Re: Tek 576 - step generator power amp - how to check?


 

It seems I found the culprit... Q241, probably open. So it was the feedback section, thanks Ozan for your help.

I made a bunch of measurements before, leading to very different readings depending on NPN or PNP mode. Below is the collection of readings.

Thats the 4th transistor I have to change since I began repairing some days ago... and I have the impression that 3 of them were not faulty at the beginning. How can that be? Is my presence that corrosive?

cheers
Martin

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On the 0 Volt line:
+12V in NPN
+1V in PNP
(now its 0,000V)

The base of Q241 was reading -12V

The collector of Q235 was reading
+ 67V in NPN
+ 51V in PNP

The emitter of Q233 was reading
- 0,42V in NPN
- 0,58V in PNP, rising to -8,8V beginning at 5uA/Step until 200mA/Step


On the output of the Step Generator main amp, readings with a 3k load:
#NPN current mode:
+ 0V @ 0,05uA/Step
+ 10V @ 0,1mA/Step
+ 32V @ 0,1mA/Step (thats where my 30V came from)
+ 43V @ 0,1mA/Step
Voltages not regulated, decrease when increasing load, increase when increasing current limit.

#NPN voltage mode:
+ 30V @ all levels (thats where my 30V came from)
at 2V/Step the signal shows steps between 28 and 30V
Voltages not regulated, decrease when increasing load, increase when increasing current limit.

#PNP current mode:
approximately correct output, regulated

#PNP voltage mode:
- 10V @ 0,05V/Step, regulated
- 26V @ 2V/Step, regulated

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