Amp powers but has +35v on CH2 (Which is actually labeled CH4 on the front of the amp, but CH2 on the PCB).
CH2 output transistors are measuring OK but were driven to +35vDC on base and emitters. The PNP outputs have -35v on collectors. Removed outputs.
Used process of elimination to determine Q62 (2sc1845) had an open junction from E-B off the PCB. I replaced this part with MPSA42 installed with legs 2&3 flipped to match B-C-E on the board.
I then powered the amp up and the MPSA42 overheated within seconds and same E-B off the PCB opened. Voltage on Base of outputs went to -35vDC. Also Q70 which is a 2sC2682 shorted legs 1-3.
Im not sure if the MPSA42 was the cause or if there may be something else going on with this amp. All diodes, resistors, and caps are measuring OK.
CH2 output transistors are measuring OK but were driven to +35vDC on base and emitters. The PNP outputs have -35v on collectors. Removed outputs.
Used process of elimination to determine Q62 (2sc1845) had an open junction from E-B off the PCB. I replaced this part with MPSA42 installed with legs 2&3 flipped to match B-C-E on the board.
I then powered the amp up and the MPSA42 overheated within seconds and same E-B off the PCB opened. Voltage on Base of outputs went to -35vDC. Also Q70 which is a 2sC2682 shorted legs 1-3.
Im not sure if the MPSA42 was the cause or if there may be something else going on with this amp. All diodes, resistors, and caps are measuring OK.
I installed an MJE182G in Q62 and it survived a little longer but is drawing current and still +35v on output bases.
Before Q62 blew again, removed Q61 and Q60(To the left of Q61) and the amp powers up OK. I replaced Q61 with an MJE172 and it powers up OK. Anything I put into Q60 and it draws excessively. The MJE182 in Q62 eventually blew out.
Before Q62 blew again, removed Q61 and Q60(To the left of Q61) and the amp powers up OK. I replaced Q61 with an MJE172 and it powers up OK. Anything I put into Q60 and it draws excessively. The MJE182 in Q62 eventually blew out.
When the feedback loop is open (outputs removed...). The voltages on the various components in that loop are very nearly useless.
Why did you remove the outputs?
Why did you remove the outputs?
I removed them for safe keeping and to test them out of circuit. They're big TO-264 JL4315/JL4215. Should I put them back in?
Yes.
Are you sure that those are the right numbers?
Power through limiter and watch the heating of all heatsink mounted components.
Are you sure that those are the right numbers?
Power through limiter and watch the heating of all heatsink mounted components.
Im back onto working with this amp. It was playing for a bit but then CH4 got +35v across terminals again. I found Q62 shorted B-E again. I replaced the MJE182 with a more robust 2n6488, with a single sync on it. It still gets raging hot but only when amp is brought into severe no-load clipping. The amp draws 1.7A at idle, but when clipped with no load will pull about 3.25A. None of the other 3 channels have this problem. Sometimes while Q62 is very hot if I power the amp off/on R186 near the light blue cap starts to smoke.
Post a photo of the power supply driver IC and the surrounding area. I want to see if it looks familiar to point me in the right direction for a diagram.
I'm out of suggestions for now. I've looked at diagrams (trying to find something similar) for hours. The dual muting transistors look like PPI but it stops there. The R245 at 220 ohms is common in many budget amps. The 4 optocouplers looks like Zeff but the amp isn't using a 3525. I can't find anything that covers all of these features.
If the amp is worth the time, drawing up a diagram may help but there's no guarantee.
If the amp is worth the time, drawing up a diagram may help but there's no guarantee.