Notes On The Troubleshooting And Repair Of Television Sets


Chapter 21) TV Adjustment

  21.1) User picture adjustment

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

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, may not be present on PAL sets).

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.

  21.2) Internal adjustments

All of the service adjustments are accomplished either using controls inside
the set (mostly pots on the mainboard and CRT neck boards), or in most newer
TVs, mostly via a service menu accessed from the remote or by using a
manufacturer specific computer interface.

* A Web site with some information on the general objectives of video and color
  setup procedures for both direct view and projection TVs is:

  - http://www.Tru-line.com/ (Tru-line Video Technologies)

* Where actual pots are present, they may be labeled on the circuit boards or
  identified by a sticker on the TV's cover.  Otherwise, the service manual or
  Sams' for the set will be required unless their function of the relevant pot
  is obvious.

* For service menus accessed via the remote control, service information is
  almost a necessity since adjustment procedures vary widely and it is all too
  easy to totally mess things up - even to the point of inflicting serious and
  expensive damage to the set.

  For information on accessing the service menus if used on your model, see
  the section: "Setup adjustments lost - TV service codes".  However, even if
  the access procedure is known, get the service manual or Sams'!

* If a computer interface is required, you can most probably forget about
  attempting to adjust anything unless you find a friendly shop to provide the
  adapter and walk you through the procedure.  Why would they want to do this?
  Because they know you there is a good chance that you will have to pay them
  to unscramble the mess you created!

  21.3) Focus adjustment

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.  Where there are two adjustment knobs on the flyback
transformer, the top one is generally for focus and the bottom one is for G2.

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.

  21.4) Adjustment of the internal SCREEN and color controls

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:

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. (LEE)

  21.5) Optimal procedure for setting brightness/background and screen adjustments

For slight tweaks, the following is not necessary.  However, if someone
turned all the internal controls or if you are making significant changes
that affect G2 (screen), then following the procedure below is desirable
to achieve best performance and maximize life of the CRT.

The typical user controls - brightness and contrast can, of course, be set
arbitrarily, depending on video content and ambient lighting conditions.

Set the user brightness and contrast controls in the middle for the following
adjustments and let the set warm up for 20 minutes or so.

(From: Jeroen H. Stessen (Jeroen.Stessen@ehv.ce.philips.com)).

Now the screen control, that's another matter. It sets the voltage
on the second grid of the electron guns, typically between +500 and
+1000 V. You will want to use a well-isolated screwdriver for that
if it is a naked potentiometer. In the old days there used to be 3
separate potentiometers for 3 G2s, now there is generally only one.

Its purpose is to set the cutoff voltage for the guns, i.e. the
voltage between K and G1 at which the beam is just off.  The higher
you set the VG2, the higher VK - VG1 must be to cut off the beam.

If you set VG2 too low then your picture will be dark. You can
compensate for that with the brightness control, which in effect will
lower the VKs.  A disadvantage is that you will not get optimum
sharpness and peak brightness from your picture tube.

If you set VG2 too high then your picture will be bright. You can
compensate for that with the brightness control, which in effect will
raise the VKs. You might even get retrace lines which can usually
not be made to disappear with the brightness control. Another 
disadvantage is that you will not get optimum LIFETIME from your 
picture tube. With a too high cutoff voltage the cathode (electron 
emitting surface) will wear out too soon.

You will need to see the picture tube specifications (or possibly
the Sams' Photofact or service manual for the set --- sam) in order to
find the correct setting for the cutoff voltage. This is measured
as VK - VG1 (for each channel RGB) and is typically 130-160 V max.
There will be spread between the 3 channels, typically the highest
of the 3 measured values will be set against the upper limit.

The usual adjustment procedure is as follows:

* Use any low-level adjustments to set a black picture with all 3
  cathode voltages at the specified level (e.g. 130 V) above
  the VG1 voltage (may be 0 V or 12 V or 20 V ?).  (These are typically
  called RGB brightness, bias, or background level and are often on the
  little board on the neck of the CRT but not always --- sam).

* Adjust VG2 (screen) until one colour just starts too light up,
  turn it back down until the screen is just black again.

