Notes on the Troubleshooting and Repair of Computer and Video Monitors


  7.4) 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, if you are making significant changes
that affect G2 (screen), or you are setting up a new or replacement CRT for
the first time, 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 monitor warm up for 20 minutes or so.

(From: Jeroen 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 service manual for the monitor --- 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.

  7.5) Position, size, and linearity adjustment

Position and size are usually user controls on computer and video monitors
but not on TVs.  On monitors with digital controls, they may usually be set
for each resolution and (automatically) stored in non-volatile memory so they
will be retained when the monitor is turned off.  On cheaper monitors, there
may be a knobs on the front or back panel and may need to readjusted whenever
the scan rate/resolution is changed.  Sometimes, there are located internally.
There may be separate adjustments for each scan range and may or may not be
accessible through holes in the back panel.

There may also be an adjustment called 'horizontal phase' which controls the
relative timing of the horizontal sync pulse with respect to retrace.  Its
effect is subtly different than horizontal position which actually moves the
raster.  If possible, center the raster and then use H Phase to center the

In monochrome monitors (mostly), position may be set via a pair of rings on the
neck of the CRT.

Size can be set to your preference for each scan rate (if they are
independent).  For computer work, slight underscan is often preferred as
all of the frame buffer is visible.  However, any slight geometric problems
with the raster will be all too visible when compared with the straight
sides of the CRT bezel.

Note that resolutions like 640 x 480, 800 x 600, and 1024 x 768 all have a
4:3 aspect ratio.  The edge of the image will line up with the bezel on most
if not all monitors since CRTs are made to a 4:3 aspect ratio.  However,
resolutions like 1280 x 1024 and 1600 x 1280 have a 5:4 aspect ratio.  With
these, in order to get (highly desireably) square pixels, the horizontal size
must be adjusted slightly smaller than that required to fill the screen.

For normal viewing of video (television) monitors, raster size should be set
so that there is about 10-15 percent overscan all around.  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 (CUE and
REV) modes.  However, for studio use, underscan is often preferred or at least
an option to permit the entire raster to be inspected.

Modern color monitors may not have any horizontal linearity control but you
may find this on older models.  There may be an internal vertical linearity
adjustment.  I am not aware of any that have user accessible linearity
controls.   If there are internal pots or coils, you will need to go back
and forth between size and linearity as these adjustments are usually not

Of course, parameters controlling your video card also affect position and
size.  There is no best approach to reconciling the effects of monitor and
video card position adjustments.  But, in general, start with the monitor
controls centered within their range or use the memory defaults as
appropriate.  Then, use the video card setup program to optimize the
settings.  Only if these do not have enough range should you use the
monitor controls.

  7.6) Pincushion adjustments

Horizontal pincushion refers to any bowing in or out on the vertical sides of
the raster.  There is not usually any explicit vertical pincushion adjustment.
Adjustment usually uses 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

  7.7) 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 a
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.
However, this may not be possible at reasonable cost or at all for many
computer monitors.

  7.8) Why is the convergence on my monitor 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 monitor.  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 typical effective writing rate of 50,000 mph across the face of a 17"
CRT (assumed resolution: 1024x768 at 75 Hz) in a variable magnetic environment
manufactured at a price you can afford without a second mortgage!

The specifications for misconvergence have two parts: a center error and a
corner error.  The acceptable center error is always the smaller of the
two - possibly .1-.2 mm. compared to .3-.5 mm in the corners.  Very often,
you will find that what you are complaining about is well within this

  7.9) CRT purity and convergence problems

Purity assures that each of the beams for the 3 primary colors - R, G, B, -
red, green, and blue - strikes only the proper phosphor 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.  There may even be
excess non-uniformity of pure white or gray images.

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 monitor 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 monitor use
that instead.  Or, plan to do the final tilt and position adjustments after
the monitor is in position - but this will probably require access to the

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

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
to be sure that your problems are not simply due to residual magnetism.

Assuming this doesn't help, you will need to set the internal purity
and/or convergence adjustments on the CRT.

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.

Note: if your monitor 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).

