🚀 go-pugleaf

I've had this happen to two add-on monitors; one was under warrantee
and was simply swapped; the other was an out of warrantee two-page
grey scale monitor (Viking) that showed this symptom last week
immediately after I heard the sharp snap of a high voltage arc from
within.  I thought this must be a common failure mode, and watched
with interest for responses to your posting but saw none.

This weekend I did a bit of research, then opened up the case and had at
it with my scope.

Up front, let me say that if your monitor suddenly took a turn for the
worse, it is unlikely it will respond to a simple re-adjustment.  You
got video blight, guy.

If you have a voltmeter, a scope, and a basic understanding of transistors,
you might be able to debug the problem.  I did.

On most monitors, the brightness control affects overall electron emission
(bias) and the contrast control affects video amplifier gain.  

If you have a schematic (fat chance), track the video signal thru each
amplifier stage and look for decent gain at each stage.  The final
stage must put a signal of over 50V peak to peak on the video tube
control grid.

[NOTE: The video tube acts like a large capacitor, and retains a
charge of ~15-20K volts even after the monitor is turned off.  It is
not lethal but it is extremely startling.  If you have to get near the
HV lead learn how to discharge the tube. ]

If you dont have a schematic (I didn't) you can try working backwards from
the video tube pins.  Identify the video signal (if you can see anything
on your screen, it'll be there), then trace back until you find a stage
that isn't amplifying.  In my case, it was the drive transistor to the
final amplifier.  Signal in = signal out; not normal behavior for an
amplifier.  Snip out transistor and check resistances with a voltmeter.
In general, c-b and e-b resistances should show about 30:1 ratio between
forward and reverse measurements.

Lots of luck.  You have almost nothing to lose by giving it a shot.  And
it felt great when it came back up.  (By the way, if there was some
precipitating event that lead to the failure, be sure to work on
eliminating that as well.  I'm pretty sure the arc I heard went to the heat
shield on the transistor that died.  I put on a different shaped shield,
blew out dust, and sprayed the whole mess with clear Krylon plastic.

Go for it....     -dan casali