* Now adjust 2 of the 3 low-level black controls until the
  other 2 colours just light up, and then back to black again.

* Select a white picture and use 2 low-level white (RGB drive or gain, also
  generally on the neck board --- sam) controls to set the proper colour
  temperature for white to your own taste.

* Check your black calibration again, may have to iterate a bit.

  21.6) Color balance adjustment

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.

Note: Some TV designs (Zenith uses this in a few models) automatically balance
CRT cathode drive by sensing emission from the red, green, and blue guns using
a gray scale reference pulse outside the viewable picture.  If this is the
case with your set, there may be no user OR service adjustments :-(.  A color
balance problem in this case means either a failure of this circuitry or a CRT
where the emission from the 3 cathodes is so unbalanced (usually due to one
being much much weaker than the others) that compensation is not possible.

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.

  21.7) Horizontal position, size, and linearity adjustment

Horizontal position may be set via a switch or jumper, a pot, or (mostly
in B/W TVs) a set of rings on the CRT neck.

Horizontal size should be set so that there is about 10-15 percent
overscan left and right.  This will allow ample margin for power line
voltage fluctuations, component aging, and the reduction in raster size
that may occur with some VCR special effects (fast play) modes.

Many sets no longer have any horizontal size adjustments and depend on
accurate regulation of the voltage to the horizontal output stage
to control horizontal size.  There may be a B+ adjustment to perform

On those that do, the adjustment may either be done by setting the B+
voltage, by a pot, or a width coil in series with the horizontal
deflection coils.

Modern sets do not generally have any linearity control but you may find
this on older models.  You will need to go back and forth between size
and linearity as these adjustments are usually not independent.

Some of the newest sets control all these parameters via settings in
non-volatile memory and use service menus accessed via the remote control
for nearly all setup adjustments.

  21.8) Vertical position, size, and linearity adjustment

Vertical position may be set via a switch or jumper, a pot, or (mostly
in B/W TVs) a set of rings on the CRT neck.

Vertical size should be set so that there is about 10-15 percent
overscan top and bottom.  This will allow ample margin for power line
voltage fluctuations, component aging, and the reduction in raster size
that may occur with some VCR special effects (fast play) modes.

Some sets no longer have any vertical size adjustments and depend on
the accurate regulation of the voltage to the vertical output stage
to control vertical size.

On those that do, the adjustment is usually a pot in the vertical output
circuitry.  If your set has a linearity control, you will need to adjust
this in conjunction with the size control as these are usually not independent.

Some of the newest sets control all these parameters via settings in
non-volatile memory and use service menus accessed via the remote control
for nearly all setup.

  21.9) Pincushion adjustments

There may be two controls - amplitude and phase.  Pincushion amplitude
as its name implies, controls the size of the correction.  Pincushion
phase affects where on the sides it is applied.  Don't expect perfection.

If the controls have no effect, there is probably a fault in the pincushion
correction circuitry.

It is best to make these adjustments with a crosshatch or dot test pattern

  21.10) Geometry adjustment

This refers to imperfections in the shape of the picture not handled
by the pincushion and size adjustments.  These types of defects include
trapezoidal or keystone shaped raster and jogs or wiggles around the periphery
of the raster.  Unfortunately, one way these are handled at the factory is to
glue little magnets to strategic locations on the CRT and/or rotate little
magnets mounted on the yoke frame.  Unless you really cannot live with the
way it is (assuming there isn't something actually broken), leave these
alone!  You can end up with worse problems.  In any case, carefully mark the
position AND orientation of every magnet so that if this happens, you can
get back to where you started.  If the magnets are on little swivels, some
experimenting with them one at a time may result in some improvement.  Of
course, it is best to obtain a service manual and follow its instructions.

  21.11) Why is the convergence on my set bad near the edges

Very simple - nothing is quite perfect.  Perfect convergence is not
even necessarily possible in theory with the set of adjustments available
on a typical TV.  It is all a matter of compromises.  Consider what
you are trying to do: get three electron beams which originate from
different electron guns to meet at a single point within a fraction
of a mm everywhere on the screen.  This while the beams are scanning
at an effective writing rate of 20,000 mph across the face of a 27" CRT
in a variable magnetic environment manufactured at a price you can afford
without a second mortgage!

