Notes on the Troubleshooting and Repair of Computer and Video Monitors


Chapter 11) Raster, Color, and Video Problems

  11.1) Blank picture, power light on, digital controls (if any) active

Does 'blank picture' means a totally black screen with the brightness and
contrast controls having no effect whatsoever?  Or, is there is no picture
but there is a raster - light on the screen?  The direction in which
troubleshooting should proceed differ significantly depending the answer.

Verify that you computer has not simply entered power saving mode and
blanked the screen or shut off the monitor video and power circuits

Confirm that the video source is not defective or blank - try another one.

Here are some questions:

1. Is there any light on the screen at any settings of the brightness
   and contrast controls, and/or when switching channels.  Can you see any
   raster scanning lines?

2. Can you obtain a raster of any kind by adjusting the screen (G2) control
   (probably on the flyback) or master background or brightness?

3. Looking in the back of the monitor, can you see the glow of the CRT

4. Do you get that static on the front of the tube that would indicate that
   there is high voltage?  Any cracking or other normal or abnormal sounds
   or smells?

If the answer to all of these is 'no', then you have a power supply and/or
deflection problem.  Refer the the section: "No picture but indications of power".

Possible causes of no raster:

* No or low high voltage (low voltage, deflection, or high voltage power
  supply failure).

* Fault with other voltages like G1 or screen (G2) to CRT.

* Filament to CRT not getting powered.

* Drive to CRT bad/shut off as a result of fault elsewhere.  For example,
  failure of the vertical deflection may disable HV or blank the screem to
  protect the CRT from burn-in due to the very bright horizontal line that
  would result.  With some monitors, it is possible that the X-ray protection
  circuitry will blank the screen.

Possible causes of no video: problem in video input, video amplifiers, video
output, cutoff due to other fault.

It could be as simple as a bad connection - try gently prodding the boards
with an insulated stick while watching the screen.  Check for loose connectors
and reseat all internal connectors.

  11.2) Brightness control has no effect

The following assumes that the picture is fine but the brightness is
fixed - probably at too high a level.  However, there could be several
interrelated problems if a common supply voltage were missing, for example.

If it is a knob, then it should be varying the control grid (G1) voltages
relative to the cathodes (K) of the CRT.  This is not likely to be a very
complex circuit.  If you do not have a schematic, I would start by tracing
from the control, check continuity and solder connections.  Check the
control itself for proper operation with an ohmmeter.  A power supply going
to one side of the control (negative probably) may be missing.  Tbe control
grid voltage will end up on the little board on the neck of the CRT - check
there as well for bad solder connections or open resistors.

If brightness is a digital control, then you will need a schematic unless
there is an obvious bad connection.

  11.3) No color - black and white picture

This means absolutely no color - equivalent to a black and white picture.
Not even a hint of color.

If you are using a composite video input, troubleshoot the chroma circuitry
like you would a TV - see the document: "Notes on the Troubleshooting and
Repair of Television Sets".

This is an extremely unlikely failure mode for  a computer monitor
unless you are using a composite video input.  It is most likely to
a software driver or program problem.  Sometimes, the PC will think that
the monitor you have connected is not capable of color and certain programs
will then display in B/W no matter what.

In some cases this is due to an initialization problem - possibly a race
condition during the boot process - especially likely if you are using an
older video card with a new fast processor.

First, confirm that the source is actually in color - try the monitor on
another computer or vice-versa.

Check the settings of any mode switches - in rare cases there is a color/mono
switch or button.

Note that to the average person, the obvious question becomes: is my color
picture tube bad?  The answer is a definitive NO.  It is virtually impossible
for a defective CRT to cause a total loss of color.  A defective CRT can
cause a lack of a primary color - R, G, or, B which will mess up the color
but is not likely to result in a black and white picture.

