There may or may not be any indication of a picture. This may be a problem in the high voltage power supply (SCREEN, G2), loss of power or a fault in the video output drivers, other video amp problems, or a bad (shorted) CRT. Is focus still reasonably sharp? If not, try adjusting it (usually on the flyback or a separate little panel). If changing focus affects brightness significantly, there is a short between the two supplies - either in the HV power supply or CRT. See the section: "Bad focus and adjustment changes brightness". In this case, changing SCREEN (G2, also on the flyback) may also affect focus or may not do anything. Try adjusting SCREEN. If it has no affect, a problem in its power supply from the flyback is possible. If you have a high impedance voltmeter (not just a DMM, the resistance of the voltage divider supplying SCREEN is hundreds of M ohms), check it while changing the SCREEN control. If it does not change, you have found a definite problem. Assuming that adjusting FOCUS and SCREEN result in normal behavior and do not strongly interact, the problem is likely in the video circuitry or output drivers. Check the power to the CRT video output drivers on the little board on the neck of the CRT. If this failed, all three video outputs will be full on. If you have a scope, look at the video outputs - they should be varying between over 100 V and a low value. If they are missing or very low all the time, there is a problem further back in the video chain. See the other sections relating to brightness and high voltage problems as well.
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 monitor.) 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 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 terribly 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. Unplug the CRT socket! 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 feel 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!
A 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! (From: A. R. Duell (firstname.lastname@example.org)). "I once spent a morning battling with a DEC VT105 terminal with a very dim and washed out picture, and only after checking everything on the video board did I wipe over the screen. That cured it. It's amazing how dirty screens can get after a few years use." 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! 1. Old CRT. The brightness of the CRT deteriorates with filament on-time. It doesn't matter much what you are doing or if you use a screen saver. 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 sections: "Brightening a old CRT" and "Monitor life, energy conservation, and laziness". 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: "Brightness and color balance adjustment". 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 brightness 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 (and the controls are having an effect - they are not faulty), 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!). First confirm that the filaments are running at the correct voltage - there could be a marginal connection or bad resistor or capacitor in the filament power supply. Since this is usually derived from the flyback, it may not be possible to measure the (pulsed high frequency) voltage with a DMM but a service manual will probably have a waveform or other test. A visual examination is not a bad way to determine if the filaments are hot enough. They should be a fairly bright orange to yellow color. A dim red or almost dark filament is probably not getting its quota of electrons. It is not be the CRT since all three filaments are wired in parallel and for all three to be defective is very unlikely. If possible, confirm that the video output levels are correct. For cathode driven CRTs, too high a bias voltage will result in a darker than normal picture. CRT brighteners are available from parts suppliers like MCM Electronics. Some of these are designed as isolation transformers as well to deal with heater-to-cathode shorts. You can try a making a brightener. 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. Adding an extra turn 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 monitor was headed for the dumpster anyhow, you have nothing to lose. You can just add a turn to an existing winding or make your own separate filament winding as outlined in the section: "Providing isolation for a CRT H-K short". In some monitors, there is a separate filament supply on the mainboard - this should be obvious once you trace the filament wires from the video driver board). In this case, it still may be possible to increase this output or substitute another supply but a schematic will be required. There are also commercial CRT rejuvenators that supposedly zap the cathodes of the electron guns. A TV or monitor service center may be able to provide this service, though it is, at best, a short term fix.
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 monitor) are set too high. See the section on: "Brightness and color balance adjustment". Check the settings of any brightness limiter controls as well.
Assuming this is not a form of ghosting resulting from cabling and/or use of switchboxes, etc, 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 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 monitor will be tossed into the dumpster).
Setting the brightness excessively high may result in enough heating of the shadow mask to distort it. IF severe enough, the positions of the holes will shift enough to result in visible purity problems. This is less of a problem with tubes using an InVar shadow/slot mask. It should also be less of a problem for Trinitron aperture grille CRTs. The only solution is to reduce the brightness.
Actually, the intensity variation is likely to be even worse than you might think - possibly as much as 2:1 from the center to the corners. In most cases you do not notice it. With large deflection angle tubes, fewer electrons make it to phosphor dots near the edge of the screen. It is simple geometry. (From: Bob Myers (email@example.com)). It is extremely difficult for any CRT display to maintain perfect brightness and color uniformity across the entire image. Just the geometry of the thing - the change distance from the gun to the screen as the beam is scanned, the changing spot size and shape, etc. - makes this nearly impossible, and there can also be variations in the phosphor screen, the thickness of the faceplate, etc.. Typical brightness-uniformity specs are that the brightness won't drop to less than 70% or so of the center value (usually the brightest spot on the screen). On color tubes, the lack of perfect brightness uniformity is aggravated by the lack of perfect COLOR uniformity and purity. What appear to be "dark spots" on a solid gray image may actually be beam mislanding (color purity) problems, which may to some degree be remedied by degaussing the monitor. Again, *some* variation is normal; if you think you're seeing too much, you can try degaussing the thing and seeing if that helps. If it doesn't, then the question is whether or not the product meets its published specs, and that 's something you'll have to discuss with the manufacturer or distributor.
