Tektronix 2335 Scope repair - No Trace - HV missing. -> Bad bleed resistor

[BreakingOhmsLaw]

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.

[128er]

Thanks for sharing!

The 2335 was my first scope that I bought a couple of years ago. My unit is from the Bundeswehr Luftwaffe. One time I took it apart out of couriosity. And I remember that I reassembled it pretty quick, because it was all squeezed together inside. And I prayed that I never have to do a repair on it 

[David Hess]

There is no reason for R225 to have any effect.  Something else, maybe an intermittent open, was causing the high voltage inverter to fail.

[BreakingOhmsLaw]

I agree David. Looks like it's not quite fixed yet.
The display starts to pump after a couple of minutes and i can hear something arc around the HV board/CRT.
HV oscillator still looks like it's supposed to according to service manual.
Can't be an overvoltage, as the OV detection circuit apparently latches the oscillator to ground when triggered and requires a power cycle.

[David Hess]

With the cover off in a dark room, you may be able to see what is arcing.

[radioman56]

  Thank You this will help a great deal everyone here has been most kind
My Tektronix scope 2335  Has No Power completely dead purchased it for ?? cant remember maybe was given to me I forget.
any who i have the repair manual and ill do what is offered here for information on that bleeding resistor. again Thanks Much. William