Power Supply Issue in a 7934 ?

Liam Perkins

I lately picked up a working 7934, SN B021338 that turned out to have
some "UPS mods" to the rear fan housing which necessitated its removal and
disassembly to revert the "body work" to stock format.

In the meantime, and with the side covers in place, I ran the scope
several times for 5-10 minutes to check some plug-ins prior to selling them.

Finishing the last of that and with the scope running I got distracted
by a phone call, someone then came to the door, etc and forgot I'd left it
running. About an hour later I remembered and by that time the scope had
shut itself off due, I hoped, to overheating even though I couldn't find
anything even remotely hot; even the main, rear heatsink was near enough
ambient as to make no difference.

This AM it is dead in the water with the only sign of life at a power
attempt being a couple of flickering neons on a rear-facing board behind
the socket on the CRT, under a clear plastic cover that says, "CAUTION 4KV
Under Cover".

Prior to all this I noticed that, no matter the plug-in or its settings
any trace seemed a bit noisy, leading me wonder about a power supply issue.

TIA for any assistance.



Liam, intuition points to a failing and dead tantalum cap on on of the LV power bus.

The néon on the HV section are trying to start up, but lack power on other circuits causes the flickering.

Just a theory