Contents:
This could be a heater-cathode (H-K) short in the CRT or a failure
of a component in the chroma circuits or video output (driver board).
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.
* Compare the voltages for the video drive signals to the CRT on the little
board on the neck of the CRT with the CRT both connected and unplugged.
A schematic will help greatly in locating these signals.
- If there is a significant difference especially on the bad color, then the
CRT is a likely candidate. Try tapping the neck of the CRT GENTLY (with
it plugged in and while viewing a picture) to see if it is an intermittent
problem.
- If there is no significant difference, you may have a bad driver or a
problem in the chroma circuits.
* Look for bad connection/cold solder joints, probably on the little
board on the neck of the CRT. Use an insulated stick to gently prod
the board and its components in an effort to induce/cure the problem.
Look carefully for hairline cracks around the component leads.
* You can swap components between two colors and/or test with an ohmmeter
on that driver board to determine what is bad. The nice thing about
color monitors and TVs is that there three copies of each of these
components. Swapping and/or comparisons between these is an excellent
diagnostic technique.
* Another simple test: Disconnect the cathode for the full-on color from its
drive. If it is still full-on, there is probably an H-K short in the CRT
since the only way to get each color on the screen is via the cathode
connection to the CRT neck board. If it is removed and there is still that
color, the current must be taking another path inside the CRT.
* Alternatively, interchange the outputs of the bad color with a good one
by jumpering on the video driver board (on the CRT neck). If the bad
color changes, then the problem is in the circuitry and not the CRT.
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.)
- If the symptom returns = 'goes red' the CRT is shorting. (See the section:
"Providing isolation for a CRT H-K short". --- sam)
- If instead the symptom becomes 'goes green' then the red drive leg has
the fault and the CRT is probably good. (In this case, there may be bad
connections or a bad component on the CRT drive board or further back
in the chroma circuitry. --- sam)
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: * Heater to cathode (H-K). The cathode for the affected gun will be pulled to the heater (filament) bias voltage - most often 0 V (signal ground). In this case, one color will be full on with retrace lines. Where the heater is biased at some other voltage, other symptoms are possible like reduced brightness and/or contrast for that color. This is probably the most common location for a short to occur. * Cathode to control grid (K-G1). Since the G1 electrodes for all the guns are connected together, this will affect not only the color of the guilty cathode but the others as well. The result may be a very bright overloaded *negative* picture with little, none, or messed up colors. * Control grid to screen (G1-G2). Depending on circuitry can result in any degree of washed out or dark picture. * Screen to focus (G2-F). Screen (G2) and focus voltage will be the same and the controls on the flyback will interact. Result will be a fuzzy white raster with retrace lines and little or very low contrast picture. Symptoms will be similar to those of a flyback with breakdown in the focus/screen divider network. * Focus to high voltage (F-HV). High voltage will be pulled down - probably arcing at the focus spark gaps/other protective devices. Line fuse and/or HOT may blow. * Other locations between electron gun elements as feed wires. Replacing the CRT may be required 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.
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: (From: Alan Harriman (capstv@sprynet.com)). A company called KDTV/IWE carries kits (core, wire and tie) for $3.30 each. It takes all of two minutes to wind. Check out: http://www.seidata.com/~kdtv. BTW, I am just a satisfied customer.
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!
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: * Discharge both the old and new tubes before you start to be sure you won't have any unpleasant surprises. * Take extreme care when handling - at the very least, a slip can result in a broken neck and a bad and expensive day. "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: * Can I still buy new RCA 25VCXP22 picture tube? What is the approximate cost? * Any equivalent tube for direct replacement? Cost? * If no replacement picture tube is available, what is other option?" (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 (Thompson, whatever!) still make them available as well as any of a number of local CRT rebuilders.
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: 0. 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. 1. 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". 2. 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. 3. 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. 4. Improperly set SCREEN (G2) voltage (usually on flyback) or faulty divider network. See the section: "Adjustment of the internal SCREEN and color controls". 5. 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". 6. 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. 7. 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. 8. High voltage is low. However, this would likely result in other symptoms as well with focus, size, and geometry.
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.
(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.
(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")
"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.
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.
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.
Assuming this is not a form of ghosting resulting from poor reception conditions, then it could be any of the following: * Poor decoupling in the power supplies for the video drive circuits - probably on the CRT neck board. Check for bad (low uF or high ESR) filter capacitors (electrolytic mostly) on this board or the power supplies feeding it. * Insufficient CRT filament voltage. This could be a result of bad connections or a bad component in the filament power supply (probably from the flyback). Check to see if the filaments are glowing bright orange and check the voltage if possible (though this can be tricky since it is often fed from a winding on the flyback and is a pulse waveform, not DC or a sinusoid. The service manual (or Sams' Photofact) will probably have info and waveforms. * Bad CRT (more likely if only one color is affected). A weak electron gun can result in this behavior. Swap it with one that work properly. If the same color is still bad, that CRT gun is weak. The CRT will need rejuvenation or need to be replaced (more likely, the entire TV will be tossed into the dumpster).
