Notes on the Troubleshooting and Repair of Computer and Video Monitors


  11.18) Totally white screen (probably with retrace lines)

There may or may not be any indication of a picture.  This may be a problem in
the high voltage power supply (SCREEN, G2), loss of power or a fault in the
video output drivers, other video amp problems, or a bad (shorted) CRT.

Is focus still reasonably sharp?  If not, try adjusting it (usually on the
flyback or a separate little panel).  If changing focus affects brightness
significantly, there is a short between the two supplies - either in the
HV power supply or CRT.  See the section: "Bad focus and adjustment changes brightness".  In this case, changing SCREEN (G2, also on the flyback) may
also affect focus or may not do anything.

Try adjusting SCREEN.  If it has no affect, a problem in its power supply
from the flyback is possible.  If you have a high impedance voltmeter (not
just a DMM, the resistance of the voltage divider supplying SCREEN is hundreds
of M ohms), check it while changing the SCREEN control.  If it does not change,
you have found a definite problem.

Assuming that adjusting FOCUS and SCREEN result in normal behavior and do
not strongly interact, the problem is likely in the video circuitry or output

Check the power to the CRT video output drivers on the little board on the
neck of the CRT.  If this failed, all three video outputs will be full on.
If you have a scope, look at the video outputs - they should be varying
between over 100 V and a low value.  If they are missing or very low all
the time, there is a problem further back in the video chain.

See the other sections relating to brightness and high voltage problems
as well.

  11.19) Shorts in a CRT

Occasionally, small conductive flakes or whiskers present since the day of
manufacture manage to make their way into a location where they short out
adjacent elements in the CRT electron guns.  Symptoms may be intermittent or
only show up when the TV or monitor is cold or warm or in-between.  Some
possible locations are listed below:

* Heater to cathode (H-K).  The cathode for the affected gun will be pulled
  to the heater (filament) bias voltage - most often 0 V (signal ground).  In
  this case, one color will be full on with retrace lines.  Where the heater
  is biased at some other voltage, other symptoms are possible like reduced
  brightness and/or contrast for that color.  This is probably the most
  common location for a short to occur.

* Cathode to control grid (K-G1).  Since the G1 electrodes for all the guns
  are connected together, this will affect not only the color of the guilty
  cathode but the others as well.  The result may be a very bright overloaded
  *negative* picture with little, none, or messed up colors.

* Control grid to screen (G1-G2).  Depending on circuitry can result in any
  degree of washed out or dark picture.

* Screen to focus (G2-F).  Screen (G2) and focus voltage will be the same and
  the controls on the flyback will interact.  Result will be a fuzzy white
  raster with retrace lines and little or very low contrast picture.  Symptoms
  will be similar to those of a flyback with breakdown in the focus/screen
  divider network.

* Focus to high voltage (F-HV).  High voltage will be pulled down - probably
  arcing at the focus spark gaps/other protective devices.  Line fuse and/or
  HOT may blow.

* Other locations between electron gun elements as feed wires.

Replacing the CRT may be required but there are a variety of 'techniques' that
can often be used to salvage a TV that would otherwise end up in the dump
since replacing a CRT is rarely cost effective:

1. Isolation - this will usually work for H-K shorts as long as only one gun
   is involved.

2. Blowing out the short with a capacitor - depending on what is causing the
   short, this may be successful but will require some experimentation.

3. Placing the CRT (TV or monitor) face down on a soft blanket and *gently*
   tapping the neck to dislodge the contamination.  Depending on the location
   of the short, one side or the other might be better as well.  Sometimes,
   this can be done in-place while watching the picture.

A combination of (2) and (3) may be required for intermittent shorts which
don't appear until under power.  See the sections below for additional
details.  However, for shorts involving the focus and high voltage elements,
even a sharp edge can result in arcing even if there is no actual short.
There is no remedy for these types of faults.

  11.20) Providing isolation for a CRT H-K short

This procedure will substitute a winding of your own for the one that is
built in to the flyback to isolate the shorted filament from the ground
or voltage reference.  Note that if you have a schematic and can determine
where to disconnect the ground or voltage reference connection to the
filament winding, try this instead.

