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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.
It is likely that in roughly 15 years, HDTV - digitally processed and transmitted as 1s and 0s - will completely replace the current system. Acceptance in the marketplace is by no means assured but with the merging of TV and computers - with the Internet as a driving force - it would seem that the days of the stand-alone analog TV set are numbered.
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.
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.
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.)
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.
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.
(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.
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:
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.
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 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.
In addition, modern VCRs are NOT built like the Brooklyn Bridge! The weight of a TV or stereo components could affect the VCR mechanically, messing up tape path alignment or worse.
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.
"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.
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.
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!
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.
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.
However, some test equipment will be needed:
I would recommend a good used Tektronix (Tek) or Hewlett Packard (HP) scope over a new scope of almost any other brand. You will usually get more scope for your money and these things last almost forever. Until recently, my 'good' scope was the militarized version (AN/USM-281A) of the HP180 lab scope. It has a dual channel 50 MHz vertical plugin and a delayed sweep horizontal plugin. I have seen these going for under $300 from surplus outfits. For a little more money, you can get a Tek 465 or 465B (newer version but similar specifications) 100 Mhz scope ($200 to $600, sometimes cheaper on eBay or elsewhere but there is more risk than buying from a reputable dealer). I have now acquired a Tek 465B and that's what I use mostly these days. The HP-180 is still fine but I couldn't pass up a really good deal. :) The Tek 465/B or other similar model will suffice for all but the most demanding (read: RF or high speed digital) repairs.
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!
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!
Note that if you are touching the little board on the neck of the CRT, you may want to discharge the HV even if you are not disconnecting the fat red wire - the focus and screen (G2) voltages on that board are derived from the CRT HV.
WARNING: Most common resistors - even 5 W jobs - are rated for only a few hundred volts and are not suitable for the 25 kV or more found in modern TVs and monitors. Alternatives to a long string of regular resistors are a high voltage probe or a known good focus/screen divider network. However, note that the discharge time constant with these may be a few seconds. Also see the section: Additional information on discharging CRTs.
If you are not going to be removing the CRT anode connection, replacing the flyback, or going near the components on the little board on the neck of the CRT, I would just stay away from the fat red wire and what it is connected to including the focus and screen wires. Repeatedly shoving a screwdriver under the anode cap risks scratching the CRT envelope which is something you really do not want to do.
Again, always double check with a reliable voltmeter!
Reasons to use a resistor and not a screwdriver to discharge capacitors:
(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.
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! :-)
These designs may take several forms:
(1) to (3) may be found in TVs with A/V inputs and outputs.
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.
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?:
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.
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.
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 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.
However, you should check across this capacitor - usually only one and by far the largest in the set - with a voltmeter and discharge as suggested in the section: Safe discharging of capacitors in TVs and video monitors if it holds more than a few volts (or wait longer) before touching anything.
Some of these are as large as 1,000 uF charged to 160 V - about 13 w-s or a similar amount of energy as that stored in an electronic flash. This is enough to be potentially lethal under the wrong circumstances.
If you want to be doubly sure, discharge this also. However, unless you are going to be removing the HV connector/flyback, it should not bother you.
The energy stored is about 1 w-s but if you touch it or come near to an exposed terminal, due to the high voltage, you will likely be handed *all* the energy and you *will* feel it. The danger is probably more in the collateral damage when you jump ripping flesh and smashing your head against the ceiling.
Some people calibrate their jump based on voltage - about 1 inch/V. :-).
There will be some HV on the back of the circuit board on the neck of the CRT but although you might receive a tingle but accidentally touching the focus or screen (G2) pins, it is not likely to be dangerous.
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.
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.
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.
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.
For information on accessing the service menus if used on your model, see the section: Setup adjustments lost - TV service codes. However, even if the access procedure is known, get the service manual or Sams'!
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.
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)
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:
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.
(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:
Using just these concepts, you can get REAL close to a proper alignment.
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 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 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.
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
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!
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.
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 nev