Re: Problems with t935a HV supply


I am still contemplating a strategy to debug the T922 HV supply that is now
putting out about 9 KV yet is drawing excessive current (about 200 ma).
Scoping a live HV supply putting out 9KV is a bit tricky.

Maybe bring up on a variac? Can only run power for about 5 seconds before
magic smoke will escape from R458 due to excessive current.
The regulation loop around Q454 and Q446 tries to keep cathode voltage at -2000V. With a lower voltage at +100V it will try to bias Q458 harder so it may not help the high current issue.

Since you already replaced R458 and (C458?) and other components, another possibility is the HV rectifier diodes CR463 and/or CR465. If you search the forum with the query "DC restorer" you will see a lot of good information about how to test for leaky HV diodes, and recommendations. A simple diode check on a DVM is not enough, they need to be tested at a higher voltage.


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