The flyback is the thing with the fat red wire coming out of it (and perhaps
a couple of others going to the CRT board or it is near this component
if your set has a separate tripler) and may have a couple of controls for
focus and screen.  It should have some exposed parts with a ferrite core
about 1/2-3/4" diameter.
The filament of the CRT is the internal heater for each gun - it is what
glows orange when the set is on.  What has happened is that a part of the
fine wire of the bad color's filament (assuming this is indeed your problem)
has shorted to the cathode - the part that actually emits the electrons.
Normally, the heater circuit is grounded or tied to a reference voltage
so when it shorts to the cathode, the cathode voltage level is pulled to
ground or this reference.

You will need some well insulated wire, fairly thick (say #18-22).  Find a
spot on the flyback where you can stick this around the core.  Wrap two
turns around the core and solder to the CRT filament pins after cutting the
connections to the original filament source (scribe the traces on the board
to break them).  Make sure you do not accidentally disconnect anything else.

This winding should cause the filaments to glow about the same brightness as
before but now isolated from ground.  If they are too dim, put another turn
on the flyback to boost the voltage as this will result in low emission,
blooming, and possible damage to the cathodes after awhile.  (Don't go
overboard as you may blow the filament totally if you put too many turns on
the core - you then toss the monitor.)

Route the wires so that there is no chance of them getting near the high
voltage or any sharp metal edges etc.  Your picture quality may be a tad
lower than it was before because of the added stray capacitance of the
filament wiring being attached to the the (formerly bad) video signal, but
hey, something is better than nothing.

  11.21) Rescuing a shorted CRT

If the short is filament-cathode (H-K), you don't want to use the following
approach since you may blow out the filament in the process.  If this is the
case, you may be able to float the filament and live with the short (see the
section on: "Red, green, or blue full on - fog over picture".

Shorts in the CRT that are between directly accessible electrodes can
be dealt with in a more direct way than for H-K shorts.  At this point
you have nothing to loose.  A shorted CRT is not terribly useful.

If the short is between two directly accessible electrodes like cathode-grid,
then as a last resort, you might try zapping it with a charged capacitor.

Unplug the CRT socket!

Start with a relatively small capacitor - say a few uF at a couple hundred
volts.  Check to see if the short is blown after each zap - few may be needed.
Increase the capacitance if you feel lucky but have had little success with
the small capacitor.

If the fault is intermittent, you will, of course, need to catch the CRT
with the socket disconnected and the short still present.  Try some gentle
tapping if necessary.  If you do this with the charged capacitor across
the suspect electrode, you **will** know when the short occurs!

  11.22) Dark picture

A monitor with a picture that is too dark may have a fault or the CRT may
just be near the end of its useful life.

First, confirm that your video source - computer, camera, etc. - is producing
a proper signal.

Is the brightness at all erratic?  Does whacking the monitor have any effect?
If so, then you may have bad connections on the CRT driver card or elsewhere.
If the brightness tends to fade in and out over a 10 to 20 second period,
a bad filament connection is likely.  Check for the normal orange glow of
the filaments in the neck of the CRT.  There should be 3 orange glows.  If
they are excessively reddish, very dim, or fade in and out, you have located
a problem.  See the section: "Picture fades in and out".

Common causes of brightness problems:

0. Dirty CRT faceplate or safety glass.  Don't laugh.  It sounds obvious, but
   have you tried cleaning the screen with suitable screen cleaner?  It is
   amazing how dirty screens can get after a few years - especially around

   (From: A. R. Duell (ard12@eng.cam.ac.uk)).

   "I once spent a morning battling with a DEC VT105 terminal with a very
    dim and washed out picture, and only after checking everything on the
    video board did I wipe over the screen. That cured it.  It's amazing
    how dirty screens can get after a few years use."

   Wipe gently with a slightly dampened cloth - not soaking or you may end
   up with real problems when the water drips down inside and hits the

1. Old CRT.  The brightness of the CRT deteriorates with filament on-time.
   It doesn't matter much what you are doing or if you use a screen saver.

   An indication of a weak CRT would be that turning up the SCREEN (G2) or
   master brightness control only results in a not terribly bright gray raster
   before the retrace lines show up.  There may be indications of poor focus
   and silvery highlights as well.  A CRT brightener may help.  See the
   sections: "Brightening a old CRT" and "Monitor life, energy conservation, and laziness".

