Notes on the Troubleshooting and Repair of Computer and Video Monitors


  11.37) Focus drift with warmup

This could be due to a problem with the focus voltage power supply, components
on the CRT neck board, or a tired worn CRT.

Focus is controlled by a voltage of 2-8 KV DC usually derived from the flyback
transformer and includes some resistors and capacitors.  One of these could
be changing value as it warms up. (assuming nothing else changes significantly
as the unit warms up - e.g., the brightness does not decrease.)

Focus voltage is derived from a subset of the high voltage winding on the
flyback using a resistive voltage divider which includes the focus pot.
These are extremely high value resistors - 200 M ohm is common - and so
leakage of any kind can reduce or increase the focus voltage.  All other
things being ok - i.e., the picture is otherwise fine - I would suspect this
type of failure rather than the CRT.

The connection to the CRT is usually a separate wire running from the flyback
or its neighborhood to the CRT neck board.  Look for components in this
general area.  Use cold spray or a heat gun to isolate the one that is
drifting.  If you have access to a high voltage meter, you should be able
to see the voltage change as the TV or monitor warms up - and when you cool
the faulty part.  If it is in the flyback, then sometimes the part with the
adjustments clips off and can be repaired or cleaned.  Most often, you will
need to replace the flyback as a unit.

* If the optimal adjustment point of the focus control doesn't change that
  much but the best focus is simply not as good as it should be, the CRT is
  probably the problem.  However, if the optimal point produces acceptable
  focus but it changes (and possibly moves off of one end of the adjustment
  knob range) as the unit warms up, the flyback or one of the components on
  the CRT neck board are likely drifting.

* If you have a high voltage meter, you can measure the focus voltage to 
  determine if it is being changed by the focus pot and if it is in the
  ball park (2-8 KV typical).  Sometimes, the part of the flyback with the
  focus pot can be snapped off and cleaned or parts replaced but usually you
  need to replace the whole unit.  There may a capacitor or two on the PCB on
  the neck of the CRT that could have increased leakage as well thus reducing
  the focus voltage.

* To determine if the CRT is the problem, for sharp focus after the unit has
  warmed up.  Power-off for an hour or so and carefully pull the CRT neck board
  off of the CRT.  Then, power up the unit.  Let it run long enough such that
  there would have been a detectable focus drift.  Now, power-down, plug the
  CRT neck board back in, and power-up.  Watch the image as it appears on the

  - If the focus starts out fuzzy and sharpens up as the image appears and
    gradually becomes sharper as the CRT warms up the CRT is likely tired.

    The only catch here is that plugging the CRT neck board into the CRT
    results in an additional load on the flyback due to the picture beam
    current which heats it more as well.  Thus, if the problem takes a few
    minutes to appear, keep the brightness turned down except to check the
    appearance of the picture from time to time. 

    You can set the focus control for optimum when warmed up and just turn
    the monitor on in advance of when you will be needing it or add a user
    focus adjustment by drilling a hole in the plastic case for an *insulated*
    screwdriver or flyback focus knob extender :-).  The CRT may continue
    to function for quite a while so this is not impending doom.

  - If the focus is relatively stable as the image appears and increases
    in brightness *and* is about as sharp as it would be with the monitor
    warmed up, the problem is most likely in the flyback.  However, also 
    check for bad components or decayed (tan or brown) glue on the CRT neck
    board.  A drifting flyback will need to be replaced as it will probably
    get worse and fail completely.  Clean the surface of the circuit board and
    CRT socket in the vicinity of the focus and screen terminals and traces.
    Contamination or just dirt and grime can easily cause problems especially
    on humid days since the resistance of these circuits is extremely high
    (100s of M ohms).

  - If the focus is relatively stable as the image appears and increases
    in brightness *and* is similar to what it would be with the monitor cold,
    you have a very strange situation where some load on the high voltage
    power supply, perhaps, is causing a thermal problem.  This would be rare.

  11.38) About the quality of monitor focus

Question: I have 2 identical monitors.  One is razor sharp from edge to edge.
The other is blurred at the corners- not from convergence problems,
but just plain out of focus.  In this monitor, the focus adjustment on
the flyback can improve the focus at the edges, but then the center of
the screen becomes worse..My question is : Is this a problem in the
electronics and presumably a fixable flaw or is it caused by variance
in the picture tube itself and not correctable ?  Or is it some other issue?