  11.4) One color is too weak or too strong

If the problem is slight and/or has gradually gotten worse, this may just
require an adjustment of the color brightness/background/bias and/or color
gain/drive controls inside the monitor.  See the section: "Brightness and color balance adjustment".

Even if it appears as though there is an excess, this may actually be a
reduction in one of the primary colors.  For example, a magenta tinge is
represents a reduction in the strength of the green signal.

* Too high an intensity for one of the color channels will result in a tint of
  one of the primaries: red, green or blue.

* Too low an intensity for one of the color channels will result in a tint of
  the complement of one of the primaries: yellow, cyan, or magenta.

* Problems mainly in the shadows or dark areas of the picture usually represent
  a fault with brightness/bias/background.

* Problems mainly in the highlights or bright areas of the picture usually
  represent a fault with the gain/drive.

A color that that is now suddenly brighter or darker than normal resulting in
incorrect color balance or a tint in the background could be due to a number
of causes:

* Bad cable or pin bent on cable connector.

* Bad connections or bad component in video amplifier or on CRT neck board for
  that color.

* Weak gun in CRT (reduced color).

* Bad video card or incorrect software color map settings.

* For monitors with sync-on-green capability, the monitor may think you are
  using sync-on-green when in fact you have separate sync.  In particular,
  this may result in a problem with excessive green:

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

  Some monitors provide a user-selectable setup option for "sync-on-green"
  vs. separate syncs.  Sometimes, this doesn't really change where the
  sync itself is coming from.  In those cases, it's automatically detected
  but *does* change where the reference level for the video is expected
  to be.  You might try checking this setting, if you have it, and changing
  it back and forth to check the effect.  It's not likely to be the problem
  in a separate-sync system like a PC, but weirder things have happened
  and it's easy and cheap to check out.

  11.5) Psychodelic color

The means colors that are not normal and that adjustment of the user
controls is not able to correct it so that all colors of the picture
are properly displayed at the same time.  For example, you are unable
to get any yellows or blues in picture that should have these colors.

* If you are using a composite video input, troubleshoot the chroma circuitry
  as you would a TV - see the document: "Notes on the Troubleshooting and
  Repair of Television Sets".

* Confirm that the input is not a weird color video - try another software
  program or video source.  We have a draftsperson who always sets up his
  Windows color scheme in this manner - we keep wishing it is the monitor
  as **that** could be fixed!

* Verify that this is not a missing color problem - one of the primary R, G,
  or B, has disappeared.  If so, refer to the section: "Intermittent, flickering, or missing colors".

* If this is a monitor with BNC connectors and you are using them, make sure
  you had the video termination switches set correctly (75 ohms if this is
  the only monitor or the last monitor in a daisychain; HiZ if an intermediate
  monitor in a daisychain.)  A very common cause of unbalanced or blooming
  colors assuming the monitor itself is good is incorrect settings of the

* A bad connection, bad component, or short circuit in the video circuitry
  or CRT neck board could also result in strange colors.

  11.6) Monitor manufacturing quality and cold solder joints

Any intermittent problems with monitors that cause random sudden changes in
the picture brightness, color, size, or position are often a result of
bad connections.

Bad solder joints are very common in monitors due both to poor quality
manufacturing as well as to deterioration of the solder bond after numerous
thermal cycles and components running at high temperature.  Without knowing
anything about the circuitry, it is usually possible to cure these problems
by locating all bad solder connections and cleaning and reseating internal
connectors.  The term 'cold solder joint' strictly refers to a solder
connection that was either not heated enough during manufacturing, was
cooled too quickly, or where part pins were moved before the solder had
a chance to solidify.  A similar situation can develop over time with
thermal cycling where parts are not properly fastened and are essentially
being held in by the solder alone.  Both situations are most common with
the pins of large components like transformers, power transistors and
power resistors, and large connectors.  The pins of the components have
a large thermal mass and may not get hot enough during manufacturing.  Also,
they are relatively massive and may flex the connection due to vibration
or thermal expansion and contraction.

