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Many problems have simple solutions. Don't immediately assume that your problem is some combination of esoteric complex convoluted failures. For a TV, it may just be a bad connection or blown fuse. Remember that the problems with the most catastrophic impact on operation like a dead TV usually have the simplest solutions. The kind of problems we would like to avoid at all costs are the ones that are intermittent or difficult to reproduce: the occasional interference or a TV that refuses to play 'StarTrek Voyager'. If you get stuck, sleep on it. Sometimes, just letting the problem bounce around in your head will lead to a different more successful approach or solution. Don't work when you are really tired - it is both dangerous (especially with respect to TVs) and mostly non-productive (or possibly destructive). Whenever working on precision equipment, make copious notes and diagrams. You will be eternally grateful when the time comes to reassemble the unit. Most connectors are keyed against incorrect insertion or interchange of cables, but not always. Apparently identical screws may be of differing lengths or have slightly different thread types. Little parts may fit in more than one place or orientation. Etc. Etc. Pill bottles, film canisters, and plastic ice cube trays come in handy for sorting and storing screws and other small parts after disassembly. This is particularly true if you have repairs on multiple pieces of equipment under way simultaneously. Select a work area which is wide open, well lighted, and where dropped parts can be located - not on a deep pile shag rug. The best location will also be relatively dust free and allow you to suspend your troubleshooting to eat or sleep or think without having to pile everything into a cardboard box for storage. Another consideration is ESD - Electro-Static Discharge. Some components (like ICs) in a TV are vulnerable to ESD. There is no need to go overboard but taking reasonable precautions such as getting into the habit of touching a **safe** ground point first. WARNING: even with an isolation transformer, a live chassis should **not** be considered a safe ground point. When the set is unplugged, the tuner shield or other signal ground points should be safe and effective. A basic set of precision hand tools will be all you need to disassemble a TV and perform most adjustments. These do not need to be really expensive but poor quality tools are worse than useless and can cause damage. Needed tools include a selection of Philips and straight blade screwdrivers, socket drivers, needlenose pliers, wire cutters, tweezers, and dental picks. For adjustments, a miniature (1/16" blade) screwdriver with a non-metallic tip is desirable both to prevent the presence of metal from altering the electrical properties of the circuit and to minimize the possibility of shorting something from accidental contact with the circuitry. A set of plastic alignment tools will be useful for making adjustments to coils and RF transformers. A low power (e.g., 25 W) fine tip soldering iron and fine rosin core solder will be needed if you should need to disconnect any soldered wires (on purpose or by accident) or replace soldered components. A higher power iron or small soldering gun will be needed for dealing with larger components. See the document: "Troubleshooting and Repair of Consumer Electronics Equipment" for additional info on soldering and rework techniques. For thermal or warmup problems, a can of 'cold spray' or 'circuit chiller' (they are the same) and a heat gun or blow dryer come in handy to identify components whose characteristics may be drifting with temperature. Using the extension tube of the spray can or making a cardboard nozzle for the heat gun can provide very precise control of which components you are affecting. For info on useful chemicals, adhesives, and lubricants, see "Repair Briefs, an Introduction" as well as other documents available at this site.
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) a CRT".
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. WARNING: Most common resistors - even 5 W jobs - are rated for only a few hundred volts and are not suitable for the 25KV or more found in modern TVs and monitors. Alternatives to a long string of regular resistors are a high voltage probe or a known good focus/screen divider network. However, note that the discharge time constant with these may be a few seconds. Also see the section: "Additional information on discharging CRTs". 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: 1. It will not destroy screwdrivers and capacitor terminals. 2. It will not damage the capacitor (due to the current pulse). 3. It will reduce your spouse's stress level in not having to hear those scary snaps and crackles.
You may hear that it is only safe to discharge from the Ultor to the Dag. So, what the @#$% are they talking about? :-). (From: Asimov (mike.ross@juxta.mnet.pubnix.ten)). 'Dag' is short for Aquadag. It is a type of paint made of a graphite pigment which is conductive. It is painted onto the inside and outside of picture tubes to form the 2 plates of a high voltage filter capacitor using the glass in between as dielectric. This capacitor is between .005uF and .01uF in value. This seems like very little capacity but it can store a substantial charge with 25,000 volts applied. The outside "dag" is always connected to the circuit chassis ground via a series of springs, clips, and wires around the picture tube. The high voltage or "Ultor" terminal must be discharged to chassis ground before working on the circuit especially with older TV's which didn't use a voltage divider to derive the focus potential or newer TV's with a defective open divider. For more details, see the document: "TV and Monitor CRT (Picture Tube) Information.