  7.10) 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 monitor, your procedure may substitute
green for red depending on the arrangement of guns in the CRT.  The procedures
for dot-mask, slot mask, and Trinitron (aperture grille) CRTs will vary
slightly.  See you service manual!

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 monitor 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.

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

  7.11) CRT convergence adjustment

In the good old days when monitors were monitors (and not just a mass
produced commodity item) there were literally drawers or panels 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.  However, some high end monitors do have user
accessible controls for minor adjustment of static (center) convergence.

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.  For PCs
(a similar approach applies to workstations) If you do not have a test
pattern generator, use a program like Windows Paint to create a facsimile
of a crosshatch pattern and use this for your convergence adjustments.
For a studio video monitor, any static scene (from a camcorder
or previously recorded tape, for example) with a lot of fine detail will

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.  (Set any user convergence controls to their
center position).

Adjust the center set of magnets on the CRT neck 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 monitor may have a slightly
different procedure.

Dynamic convergence adjusts for coincidence at the edges and corners.

On old tube, hybrid, and early solid state monitors, dynamic convergence was
accomplished with electronic adjustments of which there may have been
a dozen or more that were not independent.  With modern monitors, 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.

Remove the rubber wedges.

Loosen the deflection yoke clamp just enough so that it can be tilted but
will remain in the position you leave it.  Rock the yoke up and down to
converge the right and left sides of the screen.  Rock the yoke from side
to side to converge the top and bottom of the screen.  The rubber wedges
can be used as pivots to minimize the interaction between the two axes but
you may need to go back and forth to optimize convergence on all sides.
Reinstall the wedges firmly and tape them to the CRT securely.  Tighten the
yoke clamp enough to prevent accidental movement.

Some monitors may use a plastic frame and set screws instead of just a clamp
and rubber wedges but the procedure is similar.

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

For additional comments on convergence adjustments, see the section: "Tony's notes on setting convergence on older delta gun CRTs".

  7.12) Tilted picture

You have just noticed that the picture on your fancy (or cheap) monitor is not
quite horizontal - not aligned with the front bezel.  Note that often there is
some keystoning or other geometric distortion 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 (at least,
not without a lot of hassle), 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 monitor picture is not perfect!  BTW,
I can sympathize with your unhappiness.  Few things are more annoying than a
just noticeable imperfection such as this.

This is probably one reason why older monitors tended not to be able to expand
the picture to totally fill the screen - it is easier to overlook imperfect
picture geometry if there is black space between the edges of the picture and
the bezel!

There are several possible causes for a tilted picture:

1. Monitor orientation.  The horizontal component of the earth's magnetic field
   affects this slightly.  Therefore, if you rotate the unit 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 monitor 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?  Did your friend just visit boasting
   about his P8-1000 screamer and point the tilt out to you?

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.

   For example, on the Sony CPD1730, you press the left arrow button and blue
   '+' button at the same time.  Then adjust the tilt with the red buttons.

5. There is an internal tilt control that is misadjusted or not functioning.
   The existence 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 monitor 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 it 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!

  7.13) Monchrome monitor size, position, and geometry adjustments

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

On a monochrome (B/W) monitor you will probably see some of the following

1. Position - a pair of rings with tabs on the neck of the CRT.
   There may be electronic position adjustements as well.

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.  Where desired,
   set sizes 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.

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

Chapter 8) Low Voltage Power Supply Problems

  8.1) Low voltage power supply fundamentals

Monitors 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 (B+)
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.

Note: we will often use the term 'B+' to denote the main DC voltage that
powers the horizontal deflection system of most monitors.

The following are a couple of the typical arrangements found in color monitors:

1.  All low voltages except for the B+ to the horizontal output transistor
    (HOT) are derived from the horizontal deflection (flyback).  This
    is the scheme used in the majority of TVs.  Some kind of startup circuit
    gets the HOT booted but then all internal logic and video amplifier
    power is obtained from various windings on the flyback transformer.
    The B+ of anywhere from 60 to 130 V (higher for countries using 220 VAC
    line voltage) is likely to be regulated and its value selected based
    on the scan rate detected.  High voltage is obtained from the flyback.