These problems are particularly common with TVs and monitors - especially
cheaper monitors.

To locate cold solder joints, use a strong light and magnifier and examine
the pins of large components for hairline cracks in the solder around the
pin.  Gently wiggle the component if possible (with the power off).  Any
detectable movement at the joint indicates a problem.  With the power on,
gently prod the circuit board and suspect components with an insulated
tool to see if the problem can be effected.

When in doubt, resolder any suspicious connections.  Some monitors may
use double sided circuit boards which do not have plated through holes.
In these cases, solder both top and bottom to be sure that the connections
are solid.  Use a large enough soldering iron to assure that your solder
connection is solid.  Put a bit of new solder with flux on every connection
you touch up even if there was plenty of solder there before.  However,
remove any obvious excess.  Inspect for solder bridges, sliver, splashes,
etc. before applying power.

  11.7) Why can't monitor manufacturers learn to solder properly?

I can think of several potential reasons - all solvable but at higher
manufacturing cost.

1. Mass of large component leads (like shields) does not get adequately
   heated during manufacture leading to latent cold solder joints.  While
   they may look ok, the solder never actually 'wetted' the heavy pins
   and therefore did not form a good mechanical or electrical bond.

2. Thermal cycles and differential thermal coefficients of circuit boards,
   traces, and solder.  While it is not easy to do anything about the
   material properties, using plated through-holes or a similar mechanical
   via would greatly increase the surface area of the joint and prevent
   the formation of cracks.

3. Vibration.  This is also directly related to the single sided circuit
   boards without plated through-holes to strengthen the joints.

4. Lack of adquate mechanical support (single sided circuit boards without
   plated through-holes (vias).

I believe that the single most significantimprovement would come about
by using plated trhough-holes but this would add to the cost and apparently
the consumer is not willing to pay more for better quality and reliability!
Some designs have used rivlets - mechanical vias instead of plated ones.
While this is good in principle, the execution has often been flawed where
cold solder joints resulted between the rivlets and the circuit board traces
due to lack of adequate process control.

Monitors, due to their generally higher cost compared to TV sets, should
be better constructed but not always.

  11.8) Intermittent, flickering, or missing colors

This is a catch-all for some of the most common monitor problems.  Most of
the causes boil down to bad connections of one form or another.  However,
defective components like bias resistors on the CRT driver board or in the
video circuitry could also be at fault.

* Does whacking the monitor have any effect?  If so, then bad connections
  are confirmed.  If the color(s) come and go suddenly, then it is most likely
  *not* a CRT problem.  The bad connections could be at the VGA cable, video
  driver board on the neck of the CRT, or elsewhere (see below).

* If the color fades in and out with a delay of about 10-15 seconds, it is
  probably intermittent power to the CRT filament for that color and probably
  means a bad CRT since the three filaments are wired in parallel inside the
  CRT.  One of the internal connections has come loose.

  Look in the neck of the CRT to make sure all three filaments are glowing
  orange.  If one is out or goes on and off, toss the monitor.  Replacing the
  CRT is probably not worth it.  However, if they all go on and off together
  (all colors would be fading in and out though perhaps not quite in unison),
  then bad connections for the CRT filaments on the CRT neck board are

Possible causes of intermittent or missing colors:

* VGA or other video input cable.  Sometimes these develop intermittent
  problems at the connector to the VGA board.  These may be internal
  to the cable in which case it will need to be replaced or if you are
  handy and have infinite patience, you can replace just the VGA connector.

  Alternatively, the male pins of the cable may not be making good contact
  with the female VGA socket.  First try contact cleaner.  If this does not
  work, gently squishing the male pins with a pair of needlenose pliers may
  provide temporary or permanent relief if the pins are a tad too small.
  However, if you go too far, you can damage or break the pins or cause the
  female socket to become enlarged and loose fitting for any other monitor
  you may use.

