B/W TV size, position, and geometry adjustments These tend to be a lot simpler and less critical than for color monitors or TV sets. On a B/W TV you will probably see some of the following adjustments: 1. Position - a pair of rings with tabs on the neck of the CRT. There may be electronic position adjustements as well though this is not that common on small TVs. 2. Width and height (possibly linearity as well) controls. There may be some interaction between size and linearity - a crosshatch test pattern is best for this. Vertical adjustments are almost always pots while horizontal (if they exist) may be pots and/or coils. Size will normally be set for 5-10% overscan to account for line voltage fluctuations and component drift. Confirm aspect ratio with test pattern which includes square boxes. 3. Geometry - some little magnets either on swivels around the yoke or glued to the CRT. If these shifted, the the edges may have gotten messed up - wiggles, dips, concave or convex shapes. There may be a doxen or more each mostly affecting a region around the edge of the raster. However, they will not be totally independent. Check at extremes of brightness/contrast as there may be some slight changes in size and position due to imperfect HV regulation. There may be others as well but without a service manual, there is no way of knowing for sure. Sams often has folders for B/W TVs. Just mark everything carefully before changing - then you will be able to get back where you started. Low Voltage Power Supply Problems Low voltage power supply fundamentals TVs require a variety of voltages (at various power levels) to function. The function of the low voltage power supply is to take the AC line input of either 115 VAC 60 Hz (220 VAC 50 Hz or other AC power in Europe and elsewhere) and produce some of these DC voltages. In all cases, the power to the horizontal output transistor of the horizontal deflection system is obtained directly from the low voltage power supply. In some cases, a variety of other DC voltages are derived directly from the AC line by rectification, filtering, and regulation. In other designs, however, most of the low voltages are derived from secondary windings on the flyback (LOPT) transformer of the horizontal deflection system. In still other designs, there is a separate switchmode power supply that provides some or all of these voltages. There are also various (and sometimes convoluted) combinations of any or all of the above. There will always be: 1. A power switch, relay, or triac to enable main power. 2. A set of rectifiers - usually in a bridge configuration - to turn the AC into DC. Small ceramic capacitors are normally placed across the diodes to reduce RF interference. 3. One or more large filter capacitors to smooth the unregulated DC. In the U.S., this is most often a voltage around 150-160 V DC. In countries with 220 VAC power, it will typically be around 300-320 V DC. 4. A discrete, hybrid, or IC regulator to provide stable DC to the horizontal deflection system. Sometimes feedback from a secondary output of the flyback or even the high voltage is used. This regulator may be either a linear or switching type. In some cases, there is no regulator. 5. Zero or more voltage dividers and/or regulators to produce additional voltages directly from the line power. This relatively rare except for startup circuits. These voltages will not be isolated from the line. 6. A degauss control circuit usually including a thermistor or Posistor (a combination of a heater disk and Positive Temperature Coefficient (PTC) thermistor in a single package). When power is turned on, a relatively high AC current is applied to the degauss coil wrapped around the periphery of the CRT. The PTC thermister heats up, increases in resistance, and smoothly decreases the current to nearly zero over a couple of seconds. 7. A startup circuit for booting the horizontal deflection if various voltages to run the TV are derived from the flyback. This may be an IC or discrete multivibrator or something else running off a non-isolated voltage or the standby power supply. 8. A standby power supply for the microcontroller and remote sensor. Usually, this is a separate low voltage power supply using a small power transformer for line isolation. However, some sets use other (probably cheaper) approaches. See below. Always use an isolation transformer when working on a TV but this is especially important - for your safety - when dealing with the non-isolated line operated power supply. Read and follow the information in the section: Safety guidelines. Standby power supplies Where the TV has a remote control (which most do nowadays), there needs to be some source of voltage(s) for the remote receiver, microcontroller, and other circuitry that watch for the power on commend. These sets are never totally off. The standby supply may consist of: • A low voltage power transformer feeding one or more sets of rectifiers, filter capacitors, and possibly regulators. A power surge could cause the primary of the transformer to open up. There may also be a thermal fuse under the outer layers of insulation which blew either due to overheating or a power surge. However, if the primary is open, it is best to replace the transformer rather than attempting repair it. • One or more voltage dividers connected directly to the AC line feeding rectifiers, filter capacitors, and possibly regulators. Open resistors and dried up capacitors are common failures since the resistors are often not rated