  21.12) CRT purity and convergence problems

Purity assures that each of the beams for the 3 primary colors - red, green,
and blue - strikes only the proper phosphor dots for that color.  A totally
red scene will appear pure red and so forth.  Symptoms of poor purity are
blotches of discoloration on the screen.  Objects will change shades of color
when the move from one part of the screen to another.

Convergence refers to the control of the instantaneous positions of the red,
green, and blue spots as they scan across the face of the CRT so that they are
as nearly coincident as possible.  Symptoms of poor convergence are colored
borders on solid objects or visible separate R, G, and B images of fine lines
or images,

Note: It is probably best to face the set East-West (front-to-back) when
performing any purity and convergence adjustments.  Since you probably do not
know what orientation will eventually be used, this is the best compromise
as the earth's magnetic field will be aligned mostly across the CRT.  This will
minimize the possible rotation of the picture when the unit is moved to its
final position but there may be a position shift.  Neither of these is that
significant so it probably doesn't really matter that much unless you are
super fussy.  Of course, if you know the final orientation of the TV in your
entertainment center - and you don't expect to be redecorating, use that
instead.  Or, plan to do the final tilt and position adjustments after the set
is in position - but this will probably require access to the inside!

First, make sure no sources of strong magnetic fields are in the vicinity of
the TV - loudspeakers, refrigerator magnets, MRI scanners, etc.  A nearby
lightning strike or EMP from a nuclear explosion can also affect purity.

Cycle power a couple of times to degauss the CRT (1 minute on, 20 minutes
off) - see the section: "Degaussing (demagnetizing) a CRT".  If the built
in degaussing circuits have no effect, use an external manual degaussing coil.

Assuming this doesn't help, you will need to set the internal purity
and/or convergence adjustments on the CRT.  Modern CRTs usually use a
combination of a series of magnetized moveable rings on the neck, and yoke
position and orientation to set purity and convergence.

First, mark the positions of all adjustments - use white paint, 'White out',
or a Magic Marker on the ring magnets on the neck of the CRT, the position
and tilt of the deflection yoke, and any other controls that you may touch
deliberately or by accident.

However, if your set is still of the type with a drawer or panel of knobs
for these adjustments, don't even think about doing anything without
a service manual and follow it to the letter unless the functions of all
the knobs is clearly marked (some manufacturers actually do a pretty good
job of this).

Note: some CRTs do not have any adjustable rings for purity (and static
convergence).  Either an internal structure in the neck of the CRT or an
external 'permalloy' sleeve is permanently magnetized at the factory and
there is not way of tweaking it in the field.  However, it may be possible
to use a normal set of magnet rings in addition to or in place of it to
correct for purity or convergence problems due to loss of magnetism due
to age or someone waving a 10 pound magnet near the CRT neck!

  21.13) CRT purity adjustment

Purity on modern CRTs is usually set by a combination of a set of ring
magnets just behind the deflection yoke on the neck of the CRT and the
position of the yoke fore-aft.  As always, mark the starting position of
all the rings and make sure you are adjusting the correct set if rings!

Use the following purity adjustment procedure as a general guide only.
Depending on the particular model TV, the following purity adjustment
procedure may substitute green for red depending on the arrangement of the
guns in the CRT.  This description is based on the Sams' Photofact for the RCA
CTC111C chassis which uses a slot-mask CRT.  The procedures for dot-mask
and Trinitron (aperture grille) CRTs will vary slightly.  See you service

Obtain a white raster (sometimes there is a test point that can be grounded
to force this).  Then, turn down the bias controls for blue and green so
that you have a pure red raster.  Let the set warm up for a minimum of
15 minutes.

Loosen the deflection yoke clamp and move the yoke as far back as it will go,

Adjust the purity magnets to center the red vertical raster on the screen.

Move the yoke forward until you have the best overall red purity.

Now, move the yoke forward until you have the best overall red purity.
Tighten the clamp securely and reinstall the rubber wedges (if you set
has these) to stabilize the yoke position.  Reset the video adjustments
you touched to get a red raster.