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 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! * A fault in the power supplies to the video amplifier and/or video output circuits. Most likely, an electrolytic capacitor has dried up and is not adequately filtering the power derived from the flyback which then has ripple at the horizontal scan rate and thus locked to the screen. The voltage decays from left-to-right between horizontal flyback pulses. The most likely location for these capacitors is in the vicinity of the flyback transformer on the mainboard or on the CRT neck board. Check the capacitors with capacitor tester or ESR meter and/or take a look at the power right at the video amplifier and video output drivers. * Horizontal linearity is bad - this may actually be a horizontal geometry problem and not a brightness problem. See if objects on left side of the screen are stretched compared to those on the right (or vice-versa). If they are, the problem is in the horizontal deflection circuits - possibly a bad S correction capacitor or linearity coil. * Inoperative degauss circuit, TV moved or rotated without degaussing, or magnetic field from some other device (like a permanent magnet) is affecting CRT - slight amounts of magnetization may reduce brightness (by moving the beams into the black space between phosphor dots) before affecting color purity (where the beams land on the wrong phosphor dots). Try deguassing manually. See the section: "Degaussing (demagnetizing) a CRT".
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. It is probably not a bad CRT as the filaments are usually wired in parallel and all would not 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.
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: * Brightness circuitry - SCREEN, master background or its power supply. Could be in or around flyback or focus/screen divider. Could perhaps be in the CRT, but probably less likely. * Video amp before or at chroma demodulator - since after this point, you would most likely get colored flashes since only one of the RGB signals would likely be effected. 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.
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. * Excessive brightness/washed out picture is often an indication of a problem with the screen (G2) supply to the CRT. May be a bad capacitor or resistor divider often in the flyback transformer assembly or on the board on the neck of the CRT. * If the excessive brightness just developed over time, then a simple adjustment of the screen or background brightness controls may keep it (and you) happy for a long time. When good, a typical value would be in the 200 to 600 VDC at the CRT. The screen (it may also be called master brightness, bias, or background) control should vary this voltage. However, it may be difficult to measure as the resistors in the voltage divider network may be quite large - hundreds of M ohms. If your unit has an external screen control (less likely these days) and it has no effect, trace out the circuitry in the immediate vicinity and check the resistors and potentiometer for opens, look for bad connections, etc. If it is built into the flyback transformer and is sealed, the entire flyback will need to be replaced unless the actual problem turns out to be a bad connection or bad component external to the flyback. * Where the brightness control has no effect, suspect a missing bias supply to the G1 (control grid) electrodes of the CRT. This is usually derived from the flyback with a simple rectifier/filter capacitor power supply. Parts may have failed (though not likely the flyback itself). Adjusting the user brightness control should vary this voltage over a typical range of 0 to -50 V with respect to signal ground. * It could also be a problem with biasing of the video output transistors. There may individual controls for background brightness on the little board on the neck of the CRT. However, we are looking for a common problem since all colors are wrong in the same way. This is likely to be a missing voltage from a secondary supply from the flyback. * A short between electrodes inside the CRT can result in brightness problems. It may be possible to check this with an ohmmeter with the power off and the CRT socket removed. Test between G1, G2, and F where all colors are affected though a short between F and G2 will result in the focus control changing brightness and vice-versa - a classic symptom. However, in some cases, it only shows up when operating and one must deduce the presense and location of the short from its affect on voltages and bias levels. See the section: "Rescuing a shorted CRT" and other related topics. First, check for bad connections/cold solder joints by gently prodding with an insulating stick. Check voltages and bias levels.
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: * If adjusting the pot results in a position of acceptable focus, you may be done. It is not unusual for the focus setting to drift a over time. * If the setting is already as good as possible but not really good enough, the CRT may be tired. Alternatively, the filament voltage may be too low. Check for bad connections in the filament circuit. * If the optimal setting is out of range of the focus pot, the problem is likely leakage in the focus divider in the flyback or one of the components on the CRT neck board. 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) and the pot has little effect: * When measured right off of the source disconnected from the CRT neck board, then the problem is probably in the focus network in the flyback (or wherever it originates). Sometimes these can be disassembled and cleaned or repaired but usually requires replacement of the entire flyback or voltage multiplier. Note: you may need to add a HV (10 KV) capacitor between the focus wire and DAG ground to provide filtering so you get a DC level for your meter. * When measured with the focus wire attached to the CRT neck board with the CRT connected but reasonable with the CRT unplugged, there is probably a short between the focus and another electrode inside the CRT. See the section: "Rescuing a shorted CRT". * When measured with the focus wire attached to the CRT neck board with the CRT unplugged, there is likely a component on the CRT neck board that is leaky or breaking down. Also, check for decayed (tan or brown) glue which may turn leaky with age.Go to [Next] segment
Go to [Table 'O Contents]