adequately and run hot, in close proximity to the capacitors. • A portion of the main (switchmode) power supply that runs all the time. Failures could be almost anything that would affect normal operation of the power supply as well as problems with the control circuitry. Typical TV power supply front end The partial schematic below is similar to those found in the majority of TVs sold in countries with 110 to 120 VAC power. Many parts are not shown including the power switch or relay, RFI bypass capacitors across the rectifier diodes, and RFI line filter. Bypass resistor Line fuse Main bridge Fusable +----/\/\-----+ _ rectifier resistor | +-----+ | H o--_ --+------|>|---+---/\/\--+---+---| REG |---+---+---o B+ | | | +-----+ | | +---|>|---+ C1 _|_ Main | _|_ Regulator 115 VAC | | 400 uF --- filter | --- output +--|---| N o---------+---| G - Power line earth ground via building wiring • The line fuse is typically 2 to 4 A, usually a normal fast blow type. Even so, it may not blow as a result of faults down the line - the fusable resistor or regulator may fail first. • The main bridge rectifier is often composed of 4 discrete diodes (similar to 1N400Xs) but may also be a single unit. Failures - usually shorted diodes - are common. • The main filter capacitor can range in size from 200 to 800 uF or more at 200 to 250 V. THIS CAN BE LETHAL! A typical TV may continue to work at normal line voltage without any noticeable degradation in performance (hum bars, hum in sound, or shutdown) even if this capacitor is reduced in value by 75%. Its uF value is therefore not critical. • The regulator is often an IC or hybrid module. Failures resulting in no or reduced output, or no regulation are common. • The regulator output capacitor is needed for the B+ regulator to function properly. If this capacitor is reduced in value or develops a high ESR, regulation may fail resulting in instability, oscillation, or excessive B+ and shutdown. • The regulator bypass resistor reduces the amount of current control needed of the regulator. Caution: even if the regulator has been pulled, the B+ line will have substantial voltage as a result of this resistor. Totally dead set This can be as simple as a bad outlet (including blown fuse or tripped circuit breaker due to some other fault), switched outlet and the switch is off, or bad cordset. • Plug a lamp into the outlet to make sure it is live. If the lamp works, then the problem is the TV. It not, the outlet is defective or the fuse is blown or the circuit breaker is tripped. There is another very simple explanation that is sometimes overlooked: This is a switched outlet. You always wondered what that wall switch was for that didnt seem to do anything and you flipped randomly. :-) Well, now you know! • Try wiggling the TVs cord both at the outlet (also push the wire toward the plug) and TV (also push the cord toward the TV) with the set on and/or while pressing the power-on button. If you can get a response, even momentarily, the cord likely has broken wires internally. Beyond these basic causes, troubleshooting will be needed inside the set to determine what is defective. Also see the section: Intermittently dead set - bad cordset. Intermittently dead set - bad cordset There are two problems which are common with the line cord on appliances. Dont overlook these really simple things when troubleshooting your vacuum cleaner - or fancy electronic equipment! If wiggling the cord has an effect, then the following are likely causes: • Repeated flexing results in the internal conductors breaking either at the plug or appliance end. If flexing the cord/squeezing/pulling results in the device going on and off, it is bad. If the problem is at the plug end, cut off the old plug a couple of inches beyond the problem area and replace just he plug. If the problem is at the appliance end, an entire new cordset is best though you can probably cut out the bad section and solder what remains directly to the mainboard. In either case, observe the polarity of the cord wires - they will be marked in some way with a ridge or stripe. It is important that the new plug be of the same type (polarized usually) and that the cord is wired the same way. • The prongs do not fit snugly into older worn outlets. This can usually be remedied by using a pointed tool like an awl or utility knife to spread apart the pair of leaves often used to form each prong of the plug. If the prongs are made of solid metal, it may be possible to spread them apart - widen the space between them. Alternatively, get a 3 to 2 prong adapter just to use as an intermediate connector. Spread the leaves of its prongs. However, a new outlet is best. • Bad connections on the mainboard. As you flex the cord, it is also stressing the attachment to the mainboard and affecting some marginal solder joints. It is important to deal with these symptoms as soon as possible as erratic power cycling can lead to much more serious and expensive problems down the road. Power button on set is flakey If the on/off (or other button) on the set itself behaves erratically but the remote control works fine, then it could be a dirty button or cable or other connections to the switch PCB, particularly if the buttons on the set itself are rarely used. There could possibly be a bad pullup resistor or something of that sort - but is it worth the effort to locate? Why not just continue to use the remote? There is no reason to suspect that it will develop similar symptoms. However, there is some risk that if the