  21.14) CRT convergence adjustment

In the good old days when TVs were TVs (and not just a picture tube with
a little circuit board attached) there were literally drawers full of
knobs for setting convergence.  One could spend hours and still end up
with a less than satisfactory picture.  As the technology progressed,
the number of electronic adjustments went down drastically so that today
there are very few if any.

Unless you want a lot of frustration, I would recommend not messing with
convergence.  You could end up a lot worse.  I have no idea what is used
for convergence on your set but convergence adjustments are never
quite independent of one another.  You could find an adjustment that
fixes the problem you think you have only to discover some other area
of the screen is totally screwed.  In addition, there are adjustments
for geometry and purity and maybe others that you may accidentally move
without even knowing it until you have buttoned up the set.

Warning: Accurately mark the original positions - sometimes you will change
something that will not have an obvious effect but will be noticeable
later on.  So it is extremely important to be able to get back to where
you started.  If only red/green vertical lines are offset, then it is
likely that only a single ring needs to be moved - and by just a hair.
But, you may accidentally move something else!

If you really cannot live with it, make sure you mark everything very
carefully so you can get back to your current state.  A service manual is

Convergence is set using a white crosshatch or dot test pattern.  If you
do not have a test pattern generator, any static scene (from a camcorder
or previously recorded tape, for example) with a lot of fine detail will
suffice.  Turn the color control all the way down so you have a B/W picture.

Static convergence sets the beams to be coincident in the exact center of
the screen.  This is done using a set of ring magnets behind the purity
magnets on the CRT neck.

From the Sams' for the RCA CTC111C: "adjust the center set of magnets to
converge blue to green at the center of the screen.  Adjust the rear set
of magnets to converge red to green at the center of the screen."  Your
set may have a slightly different procedure.

Dynamic convergence adjusts for coincidence at the edges and corners.

On old tube, hybrid, and early solid state TVs, dynamic convergence was
accomplished with electronic adjustments of which there may have been
a dozen or more that were not independent.  With modern sets, all convergence
is done with magnet rings on the neck of the CRT, magnets glued to the CRT,
and by tilting the deflection yoke.  The clamp in conjunction with rubber
wedges or set screws assures that the yoke remains in position.

From the Sams' for the RCA CTC111C: "Loosen the screws at the 6 o'clock and
10 o'clock positions to permit the yoke to be tilted vertically.  Rock yoke
up and down to converge the right and left sides of the screen.  Tighten screw
at 6 o'clock and loosen screw at 3 o'clock to permit the yoke to be tilted
horizontally.  Rock yoke from side to side to converge the top and bottom
of the screen.  Tighten screws at 3 o'clock and 10 o'clock."

Many sets simply use the main clamp which locks the yoke to the neck of the
CRT in conjunction with rubber wedges between the yoke and the funnel of
the CRT to stabilize the yoke position position.

Refer to your service manual.  (Is this beginning to sound repetitious?)

For additional comments on convergence adjustments, see the sections: "Tony's notes on setting convergence on delta gun CRTs" and "Saga and general setup for large CRT TVs".

  21.15) Tilted picture

You have just noticed that the picture on your fancy (or cheap) TV
is not quite horizontal - not aligned with the front bezel.  Note that
often there is some keystoning as well where the top and bottom or left and
right edges of the picture are not quite parallel - which you may never
have noticed until now.  Since this may not be correctable, adjusting
tilt may represent a compromise at best between top/bottom or left/right
alignment of the picture edges.   You may never sleep again knowing that
your TV picture is not perfect!  BTW, I can sympathize with your unhappiness.
Nothing is more annoying than a just noticeable imperfection such as this.
However, since TVs always overscan, the only time you will really notice a
minor tilt without going out of your way to look for it is if there is text
or graphics near the edge of the screen.

There are several possible causes for a tilted picture:

1. Set orientation.  The horizontal component of the earth's magnetic field
   affects this slgithly.  Therefore, if you rotate the TV you may be able
   to correct the tilt.  Of course, it will probably want to face the wall!

   Other external magnetic fields can sometimes cause a rotation without any
   other obvious effects - have you changed the TV's location?  Did an MRI
   scanner move in next door?

