I'm an electronic test technician for a company that makes power supplies and I have been tasked with fixing a friend's PPI PC 1800.2 mono amp. I found and replaced some burnt components but upon powerup they are trying to burn up again. I took various voltage measurements and I see the problem in the output stage but I'm having a hard time figuring out the root cause. The first time i replaced Q9,Q10,Q11 and he gave it back to me with R31 and R33 burned up but the transistors seem fine. When I apply power I get the voltages as you see in the attached picture. It seems maybe Q12 and Q13 not turning on could be part of the problem but I'm not very savvy with this Triple Darlington arrangement. Any help is appreciated.
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Moved to car audio.
(Q10 and Q11 with minus 10 volts on the base and zero on the emitter... that points to an open circuit condition internally within the transistor. Worth rechecking the readings)
I was using the center point between r31 and r33 as return for all the measurements but I rechecked and that is what I have, I guess I should have a diode drop shouldn't I? Hmm.. I guess the three transistors are dead again too?
I meant junction of r31 and r32. I took Q9, 10, and 11 out and tested them, they were all open just as you said. I guess the busy circuit design distracted me from the easy solution. I put new transistors in and it was pulling high current when I applied power. I then adjusted vr1 to have a similar voltage as on the inverting side of the circuit and the idle current fell below the 2A spec for this amp. I have a nice clean signal coming out of the output so I think it's time for a field test. Thank you for the help.
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