the SCREEN control affects focus and/or vice-versa.

    Using a suitable high voltage meter (range at least 10 kVDC, 1000 M ohm or greater input impedance), you should be able to measure it connected and disconnected. The ground return will be the outside coating of the CRT which may or may not be the same as the metal chassis parts. If the voltage is very low (less than 2 kV) or too high and the pot has little effect:

    Focus drift with warmup

    This could be due to a problem with the focus voltage power supply, components on the CRT neck board, or a tired worn CRT.

    Focus is controlled by a voltage of 2-8 kV DC usually derived from the flyback transformer and includes some resistors and capacitors. One of these could be changing value as it warms up. (assuming nothing else changes significantly as the unit warms up - e.g., the brightness does not decrease.)

    Focus voltage is derived from a subset of the high voltage winding on the flyback using a resistive voltage divider which includes the focus pot. These are extremely high value resistors - 200 M ohm is common - and so leakage of any kind can reduce or increase the focus voltage. All other things being ok - i.e., the picture is otherwise fine - I would suspect this type of failure rather than the CRT.

    The connection to the CRT is usually a separate wire running from the flyback or its neighborhood to the CRT neck board. Look for components in this general area. Use cold spray or a heat gun to isolate the one that is drifting. If you have access to a high voltage meter, you should be able to see the voltage change as the TV or monitor warms up - and when you cool the faulty part. If it is in the flyback, then sometimes the part with the adjustments clips off and can be repaired or cleaned. Most often, you will need to replace the flyback as a unit.

    Bad focus and adjustment changes brightness

    This is the classic symptom of a short between the focus and screen supplies - probably in focus/screen divider which is part of the flyback or tripler. If you have a high voltage meter, measuring the focus voltage will show that (1) it is low and (2) it is affected by the SCREEN control Similarly, the SCREEN voltage will be affected by the FOCUS control (which is what is changing the brightness.

    There is a slight possibility that this may be in the CRT as well. Measure the FOCUS and SCREEN voltage with a high voltage meter. If they are identical pull the plug on the CRT. If they are now their normal values, then a shorted CRT is a distinct possibility - see the section: Rescuing a shorted CRT.

    Charlie's comments on focus problems

    (From: Charles Godard (cgodard@iamerica.net).)

    Most true focus problems that I have encountered (when the IHVT is ok) are related to leaks or resistance on the focus output. The diming of the screen when the focus pot is adjusted leads me to think in terms of a leaky socket. I'd remove the ground from the CRT socket to the tube dag and see if it sparks. If so there may be a leak in the socket to ground. It could also be leaking to another pin, such as the screen grid. A rhetorical question: What happens to the screen voltage when the focus pot is adjusted?

    I have seen sockets that had no arching or other telltale signs, leak through the plastic housing to ground out the focus voltage.

    Look closely at the screen. If the blurring is in the form of small circles, then you have an open or hi-resistance focus electrode inside the tube. The circles may vary in visibility with brightness.

    If you still haven't found the problem, try to confirm that this is truly a focus problem. Remove the CRT socket and observe the hi-voltage. If it climbs more than about 1k, say all the way up to 25 kV, then you may have a beam current problem rather than a focus problem. In that case re-check all CRT board voltages. WARNING: Removing the CRT socket and powering the set may destroy the CRT on some models. See the section: Warning about disconnecting CRT neck board.

    If you have done all of the above and removing the socket makes no change in the high voltage, then try to determine why the high voltage is low.

    Watch the screen as the brightness, contrast, or screen control are adjusted. See if you can observe any signs of blooming. When the IHVT doesn't provide enough current to satisfy the demands of the tube for current, the the picture tends to appear to expand like a balloon. i.e., bloom. This can be caused by not enough drive to the IHVT. Carefully monitor the b+ to the horizontal drive stages to see that is is stable and correct.

    Blank picture, good channel tuning and sound

    Since the tuner and sound are ok, horizontal deflection which usually generates power for most of the set is also working.

    Does 'blank picture' means a totally black screen with the brightness and contrast controls having no effect whatsoever? Or, is there is no picture but there is a raster - scan lines on the screen? The direction in which troubleshooting should proceed differ significantly depending the answer.

    Here are some questions:

    1. As above, is there any light on the screen at any settings of the brightness and contrast controls, and/or when switching channels? Can you see any raster scanning lines?

    2. Can you hear the high pitched (15735 Hz) of the horizontal deflection?

    3. Looking in the back of the set, can you see the glow of the CRT filament?

    4. Do you get that static on the front of the tube that would indicate that there is high voltage? Any cracking or other normal or abnormal sounds or smells?

    Possible causes of no raster:

    Possible causes of no video (but a good raster): Problem in video IF, video amplifiers, video output, cutoff due to other fault.

    It could be as simple as a bad connection - try gently prodding the boards with an insulated stick while watching the screen. Check for loose connectors and reseat all internal connectors.

    Purple blob - or worse

    Have you tried demagnetizing it? Try powering it off for a half hour, then on. Repeat a couple of times. This should activate the internal degausser. See the section: Degaussing (demagnetizing) a CRT.

    Is there any chance that someone waved a magnet hear the tube? Remove it and/or move any items like monster speakers away from the set.

    Was your kid experimenting with nuclear explosives - an EMP would magnetize the CRT. Nearby lightning strikes may have a similar effect.

    If demagnetizing does not help, then it is possible that something shifted on the CRT - there are a variety of little magnets that are stuck on at the time of manufacture to adjust purity. There are also service adjustments but it is unlikely (though not impossible) that these would have shifted suddenly. This may be a task for a service shop but you can try your hand at it if you get the Sams' Photofact or service manual - don't attempt purity adjustments without one.

    If the set was dropped, then the internal shadow mask of the CRT may have become distorted or popped loose and you now have a hundred pound paper weight. :( If the discoloration is slight, some carefully placed 'refrigerator' magnets around the periphery of the tube might help. See the section: Magnet fix for purity problems - if duct tape works, use it!.

    It is even possible that this is a 'feature' complements of the manufacturer. If certain components like transformers and loudspeakers are of inferior design and/or are located too close to the CRT, they could have an effect on purity. Even if you did not notice the problem when the set was new, it might always have been marginal and now a discoloration is visible due to slight changes or movement of components over time.

    Color rings - bullseye pattern

    This probably means the degaussing circuitry is terminating suddenly instead of gradually as it should. The most likely cause is a bad solder connection to the degauss thermistor or posistor or something feeding it.

    You can confirm this by manually degaussing the screen with the TV or monitor turned on. If the problem disappears, the above diagnosis is probably valid. Check for bad solder connections in the vicinity of the degauss components and AC line input.

    Magnet fix for purity problems - if duct tape works, use it!

    The approach below will work for slight discoloration that cannot be eliminated through degaussing. However, following the procedures in the section: CRT purity adjustment would be the preferred solution. On the other hand, the magnets may be quick and easy. And, where CRT has suffered internal distortion or dislocation of the shadowmask, adjustments may not be enough.

    In any case, first, relocate those megablaster loudspeakers and that MRI scanner with the superconducting magnets.

    The addition of some moderate strength magnets carefully placed to reduce or eliminate purity problems due to a distorted or dislocated shadowmask may be enough to make the TV usable - if not perfect. The type of magnets you want are sold as 'refrigerator magnets' and the like for sticking up notes on steel surfaces. These will be made of ferrite material (without any steel) and will be disks, rectangles, flexible strips. Experiment with placement using masking tape to hold them in place temporarily. Degauss periodically to evaluate the status of your efforts. Then, make the 'repair' permanent using duct tape or silicone sealer or other household adhesive.

    Depending on the severity of the purity problem, you may need quite a few magnets! However, don't get carried away and use BIG speaker or magnetron magnets - you will make the problems worse.

    Also note that unless the magnets are placed near the front of the CRT, very significant geometric distortion of the picture will occur - which may be a cure worse than the disease.

    WARNING: Don't get carried away while positioning the magnets - you will be near some pretty nasty voltages!

    (From: Mr. Caldwell (jcaldwel@iquest.net).)

    I ended up with the old 'stuck on a desert island trick':

    I duck taped 2 Radio Shack magnets on the case, in such a way as to pull the beam back.!!!!

    A $2 solution to a $200 problem. My friend is happy as heck.

    RCA sells magnets to correct corner convergence, they are shaped like chevrons and you stick them in the 'right' spot on the rear of the CRT.

    (From: Tom Sedlemyer (wesvid@gte.net).)

    First set purity as best you can.

    Obtain some pieces of refrigerator door magnet strips from an appliance repair shop (they usually have some lying around).

    Cut the strips into 1 inch pieces. Place a strip as on the bell of the picture tube as close to the yoke as possible and in line with the corner that has the purity error. Rotate the magnet until you correct the purity error and tape it in place. Multiple magnet strips can be used and you may experiment with the size of the strips for best effect. It is very important that the strips are positioned close to the yoke or the effect will not hold. The only drawback to this method is some very slight distortion of the geometry of the raster, but it beats hell out of paying for a new CRT.

    Color TV only displays one color

    I assume that now you have no other colors at all - no picture and no raster. Let us say it is red - R.

    It is probably not the CRT. Do you have a scope? Check for the R, G, and B video signals at the CRT. You will probably find no signals for the defective colors.

    This is almost certainly a chroma circuit problem as any failure of the CRT or a video driver would cause it to lose a single color - the other two would be ok. Therefore, it is probably NOT the CRT or a driver on the little board on the neck of the CRT.

    Try turning up the SCREEN control to see if you can get a G and B raster just to confirm that the CRT is ok.

    Locate the video drive from the mainboard for the good and a bad color. Interchange them and see if the problem moves. If so, then there is a video signal problem. If not, it is on the little CRT board.

    It could be a defective chroma IC or something else in the chroma decoder.

    Disappearing Red (or other color)

    Problem: I have been given an old colour TV. The reception is good, but very often, when the contrast and brightness of the TV image is low (e.g. when a night scene is shown), the red colour slowly disappears, leaving behind the green and blue image and many red lines.

    The remaining red retrace are the giveaway that this is most likely not a CRT problem.

    (If there were no red lines, it could be the filament for the red gun of the CRT going on and off due to a bad connection inside the CRT - bad news.)

    How is a black and white picture? (Turn down the color control).

    If B/W picture is good, then the problem is somewhere back in the chroma decoder circuitry.

    Check the video input to the CRT video driver board and signals on that board. If B/W picture is also bad, then you can compare red and green signals to determine where they are becoming different. The red lines in your description sounds like the red video output circuit is drifting and messing up the background level, blanking, screen, or other setting. Could be a capacitor or other component.

    The wandering black blob on old Sony

    "I had a Sony KV-1920 TV (very old) that suddenly started to displayed a black blob on the screen. The blob was anywhere from 1" around to almost the size of the entire screen. It had a sharp, not fuzzy, outline, and it would shrink in size as the TV warmed up, usually disappearing completely in 30 minutes. It shrank in sudden jumps, not gradually. Sometimes the blob would be stationary, other times it would tumble around rapidly all over the screen."

    (From: Raymond Carlsen (rrcc@u.washington.edu).)

    Measure the regulated +130 volt line... I think you'll find it has drifted upwards just enough to trigger the "protective" blanking circuit. In those sets, if the B+ (and consequently the HV) went up, the screen was automatically blanked so you couldn't use the set. It was before HV shutdown. Older RCA TV sets used to throw the horizontal out of sync. The low voltage regulator is an analog type with a pass transistor that is probably leaky, causing the high B+. Changes in line voltage and loading (with brightness changes) cause the partially blanked picture to change (the black blob moves around, sometimes blanking the entire screen). When you replace the defective component, reset the 130 volts with the pot, and you're back in business. The pot itself may have a bad spot... just move it one way or the other to get off that spot.

    Vertical brightness or color bars

    These are typically more or less equally spaced possibly more evident at the left side of the screen. They result only in brightness or color variations, not deflection speed. Diagonal lines are straight and not squiggly.

    Note that the appearance of these bars differs from those caused by ringing in the deflection circuits where diagonal lines will show a squiggling stair-step appearance.

    The most likely cause is a dried up electrolytic capacitor in the scan derived power supply for the video or chroma circuits or video output. Check for this ripple with a scope or test/replace any suspect capacitors.



  • Back to TV Repair FAQ Table of Contents.

    Tuner, AGC, and Sync Problems

    No reception from antenna or cable

    Make sure your source is providing a signal and that the cable connectors are good (center pin not broken or bent). Try another TV if possible.

    Make sure you source select switch or mode is set correctly. Someone may have accidentally set it to direct video or AUX input.

    Are all bands affected? If so, the tuner or IF is faulty. If there is a lot of snow, then it is probably toward the front (circuitry wise) of the tuner. If it is just a black screen, then it could be in the IF or video amplifier.

    If only certain bands are bad - channels 2-6 for example, then certain parts of the tuner circuitry are faulty. However, make sure the CATV mode is set correctly as this affects reception on a band-by-band basis.

    The problems may be due to bad solder connections of the tuner shields, connectors, coils, and other components. Try prodding the tuner to see if you can make the problem come and go or at least change.

    Picture is overloaded, washed out, or noisy

    This indicates an Automatic Gain Control (AGC) problem often caused by a dried up capacitor. You will probably need a schematic to go much further. This could be a problem in the tuner, IF, or video amplifiers.

    The following assumes you are sure the signal source is strong - try a VCR or other local one (channel 3/4, not the RCA jacks).

    (From: Glenn Watkins (blueribb@mail.comcat.com).) Substitute a variable voltage source for the tuner's AGC voltage. Most of the time the range of AGC is from 1 to 7 volts. If you can get a decent snow free picture with an external AGC source, then the tuner is probably OK.

    Jumping picture on white scenes

    This could be an AGC problem if the picture appears overloaded. However, if the picture is normal except unstable, the sync separate is the place to look:

    (From: Jack Schidt (jack@wintel.net).)

    White screens are a worst case video pattern for sync separators, and will cause an erratic shift in the vertical multivibrator trigger level unless the horizontal and video information is filtered out [integrated] prior to driving the vertical sync input of the processor IC.

    This will show up with a scope as high frequency noise going into the vertical sync input.

    Look for a small electrolytic [in fact, all of them], around 1-10 uF or so near the deflection/sync processor IC. Often simply increasing the value of this cap will help.

    Interference when using VCR RF connection

    (Some of these comments also apply to use of LaserDisc players, satellite receivers, video games, or other sources with RF modulator (Channel 3/4) outputs).

    This may consist of patterns or lines in the picture.

    If this only happens on the antenna or cable, it may be a problem with these sources or the tuner in the VCR rather than the TV. As a test, try the connecting the TV directly to the antenna or cable.

    If it only happens on cable, there may be a (temporary) problem with cable transmission - contact your cable company.

    If it happens on playback of good quality (commercial) recordings, then it could be a compatibility problem between the VCR and TV.

    Make sure your patch cable connections are secure and that the cables are not damaged - in particular that the center pin is intact.

    Try fine tuning if your TV has this capability. If this does not help, try switching the channel 3/4 selector on the VCR to the opposite position and try that channel, sometimes one will be better than the other particularly if one of these or an adjacent channel is active in your area.

    If you have RCA baseband video inputs on your TV, try this connection to the VCR. These should work better in any case.

    Confirm that it is not actually a problem with the VCR - try another TV if possible.

    If you just changed your component placement, the VCR or TV may be picking up interference from another component. Turn off everything but the VCR and TV and see if that identifies the culprit. Move the TV away from the VCR so see if they are interfering with each other - the TV may be introducing interference into the VCR.

    Occasionally, the particular patch cable or its length may affect reception quality - try another one.

    If none of this helps, you VCR's RF modulator may be bad or slightly weak. Alternatively, the tuner in the TV may be faulty. If reception is generally noisy on all sources, AGC or RF/IF alignment may need adjusting. However, not all tuners are created equal. Your TV may simply be making the best of a marginal situation.

    A light dimmer on the same circuit as the TV may result in similar symptoms. If you are tuning up your motorcycle (or automobile) in the same room, this may be spark ignition interference.

    RF Interference on TV

    "I have interference lines on my TV - they are particularly heavy on 2, not so prominent on 9 - one TV is on Radio Shack Color Supreme 100 (souped up rabbit ears), other is on a roof top antenna, both have coax from antenna to TV. I have HAM operator two doors away. Is there an FAQ on interference - if so where. How do I need to describe the interference pattern in order to seek help. Thanks."

    The FCC had an online interference handbook, with color photos showing how different types of interference affect a TV's picture. Unfortunatley, this has vanished from the FCC Web site but is now on the Kyes TV Web site:

    (From: Andrew Mitchell (amitchell@sympatico.ca).)

    Probably the easiest solution is to visit your ham neighbor and describe your difficulty. Amateur radio operators are licensed by federal governments (FCC) and are required as part of their examination to demonstrate a knowledge of this type of interference. It may well be that the ham is not the source of the problem and even if this is the case I'm sure he or she will be of assistance.

    (From: Alan N. Alan, WDBJ-TV, KM4IG (alann@intrlink.com).)

    OK, as a HAM myself, I can understand this. Channel 2 is the lowest TV channel, right above the six meter band, 50-54 Mhz. Channel 9 is well into VHF above 175 Mhz. It is possible that your neighbor operates 6 meters.

    I would talk to him. First, the chances are it is YOUR equipment, and his is legal and meets FCC specs. But I would be willing to bet he will be very cooperative to help you solve your interference.

    The thing to do is talk to him, calmly, and tell him about your problem. Then, schedule a time where he can transmit his gear and see if your problem exists along with his transmissions.. If it does, you can go from there. Many ham clubs have many engineers and radio and TV people in their memberships that will jump in and help you solve your problem. Again, he is probably legal, and consumer equipment is not known for it's RF resistance. Consumer manufacturers cut corners wherever they can. This includes filtering and design.

    Problems with ground loops and video hum bars

    "I am having a problem isolating where my ground loop problem is coming from. The symptoms I see are Bars on my TV which scroll up the screen. The problem is these bars come and go, and when they are present they vary in intensity. I have verified that the cable ground is connected to the earth ground on the outside of the house, but the problem still remains. This problem is also screwing up any attempts to do video electronics experimentation. I am really tired of seeing these bars and any help you could give would be appreciated."

    (From: Paul Grohe (grohe@galaxy.nsc.com).)

    1. Do these bars show up on other TV's connected to the same cable?

    2. Is your TV connected to anything else? A/V receiver? VCR?

      If so, unplug *all* the equipment and plug it in one-at-a-time until the hum appears. If you have an AV receiver in the system, try running a jumper wire from the incoming CATV ground at the TV to the receivers chassis ground (usually the "phono ground screw").

      If you have any devices with un-polarized plugs, unplug them and rotate them 180 degrees, and plug them back in.

    3. If you connect a temporary antenna and view "off-the-air" signals, are the bars still there?

    If you still cannot eliminate the hum, try building a simple "ground isolator" out of two 75-300 ohm baluns, as described in the link below:

    Place it as close to the TV as possible.

    (From: Charles Godard (cgodard@iamerica.net).)

    This seems like a cable company problem, but you need to prove it to the cable guy before he will start climbing pole's and changing amps and couplers looking for an intermittent amp. (And I don't blame him.)

    At the main cable line to the house and remove all couplers and put a single line from the cable direct to the rf input on a single TV, then watch it for a few of days. If the problem re-occurs call the cable guy and show him what you have done and explain the problem again.

    Put yourself in the shoes of the cable guy. He comes into a house with VCR's and all the gadgets we all have hooked up to our TV's with lines running all over the house, and can't get to the back of the TV to see what's there, and he's not a TV repair guy anyway and nobody else in the neighborhood is complaining and this problem may happen when it rains but it may not. mmmhhh

    If it does not show up on the single TV, then the problem is probably yours. Add one device at a time until you find the trouble maker. Start with the your Cable AMPLIFIER.

    (From: 4real (alan69@iname.com).)

    You eliminate all of the other junk attached between your main cable input to your house and your TV to be sure it isn't the cause.

    You will definitely want to suspect a problem with the amplifier you have installed. Especially if it is one of those cheap ones. Usually when the filter capacitor in an amplifier goes bad it will cause the hum bars you are describing, and they can be intermittent. Another problem may be that you have too much signal going into the amplifier. Amps are rated to handle a certain amount of input signal (measured in db) depending on the number of channels you wish to amplify, and the gain provided by the amp. If you try to feed an amplifier with too much signal it will overdrive it and cause a venetian blind, or herringbone effect. It could also be possible that the cable company is supplying a signal with reverse tilt. That means more signal on the lower channels and less on the higher ones. The lower channels might be the ones overdriving the amplifier. The only way to tell for sure is to measure it with a signal level meter. (very expensive unless you happen to be in the business and have one handy) If this is the case (too much signal going in) you probably don't need the amplifier to feed only 2 TVs.

    The last thing I can think of and the cable guy should have checked this: They use 60 VAC on their main trunk lines to power their line amplifiers. The taps which feed the individual houses are supposed to prevent this ac from going to the individual lines. Occasionally one of these devices fails or a line guy forgets to pull a fuse and hence the ac gets sent to your TV. It won't necessarily fry your TV but can cause problems. It may even damage the TV tuners that are connected to that feed. In most cases if you touch the center conductor of the cable and a good ground you can feel the ac. It isn't enough to hurt you but you will definitely know it's there. To be on the safe side you should test it with a volt meter.

    (From: Cliff R. (craeihl1@nycap.rr.com).)

    My guess would be your cable amplifier. The fact that you see TWO bars on your screen tells me that it's 120 Hz interference - the frequency caused by ailing full wave power supplies used in these amps. Take the amp out of the line for a few days. If you don't have "snow" in the picture with it out, s...can it! If you find it was bad and can't live without it, you might try making sure all your internal cable, splitters, and connections are good quality & in good shape. Radio Shack stuff......well, it stinks! You can purchase primo splitters & cable from your cable company and its not that expensive. Certainly cheaper than an amp (which you might not need if the cabling were up to snuff).

    You could also cry to the cable company for more signal into your house. This may or may not work but it's worth a shot.

    I would put an amp in line only as a LAST resort. Most of the inexpensive amps sold are......cheap. They can easily cause more trouble than they cure. If you must, go with a primo unit from Blonder-Tongue or Jerrold.

    (From: Charles Hope (charles.hope@argonet.co.uk).)

    It sounds very like a problem that I had and solved.

    Cause: Modern TV sets antenna connector does not have true earth on the screen but is at a potential of half mains supply. It is possible to draw about 30 micro-amps from this.

    Hum bars are induced in the amplifier because there is a small resistance in the earth path between output and input giving about 1 volt drop of this stray mains signal. Worse when raining because the cable ground is better then.

    Solution: Either ground the antenna screen or fit a "braid breaker" in the screen.

    Missing or noisy channel or block of channels

    If you are unable to receive certain channels or blocks of channels, this is a tuner problem - could be as simple as bad connections - or even simpler:.

    First, check to see that the tuning mode is correct - TV, CATV, as this is the most common cause of channels 'disappearing'.

    TV channels are assigned frequencies ranging from 72 to almost 800 MHz depending on broadcast or cable channel assignment. To tune over such a wide range requires splitting it up into various bands even if these are not actually defined. If you have a varactor tuned set, then you already know about the Vl, Vh, and U bands which may use separate front-end components. Even modern quartz PLL synthesized tuners need to allocate circuitry depending on frequency range. Therefore, if a block channels is not working, it could be due to a failure of some component related to that frequency range. Aside from looking for bad connections, resoldering the shields and connector pins, prodding, pressing, praying, etc. you will need a schematic to have any chance of finding such a fault.

    There is another slight possibility. Some TVs have a parental lockout capability (pre V-chip) to prevent kids or other unauthorized access to selected channels. The channel selections may have been accidentally altered. Check your user manual for instructions on programming this feature. Even on models without this option, the same internal circuitry could be present but not normally accessible. A power surge or stray cosmic ray could have put the set in a screwy mode. Unplugging power for a minute or probably a much longer time might possibly reset such an anomaly.

    Loss of Channel after Warmup

    If there is a general loss of picture and sound but there is light on the screen, then most likely the tuner or IF stage is pooping out.

    With both no sound and no picture but a raster and static, it is most likely a problem in the tuner, power to the tuner, or its controller (if non-knob type).

    If it recovers after being off for a while, then you need to try a cold spray in the tuner/controller to identify the component that is failing. Take appropriate safety precautions while working in there!

    If it stays broken, then most likely some component in the tuner, its controller, or its power supply as failed. There is a slight chance that it could be a bad solder connection - I have seen these in the tuner modules of RCAs on several occasions (and many other manufacturers - apparently not a solved manufacturing problem even after 40+ years!

    Channel tuning drifts as set warms up

    This may be a slight drift - like someone is messing with the fine tuning or such a substantial change in tuning frequency that the channels go by as though you are surfing.

    Possible causes depend on tuner type:

    1. Quartz tuner (10 button direct access digital synthesizer) - For a slight drift, a component is probably changing value, possibly the crystal in the reference oscillator. For gross changes - flipping through channels - it is more likely to be a digital control problem - the microcontroller is misdirecting the synthesizer to change frequency.

    2. Varactor tuner (buttons but not direct channel access) - If only a single pushbutton selection is the problem, the the varactor tuning diode for that button is probably changing capacitance. If all channels in a band (Vl, Vh, U) are having a problem, it is more likely to be a drifting D/A or faulty AFT (Automatic Fine Tuning) circuit or power supply.

    3. Turret or switch tuner (Knobs) - A component like a capacitor is changing value.

    You will have to get in there with a heat gun or cold spray and track it down the old fashioned way. At least, the problem is almost certainly localized to the tuner box (and possibly the controller if applicable).

    As noted, gradual slight changes in tuning are likely due to frequency determining components drifting.

    Uncontrolled channel surfing is probably a logic problem. For the quartz tuner, this could still be marginal connections causing the microprocessor to misdirect the synthesizer to change channels.

    For the latter case, particularly, the cause may still be bad connections resulting in loss of channel memory and/or erratic behavior.

    Noise in picture and sound due to bright scene

    When a bright scene comes, the screen flashes and there is a lot of noise in the sound. When a dark scene comes, there is no flash or noise. Changing channel does not help. The noise persists even when the sound is muted.

    (The following is from: Lattuca@Midwest.net (Sam Lattuca))

    When the video detector level is adjusted too high, you will get noise in the sound while screen contains a lot of white information (i.e. letters) but won't when only dark scenes are present. The video level adjust is usually a small coil normally located near the IF section. Since your set is several years old, this wouldn't be uncommon. It can be adjusted while watching the picture and listening to the sound.

    Internal interference - switchmode power supplies and digital circuitry

    (From: Mr. Caldwell (jcaldwel@iquest.net).)

    On virtually all newer televisions and in particular Mitsubishi televisions there is a problem with interference being emitted by the switched mode power supply.

    The common symptom of this 'fault' is snake like dotted 'S' lines on channels 2-6. It doesn't matter if it's cable, antenna or satellite(channel 3/4), this symptom can occur.

    The common cause of this interference being allowed into the tuner is cabling. The super cheap 'suitable for garbage tie' cable that comes with even the most expensive VCR's is the culprit in most cases. The second is a set of rabbit ears the least common is an open or high resistance to ground connection (usually at a connector) on the incoming cable line.

    To fix this there is only one reliable solution. All cabling must be hand made RG-6 cable. Make as follows:

    1. Strip the outer sheath of the cable to expose the braid and *fold the braid* away from the end so that it covers the unstripped outer braid.