2.  Some or all of the low voltages are provided by a switchmode power supply
    (SMPS) independent of the horizontal deflection.  Additional voltages may
    be provided from flyback windings as in (1).  Sometimes, this SMPS is
    a self contained and easily tested, swapped, and repaired unit.  In
    other cases, it is built onto the mainboard making it more difficult
    to trace the circuit and troubleshoot.  High voltage may be obtained
    from the flyback or a separate HV module.

For auto-scan monitors, the low voltage power supplies can get to be
quite complex as varying voltages are required for at least the horizontal
deflection based on scan range.  Separate regulators may be used for each
range which are switched by the microprocessor or a single regulator
may be programmed for the required voltages.  This is one area where
a typical PC monitor departs significantly in design compared to a TV
or fixed scan rate studio or workstation monitor.

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.
   Alternatively, an entire switchmode power supply may be used to provide
   one or more stable voltages to the horizontal deflection and other

Items (1) to (4) may be part of a separate low voltage power supply module
or located on the mainboard.

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.
   Alternative schemes including RC delays and relays are often used in
   monitors which have degaussing buttons.

7. A startup circuit for booting the horizontal deflection if various voltages
   to run the monitor 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.  Some monitors simply take the
   video input and use this via some simple logic or amplifier circuitry
   to drive the HOT.  With these, there will be no action of any kind if
   there is no input signal.  (Remember the old IBM PC monitors?  Unplug
   the video cable and the raster collapsed to a vertical line and then

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.  If the monitor does not have digital controls and/or
   has a hard on/off switch (not s soft touch button), this section is not

Always use an isolation transformer when working on a monitor but this is
especially important - for your safety - when dealing with the non-isolated
line operated power supply section.  Read and follow the safety guidelines.
Also see the document: "Notes on the Troubleshooting and Repair of Small
Switchmode Power Supplies" for additional tips and techniques when diagnosing
and testing switchmode power supplies.

  8.2) Monitor deflection derived power supply faults

Monitors use a variety of switching supply techniques and it would be
difficult to cover every possibility but here are some comments for
those that use deflection derived approaches:

Horizontal output transistor (usually a TO3 metal or TOP3 plastic case
shorts out.  This will usually blow a fuse or fusable resistor as well.

Horizontal drive chain - horizontal oscillator, driver, or driver transformer.
Newer monitors may use an IC for the oscillator and this can fail.

Startup - There may be some kind of startup circuit which gets the whole
thing going until the auxiliary voltages are available.  This could be
as simple as a multivibrator or transistor regulator to provide
initial voltage to the horizontal oscillator chip or circuit.

Output rectifier diodes can fail shorted and load down the outputs to
the point of shutting down.

Some load could be shorted or a capacitor could be shorted leading to
overload and shutdown.

Flyback transformer can have shorted windings which load down the output.
These (primary shorts in particular) may cause the horizontal output
transistor to fail as well.  Common problem with older MacIntosh computers
and video terminals.  Some secondary faults may not be instantly destructive
but result in little or no high voltage and overheating.

Also, look for cold solder joints - monitors tend to have these as a
result of temperature cycling and bad manufacturing.  (Is this sounding
repetitive yet?)

Sometimes there is a series regulator after the filter cap and this could
be bad as well.

Without a schematic, I would attempt to trace the circuit from the main
filter cap or output of the line operated switchmode power supply assuming
that has the proper (approx. 60-120 VDC depending on scan range) voltage.

If you can locate the horizontal output transistor, see if there is voltage
on its collector, should be the same.  If there is, then there is probably
a drive problem.  If you have an ECG or similar semi cross reference,
that will help you identify the ICs and transistors and locate the relevant
portions of the circuitry.

If there is no voltage at the horizontal output transistor, then there
is probably a blown fuse or bad connection somewhere or a fault in the
line operated SMPS if there is one.  However, the fuse may have blown
due to a fault in the SMPS or horizontal deflection.

  8.3) Power button on monitor is flakey

If the on/off (or other button) on the monitor itself behaves erratically
then the most likely cause is the obvious - the button or switch is dirty
or worn.  On a momentary pushbutton, if you can get at it, some contact
cleaner may help.

