Notes On The Troubleshooting And Repair Of Television Sets


  20.3) Troubleshooting tips

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.

  20.4) Test equipment

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

  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).

  20.5) Incredibly Handy widgets

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".

  20.6) Safe discharging of capacitors in TVs and video monitors

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.

  20.7) Additional information on discharging CRTs

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)

  20.8) Safe troubleshooting techniques for line powered TVs

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

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

   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.

  20.9) The series light bulb trick

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.

  20.10) Getting inside a TV

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.

  20.11) Specific considerations before poking around inside a TV

  20.12) Specific considerations before poking around inside a TV or monitor

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

  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.

  20.13) Dusting out the inside of a TV

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.

  20.14) Troubleshooting a TV with the mainboard disconnected

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.

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Written by Samuel M. Goldwasser. | [mailto]. The most recent version is available on the WWW server http://www.repairfaq.org/ [Copyright] [Disclaimer]