    2. Strip the inner conductor to it's proper length.

    3. Install a good quality RG-6 connector *over the folded* braid.

    4. Crimp with the proper RG-6 attachment to the cable crimpers, don't use a set of pliers or other -crushing- device.

    If the cable company doesn't waterproof the outside connectors, Radio Shack sells a 'sealing tape' just for this purpose. Most cable companies use self sealing 'o-ring' connectors.

    (From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).)

    There is also interference from internal microprocessors and digital text generators (on-screen display, close captioning, teletext). And with 100 Hz digital television there is a wealth of sources ...

    Using only high quality shielded cable as described above seems like really good advise, FWIW I'd like to second that. I wish that everyone would take antenna cables as seriously as you.

    Generally, double-braided cable (using copper foil for second shield) and coaxially constructed connectors are recommended. But I think that the hand-mountable F-type connectors (Radio Shack) would be equally good, though less robust, if mounted properly.

    As far as antennas go, a decent rooftop antenna should always be better than whatever rabbit ear construction you might think of. In this case, distance counts too, the antenna WILL pick up interference.

    Those darn rabbit ears

    So you bought a high performance TV and a set of $20.00 rabbit ears and there are lines on channels 2 to 6. Go buy a set of rabbit ears that has *only* a coax connector on the back, throw the cable supplied with it in the bin for 'twist ties'. Also buy an inexpensive surge suppressor that has a cable protector, enough RG-6 cable and connectors for two cables.

    You're done and if you thought carefully you would have put the antenna near your easy chair so you can adjust the picture or put the antenna where you'll get the best reception and prevent interference. The surge suppressor was needed to ground the other end of the coax so as not to make the outer shield an antenna for the interference from the TV's power supply. This method can also help allevate 'dead spots' when using rabbit ears.

    Herringbone lines in picture

    (From: Isaac Bergen (isaac.bergen@sympatico.ca).)

    Could be interference. If the pattern slowly moves up the screen, the problem is from the 60 Hz power. A line of dots or thin lines usually means corona discharge (arcing) from a nearby power line (especially on humid days). Could also be from a bad filter capacitor in the TV's power supply. A "checkered" pattern could be from a digital type noise source like a computer, etc. If you move the TV to another room and the interference changes, that's probably it.

    EM or RFI hell?

    "About a mile from my home there are four TV (channels 2, 4, 9 and 14) and several broadcast FM transmitters, all working with powers in the 100+ Kw ERP class.

    Radio reception is a nightmare, mostly (I think) because of IM products in overloaded front end stages. In most bands there are several regions at a spacing of about 100 Khz, each 30 to 40 Khz wide with a harsh buzz stronger than anything else."

    (From: Don Klipstein (don@Misty.com).)

    If the buzz is of a frequency like the power line frequency or a harmonic thereof, then the nearby transmitters may not be the culprit. Instead, nearby corona on a high voltage power line, a nearby neon sign, or a nearby light dimmer may be the offender. Although the noise from these is usually broad-band, the noise could get concentrated into bands spaced 100 kHz apart if something resonant around 100 kHz is involved in the noise production.

    I would try temporarily turning off all fluorescent lights, neon signs, lights with dimmers, etc. and asking your neighbors to do the same to see if any of these is the offender. I have often found light dimmers to be major RF noise sources.

    Possibly, an RF noise filter for the AC power for your receiver may help things. If you isolate a single offending appliance, it may help to plug it into an RF noise filter. If you use any filters with either the offending appliance or the receiver, try all combinations of plug reversal to see what works best. Both leads of any offending appliance may not equally spew noise, and both lines in the filter may not equally block noise. Both lines of the receiver's power cord may not equally bring noise into the receiver, if this is the route the noise takes.



  • Back to TV Repair FAQ Table of Contents.

    Audio Problems

    Picture fine, no audio

    First check that any muting control is not activated. This might be a button on the remote or set itself. If you have a headphone jack, it may have dirty contacts as plugging in a headphone usually mutes the speaker.

    If the set is mono or only one channel of a stereo set is out, then check for bad connections to the loudspeaker. Test the loudspeaker by disconnecting one of the wires (with the power off!) and measuring its resistance with an ohmmeter (it should be less than 100 ohms - probably less than 8 ohms). Or momentarily touch a 1.5 volt battery to the speaker terminals - you should get a click or pop from the speaker.

    Next, trace back from the speaker output terminals to the circuit board and look for bad solder connections or a loose or dirty connector.

    If these tests do not reveal anything, you probably need a scope (or audio signal tracer) and schematic. Or at least the part number off of the chip. Is the final amp a chip also or just a transistor? Have you tested the transistor? If there is little or no buzz from the speaker, that would indicate a problem fairly near the output. If the tuner/if were bad, I would expect some noise/humm pickup from the low level audio stages. Get the part number off of the chip. If it is in a socket, check the contacts for corrosion or looseness.

    Weak or distorted audio

    Assuming you are not attempting to play it at ear shattering levels, this may be due to an alignment problem in the IF/audio demodulator, a bad audio IC or other circuitry, bad connection, or a defective speaker.

    If your TV has an earphone or audio line out jack, try this to see if it is clear. If so, then your problem is in the final audio amp or speaker(s).

    If only one channel of a stereo TV is affected, it is almost certainly the audio amp or speaker for that channel. Interchange connection to the two speakers temporarily and see if the problem moves.

    If the problem is at all intermittent - try gently whacking the TV - then it is likely a bad connection - either a cold solder joint or a dirty or tired IC socket.

    The audio amplifiers in newer TVs are almost always ICs and replacements are usually readily available. If the IC is in a socket, remove the IC, clean the pins and socket contacts and reinstall it. Sometimes, the contacts on old socket lose their springiness and do not provide solid connections. Such a socket will need to be replaced.

    If the set uses discrete transistors, it s also possible for one of these to become noisy.

    If your TV is fairly old - 10 years or so - this may be an alignment problem requiring tweaking of a coil in the sound IF. See your service manual. It may be possible to have similar problems with newer TVs but this is relatively rare.

    There could also be bad electrolytic capacitors, probably in the power supply area. Even though you might think this would result in hum and there is none (even when there is no audio in the program or the sound is turned down) dried up caps can result in distorted sound that may sound like a sort of clipping. An ESR meter is best for testing (with power off!) but carefully jumpering known good caps across suspect ones (again with power off, then turn on the set and check), will eventually find the bad one(s).

    Buzzing TV

    Do you actually mean buzz - low frequency as in 60 Hz? Or, do you really mean high pitched whine. If the latter, see the section: High pitched whine or squeal from TV with no other symptoms. Or, it may be a combination of both effects. Is the buzz through the speaker or from the inside of the set?

    BTW, when I got my new super-duper RCA Colortrak in 1980, it had a similar annoying buzz - even had a repair guy out who behaved as though this was to be expected. I did get used to it and am not even aware of it today - and still use that set.

    Additional comments:

    (From: Karen (kclark9835@aol.com).)

    Also for some audio buzz problems especially in the older units don't overlook the possibility of a misaligned trap. or a touch-up of the sound discriminator may prove helpful.

    (From: Alan (algba@ix.netcom.com).)

    If the buzz is coming from the speaker suspect a bad saw filter in the if circuit (very common).

    If it is coming from elsewhere in the set it could be the flyback transformer, line input choke, or most common on those sets - the deflection yoke. I have repaired many of these yoke by using a wooden shim and some silicone rubber. In the collar of the yoke just ahead of the lock down clamp, there are some metal strips under the plastic. These are magnet that are used for convergence correction at the top and bottom of the picture. If you disturb them too much it will throw off the convergence.

    Chattering yoke

    This is a special case of buzzing originating from the deflection yoke.

    (From: John Del (ohger1@aol.com).)

    I am assuming your talking about a NAP yoke made by Panasonic/Quasar also used by them as well. The vibration comess from the metal shunt inside the yoke which is used for "self convergence". The shunts were held by a mastic material that breaks down into a powdery substance.

    Remove the yoke from the tube and bore a small hole in the plastic on both halves (top and bottom). These holes will be at the 12 and 6 positions, as the 3 and 9 will have windings visible. Fill the holes with white glue and let the glue settle in, rotating the yoke as it does. The glue will soak into the powdery material and harden it. You will have to do this several times until the glue oozes out near the clamp. Give it a full day to dry. This is better than paying $60 to $80 for the yoke.

    High pitched whine or squeal from TV with no other symptoms

    First, make sure it is not coming from the loudspeaker itself. If it is, then we are looking at an unusual electronic interference problem rather than simply mechanical vibration.

    If it is a new set and think the sounds will drive you insane, returning it for a refund or replacement may be best alternative. However, you may get used to it in time. I don't know about returning a set to a store that doesn't take refunds (I won't even ask about that!).

    In most cases, this sound, while annoying, does not indicate an impending failure (at least not to the set - perhaps to your mental health) or signify anything about the expected reliability of the set though this is not always the case. Intermittent or poor connections in the deflection or power supply subsystems can also result in similar sounds. However, it is more likely that some part is just vibrating in response to a high frequency electric current.

    There are several parts inside the TV that can potentially make this noise. These include the horizontal flyback transformer, deflection yoke, other transformers, even ferrite beads in the horizontal deflection circuits. In addition, transformers or chokes in the switching power supply if this is distinct from the horizontal deflection circuitry. Or even a portion of the sheetmetal used for shielding if in close proximity to a magnetic component.

    You have several options before resorting to a 12 pound hammer:

    If you are desperate and want to check the inside of the set:

    Note that the deflection frequency - just over 15 kHz for NTSC and PAL - is on the border of audible for adults but will likely be loud to younger people possibly to the point of being terribly annoying - or worse. If you are over 40 (men more so than women), you may not be able to hear the fundamental at all (at least you can look forward to silence in the future!). So, even sending the TV back for repair may be hopeless if the technician cannot hear what you are complaining about!

    BTW, if you have a really old tube type TV, the power tubes (damper and horizontal output) can also whine but these sets are few and far between these days :-).

    Reducing/eliminating yoke noise

    (From: Terry DeWick (dewickt@esper.com).)

    Carefully look under vertical core next to plastic liner, on top and bottom is a plate called the astigmatism shunt, it has come loose. Work RTV, epoxy, or service cement onto it to glue it down and noise should quit.

    (From: TVman (tvman@newwave.net).)

    I have fixed a total of 27 of these sets with noisy yokes by removing the yokes and using motor armature spray sealant.

    If you carefully mark the EXACT position of everything (yoke, purity magnets), and slide the yoke off the CRT, then once the yoke has been sealed with motor armature spray sealant and has dried thoroughly, put the yoke back EXACTLY where it was, there should be no problems.

    The only thing I have had to do was set the purity on one set, but it was off a little to begin with.

    Whining when off?

    Many TVs actually run their switchmode power supplies even when off to power the standby stuff like the remote control receiver, real time clock or timer, and channel memory. Depending on the design of the regulator, the power supply may be running at a low chopper frequency due to the light load. Some people, dogs, and rodents are then annoyed. It could also be an indication of a fault like a bad capacitor or loosened transformer core if this symptom just developed - your hearing isn't likely improving :-(.

    There is so much running nowadays in 'off' electronics!



  • Back to TV Repair FAQ Table of Contents.

    Miscellaneous Problems

    General erratic behavior

    You press VOLUME UP and the channel changes or a setup menu appears all by itself just at the climax of your mystery story.

    Before you break out the screwdriver (or 12 pound hammer), cover up the IR remote sensor. Some types of electronic ballasted fluorescent lights may confuse the remote control receiver. Someone or something may be sitting on the remote hand unit or it may be defective and continuously issuing a bad command. Or, the kids across the street may have nothing better to do than to drive your TV (and you) nuts with their remote!

    There is also a slight chance power line interference (from a light dimmer or external sources) may result in similar symptoms. See the section: Wiring transmitted interference.

    Assuming this is not the source of the problem:

    Check for bad connections - see if gently whacking the TV makes any difference or triggers the errant behavior. Bad connections in the power supply, system controller, or tuner, may result in this sort of behavior. See the section: TV and monitor manufacturing quality and cold solder joints. See the sections and separate documents on problems with RCA/GE/Proscan and Sony TVs if yours is made by one of these companies.

    A microcontroller or other electronic problem is also possible. If the symptoms only develop after the set warms up, it may be heat related (though simple bad connections are more likely). Use 'circuit chiller' or a heat gun to identify the bad part.

    Wiring transmitted interference

    The power that comes from the wall outlet is supposed to be a nice sinusoid at 60 Hz (in the U.S.) and it probably is coming out of the power plant. However, equipment using electric motors (e.g., vacuum cleaners), fluorescent lamps, lamp dimmers or motor speed controls (shop tools), and other high power devices, may result in a variety of effects.

    While TVs normally include some line filtering, the noise immunity varies. Therefore, if the waveform is distorted enough, some effects may show up even on a high quality TV.

    Symptoms will usually be one or two areas of noise moving slowly up the screen.

    The source is probably local - in your house and probably on the same branch circuit - but could also be several miles away.

    Jittering or flickering due to problems with AC power

    If you have eliminated other possibilities such as electromagnetic interference from nearby equipment or a faulty video cable or problems with the video input (e.g., cable or VCR) - then noisy or fluctuating AC power may be a possibility. However, most modern TVs usually have well regulated power supplies so this is less common than it used to be. Then again, your TV may just be overly sensitive. It is also possible that some fault in its power supply regulator has resulted in it becoming more sensitive to small power fluctuations that are unavoidable.

    One way to determine if the problem is likely to be related to AC power is to run the TV on clean power in the same location connected to the same video input. For example, running it on an Uninterruptible Power Source (UPS) with the line cord pulled from the wall socket would be an excellent test. The output of the UPS's inverter should be free of any power line noise. If the TV's image has now settled down:

    1. Large appliances like air conditioners, refrigerator, or washing machines on the same circuit might cause significant power dips and spikes as they cycle.

      Plugging a table lamp into the same outlet may permit you to see any obvious fluctuations in power. What else is on the same circuit? Depending on how your house or apartment is wired, the same feed from the service panel may be supplying power to widely separated areas.

    2. For some unfathomable reason, your TV may just be more sensitive to something about the power from the circuit in that room. There may be nothing actually wrong, just different. While unlikely, a light dimmer on the same circuit could be producing line-conducted interference.

      If you have a multimeter, you could at least compare the voltages between the location where it has problems and the one where it is happy. Perhaps, the TV is sensitive to being on a slightly different voltage. This might only be a problem if some circuitry in the the TV is marginal in some respect to begin with, however.

    3. There could be a bad connection somewhere on the circuit. If your house has aluminum wiring, this is a definite possibility.

      Try a table lamp since its brightness should fluctuate as well. This should be checked out by a competent electrician as it represents a real fire hazard.

    An electrician may be able to pinpoint the cause but many do not have the training or experience to deal with problems of this sort. Certainly, if you find any power line fluctuations not accounted for by major appliances, on the same circuit this should be checked by an electrician.

    TV blows fuses or trips breakers or worse when A/V connections are made

    You have sent the TV for repair and now three times, it blows something the instant anything is connected to it in your house. Other A/V equipment operates fine.

    Assuming all the other stuff is plugged into the same outlet asn is 115 VAC equipment and that thsi happens instantly when the TV is connected:

    Next time they bring it back, measure the voltage between the A/V connector shields and the shields on your cables - I wouldn't be surprised to find some substantial fraction of 115 VAC between them. This would mean that there is an internal short in the TV (their problem - any competent service center will routinely check for signal-AC ground shorts) resulting in a connection between the non-isolated AC ground and the signal ground. When you connect your equipment, you complete a path which results in a short circuit. Depending on the design of the TV and where the fault lies, much more than a simple fuse may be destroyed. This is similar to connecting a scope probe ground to a live chassis TV - see the section: Safety guidelines.

    My TV has the shakes

    You turn on your TV and 5-10 seconds later, the display is shaking or vibrating for a second or so. It used to only occur when first turned on, but now, the problem occurs 3 times in 30 seconds. Of course, many variations on this general theme are possible.

    Some possibilities:

    1. External interference - did you change anything or move your A/V setup recently? Do you have a computer monitor nearby?

    2. Defective circuitry in TV - power supply regulation, deflection, or bad internal connections are possible.

    3. Defective video cable (unlikely) - wiggle the cables to be see if you can induce the problem.

    4. Loose trim magnets of other magnetic components on or near deflection yoke. This is somewhat rare but if the adhesive comes apart, the magnetic fields from the deflection current can cause the parts to vibrate which will result in a jitter or movement of the picture. There may even be audible crackling or snapping sounds associated with this vibration.

    Note that many of the sources of electromagnetic interference that are problems with computer monitors like transformers and power lines will not cause noticeable shaking, wiggling, or jiggling on a TV because the power line and vertical scan are at almost exactly the same frequency and any such movement would be very slow.

    TV displays black box with normal picture border

    When the set is first turned on, it works fine for about 20-30 seconds, then the picture goes away - all but about 1 inch of picture all around the outer edge of the screen. The square ring of picture that is left, is dim but otherwise normal.

    (The following from: (jack haney) jhaney@pacifier.com))

    If this is a newer set, this sounds very much like a "closed caption" box for a captioning system not being used in your area. Newer Mitsubishis do much the same thing. If the wrong caption type is selected inadvertantly, all you'll see is a large black box on screen taking up about all but an inch each way. Try turning off all closed caption. The first time I saw this I looked like a damn fool in front of a customer, took me 30 minutes to figure it out.

    (From: Sam.)

    If this doesn't help, power down completely by pulling the plug for a few minutes. That may reset the TV's microbrain. Else, there may actually be a failure of the circuitry that selects the closed caption. In that case, the TV will need repair.

    Advertising overload

    "I noted the advertsing programs put in bright several frames of overshooting white signals in purpose of attracting attention which I do not want also it gets on my nerves sometimes. *flash* *flash* Ughhh! Is there a way to cutoff the "overload" or tone down that?"

    (From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).)

    Technically, the TV takes care of its own overload protection. Or at least it *should*, on some TV's you will certainly observe the line transformer going into saturation for a while. Other than that there is not much you can do, each TV should represent its input signals with as much fidelity as possible. Change channels?

    (I already suggested not watching. :-) --- Sam.)

    Strange codes appearing on TV screen

    "I've seen this sort of thing on a TV I bought a couple of years ago. I only see it when Proctor & Gamble ads are on.

    The newer TVs are required to have Closed Caption decoding (CCD). My TV has an OFF-ON button for CCD. It also has a button labeled CH1-CH2. When pushed in I get the verbal text on the screen like I should. When the button is out, I get the funny codes from Proctor & Gamble."

    (From: Tim (jollyrgr@mc.net).)

    The code you are seeing is Closed Caption 2. My Zenith has CC 1, 2, 3, 4, as well as Text 1, 2, 3, and 4. I have seen CC 1 which is the normal closed captions. CC 2 is used for commercial logging/identification. There should be a way to completely turn off the captions. The TV, as you state, has a switch for turning off the captions and should solve your problem.

    Releasing 'demo' mode

    You really want to watch CNN but the TV insists on promoting itself:

    TV was rained on

    Was the set plugged in when the leak started? Any piece of equipment with remote power-on capability has some portions live at all times when plugged in and so there may have been damage due to short circuits etc. Substantial damage could be done due to short circuits.

    Otherwise, you may just need to give it more time to dry out. I have had devices with keypads getting wet that required more than a week but then were fine. There are all kinds of places for water to be trapped and take a long time to evaporate.

    If the set got wet while unplugged (in a leaky attic or wet basement), for example, or it has a pull or click knob on/off switch, then give it time to dry out - completely. Assuming all visible water is drained, a week represents a minimum safe time to wait. Don't rush it.

    Generally, some moisture will not do any permanent damage unless the set was on in which case you will simply have to troubleshoot it the old-fashioned way - one problem at a time.

    You may be tempted to use a hair drier or heat gun to speed the process along. But, be extra careful not to do damage to the equipment. Slightly melted laptop keyboard is an example of a bit of overkill. As far as I know, this was due to a short exposure to a properly functioning blow drier. The owner swears that the blow drier is not overheating and that she hasn't been able to set her hair on fire. I can just imagine what would have happened with a real heat gun. They just don't make those keys the way they used to! :)

    TV was dropped

    You have probably seen the TV advertisements - I don't recall what they were for - where a late model TV is dropped out a many story window on a bunjie cord to rebound once undamaged and without hitting a baby in a stroller but then smash to smithereens on the sidewalk once the stroller had moved. Needless to say, this is generally not a recommended way to treat a TV set!

    However, mishaps do happen.

    Assuming it survived mostly intact - the CRT didn't implode, you could still have a variety of problems. Immediately unplug the set!

    If you take it in for service, the estimate you get may make the national debt look like pocket change in comparison. Attempting to repair anything that has been dropped is a very uncertain challenge - and since time is money for a professional, spending an unknown amount of time on a single repair is very risky. There is no harm is getting an estimate (though many shops charge for just agreeing that what you are holding was once - say - a TV, or was it a fishtank?)

    This doesn't mean you should not tackle it yourself. There may be nothing wrong or very minor problems that can easily be remedied. The following are likely possibilities:

    1. Cracked circuit boards. These can be repaired since TVs usually have fairly wide open single or two sided boards.

    2. Broken circuit components. These will need to be replaced.

    3. Broken solder connections particularly to large heavy components on single sided boards. Reflow the solder. If the trace is cracked or lifted, repair as in (1).

    4. Broken mounting brackets. These are usually made of cheap plastic and often don't survive very well. Be creative. Obtaining an exact replacement is probably not worth the trouble and expense.

    5. Components knocked out of line on the CRT envelope or neck - deflection yoke, purity magnets, convergence magnets and coils, geometry correction magnets. These will need to be reattached and/or realigned. Some CRTs use little magnets glued to the funnel portion of the CRT envelope. If any of these have come loose, it could be quite a treat to figure out where they went and in what orientation.

    6. Internal damage to the CRT - popped or distorted shadow mask, misaligned electron guns. Unfortunately, you will probably have no way of identifying these since you cannot see inside the CRT. They will not be apparent until all other faults have been remedied and the TV set is completely realigned. At that point, extremely severe purity or convergence problems that do not respond to the normal adjustment procedure would be one indication of internal damage. Give the TV a nice funeral. :( For a large screen TV, the effects may be even more dramatic.

    If you still want to tackle a restoration:

    As noted, unplug the TV even if it looks fine. Until you do a thorough internal inspection, there is no telling what may have been knocked out of whack or broken. Electrical parts may be shorting due to a broken circuit board or one that has just popped free. Don't be tempted to apply power even if there are no obvious signs of damage - turning it on may blow something due to a shorting circuit board. If it is a portable, remove the batteries.

    Then, inspect the exterior for cracking, chipping, or dents. In addition to identifying cosmetic problems, this will help to locate possible areas to check for internal damage once the covers are removed.

    (At this point, most people will assume there is no interior damage and plug the set back in and turn it on. My recommendation is to resist this temptation since as noted, this could result in further damage making the repair more expensive if there are circuit problems. However, if the unit was on at the time of the "incident" or you are really determined to get to the conclusion and would just throw the thing in the trash if it doesn't work or blows up, go for it! But, if you're the more cautious type, continue with the systematic diagnosis and repair procedure that follows.)

    Next, remove the cover. Confirm that the main filter capacitors are fully discharged before touching anything. Check for mechanical problems like a bent or deformed brackets, cracked plastic parts, and anything that may have shifted position or jumped from its mountings. Inspect for loose parts or pieces of parts - save them all as some critical magnets, for example, are just glued to the CRT and may have popped off.

    Carefully straighten any bent metal parts. Replace parts that were knocked loose, glue and possibly reinforce cracked or broken plastic. Plastics, in particular, are troublesome because most glues - even plastic cement - do not work very well. Using a splint (medical term) or sistering (construction term) to reinforce a broken plastic part is often a good idea. Use multiple layers of Duco Cement or clear windshield sealer and screws (sheetmetal or machine screws may be best depending on the thickness and type of plastic). Wood glue and Epoxy do not work well on plastic. Some brands of superglue, PVC pipe cement, or plastic hobby cement may work depending on the type of plastic.

    Inspect for any broken electronic components - these will need to be replaced. Check for blown fuses - the initial impact may have shorted something momentarily which then blew a fuse.

    There is always a risk that the initial impact has already fried electronic parts as a result of a momentary short or from broken circuit traces and there will still be problems even after repairing the visible damage and/or replacing the broken components. This is most likely if the set was actually on but most modern TVs have some circuitry energized at all times.

    Examine the circuit boards for any visible breaks or cracks. These will be especially likely at the corners where the stress may have been greatest. If you find **any** cracks, no matter how small in the circuit board, you will need to carefully inspect to determine if any circuit traces run across these cracks. If they do, then there are certainly breaks in the circuitry which will need to be repaired. Circuit boards in consumer equipment are almost never more than two layers so repair is possible but if any substantial number of traces are broken, it will take time and patience. Do not just run over them with solder as this will not last. Use a fine tipped low wattage soldering iron and run #22-26 gauge insulated wires between convenient endpoints - these don't need to be directly on either side of the break. Double check each connection after soldering for correct wiring and that there are no shorts before proceeding to the next.

    If the circuit board is beyond hope or you do not feel you would be able to repair it in finite time, replacements may be available but their cost is likely to be more than the equipment is worth. Locating a junk unit of the same model to cannibalize for parts may be a more realistic option.

    Degauss the set as any impact may magnetize the CRT. Power cycling may work but a manual degaussing is best.

    Once all visible damage has been repaired and broken parts have been replaced, power it up and see what happens. Be prepared to pull the plug if there are serious problems (billowing smoke or fireworks would qualify).

    If there are obvious problems with color, disconnect (or disable) two of the 3 primary colors with a B/W picture (color control turned all the way down) or solid raster displayed. If the raster is not now a pure color, you have a CRT or CRT purity adjustment problem.

    Perform any purity, convergence, or other realignment as needed.

    Then proceed to address any remaining problems one at a time.

    Really cleaning a TV inside and out

    (This was written for computer monitors but applies equally well to modern TV sets.)

    (From: Dr. Ludwig Steininger (drsteininger@t-online.de).)

    Often I get defective monitors, which are more than 5 years old, and have been run in offices for 8 to 10 hours/day. So, their case and pcbs usually are very dirty and dusty.

    What do I do (it's no joke!): After removing the case I carefully put them in a bath (on a flexible layer) and let them have a intensive shower of pure cold water (for 1 to 2 minutes). Additionally, the case is cleaned with soap or a detergent containing liquid (being careful, not to spill to much of it onto the PCBs). After rinsing with fresh clear water, dust and other kinds of dirt are removed and the monitors look new again. Then I allow all drops of water to run off. This can effectively be supported by turning the monitor on another side from time to time (duration: approximately 1 hour). Before turning on AC again, I let the wet monitor dry in ambient air for about 2 days (in the sunshine this can be finished in 1 day only).

    This procedure has been applied for many monitors. I've never had any bad experiences (it's very important to wait, until the pcbs are really dry!). Considering this experience, I just can't imagine, that it might not be possible, to "save" a TV set or computer monitor, which has been drowned or some liquid has been spilled, and AC has been plugged off ASAP (although I've never had such a case). I think, that in such a case, it's important to have a rapid shower in order to prevent corrosion and deposits.