2. Bad component in filament circuit or bad connection reducing filament
   voltage.  This should be easy to check - there are only a few parts
   involved.  If it is erratic, bad connections are likely.

3. Brightness control faulty - bad pot, bad connections, or problem with its
   power supply.  Depending on specific problem, control may or may not have
   any effect.  If digitally adjusted, there could be a problem with the
   logic or control chip.  If the button or menu item has no effect at all,
   then a logic or control problem is likely.

4. Improperly set SCREEN (G2) voltage (usually on flyback) or faulty divider
   network.  See the section: "Brightness and color balance adjustment".

5. Improperly set video bias (background) levels or fault in video drive
   circuitry.  See the sections starting with: "Optimal procedure for setting brightness/background and screen adjustments".

6. Fault in video amplifiers.  With all three color affected equally, this
   would most likely be a power supply problem.  A video amplifier problem
   is likely if turning up the SCREEN (G2) or master brightness control
   results in a very bright raster before the retrace lines appear.  Cheack
   signals out of the video/chroma(IC.

7. Fault in beam or brightness limiter.  Many TVs and monitors measure the
   beam current (possibly indirectly) and limit the maximum to a safe value.
   The purpose of this may be to protect the CRT phosphors, and/or to assure
   that the power supply does not go out of regulation, and/or to limit X-ray
   emission.  If this circuit screws up, a dark picture may result.  Checking
   the signals and voltages at the CRT socket should determine if this is the

8. High voltage is low.  However, this would likely result in other symptoms
   as well with focus, size, and geometry.

  11.23) Brightening an old CRT

If performing adjustments of the internal background and/or screen
controls still results in a dark picture even after a long warmup period
(and the controls are having an effect - they are not faulty), the CRT may
simply be near the end of its useful life.  In the old days of TVs with
short lived CRTs, the CRT brightener was a common item (sold in every
corner drugstore, it seemed!).

First confirm that the filaments are running at the correct voltage - there
could be a marginal connection or bad resistor or capacitor in the filament
power supply.  Since this is usually derived from the flyback, it may not
be possible to measure the (pulsed high frequency) voltage with a DMM but
a service manual will probably have a waveform or other test.  A visual
examination is not a bad way to determine if the filaments are hot enough.
They should be a fairly bright orange to yellow color.  A dim red or almost
dark filament is probably not getting its quota of electrons.  It is not be
the CRT since all three filaments are wired in parallel and for all three to
be defective is very unlikely.

If possible, confirm that the video output levels are correct.  For cathode
driven CRTs, too high a bias voltage will result in a darker than normal

CRT brighteners are available from parts suppliers like MCM Electronics.
Some of these are designed as isolation transformers as well to deal with
heater-to-cathode shorts.

You can try a making a brightener.  Caution: this may shorten the life of
the CRT - possibly quite dramatically (like it will blow in a couple of
seconds or minutes).  However, if the monitor or TV is otherwise destined
for the scrap heap, it is worth a try.

The approach is simple: you are going to increase the voltage to the
filaments of the electron guns making them run hotter.  Hopefully, just
hotter enough to increase the brightness without blowing them out.

Voltage for the CRT filament is usually obtained from a couple of turns
on the flyback transformer.  Adding an extra turn will increase the voltage
and thus the current making the filaments run hotter.  This will also
shorten the CRT life - perhaps rather drastically.  However, if the monitor
was headed for the dumpster anyhow, you have nothing to lose.  You can just
add a turn to an existing winding or make your own separate filament winding
as outlined in the section: "Providing isolation for a CRT H-K short".

In some monitors, there is a separate filament supply on the mainboard - this
should be obvious once you trace the filament wires from the video driver
board).  In this case, it still may be possible to increase this output or
substitute another supply but a schematic will be required.

There are also commercial CRT rejuvenators that supposedly zap the
cathodes of the electron guns.  A TV or monitor service center may be
able to provide this service, though it is, at best, a short term fix.

  11.24) Color balance changes across screen from left to right

The characteristics are that a solid white screen will tend to be blue tinted
on one side and red tinted on the other.  This is usually a subtle effect and
may be unavoidable with some designs.

There are several possibilities:

1. Purity - this means the beams are landing on the wrong phosphor dots.
   This is what would be affected by moving from one location to another
   or even rotating the TV on its base without degaussing.  If the problem
   just appeared, degaussing may be needed.