2. Need for degaussing.  Most of the time, magnetization of the CRT will
   result in color problems which will be far more obvious than a slight
   rotation.  However, internal or external shields or other metal parts in
   the set could become magnetized resulting a tilt.  More extensive treatment
   than provided by the built-in degaussing coil may be needed.  Even, the
   normal manual degaussing procedure may not be enough to get close enough
   to all the affected parts.

3. You just became aware of it but nothing has changed.  Don't dismiss this
   offhand.  It is amazing how much we ignore unless it is brought to our
   attention.  Are you a perfectionist?

4. There is an external tilt control which may be misadjusted.  Newer Sony 
   monitors have this (don't know about TVs) - a most wonderful addition.
   Too bad about the stabilizing wires on Trinitron CRTs.  A digital control
   may have lost its memory accidentally.  The circuitry could have a problem.

5. There is an internal tilt control that is misadjusted or not functioning.
   The existance of such a control is becoming more common.

6. The deflection yoke on the CRT has gotten rotated or was not oriented
   correctly at the time of the set's manufacture.  Sometimes, the entire yoke
   is glued in place in addition to being clamped adding another complication.

   If the TV was recently bumped or handled roughly, the yoke may have been
   knocked out of position.  But in most cases, the amount of abuse required
   to do this with the yoke firmly clamped and/or glued would have totally
   destroyed the set in the process.

   There is a risk (in addition to the risk of frying yourself on the various
   voltages present inside as operating TV) of messing up the convergence
   or purity when fiddling with the yoke or anything around it since the yoke
   position on the neck of the tube and its tilt may affect purity and
   convergence.  Tape any rubber wedges under the yoke securely in place
   as these will maintain the proper position and tilt of the yoke while you
   are messing with it.  (Don't assume the existing tape will hold - the
   adhesive is probably dry and brittle).

7. The CRT may have rotated slightly with respect to the front bezel.
   Irrespective of the cause of the tilt, sometimes it is possible to
   loosen the 4 (typical) CRT mounting screws and correct the tilt by
   slightly rotating the CRT.  This may be easier than rotating the yoke.
   Just make sure to take proper safety precautions when reaching inside!

  21.16) B/W TV size, position, and geometry adjustments

These tend to be a lot simpler and less critical than for color monitors
or TV sets.

On a B/W TV you will probably see some of the following adjustments:

1. Position - a pair of rings with tabs on the neck of the CRT.
   There may be electronic position adjustements as well though this
   is not that common on small TVs.

2. Width and height (possibly linearity as well) controls.  There may be
   some interaction between size and linearity - a crosshatch test pattern
   is best for this.  Vertical adjustments are almost always pots while
   horizontal (if they exist) may be pots and/or coils.  Size will normally
   be set for 5-10% overscan to account for line voltage fluctuations and
   component drift.  Confirm aspect ratio with test pattern which includes
   square boxes.

3. Geometry - some little magnets either on swivels around the yoke or
   glued to the CRT.  If these shifted, the the edges may have gotten
   messed up - wiggles, dips, concave or convex shapes.  There may be
   a doxen or more each mostly affecting a region around the edge of the
   raster.  However, they will not be totally independent.

Check at extremes of brightness/contrast as there may be some slight
changes in size and position due to imperfect HV regulation.

There may be others as well but without a service manual, there is no
way of knowing for sure.  Sams' often has folders for B/W TVs.

Just mark everything carefully before changing - then you will be able to
get back where you started.

Chapter 22) Low Voltage Power Supply Problems

  22.1) Low voltage power supply fundamentals

TVs require a variety of voltages (at various power levels) to function.
The function of the low voltage power supply is to take the AC line input
of either 115 VAC 60 Hz (220 VAC 50 Hz or other AC power in Europe and
elsewhere) and produce some of these DC voltages.  In all cases, the power
to the horizontal output transistor of the horizontal deflection system
is obtained directly from the low voltage power supply.  In some cases,
a variety of other DC voltages are derived directly from the AC line by
rectification, filtering, and regulation.  In other designs, however, most
of the low voltages are derived from secondary windings on the flyback
(LOPT) transformer of the horizontal deflection system.  In still other
designs, there is a separate switchmode power supply that provides some or
all of these voltages.  There are also various (and sometimes convoluted)
combinations of any or all of the above.

