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Contents:
Don't start with the electronic test equipment, start with some analytical thinking. Your powers of observation (and a little experience) will make a good start. Your built in senses and that stuff between your ears represents the most important test equipment you have.
However, some test equipment will be needed:
 | Multitester (DMM or VOM) - This is essential for checking of power supply voltages and voltages on the pins of ICs or other components - service literature like the SAMs Photofacts described elsewhere in this document include voltage measurements at nearly every circuit tie point for properly functioning equipment. The multitester will also be used to check components like transistors, resistors, and capacitors for correct value and for shorts or opens. You do not need a fancy instrument. A basic DMM - as long as it is reliable - will suffice for most troubleshooting. If you want one that will last for many years, go with a Fluke. However, even the mid range DMMs from Radio Shack have proven to be reliable and of acceptable accuracy. For some kinds of measurements - to deduce trends for example - an analog VOM is preferred (though some DMMs have a bar graph scale which almost as good). |
| Oscilloscope - While many problems can be dealt with using just a multimeter, a
'scope will be essential as you get more into advanced troubleshooting. Basic requirements
are: dual trace, 10-20 MHz minimum vertical bandwidth, delayed sweep desirable but not
essential. A good set of proper 10x/1x probes. Higher vertical bandwidth is desirable but
most consumer electronics work can be done with a 10 MHz scope. A storage scope or digital
scope might be desirable for certain tasks but is by no means essential for basic
troubleshooting. I would recommend a good used Tektronix or HP scope over a new scope of almost any other brand. You will usually get more scope for your money and these things last almost forever. My 'good' scope is the militarized version (AN/USM-281A) of the HP180 lab scope. This has a dual channel 50 MHz vertical plugin and a delayed sweep horizontal plugin. I have seen these going for under $300 from surplus outfits. For a little more money, you can get a Tek 465 100 Mhz scope ($400-700) which will suffice for all but the most demanding (read: RF or high speed digital) repairs. |
| A video signal source - both RF and baseband (RCA jacks). Unless you are troubleshooting tuner or video/audio input problems, either one will suffice. RF sources include a pair of rabbit ears or an outdoor antenna, a cable connection, or a VCR with a working RF modulator. This will be more convenient than an antenna connection and will permit you to control the program material. In fact, making some test tapes using a camcorder or video camera to record static test patterns will allow you full control of what is being displayed and for how long. |
| Color bar/dot/crosshatch signal generator. This is a useful piece of equipment if you are doing a lot of TV or monitor repair and need to perform CRT convergence and chroma adjustments. However, there are alternatives that are almost as good: a VHS recording of these test patterns will work for TVs. A PC programmed to output a suitable set of test patterns will be fine for monitors (and TVs if you can set up the video card to produce an NTSC/PAL signal. This can be put through a VCR to generate the RF (Channel 3/4) input to your TV if it does not have direct video inputs (RCA jacks). |
These are the little gadgets and homemade testers that are useful for many repair situations. Here are just a few of the most basic:
| Series light bulb for current limiting during the testing of TVs, monitors, switching power supplies, audio power amplifiers, etc. I built a dual outlet box with the outlets wired in series so that a lamp can be plugged into one outlet and the device under test into the other. For added versatility, add a regular outlet and 'kill' switch using a quad box instead. The use of a series load will prevent your expensive replacement part like a horizontal output transistor from blowing if there is still some fault in the circuit you have failed to locate. |
| A Variac. It doesn't need to be large - a 2 A Variac mounted with a switch, outlet and fuse will suffice for most tasks. However, a 5 amp or larger Variac is desirable. If you will be troubleshooting 220 VAC equipment in the US, there are Variacs that will output 0-240 VAC from a 115 VAC line (just make sure you don't forget that this can easily fry your 115 VAC equipment.) By varying the line voltage, not only can you bring up a newly repaired TV gradually to make sure there are no problems but you can also evaluate behavior at low and high line voltage. This can greatly aid in troubleshooting power supply problems. WARNING: a Variac is not an isolation transformer and does not help with respect to safety. You need an isolation transformer as well. |
| Isolation transformer. This is very important for safely working on live chassis equipment. Since all modern TVs use a line connected power supply, it is essential. You can build one from a pair of similar power transformers back-to-back (with their highest rated secondaries connected together. I built mine from a couple of similar old tube type TV power transformers mounted on a board with an outlet box including a fuse. Their high voltage windings were connected together. The unused low voltage windings can be put in series with the primary or output windings to adjust voltage. Alternatively, commercial line isolation transformers suitable for TV troubleshooting are available for less than $100 - well worth every penny. |
| Variable isolation transformer. You don't need to buy a fancy combination unit. A Variac can be followed by a normal isolation transformer. (The opposite order also works. There may be some subtle differences in load capacity.). |
| Degaussing coil. Make or buy. The internal degaussing coil salvaged from a defunct TV doubled over to half it original diameter to increase its strength in series with a 200 W light bulb for current limiting will work just fine. Or, buy one from a place like MCM Electronics - about $15 for one suitable for all but the largest TVs. Also, see the section: "Degaussing (demagnetizing) CRTs". |
It is essential - for your safety and to prevent damage to the device under test as well as your test equipment - that large or high voltage capacitors be fully discharged before measurements are made, soldering is attempted, or the circuitry is touched in any way. Some of the large filter capacitors commonly found in line operated equipment store a potentially lethal charge.