  If this just happened after reconfiguring your system and reconnecting
  the monitor or installing a new monitor, check your video connector - you
  may have bent over or pushed in pins 1, 2, or 3 - the R, G, and B video
  signals respectively.

  If you find a bent pin, ***carefully*** straighten it with a pair of
  needlenose pliers.  If it is pushed in, try to grab onto it and pull it
  out - then put a drop of Epoxy or other adhesive at its base (don't get
  any on the part of the pin that makes contact) to prevent it from being
  pushed in again.

  There may be cold solder joints on the VGA board itself at the VGA
  connector.  These can be resoldered.

* Printed circuit board on the CRT neck.  This is a common location for
  cold solder joints.  Check with a bright light and magnifying glass
  for hairline cracks around the pins of larger parts.  Prod and tap with
  an insulated tool to see if the problem is effected.  Resolder if necessary.

* Cold solder joints elsewhere in monitor usually around the pins of
  large parts such as transformers, power transistors and resistors, and
  internal connectors.  Inspect with a strong light and magnifier if

* Internal connectors that need to be cleaned and reseated.  Remove,
  clean with contact cleaner, burnish, and replace.

* Bad filament connections inside the CRT (gradual fade in and out or
  one filament not lit).  Replace CRT or monitor.

To narrow down the problem:

* Locate the output for the bad color on the video driver board on the
  neck of the CRT.   This will probably read a significantly higher
  voltage than the corresponding pins for the good colors.  A circuit
  problem is likely - probably on this board but it could be in other
  parts of the video circuitry.

* Test components on this board for the good and bad color channels.  A
  shorted transistor or open resistor can kill one channel.  Swap parts
  between good and bad colors to confirm.

* Gently pull the CRT neck board off of the CRT and replace it.  This will
  tend to clean the contacts.

* Connect an output of the video circuit/chip that is working (i.e., a color
  that appears on the screen) to *all* three color drivers on the CRT neck

   - If you now get a more-or-less black and white picture (there may be a
     moderate color tint as the relative intensities of R,G,B may not be
     balanced), the problem is likely with the circuitry on the mainboard.

     Note: the picture will be the intensity of only one color channel so it
     will not be quite *normal* in any case.

   - If you still have missing or messed up colors, the problem is on the CRT
     neck board or with the CRT.

  11.9) Some commentary on monitor and TV whacking

Anytime that intermittent symptoms are experienced, I recommend gently
whacking the patient to determine if mechanical shock or vibration affects
the behavior.  Here are a couple of responses to this suggestion.

(From Marc Gelfond (71363.1700@CompuServe.COM)):

I just love the bit about "whacking it". It brings to mind an 
episode from the old Andy Griffith show, where a new fangled piece 
of electronics gear, was broght into Emmets repair shop. After 
many long hours of fruitless troubleshooting, out of frustration 
Emmet gave the thing a whack, and sure enough it fixed the  problem. 

As we say in the Telephony business, it "CCWT" or Came Clear While Testing.
 Another saying is that it "CCBFM" Came Clear By F------ Magic!!

(To which Gavin Adams (gaa@hopi.com) comments):

In the video industry we had a saying concerning malfunctioning gear:

"If it's broke, hit it with a hammer"
"If that doesn't fix it, paint it and sell it"

My DEC 16" monitor is case in point. Evey once in a while it would
lose sync, and smacking it would bring it back (sometimes a few
smacks).  Recently it gave up the ghost completely, and after the local
DEC office gave me a quote of $900 to fix it (Bermuda), I ordered a
new Viewsonic 17" for the same price.

I ripped the guts out of the DEC beast, painted it with a marble finish,
put plants in it, and sold it! :>

  11.10) Ghosts, shadows, or streaks in picture adjacent to vertical edges

Complaints about these kinds of problems are very common especially as
the screen resolution and necessary video bandwidth keeps increasing.
Most are due to cable and video termination deficiencies and not actual
monitor defects.

