Hi Barry,
By removing transistors and seeing +0.6V we confirmed power transistor is not responsible for the fault (at least not the -33V output issue).
Voltage across R9041 is 0.672V, this turns on the power transistor. However, in this state (output exceeding target voltage), Q7042 should be off and voltage across R9041 should be zero.
This means there is current through Q7042. It could either be faulty Q7042 or faulty Q8051. Transistor Q8051 has a good base drive, it should be able to short across Q7042 and turn it off.
Is C-E terminals of Q7042 correct, i.e. not swapped?
If you short across C-E of Q8051 (with or without Q8051 in place is OK, you can do it at the socket) does output go to ~ +0.6V? If it does Q8051 is bad. If it doesn't Q7042 is bad or C-E swapped.
Is Q7042 base voltage of -29.3V wrt to its emitter or ground? Vbe of -29.3V is unlikely and would damage the transistor.
Ozan
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On Sun, Dec 22, 2024 at 12:20 PM, Barry Breaux wrote:
Ozan,
Many thanks for your help in this matter. Glad to learn as much as I can!
I replaced all the transistors (except the pass transistor) and I still have
-32.5 at the output.
The voltages you question are below.
On Sat, Dec 21, 2024 at 05:28 PM, Ozan wrote:
- Does removing Q7042 and Q7051 make output go ~ +0.6V, if not power
transistor is bad/leaky. ... Yes it does.
When all transistors are in the socket, what are the voltages at:
- Base of Q8051 with respect to its emitter (-33.5V line). ... 0.871V
- Base of Q7042 wrt its emitter ... -29.3
- Base of Q7051 wrt to ground (not emitter) ... -10V
- Voltage across R9041 (=Vbe of power transistor) ... 0.672
- Voltage across R9051, needs to be measured at mV level. ... 0.072
Thanks again for your help!
Barry