button is dirty, you may find the TV coming on at random times in the middle of the night (of course!). I think I have an older Sylvania that does that sort of thing - dont really know as I never use the power button on the set! If power is controlled by a hard switch - a pull or click knob, or mechanical push-push switch and this has become erratic due to worn contacts, replacements are available but often only directly from the original manufacturer to physically fit and (where applicable) have the volume or other controls built in. As an alternative, consider mounting a small toggle switch on the side of the cabinet to substitute for the broken switch. This will almost certainly be easier and cheaper - and quite possibly, more reliable. TV blows fuse A blown fuse is a very common type of fault due to poor design very often triggered by power surges due to outages or lightning storms. However, the most likely parts to short are easily tested, usually in-circuit, with an ohmmeter and then easily removed to confirm. Note that it *may be* useful to replace a fuse the *first* time it blows (though it would be better to do some basic checks for shorted components first as there is a small chance that having a fuse blow the second time could result in additional damage which would further complicate the troubleshooting process). However, if the new one blows, there is a real problem and the only use in feeding the TV fuses will be to keep the fuse manufacturer in business! Sometimes, a fuse will just die of old age or be zapped by a power surge that caused no damage to the rest of the TV. However, it must be an EXACT replacement (including slo-blow if that is what was there originally). Else, there could be safety issues (e.g., fire hazard or equipment damage from too large a current rating) or you could be chasing a non-existent problem (e.g., if the new fuse is not slo-blow and is blown by the degauss circuit inrush current but nothing is actually wrong). If the fuse really blows absolutely instantly with no indication that the circuits are functioning (no high pitched horizontal deflection whine (if your dog hides under the couch whenever the TV is turned on, deflection is probably working).) then this points to a short somewhere quite near the AC power input. The most common places would be: • Degauss Posistor - very likely. • Horizontal output transistor. • Power supply regulator if there is one. • Power supply chopper (switchmode) transistor if there is one. • Diode(s) in main bridge • Main filter capacitor(s). You should be able to eliminate these one by one. Unplug the degauss coil as this will show up as a low resistance. First, measure across the input to the main power rectifiers - it should not be that low. A reading of only a few ohms may mean a shorted rectifier or two or a shorted Posistor. • Test the rectifiers individually or remove and retest the resistance. • Some sets use a Posistor for degauss control. This is a little cubical (about 1/2 x 3/4 x 1) component with 3 legs. It includes a line operated heater disk (which often shorts out) and a PTC thermister to control current to the degauss coil. Remove the posistor and try power. If the monitor now works, obtain a replacement but in the meantime you just wont have the automatic degauss. If these test good, use an ohmmeter with the set unplugged to measure the horizontal output transistor. Even better to remove it and measure it. • C-E should be high in at least one direction. • B-E may be high or around 50 ohms but should not be near 0. If any readings are under 5 ohms, the transistor is bad. The parts sources listed at the end of this document will have suitable replacements. If the HOT tests bad, try powering the set first with your light bulb and if it just flashes once when the capacitor is charging, then put a fuse in and try it. The fuse should not blow with the transistor removed. Of course, not much else will work either. If it tests good, power the set without the transistor and see what happens. If the fuse does not blow, then with the good transistor (assuming it is not failing under load), it would mean that there is some problem with the driving circuits possibly or with the feedback from the voltages derived from the horizontal not regulating properly. Look inside the TV and see if you can locate any other large power transistors in metal (TO3) cans or plastic (TOP3) cases. There may be a separate transistor that does the low voltage regulation or a separate regulator IC. Some TVs have a switchmode power supply that runs off a different transistor than the HOT. There is a chance that one of these may be bad. If it is a simple transistor, the same ohmmeter check should be performed. If none of this proves fruitful, it may be time to try to locate a schematic. A blown fuse is a very common type of fault due to poor design very often triggered by power surges due to outages or lightning storms. However, the most likely parts to short are easily tested, usually in-circuit, with an ohmmeter and then easily removed to confirm. If you find the problem and repair it yourself, the cost is likely to be under $25. Fuse blows or TV blows up when sync is disrupted This is a problem which is not going to be easy to identify. One possibility is a drive problem. The messed up sync resulting from swtiching channels, or changing input connections might be resulting in an excessively long scan time for just one scan line. However, this may be enough to cause a current spike in the horizontal