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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 monitor operating or on standby at the time? If it was switched off using an actual power switch (not a logic pushbutton), 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 it was operating or on standby or has no actual power switch, then a number of parts could be fried. Monitors 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 case (unplug it first!) and start at the line connector. 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 - more 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 less than 30 ohms. If the reading is really low, there are other problems. If the resistance checks out, replace the fuse and try powering the monitor. There will be three 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: "Monitor blows fuse".
The click probably means that the power relay is working, though there could be bad contacts. Since the fuse doesn't blow now (you did replace it with one of the same ratings, right?), you need to check for: * 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 (or the pricey HOT sacrifices itself to save the resistor.) 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 nothing tests faulty so far, clip a voltmeter set on its 500 V or higher scale 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 monitor 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 or one of its outputs. Other possible problems: bad hybrid voltage regulator, bad startup circuit, bad relay contacts as mentioned above.
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. * HV 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 like momentary high pitched deflection 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 A bad filter capacitor on the output of a series regulator may also result in excessive voltage and 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 set's behavior. It may work fine at one extreme (usually low) or the other. This might give clues as to what is wrong. Also see the section: "Dead monitor with periodic tweet-tweet-tweet, flub-flub-flub, low-low voltage".
The screen is blank with no raster at all. There are indications that the power is alive - the status LEDs are lit and you can hear the normal relay clicking sounds when you change video modes. 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)/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+ of between 60 and 150 V (typical) depending on mode (for a auto-scan monitor). 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. The flyback primary winding may be open as well. 4. If there is voltage and no deflection, 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 designs use a simple multivibrator for this - a couple of transistors. Others power the horizontal oscillator IC from a special line-derived voltage. Look for pulses at the HOT base. If there are none, trace back to the driver and oscillator. Most likely: the power for startup is missing. 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 very risky for monitors and I would not recommend it - you can all too easily blow things up if not careful (including yourself). If you hear the high pitched whine of the deflection (probably not for workstation or SVGA computer monitors unless you are a bat) 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.
Note that this may be perfectly normal for your monitor if there is no video input - confirm that there is a signal that is compatible with the monitor's scan rate(s) and sync. Make sure the sync selection is set appropriately as well. This may indicate an overloaded low voltage switching power supply. The whine is caused by the switching power supply's chopper frequency dropping down due to the overload. Test the B+ to the horizontal deflection circuits (B+ input to the flyback). If it is near zero, test the HOT for shorts and replace but continue testing with a series light bulb and/or Variac. There may be something causing the HOT to go bad like a shorted flyback or bad damper diode or snubber cap. If the voltage is not zero but is low (e.g., it should be 120 V but is only 60 V), there may be a problem with: 1. Defective low voltage power supply. Test with a substitute load like a 40 W light bulb or power resistor. If the supply now outputs full voltage, it is probably fine. For a power resistor, select a value such that the load at the expected voltage will be about 1/2 to 2/3 of the nameplate power rating of the monitor. Make sure the resistor can handle this power dissipation! 2. Flyback (LOPT) transformer - shorted windings. See the section: "Testing of flyback (LOPT) transformers". 3. Deflection yoke - shorted turns in the horizontal or geometry correction windings. Unplug the yoke if possible and very slowly bring up the monitor on a Variac. Be careful - if you now get high voltage, you may get a very bright spot in the center of the screen which will quickly turn to a very dark spot in the center of the screen - turn the screen (G2) control down and don't run in this state for more than a few seconds. The problem may not be in the yoke but in the circuitry around it. Quickly test the B+. It will probably now be normal. With the yoke connected, use an oscilloscope to monitor the collector of the HOT using a Variac to bring up the voltage slowly. This is safe since you will not be going to full B+ - just enough to look at the signal shape (don't let the collector signal go over a couple hundred volts). The waveform should be a clean pulse during retrace and nearly zero during active video. Ideally it will look like a sinusoidal positive half cycle. If there are dips, serious ringing, or other uglies, then there is likely a fault in the flyback, horizontal deflection coils, or associated circuitry. Pull off the yoke connector and repeat. If it is now clean, suspect the yoke or other components directly connected to it (geometry correction coils, caps, etc.). If you suspect the yoke perform a 'ring test'. See the section: "Testing of flyback (LOPT) transformers" since similar techniques may be used. 4. Excess load on one of the flyback's secondaries. Disconnect all secondary output pins from the flyback if possible and see if your B+ returns to normal. Of course, much of the monitor will not work without these power supplies. You will then need to test the components in each of these supplies and/or reconnected them one by one. 5. Improper drive to HOT. Inspect with an oscilloscope. The drive should match the video input - its frequency should match the horizontal rate with a high time equal to the active video - typically 75-85% of the total line time. If it is on for too long, or of incorrect frequency, the output will not be correct and there may be excessive load on the low voltage power supply.
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). For high scan rate computer monitors, the this may result in horizontal hum bars, wiggles, or other distortions that will drift up or down the screen based on the difference frequency between the power line and video refresh rate being supplied by the PC or workstation. A confirmation can be obtained by varying the scan rate and seeing if the rate of drift changes predictably. 2. 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.