   What do you have near the TV or monitor?  Loudspeakers or other devices
   which generate magnetic fields can easily cause all sorts of color purity
   problems.  Relocate the offending device(s) or the TV or monitor and then
   degauss it.

   See the section: "Degaussing (demagnetizing) a CRT".

   If the problem still persists, purity adjustment may be needed.  However,
   this isn't likely to  have changed so look for other causes before tackling
   these adjustments.

2. Unequal electron gun to shadowmask/screen distance - the electron beams for
   the red and blue video travel slightly different distances on the left and
   right sides of the screen so their intensity (due to focus not being optimal
   and other factors) in each case may differ slightly affecting color balance.

3. Doming - This would only happen in very bright areas and causes the
   shadow mask to expand and distort.  (Doming should not be a problem with
   Trinitron CRTs which use tensioned wires in their aperture grill.)  This
   would also not really affect left-right color balance in particular.

I don't really know how much of a problem (2) is in practice or whether some
manufacturers compensate for it.

  11.25) Bleeding highlights

On very bright areas of the picture, one or more colors may bleed to
the right resulting in a trail of those colors.  The difference between
this problem and the section: "Trailing lines in one or more colors" is
that in this case, only highlights are affected.

One cause of this is that the color gain, contrast, or intensity controls
(whatever they are called on your monitor) are set too high.  See the section
on: "Brightness and color balance adjustment".  Check the settings of any
brightness limiter controls as well.

  11.26) Trailing lines in one or more colors

Assuming this is not a form of ghosting resulting from cabling and/or use
of switchboxes, etc, then it could be any of the following:

* Poor decoupling in the power supplies for the video drive circuits - probably
  on the CRT neck board.  Check for bad (low uF or high ESR) filter capacitors
  (electrolytic mostly) on this board or the power supplies feeding it.

* Insufficient CRT filament voltage.  This could be a result of bad connections
  or a bad component in the filament power supply (probably from the flyback).
  Check to see if the filaments are glowing bright orange and check the voltage
  if possible (though this can be tricky since it is often fed from a winding
  on the flyback and is a pulse waveform, not DC or a sinusoid.  The service
  manual will probably have info and waveforms.

* Bad CRT (more likely if only one color is affected).  A weak electron gun can
  result in this behavior.  Swap it with one that work properly.  If the same
  color is still bad, that CRT gun is weak.  The CRT will need rejuvenation or
  need to be replaced (more likely, the entire monitor will be tossed into the

  11.27) Purity problems with bright pictures

Setting the brightness excessively high may result in enough heating
of the shadow mask to distort it.  IF severe enough, the positions of the
holes will shift enough to result in visible purity problems.  This is
less of a problem with tubes using an InVar shadow/slot mask.  It should
also be less of a problem for Trinitron aperture grille CRTs.

The only solution is to reduce the brightness.

  11.28) Why does the intensity appear so non-uniform in bright areas?

Actually, the intensity variation is likely to be even worse than you might
think - possibly as much as 2:1 from the center to the corners.  In most cases
you do not notice it.  With large deflection angle tubes, fewer electrons make
it to phosphor dots near the edge of the screen.  It is simple geometry.

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

It is extremely difficult for any CRT display to maintain perfect brightness
and color uniformity across the entire image.  Just the geometry of the
thing - the change distance from the gun to the screen as the beam is scanned,
the changing spot size and shape, etc. - makes this nearly impossible, and
there can also be variations in the phosphor screen, the thickness of the
faceplate, etc..  Typical brightness-uniformity specs are that the brightness
won't drop to less than 70% or so of the center value (usually the brightest
spot on the screen).

On color tubes, the lack of perfect brightness uniformity is aggravated
by the lack of perfect COLOR uniformity and purity.  What appear to be
"dark spots" on a solid gray image may actually be beam mislanding (color
purity) problems, which may to some degree be remedied by degaussing
the monitor.  

