I've combed through a bunch of things in this amp. It came in with PS BJTs blown, and some cap leakage/damage in the middle of the board.
Replaced:
5x PS BJTs; 2n6488
Outputs in the RIGHT channel: 2n6488/2n6491
Right channel drivers: MPSA06/56 (4 total transistors)
Right channel 5.6ohm 'din' plug resistor
2x 470u16v caps
Serviced several electrolyte burns, replaced several suspect resistors. Cleaned areas with acetone. Repaired a few disconnected traces.
So now the amp 'boots' and idles at 0.5A. Both channels have audio but the RIGHT channel has -1.3vDC. I also sense similar voltage at the new 5.6ohm resistor.
In this photo, right channel is on top.
Any clues as to why there might be -1.3vDC on right channel?
Replaced:
5x PS BJTs; 2n6488
Outputs in the RIGHT channel: 2n6488/2n6491
Right channel drivers: MPSA06/56 (4 total transistors)
Right channel 5.6ohm 'din' plug resistor
2x 470u16v caps
Serviced several electrolyte burns, replaced several suspect resistors. Cleaned areas with acetone. Repaired a few disconnected traces.
So now the amp 'boots' and idles at 0.5A. Both channels have audio but the RIGHT channel has -1.3vDC. I also sense similar voltage at the new 5.6ohm resistor.
In this photo, right channel is on top.
Any clues as to why there might be -1.3vDC on right channel?
today the negative voltage across right speaker terminal I noticed was floating between -1vDC and -6vDC. Negative voltage was observed all the way back to the input and output pins of the center mounted op-amp for that channel. I removed that opamp but still found negative DC on the pads. Combed through all the resistors and most of the capacitors and didn't find anything obvious at this point
I then flexed the pcb with my hands and all of a sudden one of the blue rectifier diodes dead shorted, FR301. They look a little discolored in the photo. Guess also there is a fantom connection somewhere.
I then flexed the pcb with my hands and all of a sudden one of the blue rectifier diodes dead shorted, FR301. They look a little discolored in the photo. Guess also there is a fantom connection somewhere.
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I believe this amp is fixed. I ended up replacing the center opamp and the vDC on the right channel dropped to a stable 0.1vDC. I also sucked and re-soldered a bunch of contacts under the board and seems to have helped. Amp plays well up to clipping on both channels now.