output circuit or an excessive voltage spike on the collector of the horizontal output transistor. Normally, the HOT current ramps up during scan. During flyback, the current is turned off. This current is normally limited and the voltage spike on the collector of the HOT is also limited by the snubber capacitors to a safe value. If scan time is too long, current continues to increase. At some point, the flyback core saturates and current goes way up. In addition, the voltage spike will be much higher - perhaps destructively so. Troubleshooting these sorts of problems is going to be tough. However, a likely area to investigate would be: • Drive circuitry for the HOT including the coupling components. • The chip that generates takes the sync input and generates the horizontal drive signal. • A bad low voltage regulator might permit the B+ to rise to excessive levels during black scenes (i.e., video mute during channel changing). Internal fuse blew during lightning storm (or elephant hit power pole) Power surges or nearby lightning strikes can destroy electronic equipment. However, most of the time, damage is minimal or at least easily repaired. With a direct hit, you may not recognize what is left of it! Ideally, electronic equipment should be unplugged (both AC line and phone line!) during electrical storms if possible. Modern TVs, VCRs, microwave ovens, and even stereo equipment is particularly susceptible to lightning and surge damage because some parts of the circuitry are always alive and therefore have a connection to the AC line. Telephones, modems, and faxes are directly connected to the phone lines. Better designs include filtering and surge suppression components built in. With a near-miss, the only thing that may happen is for the internal fuse to blow or for the microcontroller to go bonkers and just require power cycling. There is no possible protection against a direct strike. However, devices with power switches that totally break the line connection are more robust since it takes much more voltage to jump the gap in the switch than to fry electronic parts. Monitors and TVs may also have their CRTs magnetized due to the electromagnetic fields associated with a lightning strike - similar but on a smaller scale to the EMP of a nuclear detonation. Was the TV operating or on standby at the time? If it was switched off using an actual power switch (not a logic pushbutton or the remote control), then either a component in front of the switch has blown, the surge was enough to jump the gap between the switch contacts, or it was just a coincidence (yeh, right). If the TV was operating or on standby or has no actual power switch, then a number of parts could be fried. TVs usually have their own internal surge protection devices like MOVs (Metal Oxide Varistors) after the fuse. So it is possible that all that is wrong is that the line fuse has blown. Remove the cover (unplug it first!) and start at the line cord. If you find a blown fuse, remove it and measure across the in-board side of fuse holder and the other (should be the neutral) side of the line. The ohmmeter reading should be fairly high - well certainly not less than 100 ohms - in at least one direction. You may need to unplug the degaussing coil to get a reasonable reading as its resistance may be 25 or 30 ohms. If the reading is really low, there are other problems. If the resistance checks out, replace the fuse and try powering the TV. There will be 3 possibilities: 1. It will work fine, problem solved. 2. It will immediately blow the fuse. This means there is at least one component shorted - possibilities include an MOV, line rectifiers, main filter cap, regulator transistor, horizontal output transistor, etc. You will need to check with your ohmmeter for shorted semiconductors. Remove any that are suspect and see of the fuse now survives (use the series light bulb to cut your losses - see the section: The series light bulb trick. 3. It will not work properly or appear dead. This could mean there are open fusable resistors other defective parts in the power supply or elsewhere. In this case further testing will be required and at some point you may need the schematic. If the reading is very low or the fuse blows again, see the section: TV blows fuse. Fuse replaced but TV clicks with power-on but no other action The click probably means that the power relay is working, though there could be bad contacts. Since the fuse doesnt blow now (you did replace it with one of the same ratings, right?), you need to check for: • Other blown fuses - occasionally there are more than one in a TV. Replace with one of exactly the same ratings. • Open fusable resistors. These sometimes blow at the same time or in place of the fuses. They are usually low values like 2 ohms and are in big rectangular ceramic power resistor cases or smaller blue or gray colored cylindrical power resistors. They are supposed to protect expensive parts like the HOT but often blow at the same time. If any of these are bad, they will need to be replaced with flameproof resistors of the same ratings (though you can substitute an ordinary resistor for testing purposes). Before applying power, check: Rectifier diodes, horizontal output transistor, regulator pass or chopper transistor (if present), and main filter capacitor for shorts. An initial test with an ohmmeter can be done while in-circuit. The resistance across each diode and the collector to emitter of the transistors should be relatively high - a few hundred ohms