TVs are particularly dangerous with respect to troubleshooting due to the fact
that a substantial portion of their circuitry - sometimes all of it - is
directly line connected. Even if your are working in a totally unrelated
area like the sound circuits, awareness of the general design and location
of the line-connected circuits can prove to be a life saver.
These designs may take several forms:
1. Separate switchmode power supply (SMPS). In this case, only the primary
side of the power supply is line connected. The remainder of the TV is
usually isolated from the line by the high frequency transformer and
feedback device (transformer or optoisolator) of the switchmode power
supply.
2. On-board SMPS - a portion of the circuitry on the mainboard is directly
line-connected. In the best case, this is restricted to the area around
the power cord connections and well marked on both top and bottom but don't
count on it. Again, the rest of the TV may be isolated but avoiding
hazardous areas is more difficult especially in cramped quarters.
3. Flyback derived power supply - a non-isolated linear (usually) power supply
provides B+ to the horizontal deflection (and startup circuit). All other
system power is derived from secondary windings on the flyback transformer.
Similar comments to (2) above apply.
(1) to (3) may be found in TVs with A/V inputs and outputs.
4. Full hot chassis - a bridge rectifier/filter capacitor/linear regulator
provides some voltages including B+. The flyback secondaries provide the
remaining voltages. All share a common return which is at the intersection
of two of the diodes of the bridge rectifier. There is no isolation.
This type of design will never be found in a TV where there are external
connections (other than the RF antenna/cable connector which can be
capacitively isolated). (However, you may actually get an AC reading
or even sparks between the RF shield and an earth ground due to this
capacitance.)
WARNING: Never attempt to add A/V inputs or outputs to such a TV as the
signals and shields will be electrically live.
Always use an isolation transformer, whatever kind of design is used in the
equipment you are troubleshooting. There are very few situations in which
an isolation transformer will hurt. If you use it automatically, you will
never have a chance to screw up.
Identify the appropriate ground point (return) for your multimeter or scope.
These should be marked in the Sams' Photofact or service manual. There may
be several such returns such as: non-isolated, signal, and CRT. Selecting
the wrong one - even momentarily connecting to it - can ruin your whole day.
If you are not using an isolation transformer (a no-no), connecting your
scope to the wrong ground point can result in (1) blown fuses and/or blown
parts, and a very dangerous situation and (2) readings that don't make sense
generally with distorted power line frequency signals of high amplitude.
* Use the non-isolated ground (A) (with your isolation transformer on the TV
*only* for measurements of voltage on the line-connected power supply.
* Use the signal ground (B) for all measurements of tuner, IF, video, and
sound circuits.
Whenever you get a reading or waveform that is grossly wrong, confirm that
you are using the proper ground point! Note that failures of fusable
resistors in the *return* of the HOT or power supply chopper or elsewhere
can also result in points that should be near ground floating at unexpected
voltage levels.
The general arrangement of components for a typical TV using a linear B+
supply with isolated auxiliary supplies for the signal circuits is shown
below including the (linear) line-connected power supply, horizontal
deflection output (drive, horizontal output transistor, flyback), and
a typical Aux power supply output.
Line fuse Main bridge Part of flyback
_ rectifier +----------+ B+ transformer
H o--_ --+------|>|---+---| |-----------------+ |:| Aux 1
| | | Filter, | )|:| +--|>|--+--o
| +---|>|---+ | REG, etc.| )|:|( _|_
115 VAC | | | | )|:|( ---
+--|---|<|---+ +----------+ +---+ |:|( |
| | | H-drive | |:| +-------+ B
+-> N o---------+---|<|---+---------+ transformer |/ C __|__
| A _|_ || +----| Horizontal -_-
+-> G - Power line earth ground /// ||( |\ E Output Signal
via building wiring ^ ||( | Transistor ground
| || +------+ (HOT)
' A _|_
Non-isolated return --> ///
(connected points)
For this power supply, what if?:
1. You connect your scope ground clip to the non-isolated ground (A) and you
are *not* using an isolation transformer?
Answer: you blow the line fuse and/or melt your scope probe ground lead.
Other parts may be damaged as well. In effect, you have just shorted
across the bottom diode of the bridge.
2. You attempt to monitor a video signal with your scope ground connected
to the non-isolated ground (A)?
Answer: you see only a highly distorted power line waveform of roughly
100 V p-p In effect, you are measuring across one of the diodes of the
bridge rectifier, stray capacitance, etc.