    By the way: I know a German company, which uses water from cleaning PCBs of computer hardware for cleaning them after being contaminated by smoke from a fire.

    So, in case of spillage, one has nothing to loose. Just try to shower your monitor or TV set!

    Cleaning the screen on a projection set

    (From: Filip "I'll buy a vowel" Gieszczykiewicz (filipg@repairfaq.org).)

    I will assume you want to clean the front part of the screen. In some sets, the screen is not smooth as it is a Fresnel lens to help direct the light. Since it's plastic, you must NOT use any harsh chemicals like Windex (ammonia), etc. Best is to use plain water with mild soap, wet a soft sponge/cotton cloth, SQUEEZE WATER OUT OF IT, and wipe the screen using gentle, vertical strokes - repeating the wet-squeeze-wipe steps to prevent from scratching the screen. I will also presume it's just dust and grime and not kids with wax pencils that you're trying to clean.

    Where cleaning of the inner surface and/or between multiple surfaces is required, some disassembly will be needed. A service manual would be highly desirable to guide you and the optics may also need cleaning.

    Setup menus will not go away or hieroglyphics on screen

    Both these problems could be caused by a faulty microcontroller or its associated circuitry. However, bad connections in the vicinity of the controller logic could also be at fault.

    Unless you see something obvious, you will need schematics.

    Setup adjustments lost - TV service codes

    Many modern TVs have RAM, somewhat like the CMOS SETUP memory in your PC, that store all factory adjustments. When power is lost, there is power surge, lightning strike nearby, nuclear detonation or EMP, it may have put bad information into the ram and thrown it out of adjustment. There is a way to get into the service mode (depress and hold a secret button down and turn set on, special combination of buttons on the remote, etc.) and then use the remote to reinitialize and adjust the problems out.

    HOWEVER, IF YOU DON'T KNOW WHAT YOU DOING YOU COULD GIVE YOURSELF WORSE PROBLEMS. YOU COULD EVEN BLOW VERY EXPENSIVE PARTS WITH SOME SETS!

    Try not to make any unnecessary changes and document every change you make!!! That way you can go back if you do anything wrong (hopefully). However, some changes - even if nothing fails - will result in an unviewable picture thus making it extremely difficult to see what you are doing.

    The Sams' Photofact manual for your set should describe this process - you may be able to get Photofacts from a local library, or you can buy them from Radio Shack or a place like MCM Electronics or an electronics distributor. The June, 1998, issue of "Electronic Servicing and Technology" (ES&T) had an article on service mode if you have access to this publication (it won't be in your public library).

    Service Mode World Wide has listings for hundreds of TV models. Although they sell this information and software, they have some free downloads (as of March 2008) at Service Mode Downloads.

    Some examples follow. You would need to check the service information for your specific model to be sure. However, trying the procedures described below probably will not hurt. The TV will just ignore you if it doesn't like your codes! However, if you do get in, make sure you know what you are doing or your original problem may be inconsequential compared to your new ones!

    Service menu caution

    Even changing a parameter which results in the loss of the picture could require replacing the EEPROM if you cannot get the set to come on and view the service menu to reset!

    However, it may be possible to drive the HOT with an external source so you can see the menus for setup.

    CAUTION: there is some risk. Should drive the HOT with too low a frequency, it may blow due to flyback core saturation. Use a series light bulb to minimize this possibility.

    Links to TV Setup Information

    (From: Sidney (sidneybek@yahoo.com).)

    EEPROM service mode access sites:

    (In no particular order.)

    EEPROM part #'s by model/chassis lookup:

    Strange number in upper left corner in Magnavox service mode

    "When I put my Magnavox in service mode a number comes up on the top left of the screen. I see no description of it in the service manual. Is this an hours used timer? Is it actually in hours?"

    (From: Mister M. (mister-m@ix.netcom.com) and zapper (zap@mhv.net).)

    This is actually a usege timer in hexidecimal.

    (Hey, at least it is not binary. :-) --- Sam.)

    TV doesn't work after being in storage

    So the TV you carefully stuffed in a corner of the garages is now totally dead. You swear it was working perfectly a year ago.

    Assuming there was absolutely no action when you turned it on, this has all the classic symptoms of a bad connection. These could be cold/cracked solder joints at large components like transformers, power resistors, or connectors and connectors that need to be cleaned or reseated. By 'no action' I mean not even a tweet, bleep, or crackle from anything.

    To narrow it down further, if careful prodding of the circuit board(s) and various large components with a well insulated stick does not induce the set to come on, even momentarily, check the following:

    1. Locate the horizontal output transistor. It will be in a TO3 metal (most likely on an older set) or TOP3 plastic package on a heat sink. With the set unplugged, confirm that there is no voltage across C to E and then measure between them with an ohmmeter. In at least one direction it should be fairly high - 1K or more. This confirms that the HOT is probably good.

      (There is also a slight chance that there is a low voltage regulator in addition to the horizontal output, so don't get them confused. The horizontal output transistor will be near the flyback transformer and yoke connector.)

    2. Trace back from the HOT collector to the flyback and through the flyback to the B+ feed from the power supply. Clip a voltmeter between this point and the HOT emitter. Make sure the leads are well insulated and can't accidentally short to anything. (This test can be performed across C to E of the HOT but if the horizontal deflection were to start up unexpectadly, the meter could be damaged by the high voltage pulses on the HOT collector. But if you can't find the B+ source, it may be worth the risk.) Plug it in and turn it on.

      • If the problem is in the low voltage (line) power supply, there will be no substantial voltage across C to E.

        You should be able to trace from the power line forward to find the bad part though a schematic will help greatly.

      • If the problem is in the startup circuit or horizontal oscillator/driver, then there will be something on the order of 100 to 160 V across C to E.

        In this case, a schematic may be essential.

    There is also a slight chance that there is a low voltage regulator in addition to the horizontal output, so don't get them confused. The horizontal output transistor will be near the flyback transformer and yoke connector.

    Older TVs with multiple intermittent problems

    If the set is say, a GE, with a manufacturing date around 1980, it is possible you have one of those circuit boards best described as bad solder joints held together with a little copper. In this case, prodding may get the set started. The circuit boards in these sets were double sided using what were called 'rivlets' for vias. The rivlets were relatively massive - literally little copper rivets - and they were not adequately heated during assembly so there were bucketloads of cold solder joints that showed up during middle age. I repaired one of these by literally resoldering top and bottom of every one of the darn things with a high wattage iron.

    TV has burning smell

    Assuming there are no other symptoms:

    If this appears after extended operation - an hour or more - it may just be a build up of dust, dirt, and grime over the years. After understanding the safety info, some careful vacuuming inside may help. Just don't be tempted to turn any screws or adjustments!

    Dust is attracted to the high voltage section in particular - even the front faceplate of the CRT collects a lot and should be wiped with a damp cloth from time to time.

    If the symptoms develop quickly - in a few minutes or less, then there could still be a dust problem - a power resistor may be heating a wad of it but other possibilities need to be considered.

    If not dust, then probably in the power supply but realize that TVs don't have a nice metal case labeled 'power supply'. It is just a bunch of stuff scattered around the main board. Without identifying the part that is heating, a diagnosis is tough especially if the set really does work fine otherwise. However, if a series regulator were faulty and putting out too much voltage, the set could appear to work properly but in fact have excessive power dissipation in certain components. If cleaning the dust does not solve the problem, you will probably need a schematic to identify the correct voltages.

    Static discharge noise and picture tube quality

    "I bought a 29" TV a couple of weeks ago and I have noticed that after being switched on for > about 15/20 minutes, whenever the picture changes from a "light" scene to a darker scene, the set makes a crackling noise. It sounds as though there has been a build-up of static and it is being discharged. I have never noticed this in a TV before and I was wondering if this is normal and acceptable behaviour for a large-screen TV?"

    (From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).)

    It probably is normal. Whether it is acceptable is a personal matter. In some geographic areas no countermeasures are taken at all...

    When the scene changes from bright to dark, the beam current is reduced to practically zero. As a result, the high voltage rises. (The high voltage supply has a relatively high internal impedance.) The high voltage is connected to the inside layer of the picture tube. A voltage change on the inside will also cause a voltage change on uncovered parts of the outside, especially on the part of the picture tube that is hidden under the deflection coils. This causes little sparks between the picture tube surface and the inside of the deflection coils and this is accompanied by a crackling sound.

    On the better picture tubes, a dark "anti-crackle coating" is painted on the picture tube near the deflection coil. This is a very high impedance coating, dark black, much darker than the usual aquadag coating over the rest of the picture tube. You should be able to see the difference.

    If, on the other hand, the outside of the picture tube near the deflection coil is not coated then you have a problem. Then you will hear strong crackling also at switch-on and switch-off. Normally you shouldn't see such a 'cheap' picture tube on the European market...

    The area of the picture tube around the anode connector is also not coated, for obvious reasons. Normally that should not cause any significant sound. Same goes for the front of the screen and neither should the anode cable crackle.

    In a dark room you should be able to see from the tiny blue flashes where the sound comes from. This is perhaps best observed at switch-on and switch-off (with a black picture on the screen). Try and keep the back cover mounted !

    Revival of dead or tired remote control units

    There are two types of problems with hand held remote controls: they have legs of their own and they get abused or forgotten. I cannot help you with walking remotes.

    Where response is intermittent or the reliable operating distance is reduced, first check the batteries and battery contacts. If some buttons are intermittent or dead, than the most likely cause is dirty or worn contacts under the rubber buttons or on the circuit board.

    If there is no response to any functions by the TV or VCR, verify that any mode switches are set correctly (on both the remote and the TV or VCR). Unplug the TV or VCR for 30 seconds (not just power off, unplug). This sometimes resets a microcontroller that may have been confused by a power surge. Confirm that the remote has not accidentally been set to an incorrect mode (VCR instead of TV, for example). If it a universal type, it may have lost its programming - reset it. Make sure you are using the proper remote if have multiple similar models.

    Test the remote with an IR detector. An IR detector card can be purchased for about $6. Alternatively, build the circuit at the end of this document. If the remote is putting out an IR signal, then the remote or the TV or VCR may have forgotten its settings or the problem may be in the TV or VCR and not the hand unit. The following is just a summary - more detailed information is available in the companion document: Notes on the Troubleshooting and Repair of Hand Held Remote Controls.

    Problems with remote hand units:

    All except (1) and (2) require disassembly - there may be a screw or two and then the case will simply 'crack' in half by gently prying with a knife or screwdriver. Look for hidden snap interlocks.

    1. Dead batteries - solution obvious.

    2. Corroded battery contacts, Thoroughly remove chemical deposits. Clean contacts with pencil eraser and/or sandpaper or nailfile.

    3. Broken connections often between battery contacts and circuit board, possibly on the circuit board - resolder.

    4. Bad resonator or crystal - replace, but diagnosing this without an oscilloscope may be tough. Broken connections on resonator legs are common.

    5. Dirt/spills/gunk preventing keys from operating reliably. Disassemble and wash rubber membrane and circuit board with water and mild detergent and/or then alcohol - dry completely.

    6. Worn or corroded contact pads on circuit board. Clean and then use conductive Epoxy or paint or metal foil to restore.

    7. Worn or dirty pads on rubber keypad. Clean. If worn, use conductive paint or metal foil to restore.

    8. Cracked circuit board - can usually be repaired as these are usually single sided with big traces. Scrape off insulating coating and jumper breaks with fine wire and solder.

    9. Bad LED. If IR tester shows no output, remove LED and power it from a 9 V battery in series with a 500 ohm resistor. If still no output, replace with readily available high power IR LED. Otherwise, check driver circuits.

    10. Bad IC - if it is a custom chip, forget it! Failure of the IC is usually quite unlikely.

      (The following is from Duane P Mantick:)

      An awful lot of IR remotes use IC's from the same or similar series. A common series comes from NEC and is the uPD1986C which, incidentally is called out in the NTE replacements book as an NTE1758. A lot of these chips are cheap and not too difficult to find, and are made in easy-to-work-with 14 or 16 pin DIP packages. Unless you have no soldering or desoldering skills, replacement isn't difficult.

    There are a large variety of universal remotes available from $10-$100. For general TV/VCR/cable use, the $10 variety are fine. However, the preprogrammed variety will not provide special functions like programming of a TV or VCR. Don't even think about going to the original manufacturer - they will charge an arm and a leg (or more). However, places like MCM Electronics do stock a variety of original remotes - prices range from $9 - $143 (Wow $143, for just a stupid remote! It doesn't even have high definition sound or anything exotic). The average price is around $40.

    Problems with the IR remote receiver

    Although the hand unit is most likely to be the cause of any problems with the remote control, it is also possible for the IR receive module to fail or for power to it to be missing. Microcontroller problems as well can result in similar symptoms.

    First confirm that the hand unit is putting out the correct code. If it is a programmable type, try re-entering the settings for your TV. Install a set of fresh batteries. Try a different remote if possible. Use an IR detector to verify IR emissions (see the section: Revival of dead or tired remote control units).

    The IR receiver is often a self contained module connected to the rest of the TV's circuitry by 3 wires: Power (+12 V typical), Ground, and Signal Out.

    The IR receiver module will be located directly behind the IR window. Test by confirming that DC power is present. A schematic will tell you exactly what it should be but figure on 6 to 12 V if you do not have one. If this is present and you have an oscilloscope, put is on the Signal Out. You should see the demodulated data stream corresponding to whatever key is pressed on the hand unit. It should be a logic level signal swinging between 0 and the supply or +5 volts.

    If there is no power, then a bad cable connection or blown fusable resistor may be the cause. If there is correct power but no signal, a fault internal to the IR module is likely. The internal circuitry may be a combination of special ICs and discrete components. The Sams'' or service manual may or may not provide the details. There may be an adjustment for the carrier frequency but don't be tempted to touch this unless you have exhasuted other possibilities - and them mark it first!

    If the signal is present, then there may be a problem in the microcontroller or other logic on the mainboard. This will require a schematic to proceed further.

    So you lost your original remote (or it fell in the toilet)

    An exact replacement remote will be easiest to use but may do significant harm to your bank account.

    For example, you cannot add or remove channels from a typical Sony TV using the common universal remotes.

    (From: Ed Ellers (edellers@delphi.com).)

    Universal Electronics' "One For All" remote controls can reproduce these codes, and any others on any Sony TV (among others). Typically you'd press [MAGIC] and then 1-9-4 to add a channel or 1-9-0 to erase one; to start the auto program mode you'd press [MAGIC] and then 1-2-4.

    Loudspeakers and TVs

    Loudspeakers incorporate powerful magnets - the larger the speaker, the larger the magnet. However, anyone who goes ballistic when the mention is made of a loudspeaker near a TV or monitor, should take their Vallium.

    The fringe fields outside the speaker box will not be that great. They may affect the picture perhaps to the point of requiring degauss. The normal degauss activated at power-on will usually clear up any color purity problems (assuming the loudspeakers have been moved away). At worst, manual degauss will be needed. The CRT will not be damaged. The maximum field - inaccessible at the voice coil - is quite strong. However, even for non-shielded loudspeakers, the magnetic field decays rapidly with distance especially since the core structure is designed to concentrate as much of the field as possible in the gap where the voice coil travels.

    However, keeping speakers away from CRTs is a good idea.

    Now, you really should keep your superconducting magnetic resonance imager magnet at least in the next room.....

    Should I replace all the electrolytic capacitors if I find a bad one?

    When a bad capacitor is found in a TV, the question of course arises as to the likelihood of other capacitors going bad in short order. It might be worth checking (other) caps in the power supply or hot (temperature) areas but you could spend you whole life replacing **all** the electrolytics in your older equipment!

    Sweet little old ladies and TVs from attic

    Always confirm the customer's complaints first!! Then verify that everything else works or you will never know if your efforts have affected something unrelated.

    (Original request from rogerj@apex.com):

    "A sweet little old lady has duped me into repairing her old GE 13" color TV. She wanted me fix a bad volume pot. "Oh it has such a good picture", she says.

    Stupidly without even turning it on, (big mistake) I begin to open the set. After 15 to 20 min. of travail, I discover that a previous "repairman" has glued the case shut! (I wonder the sweet little old lady was in the habit of tweaking everything inside!:) --- Sam.)

    Now with the set open, I turn it on and this picture is LOUSY. Bad color, and very poor convergence. But I don't know if I'm to blame for banging it around trying to open it up. Also, no horizontal or vertical hold. (fixed that with a few caps). This thing has probably been sitting around for a few years."

    Well, you certainly did not kill the caps. Anything that sits for a few years - probably in a damp unheated attic - is suspect.

    Did you find the adjustments on the yoke assembly tight? If so, you probably did not move anything very much either. She may remember the good picture it produced before being stuffed away in the attic.

    (From: Roger.)

    "Anyway after going through all the adjustments, the convergence at the sides is still bad and the horizontal size is a tad insufficient (and no adjustment available)."

    It could be that the convergence (including pincushion) circuits are still faulty - not just misadjusted.

    Other things that can effect horizontal size while still giving you a complete picture:

    1. Voltage to horizontal output transistor low. Is there a voltage regulator in your set? The one I have has none. I assume your line voltage is ok.

    2. Increased resistance or inductance of the yoke windings. For all you know, the yoke may have been replaced with the wrong part.

    3. Yoke improperly positioned on tube neck.

    4. Excessive high Voltage. This is usually not adjustable.

    I bet the thing hasn't worked properly in 10 years! :)

    Phantom spot or blob on CRT after set is shut off

    (Portions of the following from a video engineer at Philips.)

    Why is there a splotch of colored light at the center of the CRT after I kill power to my TV? Why does this not happen if the plug is pulled instead? It seems to last for hours (well maybe minutes at least).

    A broad diffused glow (not a distinct spot in the middle of the screen) that lasts for a few seconds to minutes is called 'afterglow' and may be considered 'normal' for your model. The warm CRT cathodes continue to emit electrons due to the high voltage that is still present even though the signal circuits may have ceased to operate.

    For more sharply defined spots there are two phenomena:

    1. Thermal emission from a cathode that has not yet cooled off (and this could take several minutes) gives a more or less circular spot near the centre. It is actually 3 spots from the 3 cathodes, we at Philips call them 'Christmas balls'.

    2. Field emission from sharp whiskers on any electron gun part gives a much sharper spot, sometimes with a moon-shaped halo around it. Even with the filament off, there may be some electron emission from these sharp points on the cold cathode(s) due to the strong high voltage (HV) electric fields in the electron gun. I do not know how likely this is or why this is so.

    The shape of the spot is an inverted image of the shape of the emitting area(s) on the electron guns cathodes.

    The visibility of both effects depends in the same way on the decay time of the high voltage (HV/EHT) on the anode.

    When turned off with the remote or front panel button, you are not actually killing AC power but are probably switching off the deflection and signal circuits. This leaves the HV to decay over a few minutes or longer as it is drained by the current needed to feed the phantom spot or blob.

    When you pull the plug, however, you are killing AC input and all the voltages decay together and in particular, the video signal may be present for long enough to keep the brightness (and beam current) up and drain the HV quickly. Whether this actually happens depends on many factors - often not dealt with by the designers of the set.

    A proper design (who knows, yours may simply have been broken from day 1 or simply be typical of your model) would ensure that the HV is drained quickly or that the other bias voltages on the CRT are clamped to values that would blank the CRT once the set is off. If the problem developed suddenly, then this circuitry may have failed. On the other hand, if it has been gradually getting more pronounced, then the characteristics of the CRT or other circuitry may have changed with age.

    In most sets it is left to chance whether the picture tube capacitance will be discharged by beam current at switch-off. It may simply be due to the behaviour of the video control IC when its supply voltage drops that causes the cathodes to be driven to white and this may not be formally specified by the manufacturer of the IC. Some of of the latest sets have an explicit circuit to discharge the EHT at shutdown.

    As noted in the section: Safety guidelines, the HV charge on the CRT capacitance can be present for a long time. A service technician should be very aware of that before touching HV parts!

    Interestingly, most sets for the Asian Pacific market have a bleeder resistor built in that will discharge the EHT without the need for a white flash at switch-off. These will in fact drive the beam to black at switch-off via a negative voltage to the CRT G1 electrode. The AP market is very sensitive to proper set behaviour, they don't like a white flash.

    In short, it all depends on the demands of the particular market, the chance of the picture tube producing a spot/blob, and the mood of the designer.

    So, it may not be worth doing anything to 'fix' this unless the splotch is so bright (more so than normal video and for an extended time) that CRT phosphor damage could result. This is usually not a problem with direct view TVs but would definitely be a concern with high intensity projection tubes.

    On the other hand, your phantom blob may provide for some interesting conversation at your next party!

    Comments on bright spot at power-off

    With really old TVs, it was almost expected that when shut off, the raster would collapse to a spot which would then gradually fade away. If severe enough, the result after awhile would be a permanent dark spot in the center of the screen. Modern sets usually avoid this by forcing the CRT to be blanked for a few seconds after power-off while the high voltage decays or unblanked as the raster collapses to discharge the high voltage quickly with a white flash. However, claiming they all do this by design may be optimistic! :)

    The following was in response to questions about a Sony TV but should apply in general.

    (From: JURB6006 (jurb6006@aol.com).)

    I think in this case the filter for the supply to the final video outs has gotten to a point where is discharges faster than the tube. Sony designs usually use fast blanking at power down, but it can only blank while the +220 VDC line is alive. There might also be a cap between the 220V line and the G1 biasing circuit. Also Sony's almost always incorporate some sort of HV bleeder but if it opens up usually there is a convergence problem. If it's not any of the above, the plot thickens. Look for a bad diode, or even an active spot killer circuit that has failed.

    You might want to look into this, as I tell customers, when it collapses to a line it is 480 times as bright, it won't look 480 times as bright because the phosphor simply cant put out that much light. I've also made comments in here on how fast a CRT will burn if the yoke is unplugged. It's then the original 480X multiplied by 640. If that beam is in focus that means the "drive" to the phosphor is over 300,000 X what it should be. Even if the spot is defocused to 100X it's normal size, that is still 3,000X the current on that part of the screen. Even if yours doesn't burn the screen, in time you might expect a minor purity problem in that area on a fine pitch color CRT. This is an effect known as "doming", and it happened even on some low resolution NTSC TVs! It usually happened more at the sides, and if it became permanent you could fix it up if you used enough stick-on magnets, but you can't do that in the middle.

    I bet the thing is a real pain to work on too, so I might consider just never shutting it off, or a simple modification. If G1 is grounded, it is easy. One resistor, a cap and a diode, ba-da-bing ba-da-boom. Two caps if you like to be fastidious. There are some designs in which this will not work. It does work 99% though, if the vidouts are driven they will discharge the +220 VDC supply, what I do is to AC couple the supply to the G1. It can be done with three component, but there are enhancements I recommend to protect the CRT from excessive K-G1 voltage. Don't want to cause a short there. Five or six components will do it on almost anything.

    Disposing of dead TVs (CRTs and charged HV capacitors)

    I don't know what the law says, but for safety, here is my recommendation:

    Treat the CRT with respect - the implosion hazard should not be minimized. A large CRT will have over 10 tons of air pressure attempting to crush it. Wear eye protection whenever dealing with the CRT. Handle the CRT by the front - not the neck or thin funnel shaped envelope. Don't just toss it in the garbage - it is a significant hazard. The vacuum can be safely released (Let out? Sucked in? What does one do with an unwanted vacuum?) without spectacular effects by breaking the glass seal in the center of the CRT socket (may be hidden by the indexing plastic of the socket). Cover the entire CRT with a heavy blanket when doing this for additional protection. Once the vacuum is gone, it is just a big glass bottle though there may be some moderately hazardous materials in the phosphor coatings and of course, the glass and shadow mask will have many sharp edges if it is broken.

    In addition, there could be a nice surprise awaiting anyone disconnecting the high voltage wire - that CRT capacitance can hold a charge for quite a while. Since it is being scrapped, a screwdriver under the suction cap HV connector should suffice.

    The main power supply filter caps should have discharged on their own after any reasonable length of time (measured in terms of minutes, not days or years).

    Of course around here, TVs are just tossed intact which is fortunate for scavengers like me who would not be happy at all with pre-safed TVs!

    Shock and/or spark when connecting cable or other A/V components

    TVs with hot chassis - where signal ground is actually line connected and at some intermediate (and dangerous) voltage - will have an isolation block in between the tuner and antenna/cable connections. TVs with isolated power supplies may have some bypass capacitors between the power supply and signal grounds (including the A/V shields if there are A/V connectors). It is possible for a failure to result in a serious safety hazard where the RF (antenna/cable) or A/V connectors become electrically live.

    However, a tingle or small spark might be normal. RFI bypass caps between the AC input and shield on the connector could result in some leakage - 50 V or more might be indicated using a high impedance multimeter. This is harmless. Reversing the plug in the AC socket (if it is not polarized or if you are using an unpolarized extension cord) might eliminate or greatly reduce the effect.

    Nonetheless, it should be checked out. Measure the resistance between each side of the AC plug and the RF and AV connector shields. It should be 1 M ohm or more. Test for voltage between the cable (or other device) connector and earth ground. If there is anything significant, test the resistance on the device between its shield and its power plug as above - other devices may have RFI bypass caps or be defective as well.

    What is the deal with Macrovision copy protection?

    (From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).)

    JVC owns the patent for VHS. JVC has made a deal with Macrovision that from a certain date in the past *no* VHS recorder licenced by JVC shall be able to record any video signal that contains Macrovision's copy protection pulses. Any video recorder from before that date (VHS or other) might well work OK on the altered video signal ! The copy protection pulses upset the video-AGC and H-sync. TV's usually don't have a video-AGC. The stabilizer box removes the extra pulses and makes it into a normal video signal again. No VCR should ever know the difference, so they should all record properly again.

    At the same time, all TV's are required to ignore the copy protection pulses. As a TV-designer I can tell you that this is sometimes far from trivial. Not in the least because in the beginning we were not included in "the deal". There may be TV's around whose brightness and/or sync will be disturbed by the Macrovision pulses. Officially, this is the reason for existance of the stabilizer boxes: to view better, not to copy better. Unofficially, they are sold for copying, of course.

    The next step will be that digital-TV decoders will output an analog TV signal with Macrovision copy-protection pulses so that you may watch but not record your pay-per-view program. Same problem, same solution ...

    And I thought that PAL/Secam/NTSC were *standards*, sigh ...

    Whether they like it or not (and from personal experience I can tell you that we don't like it) it is the responsibility of the TV set-maker (in your case Sony) to build a TV that takes the Macrovision copy protection pulses without showing any side effects on the screen. Seems like they didn't do a good job on your TV :-). But they will have to fix this, your complaint is valid. I think in this case it may be the dealer's responsibility too, maybe you can trade it for a different brand ? And do try it out first ...

    Sadly, more complex TV's seem to suffer more than the simpler, old-fashioned, designs. Unfortunately, Macrovision seems to be satisfied when their pulses do not affect the majority of (mostly older) TV's. In your Sony TV, the clamping circuit seems to be affected by some ultra-black pulses in the signal. Maybe an anti-Macrovision decoder can help you, officially they are designed for *this* purpose.

    AGC and copy protection

    (From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).)

    1. RF-AGC which compensates for different signal strength at the aerial, it measures RF amplitude and is *not* sensitive to video contents because with negative modulation the sync is the peak and is constant, this AGC will not work on CVBS (baseband video) inputs.