(From: Bob Myers (myers@fc.hp.com)).

The adjustment on the flyback sets the "static" focus voltage, which is
a DC voltage applied to the focus electrode in the CRT.  However, a single
fixed focus voltage will not give you the best focus across the whole CRT
screen, for the simple reason that the distance from the gun to the screen
is different at the screen center than it is in the corners.  (The beam
SHAPE is basically different in the corners, too, since the beam strikes
the screen at an angle there, but that's another story.)  To compensate
for this, most monitors include at least some form of "dynamic" focus, which
varies the focus voltage as the image is scanned.  The controls for the
dynamic focus adjustment will be located elsewhere in the monitor, and 
will probably have at LEAST three adjustments which may to some degree
interact with one another.  Your best bet, short of having a service
tech adjust it for you, would be to get the service manual for the unit
in question.

It is also possible that the dynamic focus circuitry has failed, leaving 
only the static focus adjust.

As always, DO NOT attempt any servicing of a CRT display unless you are
familiar with the correct procedures for SAFELY working on high-voltage
equipment.  The voltages in even the smallest CRT monitor can be lethal.

  11.39) Bad focus and adjustment changes brightness

This is the classic symptom of a short between the focus and screen
supplies - probably in focus/screen divider which is part of the flyback
or tripler.  However, it could also be in the CRT.  If you have a high
voltage meter, measuring the focus voltage will show that (1) it is low
and (2) it is affected by the SCREEN control Similarly, the SCREEN voltage
will be affected by the FOCUS control (which is what is changing the

To determine if the problem is in the CRT, measure the FOCUS and SCREEN
voltage with a high voltage meter.  If they are identical pull the plug
on the CRT.  If they are now their normal values, then a shorted CRT is
a distinct possibility - see the section: "Rescuing a shorted CRT".

  11.40) Charlie's comments on focus problems

(From: Charles Godard (cgodard@iamerica.net)).

Most true focus problems that I have encountered (when the IHVT is ok) are
related to leaks or resistance on the focus output.  The diming of the screen
when the focus pot is adjusted leads me to think in terms of a leaky socket.
I'd remove the ground from the crt socket to the tube dag and see if it
sparks.  If so there may be a leak in the socket to ground.  It could also be
leaking to another pin, such as the screen grid. A rhetorical question: What
happens to the screen voltage when the focus pot is adjusted?

I have seen sockets that had no arching or other telltale signs, leak through
the plastic housing to ground out the focus voltage.

Look closely at the screen.  If the blurring is in the form of small circles,
then you have an open or hi-resistance focus electrode inside the tube.  The
circles may vary in visibility with brightness.

If you still haven't found the problem, try to confirm that this is truly a
focus problem.  Remove the crt socket and observe the hi-voltage.  If it
climbs more than about 1k, say all the way up to 25kv, then you may have a
beam current problem rather than a focus problem.  In that case re-check all
crt board voltages.

If you have done all of the above and removing the socket makes no change in
the hi-voltage, then try to determine why the hi-voltage is low.

Watch the screen as the brightness, contrast, or screen control are adjusted.
See if you can observe any signs of blooming.  When the IHVT doesn't provide
enough current to satisfy the demands of the tube for current, the the picture
tends to appear to expand like a balloon. i.e., bloom.  This can be caused by
not enough drive to the IHVT. Carefully monitor the b+ to the horizontal drive
stages to see that is is stable and correct.

  11.41) Purple blob - or worse

Have you tried demagnetizing it?  Try powering it off for a half hour, then
on.  Repeat a couple of times.  This should activate the internal degausser.
See the section: "Degaussing (demagnetizing) a CRT".

Is there any chance that someone waved a magnet hear the tube?  Remove it
and/or move any items like monster speakers away from the set.

Was your kid experimenting with nuclear explosives - an EMP would magnetize
the CRT.  Nearby lightning strikes may have a similar effect.

If demagnetizing does not help, then it is possible that something shifted
on the CRT - there are a variety of little magnets that are stuck on at the
time of manufacture to adjust purity.  There are also service adjustments
but it is unlikely (though not impossible) that these would have shifted
suddenly.  This may be a task for a service shop but you can try your
hand at it if you get the SAMs Photofact or service manual - don't attempt
purity adjustments without one.