at lest - in at least one direction (in-circuit). If there is a question, unsolder one side of each diode and check - should be in the Megohms or higher in one direction. Removed from the circuit, the collector-emitter resistance should be very high in one direction at least. Depending on the type, the base-emitter resistance may be high in one direction or around 50 ohms. If any reading on a semiconductor device is under 10 ohms - then the device most likely bad. Assuming that you do not have a schematic, you should be able to locate the rectifiers near where the line cord is connected and trace the circuit. The transistors will be either in a TO3 large metal can or a TOP3 plastic package - on heat sinks. The filter capacitor should eventually measure high in one direction (it will take a while to charge from your ohmmeter). It could still be failing at full voltage, however. If you find one bad part, still check everything else as more than one part may fail and just replacing one may cause it to fail again. Assuming everything here checks out, clip a voltmeter set on its 500 V scale or higher across the horizontal output transistor and turn the power on. Warning - never measure this point if the horizontal deflection is operating. it is ok now since the set is dead. If the voltage here is 100-150, then there is a problem in the drive to the horizontal output circuit. If it is low or 0, then there are still problems in the power supply or with the winding on the flyback transformer. Other possible problems: bad hybrid voltage regulator, bad startup circuit, bad standby power supply (dried up filter capacitor, etc.) bad relay contacts as mentioned above. However, these probably would not have blown the fuse in the first place so are less likely. Power-on tick-tick-tick or click-click-click but no other action A variety of power supply or startup problems can result in this or similar behavior. Possibilities include: • Lack of startup horizontal drive - see the section: Startup problems - nothing happens, click, or tick-tick-tick sound. The main regulator is cycling on overvoltage due to lack of load. • Excessive load or faulty power supply cycling on its overcurrent protection circuit. • High voltage shutdown, or some other system detecting an out of regulation condition. However, in this case, there should be some indication that the deflection and HV is attempting to come up - momentary whine, static on the screen, etc. • A dried up main filter capacitor or other filter capacitor in the low voltage power supply that is producing an out-of-regulation condition until it warms up. A bad filter capacitor on the output of a series regulator may result in excessive voltage and subsequent shutdown. • A problem with the microcontroller, relay or its driver, or standby power supply. One possible test would be to vary the line voltage and observe the sets behavior. It may work fine at one extreme (usually low) or the other. This might give clues as to what is wrong. . No picture or raster and no sound The screen is blank with no raster at all. There are indications that the channel numbers are changing in the display. This indicates that some of the low voltages are present but these may be derived from the standby supply. Assuming there is no deflection and no HV, you either have a low voltage power supply problem, bad startup circuit, or bad horizontal output transistor (HOT) or other bad parts in the horizontal deflection. Check for bad fuses. (If you have HV as indicated by static electricity on the front of the screen and you hear the high pitched whine of the horizontal deflection when it is turned on, then the following does not apply). 1. Use an ohmmeter to test the HOT for shorts. If it is bad, look for open fusable resistors or other fuses you did not catch. 2. Assuming it is good, measure the voltage on the collector-emitter of the HOT (this is safe if there is no deflection). You should see the B+ - probably between 100 and 150 V. 3. If there is no voltage, you have a low voltage power supply problem and/or you have not found all the bad/open parts. 4. If there is voltage and no deflection (no high pitched whine and no HV), you probably have a startup problem - all TVs need some kind of circuit to kick start the horizontal deflection until the auxiliary power outputs of the flyback are available. Some Zeniths use a simple multivibrator for this - a couple of transistors. Others power the horizontal osc. IC from a special line-derived voltage. The multivibrator type are sometimes designed to fail if someone keeps turning the set on and off (like kids playing) since the power rating is inadequate. Test the transistors if it is that type with an ohmmeter. If one is shorted, you have a problem. The usual way a TV service person would test for startup problems is to inject a signal to the base of the HOT of about 15.75 kHz. If the TV then starts and runs once this signal is removed, the diagnosis is confirmed. This is risky - you can blow things up if not careful (including yourself). See the section: Bypassing the Startup Circuit for details. If you hear the high pitched whine of the deflection and/or feel some static on the scree, confirm that the horizontal deflection and high voltage are working by adjusting the SCREEN control (probably on the flyback). If you can get a raster then your problem is probably in the video or chroma circuits, not the deflection or high voltage. Reduced width picture and/or hum bars in picture and/or hum in sound The most likely cause is a dried up main filter capacitor. Once the effective capacitance drops low enough, 120 Hz (or 100 Hz in countries with 50 Hz power) ripple will make its way into the regulated DC supply (assuming full wave rectification). Another likely cause of similar symptoms is a defective low voltage regulator allowing excessive ripple. The regulator IC could be bad or filter capacitor following the IC could be dried up. Either of these faults may cause: 1. A pair of wiggles and/or hum bars in the picture which will float up the screen. For NTSC where the power line is 60 Hz but the frame rate is 59.94 Hz, it will take about 8 seconds for each bar to pass a given point on the screen. (On some sets, a half wave recitifier is used resulting in a single wiggle or hum bar). 