The video signals for red, green, and blue (or just a single signal for
monochrome) are sent over cables which are generally 75 ohm transmission
lines.  These are coaxial cables that may be combined inside a single
sheath for VGA, SVGA, MACs, and many workstations but may be separate coaxes
with BNC (or other) connectors for other video applications.

Without going into transmission line theory, suffice it to say that
to obtain good quality video, the following conditions must be met:

1. A good quality of cable must be used.  This means one in which the
   characteristic impedance is close to the optimum 75 ohms, one which has
   low losses, and one which has good shielding.   For installations
   using BNC connectors, a good quality of 100% shielded RG59U is often used.
   The BNC connectors must be properly installed or they will contribute
   to mismatch problems.

2. Where multiple monitors are to be connected to a single video source,
   all wiring is done in a daisy chain fashion.  The only taps permitted
   are the minimum necessary to connect each monitor to the chain.  This
   usually means a BNC-T connector or a pair of connectors on the monitor
   for each video signal.  T connections with cable must be avoided.

3. Only the last monitor in the chain should be terminated in 75 ohms.  All
   of the others must be set to Hi-Z.  Monitors with BNC connectors will
   usually have one switch or a switch for each color to select termination.

Monitors for PCs, MACs, and workstations usually have built in
termination and do not offer the choice of Hi-Z.  This means that without
a video distribution amplifier, it is not possible to connect multiple
monitors of this type to a single video source with any expectation of a
good quality display.

Failure to follow these rules will result in video ringing, ghosts, shadows,
and other unsightly blemishes in the picture.  It is often not possible to
control all aspects of the video setup.  The cable is often a part of the
monitor and cannot easily be substituted for a better one.  The monitor
may not have properly designed circuitry such that it degrades the video
regardless of the cable and display board quality.  The display card itself
may not have proper drivers or source termination.

Ironically, the better the video card, the more likely that there will
be visible problems due to termination.  This is due to the very high
bandwidth and associated signal edge rates.

Some examples of common termination problems:

* Overly bright picture with trails following vertical edges, perhaps with
  periodic ringing.  This is due to a missing termination.  Check if the
  monitor is set for Hi-Z instead of 75 ohms.  If there is no switch, then
  the termination may be faulty or the monitor may need an external resistor.
  For BNC connectors, plug-on terminations are available.

* Bright ghost images adjacent to vertical lines.  This may indicate that
  the terminating resistor is greater than the impedance of the cable.
  You may be using Ethernet Thinnet cable by accident which is RG58 with
  an impedance of 50 ohms.

* Dark picture and ghost images adjacent to vertical lines.  This may indicate
  that the terminating resistor is too low - multiple monitors on a chain all
  set for 75 ohms instead of just the last one.  Or, an improper type of cable
  such as audio patch cord.

* Fuzzy vertical edges.  This may indicate a poor quality cable or a run
  which is just too long.  For high resolutions such as 1280x1024, the
  maximum cable length may be as short as 25 feet or less for poor quality
  cable.  Better cable or fiber-optic repeaters may be necessary.

* Other similar problems - check cables for defective or improperly installed
  connectors.  This is especially applicable to cables with BNC or UHF type
  connectors which require a kind of artistic talent to assembly properly and

If only 1 or 2 colors (of the R, G, and B) are effected, then look for
improper switch settings or bad connections (bad cable connectors are really
common) on the problem color cables.

  11.11) General streaks or lines to the right of bright or dark areas

The problem is that on a white background the various objects leave a shadow
to their right. Not a duplicate image but more like horizontal dark streaks
on the white background. Also it seems that high intensity colors display
very bright but low intensity colors are overly dark (almost black).  The
contrast and brightness adjustments may make no difference.

This could be a number of things but they are all in the video amplifier
and probably not the CRT driver board though this is possible.  Dried
up filter capacitors could result in video dependent ripple on the power
supply lines.  Bad coupling capacitors could result in similar symptoms
but probably for only one color, not all of them.

