Star Tech Journal

STAR*TECH
Journal
February 1 996
CD
IVIC>NITC>R REPAIR
WnHourScHEMATtcs (PART
2)
Douglas "Enzo" McCallum
Shiawassee Technical Services
Lansing, Michigan
The essence of troubleshooting is isolation of the problem.
Identify the symptom, what does and does not work. A monitor is a system.
Power goes in, signals go in, pictures come out. If is a process.
Troubleshooting is a linear process as well, in parallel with the system.
If we follow the flow of the system it will lead us to the fault.
ON TO SECTIONAL PROBLEMS.
COLOR AND VIDEO PROBLEMS
There are color or video prob-
lems that affect the picture ap-
pearance. Sweep problems in-
cluding deflection affect the
shape of the image.
Color problems have the built in
advantage of multiple circuits.
There are three color circuits so
you can compare readings and
waveforms between them. If a
color is missing or stuck on full,
suspect the driver transistors on
the neck board. Drivers are gen-
erally in a TO 126, TO220,
Power supply trouble can affect
the other two. Sync affects the
stability of the picture.
TO202, or similar tab top case.
Smaller case types generally con-
tain signal transistors, no the
power types the color guns need.
Look at these transistors with
your scope, comparing the good
color with the bad color. If the
signal on the base of the bad
color driver is about the same as
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COLOR GUN
& CRT CIRCUIT:
Note filter cap on video B+ line at
the common end of the transistor
collector load resistors. Failure of
this cap results in overbright
picture with poor contrast. Resis-
tors in series with CRT cathodes
protect the circuits in the event of
high voltage shorts within the
tube.