Wednesday, November 23, 2011

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.

No comments:

Post a Comment