Since all colors are effected, look for something common like a bad power
supply.  With a scope, this would probably be rather easy even without
schematics.  If the brightness and contrast controls do nothing, this
would suggest some fault in their general area or the IC or transistors
they control in the video amps - and that this is not a CRT problem.
Locate the video amp IC if it uses one and locate a pinout - this should
be enough to determine which signals are faulty.

First, do check carefully for bad connections and other obvious failures.

This could also be a symptom of a bad CRT but this would be unusual
with a not-ancient monitor (and not if the brightness and contrast
controls have no effect).

  11.12) Washed out picture

If you can obtain a full intensity raster by varying the brightness or screen
control, then your problem is most likely in the video amplifiers or power
for the video amplifiers.

If, however, the screen control varies the brightness but will not get
a bright raster, you probably have problems either with the HV power supply
or the filament supply for the CRT - is there the normal bright orange
glow at the base of the CRT?  If it is dim or very reddish, there may
be a marginal connection or bad component in the filament circuitry.

  11.13) Retrace lines in picture

During the time the electron beam is returning from right to left at the end
of a line and bottom to top (over the course of multiple lines), it is supposed
to be result in no visible light on the screen.  However, a number of faults
can result in visible retrace lines.

The appearance will likely be a general reduction in contrast from the visible
horizontal retrace on every scan line and two dozen or so diagonal lines lines
(lower left to upper right) resulting from the vertical retrace.

The retrace lines may be either white or gray (possibly with a slight color
tint due to unequal settings of the color adjustments) or a primary color -
red, green, or blue.  Anything in between is also possible but less likely.

  11.14) White/gray retrace lines

Where all colors are involved - the lines are essentially white or gray (or
with a slight tint due to slight unequal settings of the color adjustments),
look for something common like an incorrectly adjusted screen (G2) or master
brightness/background/bias control or a problem in one of these circuits, a
defective power supply or a problem in the blanking circuitry:

* Screen (G2) or master brightness/background/bias control - mark setting and
  then see if a slight adjustment removes the retrace lines.  See the chapter:
  "Monitor Adjustments".  Of course, if this happened suddenly, the problem is
  not due to a misadjusted control though a dirty pot is possible - turn it
  back and forth - this might clean it and restore normal operation.

* Power supply or connection to CRT neck board - insufficient voltage will
  result in the CRT never totally blanking.  Check (usually scan derived)
  power supply components (from flyback).

* General power supply - check B+ for correct value and ripple.  A main power
  supply fault might result in these symptoms (and usually many others).

* Blanking circuit - this may be a part of the video/chroma chip or separate.
  Check waveforms to determine if the blanking pulses are making it to the
  video output.

  11.15) Red, green, or blue retrace lines

Where only one color is showing, suspect an incorrectly adjusted individual
background/bias control or bad part on the CRT neck board for that color.

* Individual brightness/background/bias control(s) - mark setting of pot for
  the problem color and then see if a slight adjustment removes the retrace
  lines.  See the chapter: "Monitor Adjustments".  Of course, if this happened
  suddenly, the problem is not due to a misadjusted control though a dirty
  pot is possible - turn it back and forth - this might clean it and restore
  normal operation.

* Component or connection on CRT neck board - insufficient voltage to or
  incorrect biasing of the video driver for this color can result in the
  CRT never totally blanking.  Compare voltages and signals, and swap
  components between good and bad channels to confirm.

* Blanking circuit - this may be a part of the video/chroma chip or separate.
  Check and compare waveforms of good and bad colors to determine if the
  blanking pulses are making it to the video output.

There is a slight possibility that a bad CRT may result in visible retrace
lines.  To eliminate this possibility:

* Disconnect the filament - all evidence of a picture, raster, and retrace
  lines should disappear once the filaments/cathodes have cooled (15 seconds
  or so.  If there are still visible retrace lines, the CRT is suffering
  from cold or field emission from someplace (may not even be the cathode).