If the monitor was dropped, then it is even possible that the internal
shadow mask of the CRT has become distorted and you now have a seventy-five
pound boat anchor.  If the discoloration is slight, some carefully placed
'refrigerator' magnets around the periphery of the tube might help.  See the
section: "Magnet fix for purity problems - if duct tape works, use it!"

It is even possible that this is a 'feature' complements of the manufacturer.
If certain components like transformers are of inferior design and/or are
located too close to the CRT, they could have an effect on purity.  Even if
you did not notice the problem when the monitor was new, it might always have
been marginal and now a discoloration is visible due to slight changes or
movement of components over time.

  11.42) Color rings - bullseye pattern

This probably means the degaussing circuitry is terminating suddenly instead
of gradually as it should.  The most likely cause is a bad solder connection
to the degauss thermistor or posistor or something feeding it.

You can confirm this by manually degaussing the screen with the TV or monitor
turned on.  If the problem disappears, the above diagnosis is probably valid.
Check for bad solder connections in the vicinity of the degauss components
and AC line input.

  11.43) Magnet fix for purity problems - if duct tape works, use it!

The approach below will work for slight discoloration that cannot be eliminated
through degaussing.  However, performing the standard purity adjustments
would be the preferred solution.  On the other hand, the magnets may be quick
and easy.  And, where CRT has suffered internal distortion or dislocation of
the shadowmask, adjustments may not be enough.

In any case, first, relocate those megablaster loudspeakers and that MRI
scanner with the superconducting magnets.

The addition of some moderate strength magnets carefully placed to reduce or
eliminate purity problems due to a distorted or dislocated shadowmask may be
enough to make the monitor usable - though it will probably not be perfect.
The type of magnets you want are sold as 'refrigerator magnets' and the like
for sticking up notes on steel surfaces.  These will be made of ferrite
material (without any steel) and will be disks or rectangles.  Experiment
with placement using masking tape to hold them in place temporarily.  Degauss
periodically to evaluate the status of your efforts.  Then, make the 'repair'
permanent using duct tape or silicone sealer or other household adhesive.

Depending on the severity of the purity problem, you may need quite a few
magnets!  However, don't get carried away and use BIG speaker or magnetron
magnets - you will make the problems worse.

Also note that unless the magnets are placed near the front of the CRT, very
significant geometric distortion of the picture will occur - which may be a
cure worse than the disease.

WARNING: Don't get carried away while positioning the magnets - you will be
near some pretty nasty voltages!

(From: Mr. Caldwell (jcaldwel@iquest.net)).

I ended up with the old 'stuck on a desert island trick':

I duck taped 2 Radio Shack magnets on the case, in such a way
as to pull the beam back.!!!!

A $2 solution to a $200 problem.  My friend is happy as heck.

RCA sells magnets to correct corner convergence, they are shaped like chevrons 
and you stick them in the 'right' spot on the rear of the CRT.

(From: Tom Sedlemyer (wesvid@gte.net)).

First set purity as best you can.

Obtain some pieces of refrigerator door magnet strips from an appliance
repair shop (they usually have some lying around).

Cut the strips into 1 inch pieces.  Place a strip as on the bell of the
picture tube as close to the yoke as possible and in line with the corner that
has the purity error.   Rotate the magnet until you correct the purity error
and tape it in place. Multiple magnet strips can be used and you may
experiment with the size of the strips for best effect.  It is very important
that the strips are positioned close to the yoke or the effect will not hold.
The only drawback to this method is some very slight distortion of the
geometry of the raster, but it beats hell out of paying for a new CRT.

  11.44) Color monitor only displays one color

I assume that now you have no other colors at all - no picture and no
raster.  Let us say it is red - R.

It is probably not the CRT.  Do you have a scope?  Check for the R, G,
and B video signals at the CRT.  You will probably find no signals
for the defective colors.

This is almost certainly a chroma circuit problem as any failure of the
CRT or a video driver would cause it to lose a single color - the other
two would be ok.  Therefore, it is probably NOT the CRT or a driver on
the little board on the neck of the CRT.

Try turning up the SCREEN control to see if you can get a G and B raster
just to confirm that the CRT is ok.

Locate the video drive from the mainboard for the good and a bad color.
Interchange them and see if the problem moves.  If so, then there is
a video signal problem.  If not, it is on the little CRT board.