There will always be:

1. A power switch, relay, or triac to enable main power.

2. A set of rectifiers - usually in a bridge configuration - to turn the
   AC into DC.  Small ceramic capacitors are normally placed across the
   diodes to reduce RF interference.

3. One or more large filter capacitors to smooth the unregulated DC.  In
   the U.S., this is most often a voltage around 150-160 V DC.  In countries
   with 220 VAC power, it will typically be around 300-320 V DC.

4. A discrete, hybrid, or IC regulator to provide stable DC to the horizontal
   deflection system.  Sometimes feedback from a secondary output of the
   flyback or even the high voltage is used.  This regulator may be either
   a linear or switching type.  In some cases, there is no regulator.

5. Zero or more voltage dividers and/or regulators to produce additional
   voltages directly from the line power.  This relatively rare except for
   startup circuits.  These voltages will not be isolated from the line.

6. A degauss control circuit usually including a thermistor or Posistor
   (a combination of a heater disk and Positive Temperature Coefficient (PTC)
   thermistor in a single package). When power is turned on, a relatively
   high AC current is applied to the degauss coil wrapped around the periphery
   of the CRT.  The PTC thermister heats up, increases in resistance, and
   smoothly decreases the current to nearly zero over a couple of seconds.

7. A startup circuit for booting the horizontal deflection if various voltages
   to run the TV are derived from the flyback.  This may be an IC or discrete
   multivibrator or something else running off a non-isolated voltage or
   the standby power supply.

8. A standby power supply for the microcontroller and remote sensor.  Usually,
   this is a separate low voltage power supply using a small power transformer
   for line isolation.

Always use an isolation transformer when working on a TV but this is
especially important - for your safety - when dealing with the non-isolated
line operated power supply.  Read and follow the information in the section:
"Safety guidelines".

  22.2) Typical TV power supply front end

The partial schematic below is similar to those found in the majority of
TVs sold in countries with 110 to 120 VAC power.  Many parts are not shown
including the power switch or relay, RFI bypass capacitors across the
rectifier diodes, and RFI line filter.

                                         Bypass resistor
     Line fuse   Main bridge  Fusable    +----/\/\-----+
           _      rectifier   resistor   |   +-----+   |
     H o--_ --+------|>|---+---/\/\--+---+---| REG |---+---+---o B+
              |            |         |       +-----+       |
              |  +---|>|---+     C1 _|_ Main    |         _|_ Regulator
   115 VAC    |  |           400 uF --- filter  |         --- output
              +--|---|<|---+  200 V  |  cap     |          |  capacitor
                 |         |         |          |          |
 +-> N o---------+---|<|---+---------+----------+----------+---o Non-isolated
 |                                                                 return
 +-> G - Power line earth ground via building wiring

* The line fuse is typically 2 to 4 A, usually a normal fast blow type.  Even
  so, it may not blow as a result of faults down the line - the fusable
  resistor or regulator may fail first.

* The main bridge rectifier is often composed of 4 discrete diodes (similar to
  1N400Xs) but may also be a single unit.  Failures - usually shorted diodes -
  are common.

* The main filter capacitor can range in size from 200 to 800 uF or more
  at 200 to 250 V.  THIS CAN BE LETHAL!  A typical TV may continue to work
  at normal line voltage without any noticeable degradation in performance
  (hum bars, hum in sound, or shutdown) even if this capacitor is reduced
  in value by 75%.  Its uF value is therefore not critical.

* The regulator is often an IC or hybrid module.  Failures resulting in no
  or reduced output, or no regulation are common.

* The regulator output capacitor is needed for the B+ regulator to function
  properly.  If this capacitor is reduced in value or develops a high ESR,
  regulation may fail resulting in instability, oscillation, or excessive
  B+ and shutdown.

* The regulator bypass resistor reduces the amount of current control needed
  of the regulator.  Caution: even if the regulator has been pulled, the B+
  line will have substantial voltage as a result of this resistor.
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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]