Sam Goldwasser wrote in message
...
Jim Yanik writes:

Sam Goldwasser wrote in
:

Jim Yanik writes:

"Ancient_Hacker" wrote in
oups.com:

Oh, there's also a big voltage divider resistor (the white ceramic
thing) Any moisture, dust, or leakage there and you tend to get
unstable HV.



The HV thick-film resistor network(TEK-made) has the HV feedback for
regulation on one side,and the CRT focus divider on the other. Either
one can go out-of-tolerance,and the metal pins can crack and be
intermittent.I've never been successful in resoldering them,either.

I do not think this part would be the cause of his intensity
modulation.

It may be.

New followup: I've now got the thing open. Since it appeared heat
dependent,
I used a rubber tube to blow on various components in the HV area.
Blowing on the thick film resistor network even immeidately after
power-on and the beam appeared (already messed up) seemed to make a
big difference causing the symptoms to disappear entirely for awhile.
This could not have been heat related because it was within 10 seconds of
powering up so nothing really could get hot. Then, after touching the
thick film assembly perhaps with a few wiggles thrown in, the problem
has disappeared entirely and has not reappeared. I've since cleaned
it with alcohol and I will be running the scope off and on for a few
hours over the next few days on the bench to see what happens.

Of course, the instant I put the cover back on and replace the scope in
its
hard-to-reach spot, it will screw up again.

So the working hypothesis is that indeed either contamination or cracked
joints to this part.

If cracked joints turns out to be confirmed with the problem reappearing,
I would probably try conductive silver Epoxy first at each of the
connections to the thick film itself rather than solder. If that
didn't last, then what about fashioning a replacement from discrete parts?
It might be a bit of a challenge to get high voltage resistors to fit in
the space but seems like it could work. It's only 4 resistors, though
admittedly 3 of them are high resistance (6.57M, 25.6M, 24.5M, and 550K)
and at least 2 of them need to withstand almost 3 kV.

Any comments?

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Some woven glass sleeving , cut to required lengths and formed into a
compressable and insulated pad or more pads if you cannot localise better
and cautiosly compressed by cable tie/s around the thick film ?
I've only done similar on low V SM ICs that are covered in black epoxy ,
boards as well, so impossible to re-do the SM solder , to the ceramic
boards, and is a throw away job otherwise.

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Diverse Devices, Southampton, England
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