It could be a defective chroma IC or something else in the chroma decoder.

  11.45) Disappearing Red (or other color)

Problem: I have been given an old colour TV.  The reception is good, but very
often, when the contrast and brightness of the TV image is low (e.g. when
a night scene is shown), the red colour slowly disappears, leaving behind
the green and blue image and many red lines.

The remaining red retrace are the giveaway that this is most likely not
a CRT problem.

(If there were no red lines, it could be the filament for the red gun
of the CRT going on and off due to a bad connection inside the CRT - bad

How is a black and white picture?  (Turn down the color control).

If B/W picture is good, then the problem is somewhere back in the chroma
decoder circuitry.

Check the video input to the CRT video driver board and signals on that board.
If B/W picture is also bad, then you can compare red and green signals
to determine where they are becoming different.  The red lines in your
description sounds like the red video output circuit is drifting and messing
up the background level, blanking, screen, or other setting.  Could be a
capacitor or other component.

  11.46) Interference resulting in jiggling or wiggling

Note: similar symptoms can be the result of a monitor defect or running the
monitor at scan rate beyonds its capabilities.  However, magnetic interference
from electrical wiring, other equipment is very common and sometimes overlooked
when looking for a complex, expensive, and obscure explanation for a
misbehaving monitor (or TV).

  11.47) Interference from electrical wiring

If the wiring of normal outlets is done correctly even without a safety
ground, the currents should be balanced and you will not experience a problem.
However, many circuits, particularly those involving setups like 3-way
switches or switched outlets and wiring in older buildings can have
unbalanced currents when active.   If your monitors are close enough
to the wiring, there can be interference which will take the form of
a flickering or pulsating display.

Other than recommending moving the monitors, there is no easy solution.
They can be shielded with Mu Metal but that is expensive.  Or you could
run all displays at a 60 Hz vertical rate (or 50 Hz depending on where
you live).  However, this is inconvenient and will never be quite perfect.

If you have flexibility during construction or renovation, there are ways to
minimize the chance of unexpected behavior later:

Think of it this way: If the sum of the currents in the cable are zero, there
will be no magnetic field to worry about.  This will be the case for normal
110 VAC branch circuits.

Some sources for magnetic interference:

* Three (or more) way circuits - lamps or fixtures controlled from more than
  one location which use a 'traveler'.  In this case, a single energized wire 
  runs between switches and/or the switches and the load.

* Circuits which do not have their return in the same cable.  For example,
  ceiling fixtures controlled from a wall switch but where the Hot comes
  from another location.  Or, a string of baseboard heaters fed from opposite

* Circuits which share a Neutral but where one or more of the Hots are not in
  the same cable.  This is more likely to be found in old construction using
  knob-and-tube wiring where circuits were just connected in the most
  convenient way.

* Loops in Neutral and Ground conductors.  The way circuits are supposed to
  be wired (U.S.A. at least) is nearly always in a star sort of configuration
  where the Neutral and Ground conductors never connect at the ends of the
  'star'.  However, due to poor wiring practices, it is quite possible for
  Neutrals to be connected to other Neutrals or Grounds to be connected to
  other Grounds or for them to be cross connected at various locations - all
  without any other symptoms.  This can even happen between buildings.  See
  the section: "Interference from cross-connected buildings".  However, the
  likelihood of this sort of fault isn't that great.

First confirm that the problem is due to inside wiring - shut off all power to
the building (if possible) or at least switch off each circuit in turn to see
if the problem disappears (run the monitor from a UPS or a remote outlet).

* If the symptoms persist, check for external sources of interference
  (although there could still be a Ground-Neutral loop formed by the connection
  between G and N at the service panel or to other buildings.  In this case,
  the effect would likely be strongest near the service panel.).  See the
  section: "Interference from power lines".

* If the symptoms are gone, try to narrow down the circuit or circuits that
  are responsible by switching each one on individually.

In all cases, running the Hots and Neutrals for the circuit in the same cable
(or at least in close proximity) will avoid this problem as the total current
will sum to zero.

Realistically, you would have to be very unlucky to have a noticeable problem
in residential wiring except near the service panel or high power appliances
like baseboard heaters, equipment with large motors or transformers, etc.