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 (or in the case of a multiscan monitor, correctly selected) S correction capacitor or linearity coil. * Inoperative degauss circuit, monitor 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). See if the degauss button, if present, does anything. 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 since it will require CRT replacement. 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. Make sure it is not the video source - try another one. 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 (if composite input) - since after this point, you would most likely get colored flashes since only one of the RGB signals would likely be effected. However, a bad power connection to the video circuitry could cause all the colors to be affected. If you still get flashes, it should be quite 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.
First, confirm that these are not video source - PC - related. Try the monitor on another computer. This may be a problem with the hardware or driver (software) for the video card, the O/S, or memory or bus speed. If it is not computer related, then it could be arcing, corona, bad connections, or some electronic component breaking down. See the appropriate sections for these problems. Note that problems in absolutely fixed locations or with an extent related to pixel sizes in the video card are nearly always computer/video card related and not due to a faulty monitor.
First, make sure your scan rate is set high enough (but not beyond the capabilities of the monitor). A scan rate less than 60 Hz is likely to result in annoying flicker especially at high brightness levels. See if the flickering correlates with any processor or disk activity indicating a software driver or video card problem. Assuming neither of these applies and you are not doing your work by candlelight, a flickering image is probably due to an intermittent arc or short, probably in the high voltage section near or at the flyback transformer. However, it is also possible that it is due to a simple bad connection elsewhere. So the first thing to do will be to remove the cover and without touching anything, carefully examine for any obvious signs of bad connections, arcing, or burned areas. In particular look for: * hairline cracks around the pins of large components like power transistors, power resistors, transformers, and connectors. * any discoloration, cracking, other unusual signs on the flyback. The flyback also provides, via a high resistance divider network, the several KV for focus and several hundred V for the G2 (screen) CRT electrode. These are the voltages that may be intermittently changing and resulting in flicker. Now, with the monitor powered in a darkened room with a normal picture (use the highest resolution at which your monitor will work as this should put the most stress on it, maybe). * Look for any arcing or corona around the area of the flyback or the neck of the CRT first, then just anywhere. * Use a well insulated stick (wood or plastic) to gently prod the circuits board, components, wires, etc. to see if you can induce the problem. There will probably be a pair of adjustments on the flyback itself. One of these is FOCUS and the other is SCREEN - essentially a master brightness. * Now, with one hand in your back pocket, try turning each of these a fraction of a turn in each direction. Don't worry, you cannot hurt anything by doing this. The FOCUS should only change the sharpness of the picture. The SCREEN should only change the brightness. In both cases, this should be a smooth effect. Sometimes, these controls will simply get dirty and cause the problems you have seen. In this case, just moving them back and forth may clean them. If one affects the other - if turning focus alters brightness or vice-versa, there is a short between the focus and screen voltages, probably inside the flyback but it could be elsewhere. It is likely that all of the above tests will come out negative as you may have an intermittent short internal to the flyback which can only be fixed by replacement. However, eliminate the easy fixes first.
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.
Slight deterioration in focus can be corrected by adjusting the focus control usually located on the flyback transformer. Sometimes, this is accessible externally but usually not. On monochrome monitors, the focus control, if any, may be located on the main board. Don't expect to have perfect focus everywhere on the screen. Usually there will be some degradation in the corners. A compromise can generally be struck between perfect focus in the center and acceptable focus in the corners. If the adjustments have no effect, then there is probably a fault in the focus power supply. For most color TVs and monitors, the correct focus voltage will be in the 4-8 KDC range so you will need a meter that can go that high or some big resistors to extend its range or a HV probe. You must use a high impedance meter as the current availability from the focus power supply is very low. The pots in the flyback are sometimes accessible by removing their cover, which may snap on. However, a typical focus circuit will have a large value resistor potted inside the flyback (like 200 Megohms). Try to measure the focus in-circuit. If the value you read is very low (assuming your meter has a high enough impedance not to load the circuit appreciably), then disconnect the wire (from the PCB on the neck of the CRT or wherever) and measure again and observe any change in picture. If still low, then almost certainly there is a problem with the pot or the flyback. See if you can open it enough to measure and/or disconnect the pot. If the problem is inside the potted part of the flyback, the only alternative is a new flyback or an external divider if you are so inclined. However, once the focus network goes bad inside the flyback, there is an increased chance other parts will fail at some point in the future. If the voltages check out with the CRT disconnected, there is a chance of a bad CRT or of a shorted component on the PCB on the neck of the CRT. Look for shorted capacitors or burnt or damaged traces.
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
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