This doesn't mean that every one of the 250 capacitors in your TV need to be discharged every time you power off and want to make a measurement. However, the large main filter capacitors and other capacitors in the power supplies should be checked and discharged if any significant voltage is found after powering off (or before any testing - some capacitors (like the high voltage of the CRT in a TV or video monitor) will retain a dangerous or at least painful charge for days or longer!)
The technique I recommend is to use a high wattage resistor of about 100 ohms/V of the working voltage of the capacitor. This will prevent the arc-welding associated with screwdriver discharge but will have a short enough time constant so that the capacitor will drop to a low voltage in at most a few seconds (dependent of course on the RC time constant and its original voltage).
Then check with a voltmeter to be double sure. Better yet, monitor while discharging (not needed for the CRT - discharge is nearly instantaneous even with multi-M ohm resistor).
Obviously, make sure that you are well insulated!
| For the main capacitors in a switching power supply which might be 100 uF at 350 V this would mean a 5K 10W resistor. RC=.5 second. 5RC=2.5 seconds. A lower wattage resistor can be used since the total energy in not that great. The circuit described below can used to provide a visual indication of polarity and charge. |
| For the CRT, use a high wattage (not for power but to hold off the high voltage which
could jump across a tiny 1/4 watt job) resistor of a few M ohms discharged to the chassis
ground connected to the outside of the CRT - NOT SIGNAL GROUND ON THE MAIN BOARD as
you may damage sensitive circuitry. The time constant is very short - a ms or so. However,
repeat a few times to be sure. (Using a shorting clip lead may not be a bad idea as well
while working on the equipment - there have been too many stories of painful experiences
from charge developing for whatever reasons ready to bite when the HV lead is
reconnected.) Note that if you are touching the little board on the neck of the CRT, you
may want to discharge the HV even if you are not disconnecting the fat red wire - the
focus and screen (G2) voltages on that board are derived from the CRT HV. If you are not going to be removing the CRT anode connection, replacing the flyback, or going near the components on the little board on the neck of the CRT, I would just stay away from the fat red wire and what it is connected to including the focus and screen wires. Repeatedly shoving a screwdriver under the anode cap risks scratching the CRT envelope which is something you really do not want to do. |
Again, always double check with a reliable voltmeter!
Reasons to use a resistor and not a screwdriver to discharge capacitors:
When powering up a TV (or any other modern electronic devices with expensive power semiconductors) that has had work done on any power circuits, it is desirable to minimize the chance of blowing your newly installed parts should there still be a fault. There are two ways of doing this: use of a Variac to bring up the AC line voltage gradually and the use of a series load to limit current to power semiconductors.
Actually using a series load - a light bulb is just a readily available cheap load - is better than a Variac (well both might be better still) since it will limit current to (hopefully) non-destructive levels.
What you want to do is limit current to the critical parts - usually the horizontal output transistor (HOT). Most of the time you will get away with putting it in series with the AC line. However, sometimes, putting a light bulb directly in the B+ circuit will provide better protection as it will limit the current out of the main filter capacitors to the HOT. Actually, an actual power resistor is probably better as its resistance is constant as opposed to a light bulb which will vary by 1:10 from cold to hot. The light bulb, however, provides a nice visual indication of the current drawn by the circuit under test. For example:
| Full brightness: short circuit or extremely heavy load - a fault probably is still present. |
| Initially bright but then settles at reduced brightness: filter capacitors charge, then lower current to rest of circuit. This is what is expected when the equipment is operating normally. There could still be a problem with the power circuits but it will probably not result in an immediate catastrophic failure. |
| Pulsating: power supply is trying to come up but shutting down due to overcurrent or overvoltage condition. This could be due to a continuing fault or the light bulb may be too small for the equipment. |
The following are suggested starting wattages:
| 40 W bulb for VCR or laptop computer switching power supplies. |
| 100 W bulb for small (i.e., B/W or 13 inch color) TVs. |
| 150-200 W bulb for large color or projection TVs. |
Depending on the power rating of the equipment, these wattages may need to be increased.
However, start low. If the bulb lights at full brightness, you know there is still a major
fault. If it flickers or the TV (or other device) does not quite come fully up, then it
should be safe to go to a larger bulb.
You will void the warranty - at least in principle. There are usually no warranty seals on a TV so unless you cause visible damage or mangle the screws, it is unlikely that this would be detected. You need to decide. A TV still under warranty should probably be returned for warranty service for any covered problems except those with the most obvious and easy solutions. Another advantage of using warranty service is that should your problem actually be covered by a design change, this will be performed free of charge. And, you cannot generally fix a problem which is due to poor design!
Getting into a TV is usually quite simple requiring the removal of anywhere from 4 to 16 Philips or 1/4" hex head screws - most around the rear edge of the cabinet or underneath, a couple perhaps in the middle. Disconnect the antenna and/or antenna or cable wiring first as it may stay with catch on the rear cover you are detaching. Reconnect whatever is needed for testing after the cover is removed.