Again, *some* variation is normal; if you think you're seeing too much, you
can try degaussing the thing and seeing if that helps.  If it doesn't,
then the question is whether or not the product meets its published specs,
and that 's something you'll have to discuss with the manufacturer or

  11.29) Brightness changes from left-to-right across screen

Slight variations in brightness across the face of the CRT are not unusual.
In fact, if you used a photometer to actually measure the brightness, you
might be amazed at the actual variance even with the best TV - you just
don't notice it.  However, a major variation - usually a decay from left to
right but could be the other way indicate a component failure.  Of course,
make sure the face of the screen is clean!

* A fault in the power supplies to the video amplifier and/or video output
  circuits.  Most likely, an electrolytic capacitor has dried up and is not
  adequately filtering the power derived from the flyback which then has
  ripple at the horizontal scan rate and thus locked to the screen.  The
  voltage decays from left-to-right between horizontal flyback pulses.

  The most likely location for these capacitors is in the vicinity of the
  flyback transformer on the mainboard or on the CRT neck board.  Check the
  capacitors with capacitor tester or ESR meter and/or take a look at the
  power right at the video amplifier and video output drivers.

* Horizontal linearity is bad - this may actually be a horizontal geometry
  problem and not a brightness problem.

  See if objects on left side of the screen are stretched compared to those on
  the right (or vice-versa).  If they are, the problem is in the horizontal
  deflection circuits - possibly a bad (or in the case of a multiscan monitor,
  correctly selected) S correction capacitor or linearity coil.

* Inoperative degauss circuit, monitor moved or rotated without degaussing, or
  magnetic field from some other device (like a permanent magnet) is affecting
  CRT - slight amounts of magnetization may reduce brightness (by moving the
  beams into the black space between phosphor dots) before affecting color
  purity (where the beams land on the wrong phosphor dots).

  See if the degauss button, if present, does anything.  Try deguassing
  manually.  See the section: "Degaussing (demagnetizing) a CRT".

  11.30) Picture fades in and out

If the picture faded away on the order of 10-20 seconds (and if it comes
back, also comes up to full brightness in same time frame - possibly
with the persuasion of some careful whacking) AND with NO other
significant changes such as size, focus, etc., then take a look in the back of
the tube for the filament to be lit - the orange glow near the CRT socket.  If
there is none, then you probably have a bad solder connection on the circuit
board on the neck of the CRT.  Look for fine cracks around pins on that board.
Try prodding it with an insulating stick to see if the picture comes back.
Resolder if necessary.  It is probably not a bad CRT as the filaments
are usually wired in parallel and all would not go bad at the same time.

However, if only a single color fades in and out, then a bad connection
inside the CRT is a distinct possibility - look for only one of the
filament's glow to be coming and going.  This is probably not worth fixing
since it will require CRT replacement.

If the picture faded away with other symptoms, then there is probably
a fault in the video amplifier/output one of its power supplies -
still probably a loose connection if you are able to get it back by

  11.31) Occasional brightness flashes

These may last only a fraction of a scan line or much much longer.

Make sure it is not the video source - try another one.

This could mean an intermittent fault in a variety of places including
the video circuitry and SCREEN power supply:

* Brightness circuitry - SCREEN, master background or its power supply.
  Could be in or around flyback or focus/screen divider.  Could perhaps
  be in the CRT, but probably less likely.

* Video amp before or at chroma demodulator (if composite input) - since
  after this point, you would most likely get colored flashes since only
  one of the RGB signals would likely be effected.  However, a bad power
  connection to the video circuitry could cause all the colors to be

If you still get flashes, it should be quite easy to monitor either
the video outputs or SCREEN supply (with a HV divider on your scope) for
noise.  Then trace back to power or noise source.

  11.32) Occasional static, lines, spots, or other unsightly blemishes

First, confirm that these are not video source - PC - related.  Try the
monitor on another computer.  This may be a problem with the hardware or
driver (software) for the video card, the O/S, or memory or bus speed.

If it is not computer related, then it could be arcing, corona, bad
connections, or some electronic component breaking down.  See the
appropriate sections for these problems.

Note that problems in absolutely fixed locations or with an extent related
to pixel sizes in the video card are nearly always computer/video card
related and not due to a faulty monitor.

  11.33) Flickering monitor

First, make sure your scan rate is set high enough (but not beyond the
capabilities of the monitor).  A scan rate less than 60 Hz is likely to
result in annoying flicker especially at high brightness levels.

See if the flickering correlates with any processor or disk activity indicating
a software driver or video card problem.