    2. Video-AGC which normalizes baseband signals which enter *after* the tuner-IF. A.o. this compensates for different signal strengths when you connect two VCRs together. It measures peak-white, so it *is* sensitive to video content and thus to the Macrovision pulses.

    And: A television does NOT have a video-AGC, unless you want to call the beam current limiter circuits an AGC. (Exception: the Secam-L system with positive modulation requires an RF-AGC which measures peak-white instead of peak-sync.)

    The RF-AGC does not see the peak-white of the anti-copy pulses. If you connect the VCR to the TV via the CVBS (baseband) input, then the RF-AGC is not even in the path. Still, it may be disturbed. But the sync separator may see the extra inserted Hsync pulses, and due to the phase disturbance the video clamping may be disturbed too.

    On-screen clock runs slow or fast

    Now think about it: Do you absolutely need the TV's clock??? After all, there are probably a half dozen other clocks in the same room! :)

    Cold problems with cold TVs - or - an unhappy Christmas

    (From: Ren Tescher (ren@rap.ucar.edu).)

    My Christmas repair story wasn't so happy.

    I worked as TV repairman for an appliance store.

    On a very cold (20 below zero Fahrenheit) evening a man bought a new 25" console for the family.

    As we loaded it into the back of their pickup truck, we *told* them. "Do not plug this in until it warms back up to room temperature." They nodded and said "uh-huh, okay".

    They lived about 15 minutes away.

    25 minutes later we get a telephone call...

    "Hey! that TV you sold us don't work!"

    So we ended up loading another TV into our delivery van, drove out to their place. Unloaded it, and WAITED until the thing was warm enough to plug in.

    Needless to say, I got home late that Christmas Eve and had brand new TV console waiting for my repair back at the shop.

    F-connector broken off rear of TV or VCR

    Sometimes this just happens due to poor manufacturing and a slight tug. However, usually, there has been some severe trauma - like the TV or VCR falling off a shelf while still attached. When else would it fall off a shelf? :-)

    I recently repaired a Panasonic VCR with a dangling F-connector. It required removing the cover, main board, unsoldering the A/V block and part of the power supply, just to get at the RF modulator. Then it was a simple matter of resoldering the center conductor to the printed circuit board (fortunately, nothing else was damaged) and the shell of the F-connector to the metal box.

    From: Glenn Watkins

    We get a lot of sets with broken "F" connectors broken off - both VCR's and TV's. The job can be quite involved depending on the exact set. It may take an hour just to open it, remove the tuner or RF modulator and open the case. Then if you're lucky, there is no additional damage inside the unit due to people probing with pencils, screwdrivers, etc.

    On some TV's including Sharp, there is a antenna(cable) isolation device that connects between the tuner and antenna. This device comes in a few flavors and is very easy to install if broken. I've seen them as cheap as $4.95 each mail order.



  • Back to TV Repair FAQ Table of Contents.

    Some Model Specific Problems

    Erratic problems with older GE TVs

    Older GE TVs used double sided circuit boards with poor-man's vias - rivets soldered to the traces top and bottom. These have been called 'Rivlets' and 'Griplets'. Unfortunately, whoever did the design didn't realize that (1) the the rivets did not heat adequately during soldering and (2) the expansion coefficients of the rivets and circuit board were not qutie identical.

    Thus, erratic problems are almost a certainty with these TVs. Normal logical troubleshooting is useless. The only solution is to repair every !@#$ Griplet on all circuit boards in the TV.

    I have repaired these with a high power soldering gun used on both sides with liberal application of solder and flux. However, I do not recommend this shortcut unless you are willing to redo the repair every couple of years.

    (From: Mr. Caldwell (jcaldwel@iquest.net).)

    There are two methods of repair.

  • Method 1: Clean the paint from around each griplet on both sides of the board to expose the surrounding copper pad. Apply liquid solder flux to the cleaned copper. Solder so as to bridge the griplet to the cleaned copper.

  • Method 2: Do the above but desolder the griplet and place a wire through it so that the wire extends beyond the griplet to the copper foil and solder the wire on both sides (this was the final fix GE used in this chassis).

    If done carefully Method 1 works and is reliable.

    I would normally do this prior to any troubleshooting, it repairs most problems in this chassis. While you have the board out working on it be sure to also clean and resolder high temperature components and connections that look bad. These griplets can be on all boards, even the tuner control board.

    Erratic problems on late model GE, RCA, or ProScan TV

    Problems with bad solder connections, mostly in and around the tuner are very common with several series of late model (e.g., CTC175/176/177 chassis) RCA/GE/Proscan TVs. Ignoring these erratic and intermittent problems can lead to serious damage including failure of the EEPROM and possibly other expensive ICs. Therefore, it is essential to deal with the solder connections as soon as these symptoms appear. The repairs are straightforward though perhaps tedious. Thomson may reimburse for reasonable cost of repairs.

    Some of the common symptoms include:

    See the document: RCA/GE TV CTC175-187+ Solder Connection and EEPROM Problems for additional information on these types problems including repair procedures and approaches to getting coverage from Thomson Electronics.

    Sylvania/Magnavox/Philips - no startup

    Check the resistor supplying initial base current to the horizontal driver (not HOT) transistor. On many chassis, it is R502, 47 K. It opens for not good reason. Why it fails is a mystery as its power rating should be adequate.

    Sony TVs/monitors and Hstat

    Symptoms are that the TV or monitor will shut down possibly after a warmpup period. There can be other causes but failure of the Hstat module HV sense wire is quite likely on many Sony models.

    (From: Mr. Beanz (slin01@mail.orion.org).)

    If you've determined that the HV is fine, and the H-STAT is shutting down for no reason, it's possible to bypass. There is a little brown wire coming out the bottom of the H-STAT which goes to a 3-pin connector. Two wires have a jumper, and the brown wire goes to the other. Snip the brown wire at the H-STAT and the TV will continue to function normally. Measuring the regulated B+ to the flyback will give you a pretty good indication of the condition of the HV output. If it remains steady at rated voltage (I forget what it was, 130V or 135V) then HV is A-OK. If it slowly creeps up or is too high to begin with, you have a problem. The voltage will normally jump to 150V or so after HV shutdown is tripped.

    Ideally, you should replace the H-STAT in this case. Although taking the poor man's route will WORK, you lose any protection in the event that the HV circuitry should malfunction.

    More on Hstat

    (From: Shawn Lin (slin01@mail.orion.org or lin@science.smsu.edu).)

    The H-STAT is a plastic box that sits mounted to the picture tube's shield. It's red in color (for every SONY TV that I have owned) and has a single knob on it. The flyback's HV output wire goes into the H-STAT and another HV wire exists the H-STAT and connects to the anode cap on the picture tube. It has a dual purpose, horizontal static convergence (the control adjusts this) and HV overvoltage shutdown protection. Chances are, your HV is within spec and the H-STAT is bad, but you should make sure the regulated voltage to the flyback is steady and doesn't fluctuate before assuming the H-STAT is bad.

    H-STAT is expensive, and may not be worth replacing. My kV-1952RS is old and as a whole, not worth the cost of a new H-STAT, so I just bypassed it and didn't bother replacing it. The TV's been working great for over a year and the picture is still excellent.

    Note: On some models, the sense wires need to be connected during startup or else it will never come on.

    CAUTION: On some monitors (like the Sony CPD1302), the sense signal may be used for actual HV regulation. Thus, if the sense wire is disconnected, (or the divider inside the Hstat block fails open) there is no feedback and it is possible for the high voltage (and probably B+) to increase until the HOT (and possible other components) blow. I do not know if this applies to Sony built TVs as well.

    No video on late model Sony TVs

    (From: David Kuhajda (dkuhajda@locl.net).)

    This following assumes that the more basic TV troubleshooting has already been performed and resulted in no solution - i.e., filaments are lit up, CRT voltages are all correct but the cathodes are all in cutoff, main power supply voltages are present, etc

    The 3 most common causes of no video, but good audio are as follows:

    1. AKB blanking due to either a fault in the AKB circuit or the CRT has one gun that is too weak and the AKB won't unblank the CRT.

    2. Loss of vertical deflection. The system control must see the vertical return pulse to unblank the video.

    3. Loss of data bus communication with the Jungle IC. The Jungle IC must send the all OK signal to the micro before it will unblank the video.

    To troubleshoot:

    1. Try changing the volume with the volume control. If it changes the data bus is not the problem. This is assuming the TV is tuned to a good TV station.

    2. Mark the EXACT position of the screen/G2 control with a marker. Then turn the G2 control up.

      • If the picture unblanks and a gray scale bar pattern is used, you can usually see which CRT gun is weak. Time to replace the CRT for a long term fix. All of the temporary fixes I have tried resulted in only a few months extra life out of the tube.

      • If the picture does not unblank and it looks kind of like there is a halo near the top of the screen. The vertical circuit needs checked for proper operation and repaired. WARNING do not continue to operate the TV set in this mode for any length of time, it will crack the CRT from localized heating!!!! (because the guns are shooting into the extreme top of the tube and possible the neck of the tube) RETURN the G2 control to the exact position that was marked before you started.

    The typical failure mode of the vertical circuit in a Sony causes only one of the vertical supply resistors to open usually. Either the +15 V or -15 V. This puts a full + or -15 V on the yoke which deflects the guns into the glass envelope of the tube.



  • Back to TV Repair FAQ Table of Contents.

    Items of Interest

    Various video standards

    Different standards for analog component video:

    An informal history of X-ray protection

    (The following is from: Marty).

    Most of the old tube type color TV sets used a shunt HV regulator tube, usually a 6BK4. If it failed, or some component in the HV circuit failed, the high voltage, normally 25kV, could go up to 35kV or more, causing some X-Ray leakage from the CRT. In the early 70s when news of this radiation scare was first announced, there was a public outcry to immediately fix the problem. The feds hastily imposed a requirement on manufacturers of TV sets to somehow render a TV set "unwatchable" if the HV exceeded rated limits.

    The manufacturers first response was to follow the letter of the law and the first "HEW" circuit simply blanked the video when the HV exceeded a setpoint to make the set "unwatchable".

    It was quickly noticed that the HV was not turned off with this circuit and the CRT still could emit some radiation. Many TV sets with this feature were left on so the consumer could listen to the sound, so the feds tightened the requirement.

    By this time new TV sets were all solid state and some manufacturers experimented with HV shutdown circuits, but most of these circuits were poorly designed and not reliable.

    Zenith thought they had the answer by regulating the HV with a bank of 5 capacitors across the horizontal output transistor to "hold down" the HV to 25kV. If one capacitor opened, the HV would only rise about 2kV, not a dangerous situation. This wasn't good enough for the feds.

    The "fix" that Zenith finally came out with, was a "4 legged capacitor". Two legs were the emitter return for the horizontal output transistor, & two legs were the HV holddown capacitor (the equivalent value of the bank of 5 caps). This "fix" was accepted by HEW and millions of TVs were produced. It worked so well, that other manufacturers soon followed the lead (Magnavox, GE, etc.).

    Then the worst happened! The 4 legged monsters started failing in a large numbers. Not opening completely & not shorting out. They sometimes allowed the HV to skyrocket to over 50kV. Some of them even cut the necks off of the CRTs.

    Zenith issued a recall on those models with the problem (more than one entire model year). After several "improved" versions of the capacitor, the problem was fixed but that recall almost bankrupted the company. Other companies had failures too, but usually not as dramatic as Zenith's.

    Magnavox used the HV holddown capacitor, both single & 4 leg version in several 70s era TV sets and is a good candidate for fireworks as well.

    (From: Roy J. Tellason (rtellason@pa.net).)

    The problem was reputedly due to a capacitor maker substituting a different (cheaper) material for the one that Zenith originally specified, leading to the failures. I can't recall who the cap maker was (some company in Taiwan?) but remember hearing as to how they were the ones who ended up having to pay for all those warranty repairs, including CRT replacements.

    (From: Hugh Prescott (hugh345@adams.net).)

    The original X-Ray problem was found in some GE tube sets that used a triode as a shunt high voltage regulator.

    As long as the shunt was working properly there was no significant X-Ray production and the triode was designed to contain the very low KV (soft) X-Rays it did produce.

    It was found that there was a failure mode that allowed the HV to rise and the increased voltage was enough to generate a stronger (harder) X-Ray that could exit the shunt tube.

    As I recall the most intense area was directed down through the bottom of the case. One of the comments about it's danger was that the most likely person to receive significant exposure was the bar tender standing under a wall mounted set.

    I was part of a radiation safety team that inspected a GE plant in Illinois that produced this chassis in the middle 1960s. As I recall there was no recommendations for any changes to the production line at the plant to reduce the exposure levels as there was not a level of radiation present that required it.

    A set was badly misadjusted for us to make some measurements that confirmed the exposure levels.

    From a retired Nuke Safety Geek. :)

    What is this goop around some electrolytic capacitors and other components?

    That goop is probably glue and generally harmless - it is there to hold down the components aganst vibration. I have heard of it sometimes decomposing and shorting stuff out but I doubt you have that problem.

    Therefore, unless you find a bad cap in the focus or related circuit, we are still looking at a flyback problem.

    What does the flyback (LOPT) transformer do?

    The typical flyback or Line OutPut Transformer (LOPT) consists of two parts:

    1. A special transformer which in conjunction with the horizontal output transistor/deflection circuits boosts the B+ (120 V typical for a TV) of the low voltage power supply to the 20 to 30 kV for the CRT as well as provide various secondary lower voltages for other circuits.

      A HV rectifier turns the high voltage pulses into DC and the CRT capacitance smooths it. The HV may be developed from a single winding with many many turns of wire or a lower voltage winding and a diode-capacitor voltage multiplier.

      The various secondary voltages power the logic, tuner, video signal, vertical deflection circuits, and CRT filament. In fact, with many TV designs, the only power not derived from the flyback is for the keep-alive circuitry needed to maintain channel memory and provide startup drive to the horizontal deflection/high voltage system.

    2. A voltage divider that provides the focus and screen supplies. The pots are in this divider network - and these things fail resulting poor focus, uncontrolled brightness, or fluctuating focus and/or brightness. A total short could also result in failure of other components like the horizontal output transistor. In some TVs, the focus and screen divider and/or controls are external to the flyback and susceptible to dust and problems particularly on humid days. The resistance of these circuits is so high that dirt or other contamination can easily provide a bypass path to ground especially when slightly damp.

    Why do flyback (LOPT) transformers fail?

    While flyback transformers can on occasion be blown due to a failure elsewhere in the TV or monitor's power supply or deflection circuits, in most cases, they simply expire on their own. Why?

    Flybacks are wound with many layers of really really fine wire with really really thin insulation. This entire assembly is potted with an Epoxy resin which is poured in and allowed to cure.

    In some ways, these are just short circuits waiting to happen.

    Flybacks get hot during use and this leads to deterioration of the insulation. Any imperfections, nicks, scratches, etc. in the insulation contributes to failure. Temperature cycles and manufacturing defects result in fine cracks in the Epoxy potting material reducing the insulation breakdown particularly in the area of the high voltage windings, rectifiers, and focus/screen divider network.

    It is amazing they last as long as they do with the stress they are under. They also physically vibrate to some extent. A whole bunch of other factors are also no doubt important.

    Brief comments on testing the HOT

    For a TV with no blown fuses that will not start, here are two quicky checks to see if the HOT is good and has power and drive:

    WARNING: use an isolation transformer for the oscilloscope tests (and whenever you are probing a TV in general)!!! This part of the circuit, in particular, is usually line connected. See the sections on safety.

    CRT rejuvenation

    Where one or more electron guns in the CRT have deteriorated due to wear and tear, it is sometimes possible to give them a new, but possibly, temporary lease on life through rejuvenation using a special piece of CRT service equipment.

    (From: Gary Klechowitz (klechowi@execpc.com).)

    When I rejuvenate a tube I inform the customer that there is no warranty on the job. Rejuvenating a CRT is like when Clatuu was brought back to life by Gort in "The Day The Earth Stood Still". When asked "How long will you live"? he replied: "no one knows".

    I use a Sencore Beam Builder. If your tube is just moderately dim and blurry but still shows good cut off threshold, I would just use the auto restore mode on the beam builder rather than using the restore button. If the tube is really bad with little or no cutoff threshold, then the rejuvenator is needed but that has less than a 50% chance of fixing the tube and in many cases the tube gets worse to trashed in the process.

    Memory chips in TVs

    (From: Mark Zenier (mzenier@eskimo.com or mzenier@netcom.com).)

    Actually, they are EEPROMs. A modern TV has integrated the circuitry so that the microprocessor that controls it also sets the various adjustments like vertical height and other characteristics. The same memory that knows what channels are valid and what the brightness and other user adjustable settings are is used for factory adjustments that are set when the TV is first turned on. It's a lot cheaper to use the remote control signals that are already there than add a handful of trimmer resistors.

    For service purposes there is often a magic key sequence used with your remote control to access a service page in the on screen display than can change these. Since you can easily set something that could fry the various high power deflection circuits, getting a little too curious can void your warranty, and toast your set.

    How does Picture-In-Picture (PIP) work?

    A separate video input or tuner provides the PIP baseband signal which is then resized and stored in a frame buffer large enough to hold the X and Y dimensions of the PIP image. Readout is timed to place the PIP image in the selected area of the screen and it is substituted for the main video. What could be simpler?!

    Tony's notes on setting convergence on delta gun CRTs

    (This section from: ard12@eng.cam.ac.uk (A.R. Duell))

    The older delta-gun tubes (3 guns in a triangle, not in a line) can give **excellent** pictures, with very good convergence, provided:

    1. You've set those 20-or-so presets correctly - a right pain as they interact to some extent.

    2. The CRT is set up in the final position - this type of tube is more sensitive to external fields than the PIL type.

    Both my delta-gun sets (a B&O 3200 chassis and a Barco CDCT2/51) have very clearly set out and labeled convergence panels, and you don't need a service manual to do them. The instructions in the Barco manual are something like:

    "Apply crosshatch, and adjust the controls on the convergence board in the numbered order to converge the picture. The diagrams by each control show the effect".

    Here's a very quick guide to delta gun convergence where the settings are done using various adjustments on the neck of the CRT (if you don't have a service manual but do know what each control does, and where they all are - otherwise, follow the instructions in the service manual --- sam):

    1. Apply a white crosshatch or dot pattern to the set. Don't try and converge on anything else - you'll go insane. It's useful to be able to switch between those 2 patterns.

    2. Before you start, set the height, width, linearity, pincushion, etc. They will interact with the convergence. Also check PSU voltages, and the EHT voltage if it's adjustable. That's where you do need a service manual, I guess.

    3. Turn off the blue gun using the A1 switch, and use the red and green static radial controls to get a yellow crosshatch in the middle of the screen. These controls may be electrical presets, or may be movable magnets on the radial convergence yoke (the Y-shaped think behind the deflection yoke).

    4. Turn on the blue gun and use the 2 blue static controls (radial and lateral) to align the blue and yellow crosshatches at the center of the screen. Some manufacturers recommend turning off the green gun when doing this, and aligning red with blue (using *only* the blue controls, of course), but I prefer to align blue with yellow, as it gives a check on the overall convergence of the tube.

    5. Turn off the blue gun again. Now the fun starts - dynamic convergence. The first adjustments align the red and green crosshatches near the edges - I normally do the top and bottom first. There will be 2 controls for this, either a top and a bottom, or a shift and a linearity. The second type is a *pain* to do, as it's not uncommon for it to affect the static convergence.

    6. Getting the red and green verticals aligned near the edges is a similar process.

    7. You now have (hopefully) a yellow crosshatch over the entire screen.

    8. Now to align the blue. This is a lot worse, although the principle is the same. Turn on the blue gun again, and check the static (center) convergence

    9. To align the blue lines with the yellow ones, you'll find not only shift controls, but also slope controls. Use the shift controls to align the centers of the lines and the slope controls to get the endpoints right. These interact to some extent. You'll need to fiddle with the controls for a bit to work out what they do, even if you have the manual.

    The convergence over the entire screen should now be good....

    A word of warning here... The purity is set by ring magnets on almost all colour CRTs, but on PIL tubes, there are other ring magnets as well - like static convergence. Make sure you know what you are adjusting.

    Saga and general setup for large CRT TVs

    (Panic from: V. K.)

    "I'm having problem(s) with a brand new 40" Mitsubishi tube (direct view) TV. I'm writing this with hopes of getting some basic information so that the dealer doesn't bamboozle me.

    From first viewing (5 minutes after the delivery man departed). I noticed a discoloration patch in the top right hand corner (purple when the background is blue/greenish when background is white)."

    (From: Tony (ard12@eng.cam.ac.uk).)

    As you probably know, a colour TV produces a red picture, a green picture and a blue picture on the screen at the same time. You eyes interpret that as a coloured picture. If you look at (a normal, non-projection) TV screen through a magnifying lens, you should be able to see red, green and blue dots, and no other colours.

    Now, there are 3 basic adjustments to getting a good colour picture :

    1. Purity. This means that the red picture is only red, the green picture only green, etc. This is the one that needs setting up on your set - you have a purity problem

    2. Convergence. This means that the 3 pictures line up over the entire screen (or as much of it as possible). If this one is wrong, you'll see coloured fringes around objects in the picture.

    3. Grey scale. This sets the overall colour of the picture - it means that white is really white, etc. It varies the relative intensities of the red, green and blue pictures.

    (From: VK.)

    "I called the store in a panic and they calmly told me to press the "degauss" button to eliminate the problem (which I quickly learned was spurious magnetization, caused perhaps by storage near a speaker in the warehouse?). Result? Better but not cured."

    (From: Tony.)

    Yes, spurious magnetization (or more correctly a different magnetic field around the tube from the one present when it was set up) will cause purity problems.

    (From: V. K.)

    "The next day I visited the store, and the manager said (again) that this was an easily fixable problem, requiring a few waves of a degaussing coil. To appease me, he sends the salesman home with me with small (1 foot diameter) coil in tow. Salesman (boy, actually) waves the coil in front of and around set but can't seem to remove the discoloration."

    (From: Tony.)

    Argh... Here's what should have been done IMHO.

    1. The set should have been degaussed (a fancy word for demagnetized).

    2. They should have connected a 'pattern generator' to the set. This is a piece of equipment that generates various test signals. They should have selected 'red raster' (which will appear to you as a pure red screen), and set up the purity adjustments on that. You should ask to see the pure red raster (and pure green and pure blue if the generator will allow it), and make sure there are no strange-coloured patches. If you like, you can examine the screen through a magnifying lens to check that there are no dots of other colours appearing - I do that when I'm setting up a new TV or monitor.

    3. They should then have displayed a 'cross hatch' on the screen. This is a grid of white vertical and horizontal lines. Convergence errors are shown by the lines splitting into 2 or more colours (normally one of the 'primary colours' - red, green, or blue, and its complementary colour (cyan, magenta, and yellow).) Note, however, that it's _very_ difficult or even impossible to get perfect convergence over the entire screen on a modern tube, and that you'll not notice small errors near the corners on a TV screen. Note that some engineers prefer to set up the convergence on some other type of display (dots, for example), but you should at least be able to see a cross hatch pattern (all pattern generators provide that one)

    4. They should have then displayed a 'grey scale' test display. This is a pattern of vertical grey bars of different brightnesses, from black to white. They should all have been a neutral grey, without colouration.

    Note that convergence and purity interact to some extent, and thus if either is adjusted, both must be checked (and rechecked). Grey scale adjustments interact with nothing else.

    I would want to see the set on a pattern generator (at least the patterns I've mentioned above) and identify the problems.

    (From: V. K.)

    "To demagnetize the TV, he says that a large coil is required, that encompasses the whole unit; service rep will 'be in touch'."

    (From: Tony.)

    I've never heard of that - the correct procedure is to wipe the coil around the front, top, sides and bottom _NOT the back_ and then move it 2-3m from the set before turning it off. It doesn't matter whether the set is on or off for this, btw. I've not heard of putting a large coil round the entire set. (See the section: Degaussing (demagnetizing) a CRT.

    (From: V. K.)

    "After the sales boy leaves, I could SWEAR that the picture quality in general is decreased, with people (especially their extremities like lips and ears) appearing pinker than before, and also more general interference (fringes/noise) noticeable."

    The convergence and purity are set by ring magnets on the neck of the tube. It's possible that the degaussing procedure has slightly demagnetized these, and if so, the whole set will need to be set up. Similarly, if any part of the set was magnetized at the factory, then the adjustments may have been set up to compensate, and then after demagnetization, they'll need to be reset.

    (From: V. K.)

    "So my questions are these. Can the original problem truly be FIXED with proper sized coil and application?

    (From: Tony.)

    I don't think the size of the coil will make any difference. I would want to see that set on a pattern generator, so I could be _sure_ as to what the problems are. If the dealers don't have a pattern generator, then they're not fit to be fixing TVs IMHO.

    (From: V. K.)

    "Could I be imagining that the waving of the small coil degraded the picture quality?"

    (From: Tony.)

    It's possible, but fairly unlikely. See above

    (From: V. K.)

    "Should I demand replacement to a new set? Can I legally ask for this, or is it like a new car...you own it, now you deal with the service guys forever.

    (From: Tony.)

    I don't know US law, but in the UK, if a product is defective, you can demand a refund of the money paid (not a replacement or a repair, a refund). IMHO, a TV with incorrect colours is defective...

    Liquid coupling fluid for projection TVs

    (From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).)

    The liquid serves two purposes:

    1. It conducts the heat away from the surface of the tiny picture tube.

    2. It couples the light from the glass surface of the picture tube to the glass surface of the projection lens. Using air instead of liquid would give too much unwanted refraction at the glass-air interfaces

    I believe that the composition of the liquid is mostly water + glycol, quite similar to the anti-freeze liquid in your car's radiator, but without the colorants added that should warn you against drinking it.

    A good replacement kit contains not only a plastic bottle of liquid but also new rubber sealing rings to keep it in, where it belongs. It's always recommended to buy the original stuff, if you can find it.

    (From: markmtf@earthlink.net). I just wanted to throw my $.02 in since I was one of the original members of the design team for the first Magnavox and Sylvania PTVs.

    I don't recall the models, but essentially, there are several generations of liquid cooled/coupled PTV designs. One type consists of a set of CRTs with a liquid cell as part of that component. The other type consists of a liquid cell that directly couples the CRT faceplate to the lens.

    The liquid is a solution of DI (deionized) water and propylene glycol, with a small amount of surfactant to eliminate bubbles sticking to the glass and plastic surface. Distilled water can be used. The propylene glycol is USP grade, not commercial grade for clarity reasons. You need to order this through a lab supply company and specify USP grade. If you use a cheaper grade, the solution may become cloudy. The mixture of your solution should be somewhere between 50/50 to 80/20 water/glycol. The are many kinds of surfactant which can cause cloudiness or foaming. You should probably leave this step out if you are just adding rather than replacing liquid. If you want to experiment, you might try some fluid from a photographer darkroom supply store that is used for eliminating water marks during the film drying process. Just add a few drops to 1/2 liter. Then heat it up to check for cloudiness. If it works, then you are in business. Again, if you are careful with filling the cell so that you don't mix in air, you probably don't need any surfactant.

    I probably can't help you too much on the seals or gaskets. It is very dependent on the specific model. Both types of liquid cells loose the liquid over time due to vapor traveling through the silicone seals. The CRTs with the cooling cell used a special RTV for a seal. The CRT/Lens cooling cell used silicone gaskets. There is a tradeoff on how tightly the gaskets can be tightened down due to CRT specifications. Some manufacturers were also working on a expandable chamber to reduce buildup of pressure when the liquid expanded due to heat. The higher the pressure, the faster the liquid would evaporate through the gaskets.