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. Note: for a TV or monitor, it may be necessary (and desirable) to unplug the degauss coil as this represents a heavy initial load which may prevent the unit from starting up with the light bulb in the circuit. 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. A 50/100/150 W (or similar) 3-way bulb in an appropriate socket comes in handy for this but mark the switch so that you know which setting is which! 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. Resist the temptation to immediately remove the bulb at this point - I have been screwed by doing this. Try a larger one first. The behavior should improve. If it does not, there is still a fault present. Note that some TVs and monitors simply will not power up at all with any kind of series load - at least not with one small enough (in terms of wattage) to provide any real protection. The microcontroller apparently senses the drop in voltage and shuts the unit down or continuously cycles power. Fortunately, these seem to be the exceptions.
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.
Both electrical and mechanical dangers lurk: * Main filter capacitor(s). This is the most dangerous (not the HV as you would expect). Fortunately, these capacitors will normally discharge in a few minutes or less especially if the unit is basically working as the load will normally discharge the capacitors nearly fully as power is turned off. With TVs, the main filter capacitor is nearly always on the mainboard. Monitors are more likely to have a separate power supply module. However, you should check across this capacitor - usually only one and by far the largest in the set - with a voltmeter and discharge as suggested in the section: "Safe discharging of capacitors in TVs and video monitors" if it holds more than a few volts (or wait longer) before touching anything. Some of these are as large as 1,000 uF charged to 160 V - about 13 w-s or a similar amount of energy as that stored in an electronic flash. This is enough to be potentially lethal under the wrong circumstances. * High Voltage capacitor formed by the envelope of the CRT. It is connected to the flyback transformer by the fat (usually red) wire at the suction cup (well, it looks like one anyhow) attached to the CRT. This capacitor can hold a charge for quite a while - weeks in the case of an old tube type TV! If you want to be doubly sure, discharge this also. However, unless you are going to be removing the HV connector/flyback, it should not bother you. The energy stored is about 1 w-s but if you touch it or come near to an exposed terminal, due to the high voltage, you will likely be handed *all* the energy and you *will* feel it. The danger is probably more in the collateral damage when you jump ripping flesh and smashing your head against the ceiling. Some people calibrate their jump based on voltage - about 1 inch/V. :-). There will be some HV on the back of the circuit board on the neck of the CRT but although you might receive a tingle but accidentally touching the focus or screen (G2) pins, it is not likely to be dangerous. * CRT implosion risk. Don't hammer on it. However, it is more likely that you will break the neck off the tube since the neck is relatively weak. This will ruin your whole day and the TV or monitor but will likely not result in flying glass everywhere. Just, don't go out of your way to find out. * Sharp sheet metal and so forth. This is not in itself dangerous but a reflex reaction can send your flesh into it with nasty consequences.
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.
There are times when it is desirable to remove the chassis or mainboard and work on it in a convenient location without having to worry about the equipment which will simulate the critical functions but this is rarely an option for the doit-yourselfer. My approach is usually to do as much work as possible without removing the main board and not attempt to power it up when disconnected since there are too many unknowns. Professionals will plug the chassis into a piece of equipment which will simulate the critical functions. Note that if you have a failure of the power supply - blown fuse, startup, etc., then it should be fine to disconnect the CRT since these problems are usually totally unrelated. Tests should be valid. However, if you really want to do live testing with the main board removed, here are some considerations. There are usually several connections to the CRT and cabinet: * Deflection yoke - since the horizontal coils are part of the horizontal flyback circuit, there could be problems running without a yoke. This could be anything from it appearing totally dead to an overheating or blown horizontal output transistor. There may be no problems. Vertical and any convergence coils may or may not be problems as well. * CRT video Driver board - pulling this should not usually affect anything except possibly video output and bias voltages. * CRT 2nd anode - without the CRT, there will be no capacitor to filter the high voltage and you would certaily want to insulate the HV connector **real** well. I do not know whether there are cases where damage to flyback could result from running in thie manner, however. * Front panel controls - disconnecting these may result in inability to even turn the set on, erratic operation, and other unexpected behavior. * Degauss - you just won't have this function when disconnected. But who cares - you are not going to be looking at the screen anyhow. * Remote sensor - no remote control but I doubt that the floating signals will cause problems. * Speakers - there will be no audio but this should not cause damage. If you do disconnect everything, make sure to label any connectors whose location or orientation may be ambiguous. Most of the time, these will only fit one way but not always.Go to [Next] segment
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