  11.48) Interference from power lines

Power lines (any size from local distribution to large intercontinental
transmission lines) nearby can result in noticeable effects to monitors as a
result of the magnetic fields surrounding the individual wires - similar to
that from unbalanced inside wiring (see the section: "Interference from electrical wiring".  TVs may not be affected, at least not as much, since they
will be running at a vertical rate almost the same as the power line

The severity of the effects will vary depending on the load distribution on
the three (probably) phases, distance, orientation with respect to the
monitor, etc.  Moving the monitor as far from the offending power lines as
possible, experimenting with its orientation, and seeing if you can live with
a vertical scan rate equal to the power line frequency, are the only realistic
options other than constructing an expensive mu-metal box for it.

  11.49) Interference from cross-connected buildings

Here is a rare case where the neighbor was really at fault (in a historical
sort of way).

(From: Tuyen Tran (ttran@ziplink.net)).

Get this: my house and my neighbor's house were grounded together, so we
connected to the power company's neutral in two places. The way I understand
it, this caused a ground loop between our two panels. My neighbors used to own
this place. When they built a small house next door, instead of digging a
separate well, they just ran a 3/4 inch copper pipe between my water tank and
their new place.  (This place used to be a dairy farm, so it had plenty of
water capacity.) When they installed their panel, the electrician of course
bonded their water pipes to the panel, which then connected our two grounds
together. When they sold the place, they put in their own well, but nobody
bother to cut the original pipe linking the two houses together.  It's been
like this for at least 40 years; I'm the third owner!

So I took a pipe cutter to the thing, and no more interference.

  11.50) Interference from other equipment

Any type of equipment which uses or generates strong magnetic fields can
interfere with a monitor.  Other computer monitors or TVs, equipment with
power transformers, and electric motors will cause a pulsating or flickering
display.  Loudspeakers or other equipment with static magnetic fields will
cause color purity and/or geometric distortion problems which degauss will
not cure.

The easiest way to confirm that interference is your problem is to move
the monitor or suspect equipment to a different location.  The only real
solution is to separate the monitor and interfering device.

Note that with scan rates that are not even near the power line frequency
any more, a variety of symptoms are possible including shimmering, wiggling,
undulating (how many more adjectives can you come up with?).  The rate
of the movement will be related to the difference between the monitor scan
rate and the frequency of interference.

  11.51) My monitor is possessed!

Problems are that all graphics applications fade to black, lose their color
on parts of the screen, and there are strange pincushion problems on the
right side of the monitor?  This all came up suddenly, with no apparent
changes your my part.

You tried changing video drivers, modes, cleaning connections on 
cables and video card, even pulled the card and cleaned the edge

After cleaning up, things seemed to work (still had pincushion
problem), but next time it was powered on, same weird problems.

Voodoo might be required but more down-to-earth causes are likely:

Are you sure nothing changed in the building (like you installed a medical
MRI unit with a 2T magnet in the same room)?

All monitors have a built in degauss circuit which operates when power
is turned on after being off for at least 15 minutes or so.  This could
have failed - it is switching off suddenly instead of ramping down as it
should - and is making the problem worse or you could have a power supply
failure inside the monitor.

Gradual variations in color or brightness on the screen or over time
are almost always monitor problems, not video card, software, or cables.

It won't hurt to try manual degauss with the monitor powered, see below.
If this clears it up - possibly until you turn the power off and on again, then
it may be the internal degauss circuitry.

  11.52) Shimmering image due to vibrations

If your monitor uses a Trinitron or clone CRT, then this may be normal.
Even with the 1-3 unsightly stabilizing wires running across the screen,
the vertical aperture grille wires in a Trinitron type CRT can wiggle as
a result of mechanical shocks or vibration.  Any movement results in
momentary changes in color purity, color balance, brightness.  Gently tap
on the side of the monitor and you may see the same effect.

  11.53) Wiring transmitted interference

The power that comes from the wall outlet is supposed to be a nice sinusoid
at 60 Hz (in the U.S.) and it probably is coming out of the power plant.
However, equipment using electric motors (e.g., vacuum cleaners), fluorescent
lamps, lamp dimmers or motor speed controls (shop tools), and other high power
devices, may result in a variety of effects.

While monitors normally include some line filtering, the noise immunity varies.
Therefore, if the waveform is distorted enough, some effects may show up even
on a high quality monitor.

Symptoms might include bars of noise or distortion moving slowly or rapidly up
or down the screen or diagonally.  This noise may be barely visible as a couple
of jiggling scan lines or be broad bars of salt and pepper noise, snow, or
distorted video.