    Origin of burn spot/line on projection set

    This may be an issue where a set was repaired but with a blemish that you didn't think was there before.

    (From: JURB6005 (jurb6005@aol.comtere).)

    1. If you fire a projection set up with one yoke disconnected AND the yoke windings are in series, unless you are quick, you will have a horizontal line burnt in all three tubes.

    2. The shop may have been less than 100% competent, and even if they were 100% competent, it could have happened at the actual time of the failure, before they ever saw it.

    3. If the set was fired up by ANYONE with any of the yokes disconnected, the vertical blanking circuit should have cut off ALL beam current, preventing the burning of the CRT screens. Two exceptions apply;

      • YOU were already watching the TV with a fault, or the burn happened really quickly and you didn't notice the spot.

      • The technician turned up the G2 to the point where it overrode the blanking.

    Blue focus different for projection sets?

    On some 3-tube projection sets, you may find the blue tube to have poorer focus than red and green. Don't touch it!

    (From: Dakuhajda (dakuhajda@aol.com).)

    What you are describing is completely normal. The manufactures design in the blue picture tube so that it will not focus completely.

    In fact the service literature lists exactly how much and which way it should be out of focus. In fact the picture generally looks better with the blue CRT slightly out of focus per specification - slightly wider than the scan lines so that it completely fills in the scan lines. The slight out of focus is designed to blend in the scan lines so they are not as noticeable on the big screen.

    This is because when the standard TV signal with 262.5 lines being displayed at any given time, it looks bad to see the scan lines. The standard TV signal was never designed to be viewed on a screen larger than 25".

    It also is the nature of the phosphor that makes blue not to allow a good focus. Especially in the older sets. I have not seen any early blue projection tubes focus well even when they were new.

    Comments on color purity, set orientation, and doming

    "The problem with my TV is that bright parts of the picture change color. For example, white areas may shift towards yellow or blue depending on the orientation of the set.

    What are the possible causes of doming? I have noticed that the magnitude of the doming effect varies with TV orientation even after degaussing several times at the new orientation. Does this help identify the cause of the doming in my case?"

    (Portions from: Jeroen H. Stessen (Jeroen.Stessen@philips.com).)

    The problem with regular shadow masks is 'doming'. Due to the inherent principle of shadow masks, 2/3 or more of all beam energy is dissipated in the mask. Where static bright objects are displayed, it heats up several hundred degrees. This causes thermal expansion, with local warping of the mask. The holes in the mask move to a different place and the projections of the electron beams will land on the wrong colours: purity errors. The use of invar allows about 3 times more beam current for the same purity errors.

    Both local doming and magnetic fields compete for the remaining landing reserve. Due to improper degaussing, the doming problem may be more visible. And applying a tube designed for the wrong hemisphere may very well increase the doming complaints. It is possible to deliberately offset the nominal landing in order to get more doming reserve (the shift due to doming is always to the outside of the tube). You would do this using spoiler magnets put in the right places.

    Permanently setting the contrast lower is not a real cure because the customer might not like such a dark picture. A better picture tube (Invar shadow mask) *is* a good cure (in most cases) but there is the cost price increase. (This is mainly due to the fact that Invar metal is harder to etch.)

    Also see the section: Comments on color purity, set orientation, and doming.

    About instant-on TVs

    Most TVs built since, say, 1980 have only the microcontroller powered from a small transformer when the set is off. This permits the remote control or front panel pushbutton to switch the set on. This circuitry should be no more prone to catastrophic failure than what is in a VCR or digital clock.

    Historically, there were 'instant on' TVs which kept a substantial portion of their circuitry live all the time - especially those using vacuum tubes in at least part of the circuitry (other than the CRT). In these, there was a lot more to fail. Those tubes would continue to change their characteristics for many minutes when warming up. Circuits were also much more touchy - remember all that constant tweaking! Thus, it made sense from the users's perspective to eliminate the warmup period and keep those tubes toasty all the time.

    In modern solid state TVs, the only component to really need a warmup period is the CRT. All this means is that you have to wait 20 seconds for the picture to appear.

    About gadgets to use house wiring as TV antenna

    Note that these $10 devices usually contain a single 5 cent ceramic capacitor as their 'sophisticated electronic circuitry'. The rest of the fancy plastic case is just for show.

    (The following is from: Greg Smith (LiveTV@en.com).)

    Most people mistakening believe that the larger the antenna the better the received signal. The truth of the matter is that each element of the antenna must be cut to a precise length depending upon the frequency of the signal you are attempting to receive. Further more, each element must also be spaced a precise distance away from the others. This creates what is commonly called a "directional array". (see diagram below) By providing enhanced reception (gain) in the direction the antenna is pointed, it also provides decreased reception from the sides and back. (directivity) This prevents "ghosting" which is caused by the same signal arriving at the TV at a slightly different time because the signal bounced off of some structure on it's way to your set.

    If you use the house wiring as the antenna, the length will be random and the orientation to the received signal will also be random. Therefore it will pickup the bounced/reflected signals just as well as the primary signal. IE: lots of ghosting = very poor picture quality.

    Any kind of directional antenna, even a small one, whether inside or outside, should provide a superior quality picture to that from the device you are talking about. Even a cheap "rabbit ear" antenna mounted on top of the set allows you to orient it in the best direction.

    If you only receive the VHF channels (2-10) in your area then buy a VHF ony antenna. If you only receive the UHF (19-60+) then buy a UHF only. If you get some of each then make sure that it is a combination antenna. If your set has separate inputs for VHF/UHF make sure you also get one with the proper splitter.

           -------------------
             ------()-------
                --------
                  ----
    
                   ^
                   |
          direction of signal
    

    Can I add an S-Video input to my TV or VCR?

    Possibly, but why bother? You will most likely be limited by the TV or VCR's circuitry anyhow.

    All S-Video means is (1) a special connector and (2) separate luminance (Y) and chrominance (C) rather than composite video.

    In a VCR, you will need to bypass the input circuitry and get to the place where Y and C are separate. This may or may not be possible depending on its design.

    In a TV, they may never be separate and you will need to substitute your own circuitry for the chroma demodulator.

    It is probably not worth it as you will likely not gain much in picture quality but if you really are determined, a schematic will be essential in either case.

    If all you want to do is allow for an S-video input, there are single chips which will combine the Y and C into a normal composite video signal.

    Also, see the section: How do I add A/V inputs or outputs to a TV which does not have them built in? since there may be safety implications in the case of adding S-Video to a TV without any A/V jacks.

    How do I add A/V inputs or outputs to a TV which does not have them built in?

    For A/V inputs (video and audio) The place to do this is after the video and audio IF where baseband signals are normally separate.

    For audio, in particular, an alternative is to tap into the audio circuitry which may be elsewhere. Even the loudspeaker outputs can be used but then without additional switching, you cannot disable the internal speakers when you are using your stereo system.

    Depending on the model of TV, doing any of this may be trivial to impossible - or a serious safety hazard.

    If what you are really after is replacing a dead tuner/IF with your own tuner or converter, this may be possible but, again, may not be worth the trouble. The antenna isolation circuitry is probably external to the TV's tuner so yours could be substituted in its place. Of course, any user contact with the transplanted device would then have to be TOTALLY prevented since a serious shock hazard would be present for all metal parts and connections including shield grounds. In addition, many components would likely blow the instant power was applied if this were not done perfectly.

    Unless you intend to always use the direct A/V inputs and forgo the tuner, you will need some way of selecting between them - a switch or relay. This could be manual - you push a button or flip a switch - or automatic. There are all kinds of ways to doing the detection - mechanical, checking for a low impedance connection, looking for a signal, using a switch, etc.

    You will need a schematic - don't attempt this without one (for safety, if no other reason). Even some TVs that have A/V jacks use a live chassis and provide isolation for each signal (though this is the exception rather than the rule). So, even adding another A/V jacks to one of these would be hazardous!

    Can I add direct RGB inputs to a TV?

    The signals going into the CRT neck board are separate RGB, possibly a somewhat higher voltage level, but otherwise similar to those sent to a computer monitor. So, would it be possible to bypass the tuner and color decoding and go direct for better picture quality? After all, RGB SCART inputs are common on PAL TVs.

    Assuming the scan rate of your source is NTSC (or PAL as appropriate), in principle, yes. The RGB signals may have to be amplified from the normal .7 to 1 V p-p used for monitors to whatever is required by your TV's final video amps (unless you can find a location on the mainboard where the levels are lower) and composite sync would need to be injected separately.

    However, there are some serious safety issues. Many TV are designed with a chassis that is electrically live - not isolated from the AC line so unless an isolation transformer is used to power the TV or some means of isolation is used in the coupling of the signals, you could end up with a very shocking situation and/or blown parts all over the place. I wouldn't recommend it unless you have the complete schematics and really know what you are doing.

    Adding variable volume headphones to a TV

    "My Mother-in-Law is hard of hearing. He is not. Is there such a thing as a variable volume headset that can hooked up while maintaining normal volume on the main speakers?"

    (From: Filip "I'll buy a vowel" Gieszczykiewicz (filipg@repairfaq.org).)

    Greetings. But of course... The cost is $9-ish per set and it includes a 9 foot (yup, 9 foot) cable and a handy, in-line volume control. The setup requires an audio output somewhere on the TV itself - is there a headphones jack? If yes, all you need is a "Y" (splitter) and two sets of headphones. If not, hmmm.... Either you or the local TV repair shop will need to add one. Depending on the circuitry of the speaker amplifier inside the TV, this may be as easy as splicing in a headphone jack and drilling a hole for it on the case, or as hard as somehow matching the impedance of the speaker to that of the headphones. You *will* need to look at the schematic or measure the speaker/signal. See the section: How do I add A/V inputs or outputs to a TV which does not have them built in? for the very important safety issues.

    First, however, make sure they have a TV with a headphones jack or have one put in (or get a TV that *does* have one). The 'Y' adapter can be purchased in any Radio Shack. Any cheapo one will do - no need for gold plating (they will try to sell it to you ;-) should be under $5.

    Building a Frankenstein TV

    Here is an interesting questions:

    "I got a lot of partially gutted TVs at an auction (All the same brand) and I'm trying to build a 'Frankenstein TV'.

    I have a 13" unit with a working power supply board and tuner board in one set. I have another set with a 25" picture tube in it. I'd like to drive the big tube with the guts from the small TV. Does anyone know If I'll blow up my workbench if I attempt this sort of transplant?"

    It won't blow up your workbench but the differences are probably significant enough that the performance would be unsatisfactory if it worked at all.

    In addition, this may blow up the power supply board - kill the horizontal output transistor and/or low voltage power supply itself - as the required power levels are higher. If you have nothing to lose, power your Frankenstein initially through a series 100 W light bulb and Variac. Then you will be able to tell if you are even close with less risk of blowing expensive parts.

    Of course, this does assume that all the organs your are merging are actually good to start with. Why do you you think they unloaded those TV carcasses?

    While the same chassis may be used for 19" and 25" sets, going from 13" to 25" is likely to have many differences.

    Turning a TV (or monitor) into an oscilloscope?

    This question comes up so often and it does sound like a neat project to give a defunct TV a second life. Don't expect to end up with a Tek 465 on the cheap when you are done. However, it could be a fun learning experience.

    CAUTION: See the safety recommendations below.

    You will be severely limited in the performance of such a scope. TVs and monitors are designed to operate at a very narrow range of horizontal scan rates and the high voltage is usually derived from the horizontal deflection. So, you would need to retain the original deflection system for this purpose at least.

    1. You will need to disconnect the deflection yoke from the horizontal and vertical deflection circuits of the TV or monitor without killing the HV. (also, doing all this without killing yourself as well). Depending on the design, this may be as simple as unplugging the yoke connector. More than likely, you will need to substitute a load for the horizontal deflection coil. A coil from another sacrificial similar TV or monitor would probably suffice.

      Warning: at this point you have a really bright spot in the middle of the screen which will turn to a really black spot if the brightness is not turned way down really really quickly.

    2. For the horizontal, you need a ramped current source. You are driving a non-ideal inductor (the deflection coil) so it has both inductance and resistance. Thus the waveform is a trapezoid - a voltage ramp (for the resistive part) superimposed on a voltage step (for the inductive part). This should not be too difficult. Don't expect to be able to achieve really fast sweep. Even running at normal TV rates is non-trivial.

    3. Similarly, for the vertical you need to drive with a voltage (your signal) controlled current source. However, if you are just screwing around, then the linearity etc. for the vertical may not be that important. In this case, one way is to put a current sensing resistor in series with the deflection coil and use this in a power op amp type of feedback arrangement. (You could do this for (2) as well.

    4. There is a good chance that the original brightness control will work as an intensity adjustment. However, with some TVs and monitors, this depends on receiving a valid video signal. You may need to improvise. If you do want to control the intensity from a signal source, you should be able to tap into the drive signals going to the little board on the neck of the CRT.

    5. Don't expect high bandwidth, uniform response, or any of the other things you take for granted with a decent scope. That takes work. However, as a fun project, this certainly qualifies. Interchanging the functions of the horizontal and vertical deflection yoke (and rotating it 90 degrees) may provide a better match of horizontal and vertical bandwidth to your intended applications or experiments.

    6. With a color TV or monitor, these experiments could be quite interesting and educational but there may be color fringing effects since you are not compensating for certain aspects of dynamic convergence at all.

    7. SAFETY: Once you disconnect the deflection yoke from the TV or monitor's circuits, move the original circuits out of the way and put a barrier between between you and the rest of the TV or monitor. All you will need are connections to the deflection yoke on the CRT (unless you want to do intensity modulation in which case you will need to drive the video output(s) to the CRT cathodes. I would recommend against doing this if your unit is one of those with a totally 'live' chassis as there would be additional safety hazards and circuit complications).

    (From: Chris Crochet (ccrochet@premier.net).)

    Hehehe... Actually, I've done this one. :)

    I've got two old IBM mainframe terminals, painted like charred metal, hooked up to each channel of the 'B' speaker outputs on my stereo. It's strange looking and always an attention getter when I have guests. Not to mention, the long-persistence phosphor they use makes interesting tracers :)

    One caveat, at least on these monitors (I don't know what other monitors this might apply to). When you turn them off, the circuitry shuts down in the following order: horizontal drive first, electron gun second, and vertical drive last. Therefore, if there is no vertical deflection, which would be the case if the stereo is quiet, the active electron beam becomes perfectly stationary during the course of shutdown, thus burning a hole in the phosphor. Oops :) I found it more effective to hook the stereo into the HORIZONTAL drive, thus avoiding this problem. Not quite like your average oscilloscope.

    Another interesting effect -- if the electron gun is active during vertical blanking interval, it seems to deflect so far that it bounces off the SIDES of the picture tube, and sprays all over the phosphor, making some interesting images.

    (From: Lance Edmonds (lanceedmonds@xtra.co.nz).

    Some years ago ELEKTOR and Electronics Australia magazines published articles on a design for this. Dick Smith Electronics in both NZ & Australia used to sell the kit.

    Max Bandwidth was a startling 10 or 15Khz. Enough for elementary audio servicing.

    Those magazines also published designs for delayed sweep & trigger modules as additions to any basic 'scope. Plus, a storage scope design, logic analyzer design, and a Dual trace emulator design.

    Enough to keep the average hobbist/experimenter happy for quite a while (g).

    (From: Dale H. Cook (dhcook@rev.net).)

    Every few months someone will pop up with this question. A TV would not make a very good scope. Bandwidth would be limited and the amount of work needed to build the horizontal and vertical amplifiers, sweep and triggering circuits and so on wouldn't be worth the effort. You'd need even more work to add modern features such as delayed triggering and variable hold-off. Don't even think about multiple channels and the advantages they offer. In a time when I see used Tek 465s offered for $200 it certainly doesn't pay to try to convert a TV. If you are just looking for a challenging electronic project I can think of several that have a far better chance of yielding something useful. Now, if you were starting with an antique set that used an electrostatic CRT you might do a bit better, but a 1937 Dumont will set you back about $3,000.00 or so - a little too much of an investment.

    (From: Tony Duell (ard@p850ug1.demon.co.uk).)

    I've worked on the vector monitors that were used on some of the 1970's minicomputers. These are essentially X-Y displays (not raster scanned), and would make audio-bandwidth 'scopes if given a timebase. I would guess at a bandwidth of the order of 100kHz.

    Some of them (DEC, certainly, maybe Tektronix) were electromagnetically deflected like a TV. However, there are a couple of things to be aware of. Firstly, the output amplifier, which drives the yoke at constant current, is pretty complex. Secondly, the yoke is specially made - the 2 sets of coils are pretty similar (unlike those in a TV), and the inductance is critical.

    So, while I'll keep these monitors running, I'd not want to have to covert a TV into one :-).

    (From: David Katz (DAVEkATZ@prodigy.net).)

    If by chance what you want is an X-Y display for audio, not a (more typical) X-T, it's easy. Just put a resistor in series with each yoke (about 100 ohms, 5 W) and drive them with a stereo amp.

    (From: Steve Roberts (osteven@akrobiz.com).)

    Your best hope might be to get a older generation heart monitor from a hospital, these have a professional X-Y display module to begin with, and are surprisingly easy to hack, mine was $10 at the local surplus shop. The ultra long persistence phosphor is a pain/blessing depending on what you are doing.

    For a description of what one person did, see: Dan's Home-Built O-Scope Page.

    (From: Alan (revidyks@rocketmail.com).)

    Apparently it's pretty hard to produce a decent scope.

    It is, however, pretty easy to use the CRT as something like a scope, which I did recently with the built-in green screen monitor of a thing called a Kapro 2X. It was being thrown away, so I said I'd take it and have a look inside before throwing it away.

    I wondered what if it was possible to drive the CRT from a source other than the computer video circuitry, so I did some tests, worked out how and by what voltage the deflectors were driven, (about 1v, 0.3A measured as an AC voltage).

    Once I'd worked out that this was about the same as the output from a small stereo amp, I removed the horizontal signal from the CRT and hooked one channel of my stereo across the horizontal deflector , left the vertical deflector hooked up to it's (60Hz?, 30Hz?) signal, and switched it on. The results look pretty good, I get a full-screen moving trace of the sound wave. One other thing that I did was make the beam intensity constant by turning a knob marked 'B-SUB' a bit, this would have flooded the screen with 'white' ordinarily, but was perfect for me as I could now remove the computer motherboard all together.

    I also tried connecting the left and right channels across the horizontal and vertical deflectors respectively (first disconnecting them from their normal inputs), which produced some really cool looking lissijous (sp?) figure type things, that change and throb with the music- each CD seemed to have distinctive characteristics. Maybe I'll try two different pieces of music across the axes, could be interesting...

    I'd love to try throwing some different signals of different frequencies and shapes across the axes too, especially in combination a with musical one. The 'best' results so far, have been from music with a strong bass, simple beat (cymbals with a bass drum look great), and not too many layers of guitars, vocals, etc. (too many sounds and it's an uninteresting mess...)

    If you want more information or have any advice on or experience with this sort of thing, mail me...

    If you're thinking of trying any of this, remember (in case you don't know) that TVs/Monitors can be REALLY dangerous even when switched off and unplugged. See the section: SAFETY.

    Displaying a video signal as a picture on an oscilloscope

    I am not sure why anyone would really want to do this other than as an experiment - it would be interesting one.

    If a composite video signal is the input, you will need a sync separator.

    You will have to construct a vertical deflection voltage ramp generator which can be locked to your vertical sync signal.

    The horizontal timebase of the scope will be fine for the horizontal deflection and should easily lock to your horizontal sync pulse or (if the scope has a TV trigger mode) directly to the video signal.

    A video amplifier will be needed if your Z axis does not have an internal amplifier (you need .7 V p-p to be full brightness range.) Unless you provide automatic gain control, this will need to include offset (brightness) and gain (contrast) adjustments. Even if there is an internal amplifier, it may not have the required bandwidth for the video signal.

    However, the overall brightness may be disappointing - a scope is not designed for overall high brightness. The beam focus will not be as good as that on a little TV either.

    Use of surge suppressors and line filters

    Should you always use a surge suppressor outlet strip or line circuit? Sure, it shouldn't hurt. Just don't depend on these to provide protection under all circumstances. Some are better than others and the marketing blurb is at best of little help in making an informed selection. Product literature - unless it is backed up by testing from a reputable lab - is usually pretty useless and often confusing.

    Line filters can also be useful if power in you area is noisy or prone to spikes or dips.

    However, keep in mind that most well designed electronic equipment already includes both surge suppressors like MOVs as well as L-C line filters. More is not necessarily better but may move the point of failure to a readily accessible outlet strip rather than the innards of your equipment if damage occurs.

    Very effective protection is possible through the use of a UPS (Uninterruptible Power Supply) which always runs the equipment off its battery from the internal inverter (not all do). This provides very effective isolation power line problems as the battery acts as a huge capacitor. If something is damaged, it will likely be the UPS and not your expensive equipment. Another option is to use a constant voltage transformer (SOLA) which provides voltage regulation, line conditioning, and isolation from power spikes and surges.

    It is still best to unplug everything if the air raid sirens go off or you see an elephant wearing thick glasses running through the neighborhood (or an impending lightning storm).

    Comments on lightning damage

    Unfortunately, lightning or similar surge related damage can happen to any equipment that is plugged into ANYTHING - AC line, antenna, cable, phone. Symptoms can be almost anything but often run along the lines of the following:

    "I just recently bought a new JVC TV, about 5 months ago to be exact. A few weeks ago it got hit by lightning. Not directly though. I am trying to figure out why it will not power up now. I traced as far as I could into the power and it seems to be getting 120 all the way to some sort of parts. I know at least that it is going past the fuse, and the fuse did not even blow. But when I press the on switch still nothing happens. The switch is good, as is the wiring leading to the mainboard. From there I don't know. Does anyone know of any VERY common things to go when hit by lightning? I really don't know much about TV repair, but I feel like trying to fix it myself. I just have no idea where to look now. It is getting power, but I don't know where exactly it is stopping. Any ideas would be greatly appreciated."

    (From: Brian Frank" (jambfrank@erols.com).)

    When I was in trade school the instructor who taught the TV/VCR. repair course also had a fixit place on the side. This man knew everything about TVs, he could almost tell you what was wrong with a set by just smelling it. Once I brought a set to class that had been struck indirectly as well, the instructor told me the best thing to do was to tie a rope around it an use it as an anchor for a boat. He further went on to explain that a set struck in any way usually has so many problems that it is not worth getting in to. The one time he did try to fix a particularly expensive set it took him and two other employees three weeks to get it going. And he said there were still some problems.

    As for tips on actually fixing it you might want to ask around for junkers and cannibalize them into one set.

    GFCI tripping with TV, monitor, or other high tech equipment

    Ground Fault Circuit Interrupters (GFCIs) are very important for minimizing shock hazards in kitchens, bathrooms, outdoors and other potentially wet areas. They are now generally required by the NEC Code in these locations. However, what the GFCI detects to protect people - an imbalance in the currents in the Hot and Neutral wires caused possibly by someone touching a live conductor - may exist safely by design in high tech electronic equipment. The result - false tripping - is mostly a problem with 3 wire grounded devices with built in line filters having capacitors between Hot and Ground but may also occur with 2 wire ungrounded TVs due to the power-on surge into the highly capacitive or inductive loads of their power supplies.

    However, it's also possible for this to happen when a TV is hooked to a cable system because it's cable shield is connected to Earth ground. In such a case, this may or may not be a cause for concern.

    TVs using Hot chassis designs couple the antenna and its cable shield to the tuner with small coupling capacitors. A side effect is that some AC leakage current can flow through these capacitors, possibly enough to trip the GFCI. If the TV's plug isn't polarized, try reversing it and see if the problem goes away. If it is polarized, make sure your outlet is wired properly and you aren't using a non-polarized extension cord with the TV plugged reversed.

    It's also possible the TV is faulty and the tripping is actually taking place as a result of a legitimate problem.

    To assure the set is safe, (whether reversing the plug helps or not), a leakage test should really be performed by measuring the AC current between the TV's antenna shield and an Earth ground. Try it with both polarities of the wall plug. Anything over about 8 mA will trip a GFCI. If the leakage current is less than this with either polarity of the wall plug, the GFCI is overly sensitive. If the leakage current is much more with either polarity of the wall plug, the set is defective and should be serviced immediately. It should not be used until the problem is corrected as a serious shock hazard exists.

    Multisystem TVs

    The question often arise: can my NTSC TV modified to display PAL signals (or vice-versa). Unlike a VCR where there are substantial differences between recording of NTSC and PAL, the problem of displaying the picture is much simpler.

    The following assumes 525 line NTSC and 625 line PAL:

    The horizontal scan rates are nearly identical (15,734 Hz for NTSC and 15,625 Hz for PAL), so this is not likely to be a problem. If these differed significantly, then there would be design issues similar to those for multisync computer monitors and this would drive up cost.

    The vertical scan are slightly more of a problem with 525 line/60 Hz NTSC and 625 line/50 Hz PAL. But it is a lot easier to design vertical deflection to accommodate a modest variation in rates. TVs could be easily designed or modified to accept either.

    The color encoding techniques differ but inexpensive ICs exist that can deal with either standard. In fact, many are programmable to do either with a jumper and slight modifications to the external components.

    Displaying a monochrome - B/W - picture on the other kind of set is usually possible if the set has a vertical hold control or enough vertical range. Modifying the chroma circuitry is more complicated but it should be possible to substitute a second IC and patch it into the existing video chain.

    As far as commercial multisystem TVs are concerned, the real reason we do not see many of these (at least in the U.S.) is lack of demand. They are available if you look hard enough and are willing to pay a premium. They are readily available on the international market.

    Playing NTSC videotape on a PAL TV

    Note that you will probably need an adapter at the very least to connect a PAL TV to an NTSC RF cable. This, at least isn't a problem as suitable devices should be readily available for a couple dollars. However, that may be the easy part if the VCR doesn't do enough of a conversion!

    "Does anyone know if NTSC VCRs (NTSC is a special feature in Ireland) require an NTSC capable television too, or can they convert NTSC signals to PAL (seems unlikely)?"

    (From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).)

    There are 4 possible answers:

    1. The VCR does not convert an NTSC signal to PAL, it outputs pure NTSC and you need an NTSC-compatible TV to view it.

    2. The VCR converts NTSC 3.58 to NTSC 4.43 and you need a PAL-TV adapted to NTSC 4.43 to view it (relatively minor adaptation).

    3. The VCR converts NTSC 3.58 to PAL 4.43 but keeps the field rate at 60 Hz. That is definitely not a standard signal! Some standard PAL-TV's will permit viewing it, and some Won't! At least be prepared to see interesting artifacts and crosstalks.

    4. Conversion to real standard PAL is very expensive, thus unlikely.

    The most likely answers are (1) and (3), check the spec of the VCR. The NTSC 4.43 system has been sold to middle-east and maybe US-military.

    Buying a TV in Europe

    "I have the following question for you specialists:

    Can I buy a TV in any west-european country and use it in any other west-european country? For example, buying a TV in the Netherlands and use it in Greece or buying in France and using in England."

    (From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).)

    The general answer is: NO.

