Sam Goldwasser wrote in
:
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?
I've wondered if low melting point solder like what comes in the ChipQuik
desoldering kit might work on thick-film connections.It wasn't available to
me at Tek.The conductive silver epoxy may be a better idea. Trouble if you
have to resolder the thick-film back into the motherboard.
Alternately,you could fashion a small PCB to fit where the thick film
goes,and solder your resistors to the PCB. I've not tried this,though.
--
Jim Yanik
jyanik
at
kua.net