* Turn down the screen (G2) control on the flyback (usually).  If one color
  remains no matter how you set the control, again there is some kind of
  weird emission from the CRT.  However, if white/gray retrace lines remain,
  the problem may be in the screen supply.

See the section: "Bad CRT causing retrace lines".

  11.16) Bad CRT causing retrace lines

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

The TV which I bought last started developing retrace lines after a month or
so of use.  I took it back to the lab for warranty (special deal) and had it
examined by the real experts. They found that even with the filament supply
disconnected and VG2 at 0V the screen would still light up. They could even
see that the electrons weren't even coming from the cathode.  That was with
only the picture tube in a test rig.  So in this case the obvious conclusion
had to be that the  tube was bad, and it was replaced (32" 16:9 SF, very $$).
It had something to do with processing problems during manufacturing of the
electron guns. 

So even if this was a rare case, it *can* happen that retrace lines are due
to a bad picture tube.  It's more usual to suspect the VG2 (screen voltage)
or a defect somewhere in the RGB video path.

  11.17) Red, green, or blue full on - fog over picture

This could be a heater-cathode (H-K) short in the CRT or a failure
of a component in the chroma circuits or video output (driver board).

Don't panic - heater-cathode shorts in CRTs can often be worked around.

Note: before proceeding, it is a good idea to make sure that the screen is
degaussed - else you could be attempting to track down problems with the wrong

Some simple tests can confirm or rule out other possibilities.

* Compare the voltages for the video drive signals to the CRT on the little
  board on the neck of the CRT with the CRT both connected and unplugged.
  A schematic will help greatly in locating these signals.

  - If there is a significant difference especially on the bad color, then the
    CRT is a likely candidate.  Try tapping the neck of the CRT GENTLY (with
    it plugged in and while viewing a picture) to see if it is an intermittent

  - If there is no significant difference, you may have a bad driver or a
    problem in the chroma circuits.

* Look for bad connection/cold solder joints, probably on the little
  board on the neck of the CRT.  Use an insulated stick to gently prod
  the board and its components in an effort to induce/cure the problem.
  Look carefully for hairline cracks around the component leads.

* You can swap components between two colors and/or test with an ohmmeter
  on that driver board to determine what is bad.  The nice thing about
  color monitors and TVs is that there three copies of each of these
  components.  Swapping and/or comparisons between these is an excellent
  diagnostic technique.

* Another simple test: Disconnect the cathode for the full-on color from its
  drive.  If it is still full-on, there is probably an H-K short in the CRT
  since the only way to get each color on the screen is via the cathode
  connection to the CRT neck board.  If it is removed and there is still that
  color, the current must be taking another path inside the CRT.

* Alternatively, interchange the outputs of the bad color with a good one
  by jumpering on the video driver board (on the CRT neck).  If the bad
  color changes, then the problem is in the circuitry and not the CRT.

  Here is the procedure in more detail (example for red full on):

  (From: J. K. Emerine (jkemerine@aol.com)).

  To identify if the fault is in the crt or a control problem try this (WITH

  On the CRT board, lift the output end of the green cathode final resistor.
  Do the same with the offending red cathode's resistor.  Use short insulated
  jumpers to 'swap' drive signals - drive the red cathode with the green
  drive and the green cathode with red drive.  (Note that if this problem
  only occurs after a warmup period, color at turn on will be - well - wierd,
  but it is just a test.)

  - If the symptom returns = 'goes red' the CRT is shorting.  (See the section:
    "Providing isolation for a CRT H-K short". --- sam)

  - If instead the symptom becomes 'goes green' then the red drive leg has
    the fault and the CRT is probably good.  (In this case, there may be bad
    connections or a bad component on the CRT drive board or further back
    in the chroma circuitry. --- sam)

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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]