Hi everybody,
just repaired my Tek 2335 scope.
It died on me while doing a measurement. The screen started to pump, as if the HV or deflection voltages were failing. After power cycling the scope, it showed no more trace.
Checked all the test points, the 102V were missing, showing just 13,8Volts.
Upon further investigation, i noticed that the HV oscillator was not running properly.
Having crawled though the schematics, I ended up at the 40V Unreg supply. While it was fine with about 62V, the only fault was the bleeder resistor R225 (51kOhm) had failed. It had about 20MOhms.
Didn't give it much though at first because it's just a bleed resistor. To make a long story short: I ended up soldering a replacement piggy back onto the dead resistor. The board is a pain to remove, and I preferred not to desolder it in order to go easy on the old PCB traces.
Bingo, HV returned, trace was back.
I can't explain why the bleed resistor should cause this. Perhaps the extra ripple it causes is needed to start the HV oscillator? That would surely be some good old Tek analogue voodoo magic.
Or the voltage on the CAP might be just shy from the threshold of the overvoltage circuit without the bleeder, as the unit was designed for 220V rather then 230V. Your guess is as good as mine.
Anyway, just thought I'd place the info here in case someone encounters the same problem. Carry on.