A monitor 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+.
Shorted rectifiers in the switching supply or secondary supplies running off
the flyback common. The HFR854s (one popular type in monitors) or other high
speed high efficiency rectifiers in the output side of the switching power
supply or flyback seem to like to turn into short circuits. (I had a couple
of DOA monitors where this was the problem. so much for quality control!)
After unplugging the monitor and waiting a few minutes for the filter
capacitors to discharge (check with a voltmeter 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.
These appear commonly as black cylinders about 3/8" long by 1/4 diameter.
(Kind of like 1N400Xs on steroids). The resistance of the diodes
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. If it now tests good, there may be something else that
is shorted.
Replacements are available for about $.25 from places like MCM Electronics.
Check other power semiconductors as well, in particular, the horizontal
output transistor.
Sometimes this is an indication of an *overvoltage shutdown* due to a faulty
regulator or open load.
Summary of possible causes:
* Bad solder connections.
* Other shorted components like capacitors.
* Other problems in the power supply or its controller like bad caps.
* Bad flyback.
* Short or excessive load on secondary supplies fed from flyback.
* Short in horizontal yoke windings.
* Problem with startup drive (cycling on overvoltage).
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 light bulb for the size of
the monitor 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 monitor 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.
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 monitor 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.
Since the monitor is not totally dead - there are some signs of life - the
once a second or so tweet or flup - this often means that the switching
power supply has a short in one of its load circuits, very often a shorted
rectifier. It could also be the flyback, 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.
Other possible causes:
* Bad solder connections.
* Other shorted components like capacitors.
* Other problems in the power supply.
* Bad flyback.
* Short or excessive load on secondary supplies fed from flyback.
* Problem with startup drive (cycling on overvoltage).
A failure of the horizontal output transistor or power supply switchmode transistor will blow a fuse or fusable resistor. Look for blown fuses and test for open fusable resistors in the power circuits. If you find one, then test the HOT and/or switchmode transistor for shorts. Other possibilities: rectifier diodes or main filter capacitor. While you are at it, check for bad connections - prod the circuit board with an insulated stick when the problem reoccurs - as these can cause parts to fail.
TVs and monitors usually incorporate some kind of startup circuit to provide drive to the horizontal output transistor (HOT) until the flyback power supply is running. Yes, TVs and monitors boot just like computers. There are two typical kinds of symptoms: power on click but nothing else happens or a tick-tick-tick sound indicating cycling of the low voltage (line regulator) but lack of startup horizontal drive. Check the voltage on the horizontal output transistor (HOT). If no voltage is present, there may be a blown fuse or open fusable resistor - and probably a shorted HOT. However, if the voltage is normal (or high) - usually 60-150 V depending on scan rate (for an auto-scan monitor), then there is likely a problem with the startup circuit not providing initial base drive to the HOT. The startup circuits may take several forms: 1. Discrete multivibrator or other simple transistor circuit to provide base drive to the HOT. 2. IC which is part of deflection chain powered off of a voltage divider or transformer. 3. Other type of circuit which operates off of the line which provides some kind of drive to the HOT. The startup circuit may operate off of the standby power supply or voltage derived from non-isolated input. Be careful - of course, use an isolation transformer whenever working on TVs and especially for power supply problems. Note that one common way of verifying that this is a startup problem is to inject a 15 KHz signal directly into the HOT base or driver circuit (just for a second or two). If the TV then starts up and continues to run, you know that it is a startup problem. Caution: be careful if you do this. The HOT circuit may be line-connected and it is possible to destroy the HOT and related components if this is not done properly. I once managed to kill not only the HOT but the chopper transistor as well while working in this area. An expensive lesson. I have also seen startup circuits that were designed to fail. Turning the TV on and off multiple times would exceed the power ratings of the components in the startup circuit. Some Zenith models have this 'feature'. When this situation exists, it could be that the circuit is not providing the proper drive or that due to some other circuit condition, the drive is not always sufficient to get the secondary supplies going to the point that the normal circuits take over. I would still check for bad connections - prod the circuit board with an insulated stick when the problem reoccurs.
If you can turn it back on with the s momentary key or power button: When it shuts off, do you need to push the power button once or twice to get it back on? Also, does anything else about the picture or sound change as it warms up? 1. If once, then the controller is shutting the TV down either as a result of a (thermally induced) fault in the controller or it sensing some other problem. Monitoring the voltage on the relay coil (assuming these is one) could help determine what is happening. The controller thinks it is in charge. 2. If twice, then the power supply is shutting down as the controller still thinks it is on and you are resetting it. A couple of possibilities here would be low voltage or high voltage regulation error (excessive high voltage is sensed and causes shutdown to prevent dangerous X-ray emission). A partially dried up main filter capacitor could also cause a shutdown but there might be other symptoms like hum bars in the picture just before this happened. Clipping a good capacitor across the suspect (with power off!) would confirm or eliminate this possibility. If it uses a hard on/off switch, then this may be like pulling the plug and would reset any abnormal condition.