Assuming neither of these applies and you are not doing your work by
candlelight, a flickering image is probably due to an intermittent arc
or short, probably in the high voltage section near or at the flyback
transformer.  However, it is also possible that it is due to a simple
bad connection elsewhere.

So the first thing to do will be to remove the cover and without touching
anything, carefully examine for any obvious signs of bad connections, arcing,
or burned areas.  In particular look for:

* hairline cracks around the pins of large components like power transistors,
  power resistors, transformers, and connectors.

* any discoloration, cracking, other unusual signs on the flyback.  The flyback
  also provides, via a high resistance divider network, the several KV for
  focus and several hundred V for the G2 (screen) CRT electrode.  These are
  the voltages that may be intermittently changing and resulting in flicker.

Now, with the monitor powered in a darkened room with a normal picture
(use the highest resolution at which your monitor will work as this should
put the most stress on it, maybe).

* Look for any arcing or corona around the area of the flyback or the neck
  of the CRT first, then just anywhere.

* Use a well insulated stick (wood or plastic) to gently prod the circuits
  board, components, wires, etc. to see if you can induce the problem.

There will probably be a pair of adjustments on the flyback itself.  One of
these is FOCUS and the other is SCREEN - essentially a master brightness.

* Now, with one hand in your back pocket, try turning each of these a fraction
  of a turn in each direction.  Don't worry, you cannot hurt anything by
  doing this.  The FOCUS should only change the sharpness of the picture.
  The SCREEN should only change the brightness.  In both cases, this should
  be a smooth effect.  Sometimes, these controls will simply get dirty and
  cause the problems you have seen.  In this case, just moving them back
  and forth may clean them.  If one affects the other - if turning focus
  alters brightness or vice-versa, there is a short between the focus and
  screen voltages, probably inside the flyback but it could be elsewhere.

It is likely that all of the above tests will come out negative as
you may have an intermittent short internal to the flyback which can only
be fixed by replacement.  However, eliminate the easy fixes first.

  11.34) Excessive brightness and/or washed out picture

There are a number of possibilities including incorrect screen (G2) or bias
(G1) voltages, or a problem in the video or blanking circuitry.  Any of these
could be the result of bad connections as well.  A short in the CRT can also
result in these symptoms.

* Excessive brightness/washed out picture is often an indication of a
  problem with the screen (G2) supply to the CRT.  May be a bad capacitor
  or resistor divider often in the flyback transformer assembly or on 
  the board on the neck of the CRT.

* If the excessive brightness just developed over time, then a simple
  adjustment of the screen or background brightness controls may keep
  it (and you) happy for a long time.

  When good, a typical value would be in the 200 to 600 VDC at the CRT.  The
  screen (it may also be called master brightness, bias, or background) control
  should vary this voltage.  However, it may be difficult to measure as the
  resistors in the voltage divider network may be quite large - hundreds of M
  ohms.  If your unit has an external screen control (less likely these days)
  and it has no effect, trace out the circuitry in the immediate vicinity and
  check the resistors and potentiometer for opens, look for bad connections,
  etc.  If it is built into the flyback transformer and is sealed, the entire
  flyback will need to be replaced unless the actual problem turns out to be a
  bad connection or bad component external to the flyback.

* Where the brightness control has no effect, suspect a missing bias supply to
  the G1 (control grid) electrodes of the CRT.  This is usually derived from
  the flyback with a simple rectifier/filter capacitor power supply.  Parts
  may have failed (though not likely the flyback itself).  Adjusting the user
  brightness control should vary this voltage over a typical range of 0 to -50
  V with respect to signal ground.

* It could also be a problem with biasing of the video output transistors.
  There may individual controls for background brightness on the little board
  on the neck of the CRT.  However, we are looking for a common problem since
  all colors are wrong in the same way.  This is likely to be a missing voltage
  from a secondary supply from the flyback.

* A short between electrodes inside the CRT can result in brightness problems.
  It may be possible to check this with an ohmmeter with the power off and the
  CRT socket removed.  Test between G1, G2, and F where all colors are
  affected though a short between F and G2 will result in the focus control
  changing brightness and vice-versa - a classic symptom.