    There are multi-standard TV's that cover more countries, but a TV that covers them all is extremely rare. Most countries now have PAL-BG, including all of Germany. England has PAL-I, the analog sound is at a different frequency and the digital sound is their own variety of Nicam. France has Secam L-L', mostly incompatible with anything else. I don't know about Greece, probably PAL-BG. Most Philips high-end sets can do PAL-BG, Secam-BG and NTSC (the latter from the baseband video inputs only).

    (From: Allan Mounteney (allan@amounten.demon.co.uk).)

    The answer is YES. Well, at least one.

    Reason I know is that I was with a company that made computers with TV-OUT for world wide use and wanted something that could show that the TV Out worked for various countries.

    This ONE and ONLY one we could find Three years ago came from Germany and covered PAL, SECAM and the American NTSC systems and came with a note that said from the time of making/selling that set it would not work in just one small country in South America. All features (including audio) were adjustable from the front panel Menu and it was a Grundig 17" job. I am advised that there is a load of others on the market now.

    The company who seemed to know all about these international sets and gave us good service at that time was Andrew McCulloch Ltd in Cambridge UK. Phone #44(0)1223-351825

    Could a TV be modified for 3D (stereo) display?

    The whole idea of stereo 3-D vision to put the left and right views to the appropriate eyeball. There are two common ways of doing this:

    1. Use different colors for the two views with color filters in from of each eye to separate the views. This is what were often used for the really bad (content wise) sci-fi movies of the '50s.

    2. Display alternate views on the same monitor screen but use LCD shutter glasses to allow each eye to only see the appropriate view. This requires increasing the refresh rate to avoid unacceptable flicker.

    The first approach can be used with any TV and a pair of monochrome video cameras. Of course, true color cannot be used since pure colored images are needed to separate the stereo views.

    Alternating views with synchronized LCD glasses is a possibility but on a standard TV, the resulting refresh rate would be 30 Hz with a 50% duty cycle which is likely to be useful only as a short experiment - else your viewers will likely develop splitting headaches.

    Displaying TV on a computer monitor

    My general recommendation is that if you have the space, buy an inexpensive TV - the quality in the end may in fact be better. And, it will be usable without tying up your expensive monitor and (maybe) PC.

    While various convertors are advertized to use a computer monitor with video from a VCR or other source, keep in mind that if it sounds too good to be true, it probably is like the claim of a $200 box for this:

    OK, let me get this straight - this card/box will enable a 31.4 kHz horizontal scan rate monitor (VGA) be used as a TV - yes or no? It thus includes a video A/D, full screen frame buffer, D/A, and all the other tuner stuff for under $200? I don't think so. A scan doubler - which is a subset of the above - will not result in a high quality picture since it will display pairs of lines interleaved. Or does the impressive advertisement leave out the key requirement that the monitor sync at the NTSC horizontal scan rate of 15.734 kHz (most newer monitor do not)? Or is it a board that plugs into a PC and indeed does use the resources of the PC including the VGA card and bus?

    In any case, get a written money back satisfaction guarantee.

    Displaying computer video on a TV

    Assuming this means NTSC:

    1. You need to convert RGB to NTSC - there are single chips for this. Try Sony, Philips, Motorola, and others. These will combine the R, G, B, H sync, and V sync into a single composite video signal using a minimum of additional components.

    2. You need to match the scan rate to NTSC - 15.734 kHz horizontal. Even basic VGA is twice this - 31.4 kHz. If your video card can be programmed to put out interlaced NTSC rate video then this is easy. If not, it is more difficult. If you want to use anything higher res than VGA, it is a very non-trivial problem requiring the construction of a scan convertor which includes a video A/D, full frame store, interpolator/readout timing, video D/A. Unless you are an experienced digital/analog designer, you really do not want to tackle any of this.

    For the special case of VGA->NTSC, you may be able to get away with just storing a single scan line since the horizontal frequency is (almost) exactly twice the NTSC horizontal of 15.734 kHz. A double buffer where one buffer is storing while the other is reading out at approximately half the VGA pixel rate should work. With appropriate timing, even lines become the even field for NTSC and odd lines become the odd field (I may have this backwards). It is still not a trivial undertaking. Also, keep in mind that the quality you will get on NTSC will be poorer than the VGA due to fundamental NTSC bandwidth limitations. Also, flicker for line graphics will be significant due to the interlacing at 30 Hz. Even this is a non-trivial undertaking.

    The requirements for PAL are very similar. For 625 lines systems, the 800x600 is the format that most closely matches the TV resolution.

    You can also buy little boxes to do this. Quality is general not great as you are seriously limited by NTSC/PAL and the VCR. Except for presentations on existing TV rate equipment, it is probably not worth the effort. This is totally useless for any serious computer applications.

    For professional presentations, modern video projectors are available that use high resolution LCD panels and real-time scan conversion. However, they are still relatively expensive).

    How can I couple 4 TV screens to make them act like only one?

    "I've being thinking about how people do these kind of things? Is this analog stuff or do they use some kind of digitized signal which is then divided to each TV?"

    It is mostly digital. The original master signal is digitized and stored in memory. Control codes specify the readout of a (probably double buffered) frame store. 9 and 16 screen versions are common. If you look closely, you will note that the resolution of pictures that differ is always lower indicating that the whole affair is driven from a single tape source with appropriate decoding. Where the pictures are the same, they may be at full resolution. Sub blocks of identical pictures may be at some intermediate resolution.

    What is Scan Velocity Modulation?

    (From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).)

    Scan velocity modulation occurs around the transients in the luminance signal. The beam is sped up just before and just after the edge and it is slowed down during the edge. This makes for a sharper edge. On an alternating B/W pattern (stripes, checkerboard) you will see that the white parts get smaller and the black parts get whiter. This geometry error is a side-effect. Some say that this is the main intended effect of SVM.

    SVM is *supposed* to be used to compensate for the spot blowup at high beam current. Peaking does not help to improve sharpness because the higher peak beam current also gives a fatter spot. SVM *can* work in that case. Unfortunately it is often misapplied, too much SVM will give a very unnatural picture, with obvious horizontal geometry errors.

    If applied properly, SVM can improve the picture. Unfortunately there has been a rat race, led by Japanese, suggesting that more is better.

    Some people will simply advise turning the contrast down. At low beam current the spot size will be acceptably small and SVM is not needed. In most, if not all, cases they will disable the SVM circuit, usually by pulling the supply connector to the SVM panel. That panel is often fixed to the neck of the picture tube, behind the video amplifier panel.

    About Automatic Black Level Stabilization and possible problems

    This is how some Philips TVs automatically compensate for component drift.

    (From: Jeroen Stessen (Jeroen.Stessen@philips.com).)

    Check datasheets of TDA4580, TDA4680, TDA4780, TDA8390:

    These video control ICs have automatic black level stabilization. At the top of each field it generates 3 measurement lines (R,G,B). Through a BCI beam current info wire it measures the beam current. It adjusts the amplitude of the measurement pulses so that it gets a fixed current level back, usually around 10 uA. Actually it measures a delta-voltage, the current is determined by resistors on the picture tube panel. The result is used as an offset for the video signals so that black voltage is put at zero beam current. (3 capacitors near the IC store 3 voltages, a 4th capacitor is used to store an ultra-black reference to compensate for leakage current.)

    The pulses are easily seen on the screen if you turn down the vertical amplitude (height), you'll see 3 dim lines at the top.

    When the set first starts up, the measurement loop can not close because cold cathodes can not produce any beam current. That would cause the loop to increase the black level to maximum. To prevent that, first it is measured whether the cathodes are warm yet. Instead of the 3 small measurement pulses a much larger (peak-white level) warm-up pulse is generated. This looks like a big fat white line at the top (above the screen edge).

    The video control IC waits for the beam current to increase (to approx. 5 mA for the 3 guns together) before it will release the black level control loop. This is measured over the same BCI line, a diode adds an extra load resistor to this line so that a large beam current is required to cross the detection level. After this level is reached (was it +8 or +9 V ?) the black level stabilization starts, and after almost a second the picture is unblanked. It should be correct at once.

    (I have often argued that this long delay is not pleasing to the customer who would rather have a faint picture earlier.)

    Reasons why the warm-up trip level may not be reached are:

    If there is too little beam current in general then I would start with re-adjusting the VG2 screen voltage. Because of the automatic black level stabilization this will NOT have any obvious effect on the picture. The stabilization just follows the VG2.

    If VG2 is too low then the peak drive limiter will kick in too early and you can't get a decent contrast. Also the picture will be less sharp because at a lower cutoff voltage a smaller cathode area is used plus there's your problem with the warm-up detector.

    If VG2 is too high then the video signal can not achieve blanking level anymore and you will see retrace lines. Also the cathodes will wear out faster because at a higher cutoff voltage a smaller cathode area is used, you'll have a sharper picture but not for long.

    The proper adjustment procedure for VG2 is in the service doc, you need to observe the measurement pulses on an oscilloscope. But a little twist won't hurt too much, especially if you have already written off the CRT.

    (I have the advantage of having known the people who invented this system many years ago, they were once my teachers.)

    What is Kell factor with respect to interlaced displays?

    (The following is from Bob Myers (myers@fc.hp.com).)

    The Kell factor - which has to do with the fact that we're often undersampling an image from the standpoint of the Gospel According to St. Nyquist - IS a factor in the reduction of vertical resolution, but interlacing plays a part as well. This comes from at least two factors:

    1. The receiver usually cannot precisely interleave the two fields.

    2. More importantly, there are steps taken to reduce the interline flicker which reduce the effective vertical resolution. This includes running the line width of the display somewhat larger than would otherwise be the case, and in interlaced cameras, discharging the entire screen (including the lines from the "other" field) after every field scanned.

    Interlace is particularly troublesome on moving images, where you will often perceive momentarily "missing" details. There was a LOT of discussion regarding the gory details of interlacing in the recent HDTV debates within SMPTE and other groups.

    Homemade V-chip (or at least viewing limiter)

    Here is an interesting questions:
    "I would like a control box of some sort that controls the cable signal that comes into the TV. I want to be able to control the total time a particular child has in his account to watch, plus the actual channels that he is allowed to watch (no Playboy or MTV), PLUS the time of day that he can watch (not during home work time). Programmable by channel, cumulative time, hour of the day, and day of the week. I also need a master pass word for parental programming of the kids accounts, plus be able to watch what I want to at any time. The kids could use either an individual account number or an individual "card" of some kind with a PIN like our ATM cards. This "box" should be secure so that a 14 year old boy can't bypass it very easily. At least without doing come major damage so that I'd know it when he did it. I know that this is a lot to ask, but I'm very familiar with computer programming and chipset technology, I do know that such a thing can be done. I just don't have the electronics knowledge to do it."

    The following probably won't help you build such a gizmo but here are some thoughts:

    First, I would not attempt to build any of the RF/cable switching stuff - there are too many variations. I would suggest trying to control the control of what you have. With a cable box, this would be relatively easy - just put the box and an IR transmitter in the same sealed enclosure. If you have only a cable ready TV, you could substitute or intercept the remote detector signal inside the set and disable the front panel controls.

    Then you need:

    Each account would have a means of adding to the balance, password authentication, etc.

    You would have a superuser account for your own watching as well as changing any of the individual account settings.

    Too bad I don't still teach my intro to computer design courses - this would make a nice term project.

    If you have a junker PC, this would be a simple bit of programming (but quite wasteful of power even for an 8088 based PC).

    What is Aquadag?

    You may see the term 'Aquadag' referring the the black paint covering the outside of most of the funnel section of the CRT.

    (From: Nicholas Bodley (nbodley@tiac.net).)

    Aquadag used to be a trademark of Acheson Colloids [Corp.?], I think around Niagara Falls or Buffalo, NY. It was one of many "-dag" colloidal graphites; they also made Oildag, Gredag (grease), and Alcoholdag, as I recall. Unfortunately, it's probably sold in 55-gallon drums minimum. I hope you can find smaller quantities. Are there any CRT rebuild shops around the USA? See the Thomas Catalog (ThomCat) in a library to find Acheson.

    I am pretty sure there's nothing magic about the graphite. If you can find some reasonably-priced nickel-flake or copper-flake paint (be sure it's conductive!), you might have an affordable (?) coating. How about plain metal foil, maybe even ordinary aluminum foil? You surely don't need current-carrying capacity; you would need a decent adhesive, though. How to make sure you have continuity between pieces, I'm not so sure; shoot for really tight crimps that deform the metal and are gas-tight. (This might, however, be quite unnecessary.)

    Combined computer monitor and TV

    "This is a 27" VGA monitor which should also be able to be used as an NTSC television monitor. Can anybody comment on it?"

    IMO, I think the entire idea of a combined TV/computer monitor is silly especially when the likely cost premium is taken into account. Watching the boob tube will tie up your entire PC. The optimal size for TV and computer use is not the same nor are the requirements in terms of scan rate, resolution, brightness, and sharpness. Thus, the design will be inherently more expensive and include more compromises. So, I will probably be proved wrong by record sales of these things but my only justification for such a hybrid would be if there were absolutely no space for both.

    Interesting TV Switch Mode Power Supply

    The following was found in a Sony TV:

                             Q1 switching/reg
                                transistor
                      (+)            ---
                   +---+-------------| |---------------+-----------+
             +--+  |  _|_            ---               |          _|_
       AC >--|  |--+  --- C1          |                |    diode /_\ D2
             |  |      |              |                |           |
       AC >--|  |------+---+    +-----+------+         |           |
             +--+     (-)  |    | Reg. Drive |         C           |
              D1           |    +-----+------+         C choke     +---- gnd
                           |          |                C          _|_
                           |          |                | +135 cap --- C2
                           |          |                |  I--->    |+
                           +----------+----------------+-----------+---> + 135
    
    

    Although at first this appears to short out the line supply, when drawn like this it turns out to be a valid switching regulator:

    It would appear to fail and run away under the following circumstances:

    1. Inductance is too low and choke cannot store enough energy even at high duty cycle to supply load. Too high a duty cycle and core saturates at which point transistor blows up.

    2. Inductance is too high relative to switching frequency so that choke does not have time to discharge (its current) before next current pulse - DC current will just keep increasing until core saturates. This could only really happen if the switching frequency were too high for some reason unless someone changed core material or something like t.

    3. Load is too great due to fault elsewhere.

    Unfortunately for reliability and troubleshooting, many modern power supply designs have similar failings.

    When attempting to diagnose problems with these types of circuits where the natural outcome of a fault is for one or more expensive parts to fail catastrophically, it is wise to either use a Variac to bring up the input voltage slowly and carefully observe the behavior hopefully before too late or put a load in series with the line such as a 100W light bulb to limit the current (though this will change the behavior in various ways).

    The horizontal output transistor substitution jig trick

    (From: penguin@datastar.net).

    Okay, here's a good trick you can use for almost all TV work. Mount a TO-3 transistor socket on a heatsink that has about as much surface area as the skin of both fists balled up, actually the bigger the heatsink the better. Then mount a horizontal output transistor in the socket. Use an ECG238 or equivalent. Make sure you use a good mica insulator, as there will be over a thousand volts on the collector.

    Solder a 1.5 foot red wire (18 gauge or bigger) to the collector, an equal but yellow wire to the base and an equal but green wire to the emitter (or use your own color codes). You may be able to salvage a ready made heatsink with socket out of an old receiver or TV. Mine came out of some old Curtis Mathes TV's. Solder a damper diode with the cathode to the collector and the anode to the emitter. Add a 200 ohm 1/4 watt resistor from base to emitter. Add these parts to the socket not the transistor, so the transistor can easily be changed if you ever need to.

    Now you have a very useful test jig. If you are ever working on a TV that has a blown HOT (horizontal output transistor) you can pull out the bad part and connect this jig. Then you can run the set at low voltage.

    If you have a set where the HOT is running too hot, this method often will give you some running time, hopefully enough time to find out what the problem is. Often the bad parts themselves will self destruct or heat up to where identifying them is easy. Usually a bad flyback will crack and smoke proving itself to be bad. Once your satisfied that the problem is cured, you can put the original HOT in knowing it will be safe.

    Usually when I power a TV in this way (using a Variac) I'll bypass the series pass regulator with a jumper. This is easily done by finding the 180 to 330 ohm 15 to 20 watt regulator bypass resistor and putting a jumper across it. With the regulator bypassed the power supply will go to 160 volts this is why it is necessary to use a Variac and only run it at about 60 volts. It is necessary when using this brute force approach, to make sure that all of the low voltage supplies coming off the flyback are fused with fusible resistors. Most models do fuse the LV supplies, but some don't. (One particular RCA comes to mind.)   Another good trick, if your out in the field and don't have a varactor handy, is to simply pull the series pass regulator (e.g. STR30130 etc.) and let all the power just come through the 20 w bypass resistor. If the flyback is okay the B+ to the flyback will come up to about 60 to 90 volts, and in many cases you can even see a dim picture.  Anything less than 50-60 volts and the flyback is probably bad. Don't run it this way for more than a minute or two as the the resistor will be dissipating close to or more than its rated power under these circumstances. Of coarse you obviously can't use this method with switching power supplies.

    Here's a good trick for the Sony TVs that use the SG-613, even though this device is a gate controlled SCR you can sub a regular HOT like the ECG238 on your test jig. I used to blow out these buggers to the tune of about $20 dollars a pop til I figured out how to use the "HOT Heatsink Jig". Now with the jig connected, the horizontal width may not open up all the way, but you can run these old Sony's like this for about 5 minutes before the HOT jig starts to get too hot. Usually if they run this long they're ok and then you can put the SG-613 in knowing that you aren't going to see a bunch of $$$ go up in smoke. I had a Sony that would run for weeks then blow the SG-613 finally put this one to rest when I changed out the horizontal output transformer which was separate from the flyback on this oldie.

    Always be cautious of the high voltage on the collector of the HOT JIG.

    Ken's comments on (TV) SMPS repair

    (From Kenneth Aaron (kennetha@geocities.com).)

    1. NEVER NEVER NEVER power on a SMPS without load, the newer ones especially in TVs (not so much VCRs) self destruct when not loaded.

    2. The light bulb test is great to see if your SMPS can handle the load of the horizontal circuits - when the set does not turn on - you get the initial power on then it goes off - best is to disconnect the collector of the horizontal output transistor and plug in a light bulb and see if it goes on.

    3. Chinese TVs have poor quality capacitors - the latest models mostly have the same SMPS so this applies for all. There is a small electrolyte cap in the supply feedback about 22 to 47uF at 50V. This dries up real fast and after 2 years or so the voltage B+ goes up from 110 V to anything up to 300 V! (not joking here). I got a TV where the end of the tube was blown off from a supply that reached 296 V instead of 105 V. that's over 75kV into the screen!

    4. The newer Philips and copycat Chinese models use the main supply to power the horizontal. The flyback is driven by a transistor which has over 2500 V on it's collector. The flyback is built differently as well. These transistors are called 2DS... - do not use a replacement - I did use one with a higher voltage and it fried - this is because the ceramic capacitors around it along with it's different characteristics changed the 'on' time and it overheated after a few minutes.

    5. You can always use a BU208D instead of a BU208A, BU508D instead of BU508A, etc, the extra diode will do no harm.

    IR detector circuit

    This IR Detector may be used for testing of IR remote controls, CD player laserdiodes, and other low level near IR emitters.

    Component values are not critical. Purchase photodiode sensitive to near IR - 750-900 um or salvage from optocoupler or photosensor. Dead computer mice, not the furry kind, usually contain IR sensitive photodiodes. For convenience, use a 9V battery for power. Even a weak one will work fine. Construct so that LED does not illuminate the photodiode!

    The detected signal may be monitored at the collector of the transistor (Q1) with an oscilloscope.

    
     Vcc (+9 V) >-------+---------+
                       |         |
                       |         \
                       /         /  R3
                       \ R1      \  500
                       / 3.3K    /
                       \       __|__
                       |       _\_/_  LED1 Visible LED
                     __|__       |
           IR ---->  _/_\_ PD1   +--------> Scope monitor point
             Sensor    |         |
           Photodiode  |     B |/ C
                       +-------|    Q1 2N3904
                       |       |\ E
                       \         | 
                       / R2      +--------> GND
                       \ 27K     |
                       /         |
                       |         |
          GND >--------+---------+
                      _|_
                       -
    
    

    UK Satellite TV information

    (From: Martin Pickering).

    Here is a list of the FAQs and other documents related to UK satellite TV available at:

    by adding the appropriate file name to the URL, above, or following the links.

    aegir.htm      The Aegir/Dixi/Lenco/Oritron Jupiter D2Mac decoder FAQ
    churchil.htm   The Alfaglade Churchill D2Mac decoder FAQ
    d2mac.htm      A general discussion about buying D2Mac decoders
    digifaq.htm    Chris Moore explains Digital Satellite Receivers
    diseqc.htm     An explanation of DiSeqC
    dummies.htm    Satellite TV for the beginner
    filmnet.htm    Philips BBD-901 FilmNet D2Mac decoder FAQ
    galaxis.htm    Galaxis digital receiver specifications
    grd150.htm     Grundig GRD150/200/280/300 receiver FAQ
    interf.htm     A discussion about picture interference problems
    jack.htm       Lots of humorous stories originally published in magazines
    lnb.htm        A discussion about various LNBs and frequencies
    money.htm      How to make money from Satellite TV!
    mrd920.htm     Pace MRD920 D2Mac receiver FAQ
    mss200.htm     Pace MSS200/Apollo receiver FAQ
    mss500.htm     Pace MSS500/1000 receiver FAQ
    nimbus.htm     Mimtec Nimbus receiver FAQ
    prdkits.htm    Upgrade kits for Pace PRD receivers explained
    products.htm   Index page for SatCure products
    reliable.htm   A discussion about making your receiver more reliable
    sat1700.htm    Nokia SAT1700 receiver FAQ
    sataccs.htm    SatCure accessories page
    satbooks.htm   SatCure books page
    satfaqs.htm    Index page for all FAQs
    satkits.htm    Satcure repair/upgrade kits page
    satwalk.htm    A discussion about the SatWalker and other motorised units
    scarts.htm     A discussion about Scart connectors - which is best?
    spares.htm     A complete price list of SatCure component spares
    sparkly.htm    A discussion about "sparlies" ("fischen") and the cures
    sr5500.htm     Echostar SR5500 receiver FAQ
    srd400.htm     Amstrad SRD400 receiver FAQ
    srd500.htm     Amstrad SRD500 receiver FAQ
    srd510.htm     Amstrad SRD510 receiver FAQ
    srd600.htm     Amstrad SRD600 receiver FAQ
    srx200.htm     Amstrad SRX200 receiver FAQ
    ss9kits.htm    Upgrade kits for Pace SS9xxx receivers explained
    svs250.htm     BT-SVS250 receiver FAQ
    tools.htm      A discussion about repair tools
    uniden.htm     Uniden UST- receiver models FAQ
    which.htm      Which receiver shall I buy?
    advice.htm     What to do if your receiver will not work?
    ard200.htm     Cambridge ARD200, BT-SVS200, JVC TU-AD1000 receiver FAQ
    ctu900.htm     Philips CTU900 D2Mac decoder FAQ
    mss100.htm     Pace Prima/MSS100 receiver FAQ
    prd.htm        Pace PRD800/900 receiver FAQ
    rd480.htm      Cambridge/Matsui RD480 Extra receiver FAQ
    srd6.htm       Ferguson SRD6 receiver FAQ
    srd700.htm     Amstrad/Fidelity SRD700, SR950, SR950+ receiver FAQ
    ss9.htm        Pace SS9xxx receiver FAQ
    svs300.htm     BT-SVS300 receiver FAQ
    why.htm        "Why am I in business" - a discussion for when you're down!
    yourfaqs.htm   Copies of questions and answers - let's have more!
    



  • Back to TV Repair FAQ Table of Contents.

    International Color Television Standards

    Brief description of international color TV standards

    (The following is from Bob Myers: (myers@fc.hp.com).)

    Well, the joke was that SECAM stands for System Essentially Contrary to the American Method....:-)

    The basic, oversimplified description of the three common encoding methods is as follows:

    NTSC: Used in North America, Japan, and a few other areas. Luminance ("black and white" information) is sent just as it was before color, and color information is provided in two "color difference" signals (actually, derived along with the luminance (Y) signal) via matrix multiplication) which are carried on a "color subcarrier". The chroma (color) signals are severely band-limited compared to the luminance signal, which is one reason you can never fully recover proper RGB from an NTSC-encoded signal. The color information itself is encoded such that the PHASE of the chroma signals, relative to the reference signal, is important in recovering the color. As used in the U.S., the broadcast standard provides a line rate of approx. 15,734.26 Hz, and a field rate of 59.94+ Hz*

    PAL: Very similar to NTSC, with the exception that the phase of the color subcarrier is reversed on alternate lines; this tends to cancel some of the more common color errors seen in the NTSC system. (The color signals of PAL are also simple color-difference signals, rather than using the more involved RGB -> YIQ matrix of NTSC). In the most common European PAL broadcast systems, a line rate of 15,625 Hz and a field rate of 50.00 Hs are used*.

    SECAM: This system is very different from both NTSC and PAL. Luminance and color-difference signals are still used, but the color difference signals are sent separately, on successive lines. This requires at least a one-line memory or delay line be provided in the receiver for proper color decoding. The broadcast SECAM systems usually use similar line/field rates as for the PAL broadcast standards noted above*.

    Note: In all three cases, the terms "NTSC", "PAL", and "SECAM" technically refer only to the COLOR-ENCODING systems described above; they do not specifically imply a set of timing standards or frequencies. The one possible exception to this is the use of the term "NTSC", since the U.S. National Television Standards Committee ALSO came up with various timing standards for U.S. television. But in all cases, the color encoding method is not *strongly* tied to a specific line/field timing. For example, there is at least one broadcast system (Brazil's) which uses NTSC encoding, but at the line/field rates more commonly seen in the European systems.

    Some questions and answers about TV standards

    (Responses from: Steve McKinty: (smckinty@france.sun.com))

    1. What are the most common TV standards in the world?

    NTSC: National Television Standards Committee PAL: Phase Alternate Line SECAM: SEquential Couleur Avec Memoire (Sequential colour with memory)

    There are other differences though. Strictly they are just different colour systems, but most countries which use PAL have 625 lines in a picture and send 25 full pictures/second, most NTSC countries have 525 lines and send 30 full pictures/second (mostly for historical rather than technical reasons). That complicates things.

    2. Who devised them, and when? and why? Are they as old as television?

    The first serious TV experimenting was done in several countries around the period 1900-1930, mostly black & white. The BBC started a regular service in 1936, other countries followed soon after, but since the technology was developing very rapidly there were always improvements being made. The BBC started with 405 lines, the US started a service a couple of years later with 525, by the time other European countries started the technology allowed 625 lines. France even tried 819 lines.

    All those system were black & white, but people wanted to have colour. During the 1940's much of Europe was at war, and technological development for entertainment slowed down, but in the US they were able to continue and devised a colour system which was compatible with the existing black & white one.

    By compatible I mean that a black & white TV got a black & white picture, a colo(u)r one got a colour picture. No need to make people throw away their B&W TVs. This system was endorsed by the American National Television Standards Committee, and was named after it => NTSC.

    After the war other countries started to look at colour. NTSC was a very clever system, but it had some flaws. Engineers in various countries tried to improve on it, and Telefunken in Germany came up with a simple modification which improved colour stability. It was named PAL because they reversed the Phase of the colour signal on Alternate Lines.