As you pull the cover straight back (usually) and off, make sure that no other wires are still attached. Often, the main circuit board rests on the bottom of the cover in some slots. Go slow as this circuit board may try to come along with the back. Once the back is off, you may need to prop the circuit board up with a block of wood to prevent stress damage and contact with the work surface.
Most TVs can still be positioned stably on any of three sides (left, right, bottom) even without the rear cover. However, some require the cover for mechanical strength or to not easily fall over. Be careful- larger TVs, in particular, are quite heavy and bulky. Get someone to help and take precautions if yours is one of the unstable variety. If need be, the set can usually safely be positioned on the CRT face if it is supported by foam or a folded blanket.
Reassemble in reverse order. Getting the circuit board to slide smoothly into its slots may take a couple of attempts but otherwise there should be no surprises.
The first thing you will notice when you remove the cover is how super dusty everything is. Complements to the maid. You never dreamed there was that much dust, dirt, and grime, in the entire house!
Use a soft brush (like a new paintbrush) and a vacuum cleaner to carefully remove the built up dust. Blowing off the dust will likely not hurt the TV unless it gets redeposited inside various controls or switches but will be bad for your lungs - and will spread it all over the room. Don't turn anything - many critical adjustments masquerade as screws that just beg to be tightened. Resist the impulse for being neat and tidy until you know exactly what you are doing. Be especially careful around the components on the neck of the CRT - picture tube - as some of these are easily shifted in position and control the most dreaded of adjustments - for color purity and convergence. In particular, there will be a series of adjustable ring magnets. It is a good idea to mark their position in any case with some white paint, 'white out', or a Magic Marker so that if they do get moved - or you move them deliberately, you will know where you started.
For general viewing, subdued lighting but not total darkness is probably best. However, for most dramatic impact, a darkened environment may be preferred. Make the following adjustments under the expected viewing conditions.
Tune to a strong channel or play a good quality tape.
Turn the brightness, contrast, and color controls all the way down. Center the tint control (NTSC).
Increase the brightness until a raster is just visible in the darkest (shadow) areas of the picture.
Increase the contrast until the desired intensity of highlights is obtained.
Since brightness and contrast are not always independent, go back and forth until you get the best picture.
Initially adjust the color control for pastel shades rather than highly saturated color. Set the tint control for best flesh tones. Then, increase the color control to obtain the desired degree of color saturation.
On a decent TV, you should be able to make out the individual scanning lines. If they are fuzzy, especially in bright areas, then focus may need to be adjusted.
The focus pot is usually located on the flyback transformer or on an auxiliary panel nearby. The focus wire usually comes from the flyback or the general area or from a terminal on a voltage the multiplier module (if used). It is usually a wire by itself going to the little board on the neck of the CRT.
Let the set warm up for at least half an hour. Display a good quality signal. Turn the user color control all the way down and the brightness and contrast controls all the way up. This will be the worst case. Adjust the focus control for best overall sharpness - you may not be able to get it perfect everywhere - center as well as corners. If best focus is at one end of the focus pot's range and still not good enough, there may be a problem in the focus divider, focus pot, or some related component.
The screen should be adjusted with a white pattern (snow from the tuner should do or turn the user COLOR control all the way down to get a black and white picture). Put the set in Service mode (horizontal line) if it has such a switch in the back or inside. If not, just use the raster in a darkened room. Adjust screen for a dim white line (raster). If the line is not white at its dimmest point, you will need to adjust the drive and cutoff controls for R, G, & B.
Alternatively, you can use the following procedure:
(From LEE)
Turn R, G, and B screen (or background) controls down. Now turn color control fully counterclockwise -- off. Now turn up red screen until the screen just shows a red hue. Now turn red gun down until red tint just goes away. Now do the same with the green and blue screen controls. Now adjust the two DRIVE controls for the best black and white picture. That's all there is to it. I don't like to work with just a thin "SETUP" line. Cartoons seem to be the best thing to have on while doing the above procedure. You can also use just plain snow (no program) if you prefer. If you can obtain a good B&W pic. when you're done, the tube is good and the set if most likely functioning properly. Be patient and go slow while watching the large mirror that you are using during this procedure.
Color balance needs adjustment if the highlights and/or shadows of a black and white picture (turn the color control all the way down) are not a perfectly neutral gray.
To adjust the color balance: Turn the color control all the way down so that you get what should be a B/W picture. Set the user brightness and contrast controls about mid-range. The tint control should not matter (if it does at this point, you have other chroma problems or an 'autocolor' switch is on limiting the range of some controls).
Adjust the sub-brightness controls (may be called color screen, background, or the like) so that the dark areas of the picture are just visible and neutral gray. Then, adjust the color gain controls until the brightest areas are neutral white but not so bright that there is 'color bleeding' in the highlights.
This should get you close. If something is still shifting after warmup and get some cold-spray or even a little blower and try to locate the component that is drifting. Most likely a transistor or capacitor.
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