  However, in some cases, it only shows up when operating and one must deduce
  the presense and location of the short from its affect on voltages and bias

  See the section: "Rescuing a shorted CRT" and other related topics.

First, check for bad connections/cold solder joints by gently prodding
with an insulating stick.  Check voltages and bias levels.

  11.35) Focus problems

Slight deterioration in focus can be corrected by adjusting the focus
control usually located on the flyback transformer.  Sometimes, this
is accessible externally but usually not.  On monochrome monitors, the
focus control, if any, may be located on the main board.

Don't expect to have perfect focus everywhere on the screen.  Usually there
will be some degradation in the corners.  A compromise can generally be
struck between perfect focus in the center and acceptable focus in the

If the adjustments have no effect, then there is probably a fault in the
focus power supply.

For most color TVs and monitors, the correct focus voltage will be in the
4-8 KDC range so you will need a meter that can go that high or some big
resistors to extend its range or a HV probe.  You must use a high impedance
meter as the current availability from the focus power supply is very low.

The pots in the flyback are sometimes accessible by removing their cover,
which may snap on.  However, a typical focus circuit will have a large
value resistor potted inside the flyback (like 200 Megohms).

Try to measure the focus in-circuit.  If the value you read is very low
(assuming your meter has a high enough impedance not to load the circuit
appreciably), then disconnect the wire (from the PCB on the neck of the
CRT or wherever) and measure again and observe any change in picture.

If still low, then almost certainly there is a problem with the pot or
the flyback.  See if you can open it enough to measure and/or disconnect
the pot.  If the problem is inside the potted part of the flyback, the
only alternative is a new flyback or an external divider if you are so
inclined.  However, once the focus network goes bad inside the flyback,
there is an increased chance other parts will fail at some point in the future.

If the voltages check out with the CRT disconnected, there is a chance of a
bad CRT or of a shorted component on the PCB on the neck of the CRT.  Look
for shorted capacitors or burnt or damaged traces.

  11.36) Bad focus (fuzzy picture)

Focus voltage on the CRT is usually in the range of 2-8 KV DC and should
be controllable over a fairly wide range by the focus pot - usually located
on the flyback or a little panel in its vicinity:

* If adjusting the pot results in a position of acceptable focus, you may be
  done.  It is not unusual for the focus setting to drift a over time.

* If the setting is already as good as possible but not really good enough,
  the CRT may be tired.  Alternatively, the filament voltage may be too low.
  Check for bad connections in the filament circuit.

* If the optimal setting is out of range of the focus pot, the problem is
  likely leakage in the focus divider in the flyback or one of the components
  on the CRT neck board.

Also see the sections: "Focus adjustment" and "Focus drifts with warmup".

The focus wire usually comes from the flyback or if the general area or from a
terminal on a voltage multiplier module in some cases.  It is usually a wire
by itself going to the little board on the neck of the CRT.

If a sparkgap (a little 2 terminal device with a 1/8" gap in the middle)
is arcing with power on, then the resistive divider has shorted inside
the flyback, focus board, or HV multiplier - whatever you TV has - and
the this unit will need to be replaced.  Ditto if the SCREEN control affects
focus and/or vice-versa.

Using a suitable high voltage meter (range at least 10 KVDC, 1000 M ohm or
greater input impedance), you should be able to measure it connected and
disconnected.  The ground return will be the outside coating of the CRT which
may or may not be the same as the metal chassis parts.  If the voltage is very
low (less than 2 KV) and the pot has little effect:

* When measured right off of the source disconnected from the CRT neck board,
  then the problem is probably in the focus network in the flyback (or wherever
  it originates).  Sometimes these can be disassembled and cleaned or repaired
  but usually requires replacement of the entire flyback or voltage multiplier.
  Note: you may need to add a HV (10 KV) capacitor between the focus wire and
  DAG ground to provide filtering so you get a DC level for your meter.

* When measured with the focus wire attached to the CRT neck board with the
  CRT connected but reasonable with the CRT unplugged, there is probably a
  short between the focus and another electrode inside the CRT.  See the
  section: "Rescuing a shorted CRT".

* When measured with the focus wire attached to the CRT neck board with the
  CRT unplugged, there is likely a component on the CRT neck board that is
  leaky or breaking down.  Also, check for decayed (tan or brown) glue which
  may turn leaky with age.

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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]