    At the same time Henri de France, in France, fixed the same flaw in a different way. His design (SECAM) needed a memory inside the set which made it more expensive. PAL gave as good a result, so most countries opted for that. France stayed with SECAM, possibly because in the De Gaulle era of the 50's memories of German occupation were still fresh, and dropping a French system in favour of a German one would have been unpopular. Rumour has it that the French government subsidized Thomson to make memory affordable.

    Since Britain went PAL, France went SECAM, and the US went NTSC, any colonies or dependencies of those countries tended to get the same system. India/Pakistan got PAL, Algeria got SECAM, and since the US helped rebuild Japan after WW2 it got NTSC, etc.

    3. What's the difference?

    To squeeze a colour signal into the same space as a black & white one, and stay compatible, the NTSC designers separated the colour and brightness information. The human eye is less sensitive to colour, so they were able to reduce the bandwidth of that signal (make it take up less space in each channel), 'hiding' it at the high-frequency end of the video. That meant they didn't need to make the channels bigger, and incompatible.

    To do that, they used the fact that you can represent most colours with a combination of Red, Green and Blue. If you film a scene with three cameras, one for each colour, then add all the outputs together you get a black & white image. This signal is called luminance, usually represented by 'Y'. Mathematically Y = R + G + B. (Actually, not all the contributions are equal).

    They then transmitted the Y signal just as for a black & white TV, and also transmitted the R and B in the extra colour signal. B&W TV's only saw Y, and colour TV's got Y, R and B. Since Y = R + B + G, G can be obtained as Y - (R+B), so they didn't need to transmit all three.

    To get both R and B into one signal, they use a combination of Phase and Amplitude modulation (think of it as AM and FM at the same time). Its called quadrature modulation, and works very well, but is susceptible to phase changes as it passes along cables, etc. If the signal gets +10 degrees phase change the colour will visibly change, which is why NTSC TV's have a tint control.

    PAL overcomes that by sending R and +B on one line, then R and -B on the next. That way a +10 phase shift on one line becomes -10 on the next, and small differences will cancel out. PAL TV's don't need tint controls. (Some old PAL sets may have a one, however).

    SECAM doesn't send both R & B together, it sends R on one line, B on the next. No fancy modulation, so no phase problems, but you need a 'memory' in the set to save up the signal from the previous line, since both R & B are required together for processing.

    4. Why do you need different TVs?

    Mostly because of the different numbers of lines. Its quite easy to make one colour decoder which can cope with all the systems, but making a TV which can do 625 and 525 lines, 25 and 30 pictures/second, gets expensive. Consumers shop on price, no-one will buy a SECAM TV in the USA even if it only costs $20 more, since there aren't any SECAM channels.

    5. Why do you need different VCRs? Why can't one VCR record the same "output"?

    Some can, but like TVs it costs more to make them adjust. The motor speed varies with the number of pictures transmitted per second, for example. (This is covered in more detail in the document: Notes on the Troubleshooting and Repair of Video Cassette Recorders.)

    6. Why did different systems evolve? Is one cheaper? Is one better?

    When originally developed, expense was considered based on contemporary technology. As noted, politics may have been equally important.

    As to which has better quality, its all rather subjective. The 625-line system adopted in Europe has better vertical resolution than the 525-line US system, but some people find the 50Hz field rate still produces some flicker. NTSC/PAL/SECAM are all equally capable of excellent colour reproduction, but under poor signal conditions NTSC can degrade more quickly.

    7. Are there other systems besides the ones I've mentioned? Why?

    Some others, like MAC where the colour and luminance are completely separated. That gets rid of interference (ever see the strange colours which appear on very fine check patterns?) but is more expensive and really only possible due to modern electronics.

    8. Are there going to be more or less systems in the future?

    That is THE question! There are certainly going to be different systems, more lines, better sound, etc.

    9. Is there any way to convert a PAL tape to NTSC or vice versa?

    Yes. If the PAL tape has 625 line pictures and the NTSC one has 525 line then you normally need a computer which can read in one format and re-adjust things. Not cheap, but becoming cheaper, several companies offer that sort of service. Some PAL VCRs can do a half-conversion, enough to fool most PAL TVs into thinking its got a PAL signal.

    10. Do they teach this stuff in electrical engineering courses?

    Sometimes. Some of it, depends a lot on the course and school.

    Politically Correct TV Standards

    (The following is from: Robert Rolf).

           SECAM:
                   Used by France and the former Soviet union.
                   No tint control. No color control.
                   Full socialism. The state knows exactly what color you
                   should see, and how strong that color should be.
    
           PAL:
                   Used by Germany & UK, Australia etc.
                   No tint control. A color control.
                   Partial socialism. The state knows exactly what color you
                   should see, but you get a choice as to how strong it can be.
    
           NTSC:
                   Used in USA and Canada, Japan etc.
                   A tint control, A color control.
                   Uncontrolled socialism. The state lets you chose what color you
                   see and how strong it can be. They then tax you regardless.
    

    Just another way of looking at it....

    Variations on a 'standard' - the PAL system

    In the U.S., when PAL is mentioned, it is usually assumed to be 625 line/50 Hz as used in the UK and man other places. However, there are several variations on the PAL system.

    (The following from: Ed Ellers I can think of five major groups right off:

    PAL B/G/H - 625-line 50 Hz systems used in most European countries and many countries in Africa, the Middle East and Asia.

    PAL I - Again 625 lines at 50 Hz, but with a different sound carrier frequency. Used in the United Kingdom, the Republic of Ireland, Hong Kong, South Africa and a few other places I've forgotten.

    PAL D/K - Yet another 625-line system with a different sound carrier. Used in some former Warsaw Pact countries as well as mainland China; this may become more popular as PAL/SECAM receivers become common in the CIS republics and other former Soviet bloc countries, since those countries now use SECAM D/K.

    PAL M - This one's 525 lines at 59.94 Hz; it's just like NTSC except for PAL-type color encoding. It's used mainly in Brazil.

    PAL N - A real oddity, with 625-line 50 Hz video but a *lower* sound carrier and a lower color subcarrier than the B/G/H, I and D/K varieties. It's used in Argentina and a few other places.

    What about PAL sets WITH a tint/hue control?

    Here is one for the record books - a Sony PAL TV that really wants to be NTSC!

    (From: Tony Duell (ard@p850ug1.demon.co.uk).)

    Although this very old Sony set (KV-1300) receives PAL signals, it's much closer to an NTSC set inside. In fact it's one of the strangest PAL decoders that I have ever seen. As you know, in the PAL system, the phase of one of the colour signals is inverted on each line, and in the receiver there's a bistable which switches at half the line rate to re-invert the colour signal on alternate lines. Well, to avoid a patent, the Sony set only uses (say) the in-phase colour signal that's received on alternate lines. For the lines in between it uses the previous line's colour signal (ignoring the incoming inverted one), which has been stored in a delay line.

    This approach avoids the main patent on the PAL system. It also means that this set doesn't automatically correct for phase errors in the colour signals - it's almost an NTSC decoder. Hence the hue control (which is also on my kV1320UB schematic). It's just about the only set like that.

    TV, shortwave, power worldwide

    (From: Mark Zenier (mzenier@netcom.com).)

    A book, "The World Radio TV Handbook" published by Billboard that covers TV, along with where all the world's shortwave radio transmitters are, and what sort of power comes out of the wall plug all around the world. It has a new edition each year and costs around $25 to $30.

    Color television standards worldwide

    (The following is from EDMUNDO, Design Engineer Ten-Lab. This and additional information are available at: http://www.tenlab.com/).

    We at Ten-Lab have put together the following chart listing countries and their corresponding color TV standards.

    We are trying to be as accurate as possible, but we need your feedback to refine and correct the information. We are doing the best we can in spite of inherent problems such as:

    1. Some of the literature and charts are contradictory; even some books and manuals contradict each other more than they agree.

    2. Many countries have changed their names during the last few years.

    3. Some countries have one broadcast TV system, but also receive programs in a different system from beyond their borders. This creates some confusion about the format(s) used locally.

    INTERNATIONAL TV STANDARDS CHART by TEN-LAB  (UPDATED Jan 19, 1996)
    COUNTRY			VHF STANDARD	UHF STANDARD
    

    AFGANISTAN PAL/SECAM B ALBANIA PAL B PAL G ALGERIA PAL B PAL G ANGOLA PAL I ARGENTINA PAL N PAL N AUSTRALIA PAL B PAL G AUSTRIA PAL B PAL G AZORES PAL B BAHAMAS NTSC M BAHRAIN PAL B PAL G BANGLADESH PAL B BARBADOS NTSC M BELGIUM PAL B PAL H BERMUDA NTSC M BOLIVIA NTSC M BOTSWANA PAL I BOURKINA FASO SECAM K1 BRAZIL PAL M PAL M BRUNEI PAL B BULGARIA SECAM D SECAM K BURMA NTSC M BURUNDI SECAM K1 CAMBODIA NTSC M CAMEROON PAL B PAL G CANADA NTSC M NTSC M CANARY ISLANDS PAL B CHAD SECAM K1 CHILE NTSC M NTSC M CHINA PAL D COLOMBIA NTSC M NTSC M COSTA RICA NTSC M NTSC M CUBA NTSC M NTSC M CYPRUS PAL G PAL G CZECHOSLOVAKIA: now CZECH REPUBLIC PAL PAL SLOVAK REPUBLIC PAL PAL DAHOMEY SECAM K1 DENMARK PAL B PAL G DJIBHOUTI SECAM B SECAM G DOMINICAN REP NTSC M NTSC M ECUADOR NTSC M NTSC M EGYPT SECAM B SECAM G EL SALVADOR NTSC M NTSC M EQUATORIAL GUINEA PAL B ETHIOPIA PAL B PAL G FIJI PAL B FINLAND PAL B PAL G FRANCE SECAM L SECAM L FRENCH POLYNESIA K1 GABON SECAM K1 GAMBIA PAL I GERMANY PAL B PAL G GHANA PAL B PAL G GIBRALTAR PAL B PAL H GREECE SECAM/PAL B SECAM/PAL G GREENLAND NTSC M/PAL B GUADELOUPE SECAM K1 GUAM NTSC M GUATEMALA NTSC M NTSC M GUANA (FRENCH) SECAM K1 GUINEA PAL K HONDURAS NTSC M NTSC M HONG KONG PAL I HUNGARY SECAM D/PAL SECAM K/PAL ICELAND PAL B PAL G INDIA PAL B INDONESIA PAL B PAL G IRAN SECAM B SECAM G IRAQ SECAM B IRELAND PAL I PAL I ISRAEL PAL B PAL G ITALY PAL B PAL G IVORY COAST SECAM K1 JAMAICA NTSC M JAPAN NTSC M NTSC M JORDAN PAL B PAL G KENYA PAL B PAL G KOREA NORTH SECAM D KOREA SOUTH NTSC M NTSC M KUWAIT PAL B LEBANON SECAM B SECAM G LIBERIA PAL B PAL H LIBYA SECAM B SECAM G LUXEMBOURG PAL B PAL G/SECAM L MADAGASCAR SECAM K1 MADEIRA PAL B MALAGASY SECAM K1 MALAWI PAL B PAL G MALAYSIA PAL B MALI SECAM K1 MALTA PAL B PAL H MARTINIQUE SECAM K1 MAURITANIA SECAM B MAURITIUS SECAM B MEXICO NTSC M NTSC M MONACO SECAM L MONGOLIA SECAM D MOROCCO SECAM B MOZAMBIQUE PAL B NAMIBIA PAL I NEPAL PAL B NETHERLANDS PAL B PAL G NETH. ANTILLES NTSC M NTSC M NEW CALEDONIA SECAM K1 NEW GUINEA PAL B PAL G NEW ZEALAND PAL B PAL G NICARAGUA NTSC M NTSC M NIGER SECAM K1 NIGERIA PAL B PAL G NORWAY PAL B PAL G OMAN PAL B PAL G PAKISTAN PAL B PANAMA NTSC M NTSC M PARAGUAY PAL N PERU NTSC M NTSC M PHILIPPINES NTSC M NTSC M POLAND SECAM D/PAL SECAM K/PAL PORTUGAL PAL B PAL G PUERTO RICO NTSC M NTSC M QATAR PAL B REUNION SECAM K1 RUMANIA PAL D PAL K RUSSIA SECAM D SECAM K RWANDA SECAM K1 SABAH/SARAWAK PAL B ST. KITTS NTSC M NTSC M SAMOA NTSC M SAUDI ARABIA SECAM B/PAL B SECAM G SENEGAL SECAM K1 SEYCHELLES PAL B PAL G SIERRA LEONE PAL B PAL G SINGAPORE PAL B PAL G SOMALIA PAL B PAL G SOUTH AFRICA PAL I PAL I SPAIN PAL B PAL G SRI LANkA PAL B SUDAN PAL B PAL G SURINAM NTSC M NTSC M SWAZILAND PAL B PAL G SWEDEN PAL B PAL G SWITZERLAND PAL B PAL G SYRIA SECAM B TAHITI SECAM K1 TAIWAN NTSC M NTSC M TANZANIA PAL B PAL B THAILAND PAL B TOGO SECAM K TRINIDAD Y TOBAGO NTSC M NTSC M TUNISIA SECAM B TURKEY PAL B PAL G UGANDA PAL B PAL G UNITED ARAB EMIRATES PAL B PAL G UNITED KINGDOM PAL I UPPER VOLTA SECAM K1 URUGUAY PAL N PAL N USA NTSC M NTSC M VENEZUELA NTSC M NTSC M VIETNAM PAL B PAL G YEMEN PAL B YUGOSLAVIA PAL B PAL G ZAIRE SECAM K1 ZAMBIA PAL B PAL G ZIMBABWE PAL B PAL G

    Cable channel allocation

    (The following is from: (kruskal@watson.ibm.com (Vincent Kruskal).)

    The following table gives the definitions of the numeric cable channels defined in the EIA (Electronic Industries Association) Interim Standards, IS-6 (CP), May 1983 and associated information that has been gathered. Definitions appear at the end.

              HRC
            Picture
    Channel  Carrier Cable Band  Other Names
    -------- ------- ----------- --------------------------
    

    1 72 MHz Low (VAR) A-8, C54, J54, G64, 4+, 5A 2 54 Low 3 60 Low 4 66 Low 5 78 Low A-7, C55, J55, G65 6 84 Low A-6, C56, J56, G66

    7 174 High 8 180 High 9 186 High 10 192 High 11 198 High 12 204 High 13 210 High

    14 120 Mid A 15 126 Mid (ATC) B 16 132 Mid (ATC) C 17 138 Mid (VAR) D 18 144 Mid (VAR) E 19 150 Mid (VAR) F 20 156 Mid (VAR) G 21 162 Mid (VAR) H 22 168 Mid (VAR) I 23 216 Super (VAR) J 24 222 Super (VAR) K 25 228 Super L 26 234 Super M 27 240 Super N 28 246 Super O 29 252 Super P 30 258 Super Q 31 264 Super R 32 270 Super S 33 276 Super T 34 282 Super U 35 288 Super V 36 294 Super W 37 300 Hyper AA, W+1 38 306 Hyper BB, W+2 39 312 Hyper CC, W+3 40 318 Hyper DD, W+4 41 324 Hyper EE, W+5 42 330 Hyper FF, W+6 43 336 Hyper GG, W+7 44 342 Hyper HH, W+8 45 348 Hyper II, W+9 46 354 Hyper JJ, W+10 47 360 Hyper KK, W+11 48 366 Hyper LL, W+12 49 372 Hyper MM, W+13 50 378 Hyper NN, W+14 51 384 Hyper OO, W+15 52 390 Hyper PP, W+16 53 396 Hyper QQ, W+17 54 402 Hyper RR, W+18, C62 55 408 Hyper SS, W+19, C63 56 414 Hyper TT, W+20, C64 57 420 Hyper (HAM) UU, W+21, C65 58 426 Hyper (HAM) VV, W+22, C66 59 432 Hyper (HAM) WW, W+23, C67 60 438 Hyper (HAM) AAA, W+24, C68 61 444 Hyper (HAM) BBB, W+25, C69 62 450 Hyper (HAM) CCC, W+26, C70 63 456 Hyper DDD, W+27, C71 64 462 Hyper EEE, W+28 65 468 Ultra U14, FFF, W+29 66 474 Ultra U15, GGG, W+30 67 480 Ultra U16, HHH, W+31 68 486 Ultra U17, III, W+32 69 492 Ultra U18, JJJ, W+33 70 498 Ultra U19, KKK, W+34 71 504 Ultra U20, LLL, W+35 72 510 Ultra U21, MMM, W+36 73 516 Ultra U22, NNN, W+37 74 522 Ultra U23, OOO, W+38 75 528 Ultra U24, PPP, W+39 76 534 Ultra U25, QQQ, W+40 77 540 Ultra U26, RRR, W+41 78 546 Ultra U27, SSS, W+42 79 552 Ultra U28, TTT, W+43 80 558 Ultra U29, UUU, W+44 81 564 Ultra U30, VVV, W+45 82 570 Ultra U31, WWW, W+46 83 576 Ultra U32, AAAA, W+47 84 582 Ultra U33, BBBB, W+48 85 588 Ultra U34, CCCC, W+49 86 594 Ultra U35, DDDD, W+50 87 600 Ultra U36, EEEE, W+51 88 606 Ultra (RA) U37, FFFF, W+52 89 612 Ultra U38, GGGG, W+53 90 618 Ultra U39, HHHH, W+54 91 624 Ultra U40, IIII, W+55 92 630 Ultra U41, JJJJ, W+56 93 636 Ultra U42, KKKK, W+57 94 642 Ultra U43, LLLL, W+58 95 90 Low (FM) A-5, C57, J57 96 96 Low (FM) A-4, C58, J58 97 102 Low (FM) A-3, C59, J59 98 108 Low A-2, C60, J60, G60 99 114 Low A-1, C61, J61, G61 100 648 Ultra U44, MMMM, W+59 101 654 Ultra U45, NNNN, W+60 102 660 Ultra U46, OOOO, W+61 103 666 Ultra U47, PPPP, W+62 104 672 Ultra U48, QQQQ, W+63 105 678 Ultra U49, RRRR, W+64 106 684 Ultra U50, SSSS, W+65 107 690 Ultra U51, TTTT, W+66 108 696 Ultra U52, UUUU, W+67 109 702 Ultra U53, VVVV, W+68 110 708 Ultra U54, WWWW, W+69 111 714 Ultra U55, AAAAA, W+70 112 720 Ultra U56, BBBBB, W+71 113 726 Ultra U57, CCCCC, W+72 114 732 Ultra U58, DDDDD, W+73 115 738 Ultra U59, EEEEE, W+74 116 744 Ultra U60, FFFFF, W+75 117 750 Ultra U61, GGGGG, W+76 118 756 Ultra U62, HHHHH, W+77 119 762 Ultra U63, IIIII, W+78 120 768 Ultra U64, JJJJJ, W+79 121 774 Ultra U65, KKKKK, W+80 122 780 Ultra U66, LLLLL, W+81 123 786 Ultra U67, MMMMM, W+82 124 792 Ultra U68, NNNNN, W+83 125 798 Ultra U69, OOOOO, W+84

    Notes on cable and broadcast frequencies

    (The following is from: (kruskal@watson.ibm.com (Vincent Kruskal).)

    RF band: To get the band, subtract 1.25 from picture carrier (low end) and add 4.75 (high end).

    Color subcarrier: Add 3.58... to picture carrier.

    Sound carrier: Add 4.5 to picture carrier.

    HRC: Harmonically Related Carrier. Makes both second- and third-order beats invisible by making them fall directly on the picture carrier of other channels. That is, multiplying the picture carrier by two or three will yield exactly another picture carrier.

    IRC: Incrementally Related Carrier, add 1.25 to HRC frequency. A General Instruments (Jerrold) catalog said that IRC makes third-order (more important than second-order) beats invisible by making them fall directly on the picture carrier of other channels. But it is not true that multiplying an IRC picture carrier by two or three yields another IRC picture carrier. This contradiction has not been resolved. The reason third-order harmonics are more important is that oscillators and amplifiers tend to generate odd-order harmonics far more than even-order ones.

    Broadcast frequency: Add 1.25 to HRC frequency except for channels 5 and 6. For them, subtract 0.75. But these are just nominal frequencies. The FCC actually has three channel designations for each number as in 5, 5- and 5+. The minus channels are 10 kHz below the nominal value and the plus channels are 10 kHz above. For example:

    How did the (vertical) frame rate get chosen

    Some people think that TVs are synchronized to the local power line since the vertical scan rate is around 60 Hz (or 50 Hz). This is not correct.

    No TV (at least once the broadcast standards were defined - some experimental schemes did) ever used the power line for synchronization. However, older TVs had line frequency power transformers (no SMPSs) whose stray magnetic fields could affect the CRT deflection slightly. So it made sense (well, this is one justification at least) to make the vertical scan rate (field rate) equal to the power line frequency. Otherwise, there would be a jiggle or wiggle in the picture due to the stray magnetic field affecting the deflection of the beam inside the CRT. Since it was thought at the time (and for other reasons as well like cost) that 60 Hz was adequate to produce an acceptable amount of flicker, this all fit together nicely.

    In the good old days before color TV, the frame/field rate was exactly 30/60 Hz (or 25/50) Hz. With color, it had to be changed slightly (see the section: Why is the NTSC color subcarrier such a weird frequency?) but since TVs no longer use line power transformers, there would not even be a slow position shift (period of several seconds) due to this so it didn't matter.

    Why is the NTSC color subcarrier such a weird frequency?

    (The following is from Bob Myers (myers@fc.hp.com).)

    Actually, if we wanted to define the rates to the Nth degree, the important starting point is the field rate. The NTSC color frame rate was defined as (60 * 1000/1001) Hz, which is a bit more than 59.94 Hz. From this rate, all the others in the system are defined. The line rate is 262.5 times this, and the color subcarrier is defined as 455/2 times the line rate. This is often given as simply 3.579545 MHz, but the the color subcarrier was actually derived from the line/field rates rather than the other way around.

    The whole thing was done so as to avoid (or at least minimize) interaction between the luminance, chrominance, and audio subcomponents in the standard color signal. This could have been achieved by moving either the audio subcarrier or adjusting the line and frame rates as described above. Unfortunately, the latter route was chosen, leaving us with this very strange looking set of rates.

    The precise color burst frequency winds up being 3.579545.4545... under this definition, but giving it to the nearest Hz is within the tolerances of the system.

    What is the maximal allowed deviation of the horizontal frequency?

    (The following is from: Peter Bennett VE7CEI (bennett@triumf.ca))

    In Canada and the US (525 line, 60 HZ, NTSC), the horizontal frequency is 15,734.264 Hz. The colour subcarrier is 455/2 times the horizontal frequency which should come to 3.579545 MHz. I believe the tolerance on the subcarrier frequency is +/- 10 Hz.

    Informal comparison of TV standards

    Q: I heard that TV in certain part of Europe has more quality in Europe then here in North America. I'd like to know the differences between the two systems. Is that why we cant use video tapes from there?

    (Responses from: Mark Zenier (mzenier@netcom.com or mzenier@eskimo.com))

    The first difference is that a lot of the world runs on 50 Hz power as opposed to North America's 60 Hz power. In the olden days, before active power supply regulators got cheap, it was decided that the vertical scan rate match the power supply frequency, so that ripple in the power supply wouldn't produce obnoxious visual effects.

    So the PAL/SECAM signals have 50 vertical scans per second.

    I don't know the exact reasoning, but the horizontal scan rate is close to the same. 15750 (now 15734) for 60 Hz, and 15625 for 50 Hz systems. My guess is the tradeoff between cost (50 years or so ago) and audibility for a large portion of the population.

    So 50 Hz systems have more lines - 625 vs. 525 lines for 60 Hz systems.

    The second difference is that European TV channels are wider. 7 or 8 MHz compared to the North American 6 MHz. Video bandwidth is limited to 4.2 MHz in a 6 MHz channel, but can be as much as 6 MHz in some of the 50 Hz systems. (Note: Systems is plural. There are many different European systems with incompatible color and sound transmission methods.)

    As for the quality, if you move a little farther away, so that a pixel on each system subtends the same angle, NTSC doesn't have a poorer picture, just a smaller one.

    PAL-plus

    "I wonder if you could tell me about PAL-Plus. The last time I was in Germany was in '84 so I've been out of touch with them."

    (From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).)

    Oh boy, here goes another long story:

    PAL-plus is an attempt to extend the life-cycle of terrestrial PAL transmissions by including compatible wide-screen (16:9) transmissions. It is an advanced variant of the letterbox format, this means that when you receive a PAL-plus widescreen program on an older 4:3 receiver you will see black bars top and bottom. It was originally developed in Germany (university of Dortmund in cooperation with German terrestrial broadcasters and some setmakers). Later a large consortium of European and Japanese setmakers took over and finished the job. Strangely, the German broadcasters seem to use PAL-plus only very rarely.

    The PAL-plus standard comprises three extensions to the PAL-standard:

    1. Vertical helper. In order to compensate for the fact that 1/4 of the video lines are not used, which would deteriorate vertical resolution for the widescreen viewer, the missing vertical information has been coded into the black lines in a manner as to be nearly invisible on a 4:3 receiver (you see some dark blue). The 16:9 PAL-plus receiver combines 432 visible lines plus 144 helper lines into 576 new visible lines.

    2. Colour-plus. The PAL colour carrier is modulated in a slightly different way (making use of correlation between 2 fields) in order to give a cleaner Y/C separation in the PAL-plus receiver.

    3. Signaling bits from which the receiver can conclude whether the transmission is 4:3/16:9/PAL-plus and adapt the display format accordingly. The bandwidth of these bits is low enough to survive recording on a VHS recorder.

    In order to enable a poor-man's PAL-plus receiver, the standard permits using the mark "PAL-plus" if at least the vertical helper reconstruction is included. Colour-plus is optional, so you will find sets on the market with only half of the PAL-plus extension.

    PAL-plus may also be combined with teletext, ghost cancellation reference, digital Nicam stereo, VPS, PDC and what-you-have more. Theoretically it can be broadcast over a satellite channel too, but it was not designed for that and some aspects of a satellite channel do indeed give interesting technical problems.

    There are also sets marketed as "PAL-plus compatible". These are mostly widescreen sets without any PAL-plus processing at all, but they allow switching of the display format between 4:3 and 16:9. They may well do that automatically, based on the signaling bits.

    There are 2 methods for displaying a 4:3 letterboxed signal on a 16:9 display, without using the PAL-plus helper lines:

    1. Increase of the vertical deflection amplitude to display only the centre 432 lines.

    2. Vertical interpolation without using the helper, to convert 432 lines into 576 lines and display on a 576 lines display.

    Both modes may be called "movie expand". Only when you really convert to full-resolution widescreen will it be called "widescreen".

    And there are 4 methods for displaying a regular 4:3 signal on a 16:9 display (regular PAL, has nothing to do with PAL-plus):

    1. Decrease of the horizontal deflection amplitude, this gives black bars left and right.

    2. Horizontal interpolation, to convert N pixels to 3/4*N pixels. Both modes may be called "4:3" or "normal".

    3. Non-linear horizontal deflection waveform, called "Panorama mode" by JVC, works by increasing the S-capacitor value.

    4. Non-linear horizontal interpolation, called "Superwide" by Philips, works with an advanced sample-rate converter.

    With both modes, the left and right edges of the picture will be stretched to fill the left and right bars, but the aspect ratio of the centre part of the picture will hardly be affected.