The source is probably local - in your house and probably on the same branch
circuit - but could also be several miles away.

* One way to determine if the problem is likely to be related to AC power
  is to switch your vertical scan rate to match the power line frequency:
  60 Hz in the U.S., 50 Hz in most European countries, etc.  If the pattern
  of noise or distortion is now stationary (or at most slowly drifting up
  or down the screen), the interference is likely power line related:

  - A single bar would indicate interference at the power line frequency.

  - A pair of bars would indicate interference at twice the power line

  Either of these are possible.

* Try to locate the problem device by turning off all suspect equipment to see
  if the problem disappears.

* The best solution is to replace or repair the offending device.  In the
  case of a light dimmer, for example, models are available that do a better
  job of suppressing interference than the typical $3 home center special.
  Appliances are supposed to include adequate noise suppression but this is
  not always the case.

  If the source is in the next county, this option presents some significant
  difficulties :-).

* Plugging the monitor into another outlet may isolate it from the offending
  device enough to eliminate or greatly reduce the interference.

* The use of a line filter may help.  A surge suppressor is NOT a line filter.

* Similar symptoms could also be produced by a defective power supply in the
  monitor or other fault.  The surest way of eliminating this possibility is
  to try the monitor at another location.

  11.54) Jittering or flickering due to problems with AC power

If you have eliminated other possibilities such as electromagnetic
interference from nearby equipment or electric wiring or a faulty video
card or cable - or software - then noisy or fluctuating AC power may be
a possibility.  However, modern monitors usually have well regulated power
supplies so this is less common than it used to be.  Then again, your
monitor may just be overly sensitive.  It is also possible that some
fault in its power supply regulator has resulted in it becoming more
sensitive to minor power line fluctuations that are unavoidable.

One way to determine if the problem is likely to be related to AC power
is to run the monitor on clean power in the same location on the same
computer.  For example, running it on an Uninterruptible Power Source
(UPS) with the line cord pulled from the wall socket would be an excellent
test.  The output of the UPS's inverter should be free of any power line
noise.  If the monitor's image has now settled down: 

1. Large appliances like air conditioners, refrigerator, or washing machines
   on the same circuit might cause significant power dips and spikes as they

   Plugging a table lamp into the same outlet may permit you to see any obvious
   fluctuations in power.  What else is on the same circuit?  Depending on
   how your house or apartment is wired, the same feed from the service panel
   may be supplying power to widely separated areas.

2. For some unfathomable reason, your monitor may just be more sensitive to
   something about the power from the circuit in that room.  There may be
   nothing actually wrong, just different.  While unlikely, a light dimmer
   on the same circuit could be producing line-conducted interference.

   If you have a multimeter, you could at least compare the voltages
   between the location where it has problems and the one where it is
   happy.  Perhaps, the monitor is sensitive to being on a slightly
   different voltage.  This might only be a problem if some circuitry
   in the monitor is marginal in some respect to begin with, however.

3. There could be a bad connection somewhere on the circuit.  If your house
   has Aluminum wiring, this is a definite possibility.

   Try a table lamp since its brightness should fluctuate as well.  This
   should be checked out by a competent electrician as it represents a real
   fire hazard.

An electrician may be able to pinpoint the cause but many do not have
the training or experience to deal with problems of this sort.  Certainly,
if you find any power line fluctuations not accounted for by major
appliances, on the same circuit this should be checked by an electrician.

  11.55) My monitor has the shakes

You turn on your monitor and 5-10 seconds later, the display is shaking or
vibrating for a second or so.  It used to only occur when first turned on,
but now, the problem occurs 3 times in 30 seconds.   Of course, many
variations on this general theme are possible.

Some possibilities:

1. Defective degauss circuit - this would normally cause a shaking or
   vibration when you first turn it on but you normally do not notice it
   since the CRT is not warmed up.  The degauss circuit may have developed
   a mind of its own.

2. Other defective circuitry in monitor - power supply regulation, deflection,
   or bad internal connections.

3. External interference - did you change anything or move your setup
   recently?  See the sections on: "Interference from other equipment",
   "Interference from electrical wiring", and "Interference from power lines".

4. Defective video cable (unlikely).  Wiggle the VGA cable to be see if
   you can induce the problem.

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