If power is controlled by a hard switch - a mechanical push-push latching
switch and this has become erratic due to worn contacts or just plain broken,
replacements may be available but often only directly from the original
manufacturer to physically fit.  As an alternative, consider mounting a small
toggle switch on the side of the cabinet to substitute for the defective one.
This will almost certainly be easier and cheaper - and quite possibly, more

  8.4) Monitor blows fuse

If the fuse really blows absolutely instantly with no indication that the
circuits are functioning (no high pitched horizontal deflection whine (if
your dog hides under the couch whenever the monitor is turned on, deflection
is probably working)), then this points to a short somewhere quite near
the AC power input.  The most common places would be:

        * Degauss Posistor - very likely.
        * MOV or other surge suppressor.
        * Horizontal output transistor (if deflection derived power supply)
        * Power supply regulator if there is one.
        * Switchmode power transistor if there is a line operated SMPS.
        * Diode(s) in main bridge
        * Main filter capacitor(s).

You should be able to eliminate these one by one.

Unplug the degauss coil as this will show up as a low resistance.

First, measure across the input to the main power rectifiers - it
should not be that low.  A reading of only a few ohms may mean a
shorted rectifier or two, a shorted Posistor, or a fried MOV.

* Test the rectifiers individually or remove and retest the resistance.

* Some monitors use a Posistor for degauss control.  This is a little cubical
  (about 1/2" x 3/4" x 1") component with 3 legs.  It includes a line
  operated heater disk (which often shorts out) and a PTC thermister to
  control current to the degauss coil.  Remove the posistor and try power.
  If the monitor now works, obtain a replacement but in the meantime you
  just won't have the automatic degauss.

If these test good, use an ohmmeter with the monitor unplugged to measure
the horizontal output transistor or SMPS switchmode transistor.  Even
better to remove it and measure it.

* C-E should be high in at least one direction.  (Both directions should be
  high if the transistor does not have an internal damper diode).

* B-E should be high in one direction and not shorted in the other or around
  50 ohms in both directions (typical for transistors with internal damper
  diodes) but should not be near 0.

If any readings are under 5 or 10 ohms, the transistor is bad.  The parts
sources listed at the end of this document will have suitable replacements.

If this tests bad, try powering the monitor first with your light bulb and if
it just flashes once when the capacitor is charging, put a proper fuse in
and try it.  The fuse should not blow with the transistor removed.

Of course, not much else will work either.

Install a new transistor and power the monitor using your series light bulb.
If the bulb now flashes once and then settles down to a low brightness level,
the monitor may be fine.  See if there is an indication of deflection and
HV - look for the glow of the CRT filaments and turn up the brightness to
see if there is any indication of a raster.  With the light bulb, not
everything will be normal but some life would be a good sign.  Even a
pulsating light bulb may just mean that the light bulb is too small for
the monitor power requirements.  It may be safe to try a higher wattage bulb.

If the bulb glows at close to full brightness, there is probably still some
fault elsewhere.  Don't be tempted to remove the light bulb just yet.
There could be problems with the driving circuits, flyback, secondary loads,
or with the feedback from the voltages derived from the horizontal not
regulating properly.

See if you can locate any other large power transistors in metal (TO3) cans
or large plastic (TOP3) cases.  There may be a separate power transistor
that does the low voltage regulation or a separate regulator IC or hybrid.
As noted, some monitors have a switchmode power supply that runs off
a different transistor than the HOT.  There is a chance that one of these
may be bad.

If it is a simple transistor, the same ohmmeter check should be performed.

If none of this proves fruitful, it may be time to try to locate a schematic.

A blown fuse is a very common type of fault due to poor design very often
triggered by power surges due to outages or lightning storms.  However,
the most likely parts to short are easily tested, usually in-circuit, with
an ohmmeter and then easily removed to confirm.

Occasionally, fuses simply tire of life and just replacing the fuse will
be all that is needed.

Even if it is more involved than this, if you find the problem and repair
it yourself, the cost is likely to be under $25.

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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]