The monitor may do nothing, cycle on and off for a while, power up and then shutdown in an endless cycle - or at least for a while. Then it comes on and operates normally until it is turned off. A couple of possibilities: 1. The main filter capacitor or other filter capacitors in the low voltage power supply is dried up and this can cause all kinds of regulation problems. 2. The power supply regulator is defective (or marginal) allowing excessive voltage on its output and then the X-ray protection circuitry shuts you down. If you can get access to a Variac, it would be worth bringing up the input voltage slowly and seeing if there is some point at which it would stay on. If there is, then if the picture has serious hum bars in it the main filter cap could be bad. If more or less a decent picture with minor hum bars then it could be the regulator.
So, what else is new? In the old days, a TV or monitor was expected to take a few minutes (at least) to warm up. We are all spoiled today. Of course, you usually maintained a full time technician or engineer to fiddle with the convergence adjustments! If it just takes a while for the picture to become as bright as you like, this is probably just a result of an old tired CRT (see the section: Monitor life and the care and feeding of CRTs" and "Brightening an old CRT". If, however, nothing happens for a few minutes, then some component needs to be powered for a while before it starts cooperatings. This is probably a dried up capacitor in the power supply since that is drifting with temperature and needs to be located with cold spray or a heat gun.
This is probably a protection circuit kicking in especially if turning power off or pulling the plug is required to restore operation. The detection circuit could be in the power supply or horizontal deflection output circuit. It may be defective or the current may be too high for some other reason. A couple of tests can be performed to confirm that it is due to beam current: * Determine if behavior is similar when adjusting the user brightness control and the screen (G2) pot (on the flyback) or master brightness control. If the monitor quits at about the same brightness level, overcurrent protection is likely. * Disconnect the filaments to the CRT (unsolder a pin on the CRT socket) and see if it still shuts down under the same conditions. If it is overcurrent protection, shut down should now *not* take place since there is no beam current.
This describes problems like turning up the brightness causes a loss of sync or adjusting height also affects width. These may be caused by poor regulation in one or more low voltage power supplies or and interaction between the high voltage and low voltage power supplies - possibly a dried up capacitor if it is relatively old, bad connections, or another faulty component. Measure the B+ to the horizontal deflection (to the flyback, not the horizontal output transistor). If it is changing with the problem, then a regulation problem is confirmed. If this voltage is solid, you will need to check the others to see which one is actually changing.
What exactly is the purpose of such a relay ... i.e., why doesn't the power switch on the monitor just apply power directly instead of through a relay? On a TV, the usual reason for a relay instead of a knob switch is to permit a remote control to turn power on and off. If your TV does not have a remote, then it is simply the same chassis minus 24 cents worth of circuitry to do the remote function. Isn't marketing wonderful? On a monitor without any remote control, there can be two likely reasons: 1. Reduce the needed capacity of the on/off switch. High resolution monitors do consume a fair amount of power. A soft touch button may be more elegant or cheaper. 2. Allow for automatic power saving 'green' operation. When replacing a relay, only unknown is the coil voltage. It is probably somewhere in the 6-12 volt range. You should be able to measure this on the coil terminals in operation. It will be a DC coil. However, the relay controls the 125 VAC (or 220) which you should treat with respect - it is a lot more dangerous than the 25KV+ on the CRT! Almost certainly, the relay will have 4 connections - 2 for power and 2 for the coil. If it is not marked then, it should be pretty easy to locate the power connection. One end will go to stuff near the AC line and the other end will go to the rectifier or maybe a fusable resistor or something like that. These will likely be beefier than the coil connections which will go between a transistor and GND or some low voltage, or maybe directly into a big microcontroller chip. Of course, the best thing would be to get the schematic but with monitors this may not be easy. Once you are sure of the AC connections - measure across them while it is off and also while it is on. While off, you should get 110-125 VAC. While on and working - 0. While on and not working either 110-125 VAC if the relay is not pulling in or 0 if it is and the problem is elsewhere. We can deal with the latter case if needed later on. Note the even if the relay contacts are not working, the problem could still be in the control circuitry not providing the correct coil voltage/current, though not likely. It may be expensive and/or difficult to obtain an exact replacement, but these are pretty vanilla flavored as relays go. Any good electronics distributor should be able to supply a suitable electrical replacement though you may need to be creative in mounting it.
A posistor is a combination of a PTC (positive temperature coefficient) resistor and another resistor-element to heat it up and keep it hot. Sometimes, these will go by the name posister or thermister. The heater is a disk shaped resistor across the power line and the themister is a disk shaped device in series with the degauss coil. They are in clamped together to be in close contact thermally. You can pry off the lid and see for yourself. The most common failure mode is for the part to short across the line. Its function is to control degauss, so the only thing you lose when you remove one of these is the degauss function on power-on. When you turn the TV or monitor on, the PTC resistor is cold and low resistance. When heated, it becomes very high resistance and turns off the degauss coil but gradually - the current ramps down to zero rather than being abruptly cut off.. Computer Component Source stocks a wide variety, I believe but it may be cheaper to go direct to the manufacturer if they will sell you one.Go to [Next] segment
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