There is a small PCB that is edge-mounted near the rear of the motherboard
PCB and from what I can discern, it is some sort of High Voltage Detect
circuit. There is a pot on it, but I didn't notice it earlier because it was
completely covered with glue to seal it from adjustment attempts.

Not saying that I am even close to the problem, but this particular area is
looking mighty interesting... I tapped on that little PCB with the handle of
a small screwdriver, and the HV kicked off.

The next thing I did was try to remove some of the glue. It's not hot-melt
glue and it has a considerably resistance to heat---even from a 140W Weller
soldering gun, which I used as a heat source to try and melt off the glue. I
ended up chipping away at it a little. Just enough to free up the pot a
little. I got about 10º of rotation out of it, to the CW direction. And then
I powered up the monitor again. This time, tap on the PCB and it doesn't
affect it.

Next test was a 'sanity test'--rotate the pot back to where it was, and tap
on it while monitor running. And... the HV kicked off again. Rotate the pot
CW just a bit, power up and tap on it... doesn't kick off. 'Could be sheer
coincidence, but at the moment the notion of HV Detect being on this little
PCB and it's apparent sensitivity to vibration seems to suggest that this
board may have been tweaked to the edge of sanity, and just modest aging has
pushed the detection value over the threshold. So turning the pot moves us
back into moderate territory. The raster got a bit brighter too, so I turned
down the Screen pot on the FB to compensate.

Well it did kick off once, since playing with that adjustment, but
restarting it took drasticly less effort (1-2 cycles as opposed to 50 cycles
of off/on switching to get it to stay on.

I have a hunch this little PCB is an X-ray safety precaution. It probably is
there to shut down the HV if it becomes high enough to emit X-rays. So the
question is, why is the voltage excessive as to trip this safety mechanism?
Or perhaps the mechanism is falsely-triggering. I might take the other PCB
from the "parts" monitor and install it in this one and see what happens.
Killed an entire day trying to solve this bugger of a problem.


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Best Regards,

Mark A. Weiss, P.E.
www.mwcomms.com
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