    Interesting, huh?

    More information about watching NTSC video on PAL systems

    (From: Aaron Smart (spectrum_kid@hotmail.com).)

    Most VCRs of remote quality sold in the British Isles (UK & Ireland) and probably other PAL regions (and definitely in the Middle East) in the past few years say something along the lines of "NTSC playback on PAL TV". It's usually automatic (it often takes the VCR about a second to adjust to NTSC speed), but some older VCRs have a switch. They don't usually go into much detail about how it's done in the manuals, but I'm pretty sure they usually just convert the signal to NTSC 4.43. Many decent TVs made in the last decade or so can handle this well enough, but some (most noted are Philips sets) will only display it in a kind of 'meshed' black and white. However, use of an RGB connection (e.g. SCART) removes this problem.

    All DVD players sold in PAL regions (I think) are equipped to play NTSC video (too bad that most of them only play Region 2 discs, which means the only NTSC discs they can play are Japanese or region-free titles). I don't know exactly how every player works of course, but with ones I have used, they either just play NTSC discs in NTSC 4.43 or have the option to do that or play them in 'PAL' which I assume is PAL 4.43 at 60Hz which seems to be the same, except it won't turn out in blank and white with a composite connection as it does on some sets.

    Also, certain newer videogame consoles (Sega Dreamcast and Nintendo Gamecube mostly) have many games that are allowed to be played in either 60 or 50Hz modes (since most games originate from NTSC regions, and 50Hz versions are usually slowed down and contain sometimes MASSIVE 'letterbox' borders). I don't know about other consoles, but the Dreamcast displayed the 60Hz video modes in PAL.

    Panasonic TV manuals call this system "PAL 60/525", and I've heard it called "PAL 60" before, so it must be standardised to some extent.

    I've tried a few PAL TVs with displaying 60Hz (PAL or NTSC), and here's some vague results of what happened: (O = yes, X = No. 'M' under NTSC only means black and white with composite or RF connection - not tested with RGB. Sorry I don't know the model numbers for all the tested sets. All models are Irish models unless stated)

     TV Make/Model (if known)         PAL 60    NTSC 4.43
    --------------------------------------------------------
     Panasonic TX-21S4TL (1998)          O RGB only
     Panasonic TC-14JR1L (2000)          O          ?
     Hitachi C1405R (UK, 1994)           O          ?
     Philips 21"(mid 90s)                ?          M
     BPL 14" (early 90s)                 X          X
     Panasonic 21" (early 90s?)          O          ?
     Mitsubishi 21" (late 80s?)          X          X
     Ferguson 14" (late 90s)             O          O
     Akura 14"(Daewoo tube, early 90s)   X          X
     Salora 21" (circa 1990)             O          ?
     Philips 28" (circa 2000)            O*         ?
    
    * The colour was screwy at the top of the screen and was kind of unstable.
    

    I've noticed that on a lot of sets that 60Hz pictures go off the screen a bit (vertically), and the OSD is stretched with it.

    As for multi-region TV sets, most sets sold in the British Isles only receive PAL I RF transmissions, but I've seen some cheaper TVs sold in Ireland with PAL G as well. However, I have seen a widescreen Philips set (circa 2000, 32"?) that actually had a country selection menu - and it was definitely changing to the different PAL systems and to SECAM when I put it on France. I personally haven't seen other TVs like this, but there probably are others.

    In the Middle East, though, it's a totally different story. In Saudi Arabia, anyway, all TV sets and VCRs had at least PAL (dunno which one) and MESECAM support. Most VHS VCRs had NTSC 4.43 playback, you could get multi-region TVs which had support for PAL, SECAM, NTSC 4.43 and NTSC 3.58. I had a multi-region Daewoo TV which had 4 LEDs below the screen to tell you which system you were watching - how nice!



  • Back to TV Repair FAQ Table of Contents.

    Service Information

    Advanced TV troubleshooting

    If the solutions to your problems have not been covered in this document, you still have some options other than surrendering your TV to the local service center or the dumpster.

    Also see the related document: Troubleshooting and Repair of Consumer Electronic Equipment.

    Manufacturer's service literature: Service manuals are available for a great deal of consumer electronics. Once you have exhausted the obvious possibilities, the cost may be well worth it. Depending on the type of equipment, these can range in price from $10-50 or more. Some are more useful than others. However, not all include the schematics so if you are hoping to repair an electronic problem try to check before buying.

    Inside cover of the equipment: TVs often have some kind of circuit diagram pasted inside the back cover. In the old days, this was a complete schematic. Now, if one exists at all, it just shows part numbers and location for key components - still very useful. Some TVs - as late as 10 years ago, maybe even now - included a complete schematic with the product information and owner's manual. I have a 1984 Mitsubishi which has this. It is, however, the exception rather than the rule anymore.

    Sams' Photofacts (http://www.samswebsite.com/): These have been published for over 45 years (I don't know for how long but I have Sams' for a 1949 portable (3 inch) Pilot TV - about as portable as an office typewriter (if you remember what one of those was like) and are generally the best most consistent source of service info for TVs (nearly every model that has ever been sold), radios, some VCRs and other consumer electronics. For TV servicing, they are indispensable.

    There are some Computerfacts but the number of these is very limited. The VCRfacts are also somewhat limited and the newer ones tend to have strictly (obvious) mechanical information.

    Even if they don't list your model, they may have a folder for one using the same chassis so search by chassis number as well. Even if this doesn't help, there still may be a folder for models that are similar enough to be of value (though you really have to be in the library to be able to determine this by looking at the circuit diagrams or photos) so check out folders for other model numbers that are close to the one you really want.

    You can search this site to determine if they have a folder for your model. Service info (EFacts) for most models manufactured after 1992 is available in electronic form (currently) about $11. These are similar to the print PhotoFacts but may be ordered on-line and will arrive via email within 1 business day. The hard copy versions are also available from major electronics distributors but may be more expensive than the EFacts. However, I don't know how quickly the Photofact is created after a new model is introduced. So, if your set is just out of warranty, it's possible that none is available.

    Sams' Photofacts are also often available (for photocopy costs) from your local large public library which may subscribe to the complete series.

    One advantage of the Sams' info is that it is compiled in a very consistent format so that once you are familiar with one model TV, it is easy to transfer that knowledge to any other. They provide waveforms at key locations and DC voltage measurements almost everywhere. Additional info such as IC pin to ground and coil resistances are often provided as well. The manufacturer's service manuals are generally not nearly as complete.

    Note that I have no connection with Sams Technical Publishing (formerly Howard Sams).

    Elsewhere around the world, libraries may also have Sams' or other service information:

    (From: Chris Laudan (chris.laudan@zetnet.co.uk).)

    This is true here in UK too, though not Sams photofacts, just schematics issued by manufacturers. Go to the reference library and ask for Video and TV service manuals, most main libraries carry a good selection.

    (From: Michael Covington (mcovingt@ai.uga.edu).)

    Concerning Sams' Photofacts, here is a bit of history that seems to be very little known:

    The name "Sams" is not an acronym, nor is it "Sam." It's the last name of Howard W. Sams, who founded the Howard Sams publishing company (now Sams Technical Publishing). Accordingly, they are "Sams' Photofacts" rather than any of various other spellings that we often see on the net.

    And they're a great product. Apparently "Howard W. Sams & Co., Indianapolis, Indiana" is a sufficient address to reach them. So is 1-800-GAT-SAMS.

    Service manuals for really old TVs

    Try your large public library for Sams' photofacts. I found a 3" TV of from around 1948 at a yard sale. There was no problem finding a complete set of Sams' service information including full schematics, parts list (of course, finding suitable parts like tubes in the 1990s may be a bit more difficult than when this set was new!), troubleshooting procedures, etc. Someday, I may even get around to fixing it. All the paper capacitors are leaky (for starters).

    BTW, the case for this 3" TV is about 18" x 10" x 15" and it includes a handy option: a 6 inch semispherical water filled magnifier. The CRT is an oscilloscope tube. Not your modern portable!

    How to locate service info when all the little stickers have fallen off

    On newer TVs, the chassis number may be printed on the mainboard.

    (From: "Clifton T. Sharp, Jr."

    Somewhere on the chassis there may be an inked chassis number; an example for one color set was TS-914. A chassis number will at least get you some service information, and should be all you need unless you plan to try to get cabinet parts for it (chortle guffaw titter). Number should be findable on the back of the chassis (i.e. where the pots are mounted through the chassis).

    Techical assistance help numbers

    Here are contact numbers for some TV manufacturers:

       Curtis Mathis               1-800-949-4999  Steve Long
    
       Mitsubishi                  1-800-552-8324
    
       NAP                         1-900-896-8324
    
       Zenith                      1-800-874-1930  ext 1065 or 1066
                                   1-800-856-0981
                                   1-312-745-5154
    
       Sanyo                       1-800-877-5032
    
       Panasonic                   1-201-348-7957
                                   1-201-348-7958
                                   1-201-392-6961
                                   1-201-392-6992
    

    Web resources

    Many manufacturers are now providing extensive information via the World Wide Web. The answer to you question may be a mouse click away. Perform a net search or just try to guess the manufacturer's home page address. The most obvious is often correct. It will usually be of the form "http://www.xxx.com" where xxx is the manufacturers' name, abbreviation, or acronym. For example, Hewlett Packard is hp, Sun Microsystems is sun, Motorola is, you guessed it, motorola. Electronic parts manufacturers often have detailed datasheets for their product offerings. For example:

    Parts information

    I have found one of the most useful single sources for general information on semiconductors to be the ECG Semiconductors Master Replacement Guide, about $6 from your local Philips distributor. STK, NTE, and others have similar manuals. The ECG manual will enable you to look up U.S., foreign, and manufacturer 'house' numbers and identify device type, pinout, and other information. Note that I am not necessarily recommending using ECG (or other generic) replacements if the original replacements are (1) readily available and (2) reasonably priced. However, the cross reference can save countless hours searching through databooks or contacting the manufacturers. Even if you have a wall of databooks, this source is invaluable. A couple of caveats: (1) ECG crosses have been known to be incorrect - the specifications of the ECG replacement part were inferior to the original. (2) Don't assume that the specifications provided for the ECG part are identical to the original - they may be better in some ways. Thus, using the ECG to determine the specifications of the parts in your junk bin can be risky.

    Other cross reference guides are available from the parts source listed in the section: Repair parts sources.

    Suggested references

    Some good sources for technology information:

    For a technical reference on the various flavours of NTSC, PAL, and SECAM used around the world, I suggest:

    The following book is for PAL TV-standard specifications:

    The following is more directed toward digital video but may still contain some information useful for understanding analog TV technology:

    There don't seem to be nearly as many TV repair books for modern solid state TVs as I recall for old tube sets. Here are a couple which you may find (or its predecessor) at your local public library (621.384 if you library is numbered that way) or a technical book store. MCM Electronics has the Davidson as well.

    The following is a recent publication:

    FCC ID Numbers of TVs

    Only a few manufacturers actually produce the vast majority of TVs. For example, Radio Shack, Magnavox, and Emerson do not make their own TVs (I can tell you are not really surprised!).

    How do you determine the actual manufacturer? For most types of consumer electronic equipment, there is something called an 'FCC ID' or 'FCC number'. Any type of equipment that may produce RF interference or be affected by this is required to be registered with the FCC. This number can be used to identify the actual manufacturer of the equipment.

    A cross reference and other links can be found at:

    Interchangeability of components

    The question often arises: If I cannot obtain an exact replacement or if I have a monitor, TV, or other equipment carcass gathering dust, can I substitute a part that is not a precise match? Sometimes, this is simply desired to confirm a diagnosis and avoid the risk of ordering an expensive replacement and/or having to wait until it arrives.

    For safety related items, the answer is generally NO - an exact replacement part is needed to maintain the specifications within acceptable limits with respect to line isolation, X-ray protection and to minimize fire hazards. Typical parts of this type include flameproof resistors, some types of capacitors, and specific parts dealing with CRT high voltage regulation. However, during testing, it is usually acceptable to substitute electrically equivalent parts on a temporary basis. For example, an ordinary 1 ohm resistor can be substituted for an open 1 ohm flameproof resistor to determine if there are other problems in the horizontal deflection circuits before placing an order - as long as you don't get lazy and neglect to install the proper type before buttoning up the monitor or TV.

    For other components, whether a not quite identical substitute will work reliably or at all depends on many factors. Some deflection circuits are so carefully matched to a specific horizontal output transistor that no substitute will be reliable.

    Here are some guidelines:

    1. Fuses - exact same current rating and at least equal voltage rating. I have often soldered a normal 3AG size fuse onto a smaller blown 20 mm long fuse as a substitute.

    2. Resistors, capacitors, inductors, diodes, switches, potentiometers, LEDs, and other common parts - except for those specifically marked as safety-critical - substitution as long as the replacement part fits and specifications should be fine. It is best to use the same type - metal film resistor, for example. But for testing, even this is not a hard and fast rule and a carbon resistor should work just fine.

    3. Rectifiers - many are of these are high efficiency and/or fast recovery types. Replacements should have at equal or better PRV, Imax, and Tr specifications.

    4. Posistors - many of these are similar. Unfortunately, the markings on the devices are generally pretty useless in determining their ratings. Note, however, that the prices for replacement posistors may be quite reasonable from the original manufacturer so it may not make sense to take the risk of using an unknown part.

      (From: Stefan Huebner (Stefan.Huebner@rookie.antar.com).)

      In most cases you can use a standard 3-terminal-device, the resistance of the temperature dependent resistors in it are nearly identical. Here is a list of possible replacement devices:

      380000-01, 24340521, 2199-603-1201, 163-024A, 163-035A, CO2200-N66, C8ROH, QX265P05503, 32112026, 4822-A1-11240148, 02199-003-120, 15-08-001A, 5391560067, F400001.

    5. Transistors and thyristors (except HOTs and SMPS choppers) - substitutes will generally work as long as their specifications meet or exceed those of the original. For testing, it is usually OK to use types that do not quite meet all of these as long as the breakdown voltage and maximum current specifications are not exceeded. However, performance may not be quite as good. For power types, make sure to use a heatsink.

      Also see the section: Replacement power transistors while testing.

    6. Horizontal output (or SMPS) transistors - exact replacement is generally best but except for very high performance monitors, generic HOTs that have specifications that are at least as good will work in many cases. Make sure the replacement transistor has an internal damper diode if the original had one. For testing with a series light bulb, even a transistor that doesn't quite meet specifications should work well enough (and not blow up) to enable you to determine what else may be faulty. The most critical parameters are Vceo/Vcbo, IC, and Hfe which should all be at least equal to the original transistor. I have often used by favorite BU208D as a temporary substitute for other HOTs and SMPS (chopper) transistors. Make sure you use a heatsink and thermal grease in any case - even if you have to hang the assembly by a cable tie to make it fit.

      For that matter, you can usually substitute a similar HOT with the D suffix instead of the A (or no) suffix. These have a built-in damper diode and two in parallel (the external one) will not hurt (or remove it). Naturally, the reverse is not true since a damper diode IS essential and the HOT will probably not last beyond the click of the power relay without one!

      On SVGA monitors, there will likely be additional circuitry between the HOT and the damper so this trick doesn't work for them.

      However, using a HOT with much better specs may actually result in early failure due to excessive heating from insufficient and/or suboptimal base drive. See the document: "TV and Monitor Deflections Systems" for more info.

      For more information, see the document: TV and Monitor Deflection Systems.

    7. Deflection yokes - in the old days, particularly for B/W TVs, all of these were quite similar. It was common to just swap with one that fit physically and at most need to adjust or change a width coil. With color TVs and high performance multiscan monitors, this is no longer the case. Sometimes it will work but other times the power supply won't even be able to come up as a result of the impedance mismatch due to different coils and pole piece configurations. In addition, there may be other geometry correction coils associated with the yoke that could differ substantially.

      However, if you are really determined, see the section: Swapping of deflection yokes.

      Also see the section: Replacement power transistors while testing.

    8. Standby power transformer - this most likely only has a single secondary so locating a standard UL approved (for safety reasons) power transformer with the same output voltage should not be difficult.

      Check the service manual or the Sams' Photofact for the set to determine the required output voltage and if a centertap is needed. Current should be quite low.

    9. CRTs - aside from the issues of physical size and mounting, many factors need to be considered. These include deflection angle, neck diameter, base pinout, focus and screen voltage requirements, purity and convergence magnets, etc. Color CRT replacement is rarely worth the effort in any case but trying to substitute a different CRT is asking for frustration. For monochrome CRTs, there is less variation and this may be worth a try.

    10. The following are usually custom parts and substitution of something from your junk box is unlikely to be successful even for testing: flyback (LOPT) and SMPS transformers, interstage coils or transformers, microcontrollers, and other custom programmed chips.

    Horizontal output transistor pinouts

    You will nearly always find one of two types of horizontal output transistors in TVs and monitors:

  • Metal can - TO3 package:
                  _
                / O \         View from bottom (pin side)
              / o   o \
             (  B   E  )      B = Base, E = Emitter, C = Collector
              \       /
                \ O / C       The metal case is the Collector.
    
    

  • Plastic tab - TO3Pn (n = several suffixes) package:

                _____
               /     \
              |   O   |      View from front (label side)
              |       |
              |       |      B = Base, E = Emitter, C = Collector
              |_______|
                | | |        If there is an exposed metal tab, this is the
                | | |         Collector as well.
                B C E
    

    Some other transistor types use the same pinout (TO66 for metal can, TO218 and TO220 for plastic tab) but not all. However, for horizontal output transistors, these pinouts shouuld be valid.

    Note that those with a built in damper diode may read around 50 ohms between B and E (near 0 on the diode test range) - this is normal as long as the resistance is not really low like under 10 ohms.

    How do you locate the HOT

    Well, it is usually the LARGEST transistor in the set near the LARGEST transformer in the set (flyback - the thing with the FAT red wire connecting to the picture tube) on the LARGEST heat sink in the set.

    Got that? :-)

    Or, in the good old days....

    (From: Don Wall (d.wall@nunet.neu.edu).)

    Sure, it's usually the largest tube in the set, has a top cap, runs very hot, and is often a 6BQ6G or some such. (tongue firmly in cheek) Actually, back in the days of yore, the Horizontal Output Tube was frequently referred to as the HOT; guess some things don't change!

    Replacement power transistors while testing

    During testing of horizontal deflection circuits or switchmode power supplies, particularly where the original failure resulted in the death of the HOT or chopper, overstress on replacement transistors is always a possibility if all defective components have not be identified.

    Therefore, using a part with better specifications may save you in the long run by reducing the number of expensive blown parts. Once all other problems have been located and repaired, the proper part can be installed.

    However, this is not always going to work. In a TV and especially a high performance monitor, the HOT may be closely matched to the drive and output components of the deflection circuits. Putting in one with higher Vce, I, or P specifications may result in overheating and failure due to lower Hfe.

    Where possible, a series load like a light bulb can be used limit the maximum current to the device and will allow you to power the equipment while checking for other faults. Some designs, unfortunately, will not start up under these conditions. In such cases, substituting a 'better' device may be the best choice for testing.

    (From: Glenn Allen (glenn@manawatu.gen.nz).)

    I been repairing SMPS of all types but when I started on those using MOSFETs I was blowning a few of them when replaced because something else was faulty.

    Ever since I have been using a BUZ355 on a heat sink I haven't blown it. It is rated at 800 V, 6 A, and 220 W. it is a TO218 case bigger than a T0220. It seems the higher ratings allows you to do repair where as a something like a 2SK1117 or MTP6N60 will just blow.

    Testing of replacement HOTs

    The following is useful both to confirm that a substitute replacement HOT is suitable and that no other circuit problems are still present. However, single scan line anomalies (particularly when changing channels and/or where reception is poor with a TV or when switching scan rates and/or when no or incorrect sync is present with a monitor) resulting in excessive voltage across the HOT and instant failure are still possible and will not result in an HOT running excessively hot.

    (From: Raymond Carlsen (rrcc@u.washington.edu).)

    After installing a replacement HOT in a TV set or monitor, I like to check the temperature for awhile to make sure the substitute is a good match and that there are no other problems such as a weak H drive signal. The input current is just not a good enough indicator. I have been using a WCF (well calibrated finger) for years. For me, the rule of thumb, quite literally, is: if you can not hold your finger on it, it's running too hot, and will probably fail prematurely. Touching the case of the transistor or heat sink is tricky....

    Metal case transistors will be connected to the collector and have a healthy pulse (>1,200 V peak!) and even with plastic case tab transistors, the tab will be at this potential. It is best to do this only after the power is off and the B+ has discharged. In addition, the HOT may be hot enough to burn you.

    A better method is the use of an indoor/outdoor thermometer. I bought one recently from Radio Shack for about $15 (63-1009). It has a plastic 'probe' on the end of a 10' cable as the outdoor sensor. With a large alligator clip, I just clamp the sensor to the heat sink near the transistor and set up the digital display near the TV set to monitor the temperature. The last TV I used it on was a 27" Sanyo that had a shorted H. output and an open B+ resistor. Replacement parts brought the set back to life and the flyback pulse looked OK, but the transistor was getting hot within 5 minutes... up to 130 degrees before I shut it down and started looking for the cause. I found a 1 uF 160 volt cap in the driver circuit that was open. After replacing the cap, I fired up the set again and monitored the heat sink as before. This time, the temperature slowly rose to about 115 degrees and stayed there. I ran the set all day and noticed little variation in the measurement. Test equipment doesn't have to cost a fortune.

    Removing and replacing the deflection yoke

    Should you need to remove the deflection yoke on a color CRT, some basic considerations are advised both to minimize the needed purity and convergence adjustments after replacement as well as to prevent an unfortunate accident.

    The position and orientation of the yoke (including pitch and yaw) and magnet assembly (purity and static convergence rings, if used) are critical. Use paint or White-Out(tm) to put a stripe across all of the magnet rings so you will know their exact positions should they accidentally shift later. If there are rubber wedges between the yoke and the funnel of the tube, assure that they are secure. Tape them to be doubly sure as adhesive on old tape dries up with age and heat and becomes useless. This will avoid the need for unecessary dynamic convergence adjustments after reassembly.

    The neck is the most fragile part of the CRT so do not apply any serious side-ways force and take care not to bend any of the pins when removing and replacing the CRT socket.

    The yoke and purity/static convergence assemblies will be clamped and possibly glued as well. However, the adhesive will probably be easily accessible - big globs of stuff like hot melt glue and/or RTV silicone. Carefully free the adhesive from the glass neck of the CRT. Loosen the clamps and gently wiggle the magnets and yoke off the neck. They may appear stuck from age and heat but should yield with gently persuasion.

    Once the yoke is replaced, some fine adjustments of the picture rotation, purity, and static and dynamic convergence may be needed but hopefully with your most excellent diagrams, these will be minimal.

    Similar comments apply for monochrome CRTs but there are far fewer issues as the yoke is positioned firmly against the funnel of the CRT and rotation and centering are usually the only adjustments. However, there may be magnets located on swivels or glued to strategic locations on the CRT envelope to correct for geometric distortion.

    Swapping of deflection yokes

    This should work with identical TVs or monitors. Your mileage will vary if you are attempting a swap between monitors with similar specifications. Chances of success for monitors with widely different screen sizes or scan rate specifications is close to zero.

    One indication of compatibility problems would be major differences in resistance readings for the corresponding yoke windings, CRT HV and other bias levels, etc.

    Before you do the transplant, see the section: Swapping of deflection yokes for procedures and precautions to minimize problems in realignment.

    Make a precise diagram of everything you do.

    Keep the purity/static convergence magnet assembly with the original CRT if possible and install it in the same or as nearly the same position as possible when you replace it.

    Once you are sure of the connections, power it up carefully - there is no assurance that your yokes are compatible. A yoke with a much lower resistance or inductance than the original may overstress components in the power supply.

    You will then need to go through all the adjustments starting with purity and convergence.

    Swapping of CRTs

    Given the problems of just replacing a CRT with an identical new one, it isn't surprising that attempting to substitute a CRT which is not the same type will result in difficulties - to say the least. Obviously, the closer in size, scan rate (for monitors), and deflection angle, the more likely the chances of success. Where the alternative is to junk the TV or monitor, it may be worth a shot - and you may get lucky!

    It may be best to transfer as much as possible with the CRT - yoke and purity and convergence magnets. The connectors to the yoke may need to be changed but this may be the least of your problems. Difference in yoke impedance and other characteristics may result in anything from incorrect size to a truly spectacular melt-down! The latter is much more likely with SVGA monitors compared to similar size/deflection angle TVs.

    Where the neck size is the same, the yoke can be moved from one CRT to the other but you will have to do a complete purity and convergence set up and even then you may have uncorrectable convergence errors. See the section: Swapping of deflection yokes.

    (From: J. G. Simpson (ccjgs@cse.bris.ac.uk).)

    Monitors are generally designed by choosing a CRT, then the EHT, then designing a yoke to scan the CRT, then designing a driver circuit to drive the yoke.

    In a CRT test lab it's common to have variable supplies for EHT and other voltages, a small selection of yokes, and variable amplitude drive circuits.

    EHT affects scan sensitivity, brightness, spot size. You can't get high brightness and small spot size on a large monitor with 3 kV of EHT. Virtually every variable has some effect on convergence. Spot size is important, in as much as you want most of it on the phosphor and not the shadow mask.

    Provided the neck size is the same you can swap tubes in yokes but don't expect it to work very well. Different tube manufacturers may use radically different gun structures. A given yoke and its driver may give underscan or overscan and it's pretty well certain that convergence will be way off.

    The military spends a small fortune on trying to get the drop into the yoke and it flies with no adjustment or convergence CRT. For the rest of us swapping a CRT is a pain in the butt.

    Decayed glue in electronic equipment

    Larger components like electrolytic capacitors are often secured to the circuit board with some sort of adhesive. Originally, it is white and inert. However, with heat and age, some types decay to a brown, conductive and/or corrosive material which can cause all sorts of problems including the creation of high leakage paths or dead shorts and eating away at nearby wiring traces.

    The bottom line: Most of the time, this stuff serves no essential purpose anyhow and should be removed. A non-corrosive RTV or hot-melt glue can be used in its place if structural support is needed.

    Repair parts sources

    For general electronic components like resistors and capacitors, most electronics distributors will have a sufficient variety at reasonable cost. Even Radio Shack can be considered in a pinch.

    However, for consumer electronic equipment repairs, places like Digikey, Allied, and Newark do not have the a variety of Japanese semiconductors like ICs and transistors or any components like flyback transformers or degauss Posistors.

    See the document: Major Service Parts Suppliers for some companies that I have used in the past and others that have been recommended.

    For those hard-to-find or overpriced TV replacement boards, modules, or other parts, try:

    File Edit Options Buffers Tools HTML SGML Help

    (From: Len.)

    "Salvage King TV and VCR Parts" (2818 Odonnell Street, Baltimore, MD 21224) operates also as O'Donnell TV. It has turned into a TV/VCR parts boneyard. Donnie, the owner, will actually dig out a chassis from the tons he has and cut out a part for you! He also rents Sams' Photofacts! It's been a while since I was there so I hope this info is still current!"

    Also see the documents: Troubleshooting of Consumer Electronic Equipment and Electronics Mail Order List (this one is quite dated though) for additional parts sources.



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