2. Hum in the sound. This may or may not be noticeable with the volume turned down. 3. Possible regulation problems resulting in HV or total shutdown or power cycling on and off. The best approach to testing the capacitors is to clip a good capacitor of approximately the same uF rating and at least the same voltage rating across the suspect capacitor (with the power off). A capacitor meter can also be used but the capacitor may need to be removed from the circuit. Once the capacitors have been confirmed to be good, voltage measurements on the regulator should be able to narrow down the problem to a bad IC or other component. Excessive B+ from fixed regulator like STR30123/STR30130/STR30135 These are fixed regulators that do fail but the problem may be elsewhere. If the B+ goes to high, the X-ray protection circuitry may kick in and shut down the horizontal deflection. If there is little or no load (horizontal deflection not running at all), all bets are off as well - the resistor that is likely across input-output will dominate and boost the voltage above the proper output for the regulator chip. Use a Variac to bring up the voltage to the TV. If the deflection does not start up at any voltage even with the B+ ramping up past its normal value, the problem is probably in the horizontal deflection/startup circuitry, not the regulator. Some of these may go out of regulation if the output electrolytics are dried up. There might a a 10 uF 200 V or so electrolytic across the output to ground. Test it or substitute a known good one of about the same uF rating and at least equal voltage rating. If you can get the TV to work at reduced voltage using a Variac (but possibly with hum bars in the picture and hum in the audio), check the output capacitor. Otherwise, it could be the regulator or one of its biasing components (sets current to B input - the voltage at this input should be close to the output voltage value). Also check to be sure the input voltage is solid - main filter capacitor is not dried up. TV power cycling on and off The power light may be flashing or if you are runing with a series light bulb it may be cycling on and off continuously. There may be a chirping or clicking sound from inside the set. (Note: using too small a series light bulb load during testing for the size of the TV may also result in this condition.) If there is a low voltage regulator or separate switching supply, it could be cycling on and off if the horizontal output, flyback, or one of its secondary loads were defective. Does this TV have a separate low voltage regulator and/or switching power supply or is it all part of the flyback circuit? For the following, I assume it is all in one (most common). Some simple things to try first: Verify that the main filter capacitor is doing its job. Excessive ripple on the rectified line voltage bus can cause various forms of shutdown behavior. An easy test is to jumper across the capacitor with one of at least equal voltage rating and similar capacitance (make connections with power off!). Use a Variac, if possible, to bring up the input voltage slowly and see if the TV works at any point without shutting down. If it does, this could be an indication of X-ray protection circuit kicking in, though this will usually latch and keep the set shut off if excessive HV were detected. Dead TV with periodic tweet-tweet, flub-flub, or low-low voltage A TV which appears to be dead except for a once a second or so tweet or flub usually indicates an overload fault in the power supply or a short in one of its load circuits. In some cases, the low voltage (including B+) will just be reduced to a fraction of their normal value as a result of an overload on one of the outputs - usually the main B+. This may be caused by a shorted rectifier in the power supply, flyback, or even the yoke, but check the the loads first. Wait a few minutes for the filter caps to discharge (but stay away from the CRT HV connector as it may retain a dangerous and painful charge for a long time), use an ohmmeter across the various diodes in the power supply. Using an ohmmeter on the rectifier diodes, the resistance in at least one direction should be greater than 100 ohms. If it is much less (like 0 or 5 ohms), then the diode is probably bad. Unsolder and check again - it should test infinite (greater than 1M ohms) in one direction. Summary of possible causes: • Bad solder connections. • Other shorted components like capacitors. • Other problems in the power supply or its controller. • Bad flyback. • Short or excessive load on secondary supplies fed from flyback. • Short in horizontal yoke windings. • Problem with startup drive (cycling on overvoltage).
Posted on: Wed, 13 Nov 2013 15:02:43 +0000