I'm working on a Micro-Tech 2400 that had a channel out. I located a bad O/P PNP on the "LOW SIDE" output. It had a direct CE short, but base was not shorted. I replaced all three PNPs of the bank because the original "sort" was not available. After re-assembly, amp worked fine for a while. While pushing around 100W into 4ohms, the channel quit again. This time, a PNP in the "HIGH SIDE" went with CE short. Now, on power-up, after a few seconds, speaker hums and lights dim for a second or two. I replaced that PNP with one of the left-overs from the "LOW side" and it worked fine for a while. Again, however, while pushing just a few watts, one of the new "LOW SIDE" PNPs went, CE short. No hum, or dimming lights though. Fortunately, there was a spare PNP of the new "sort" so I replaced the bad one. I'm out of new PNPs so am nervous about pushing any power. I compare the voltages on the 12-pin ribbon connectors between the good and bad channels, and the voltages match. I read the voltage across the emitter power resistors during idle and there is considerable variation on both channels from zero to maybe 11mV on both channels. Anyone seen this problem before? I'm not sure how much more info to provide, so please ask.
I found the service manual here, but there is no schematic. Does anyone have a pdf?
Also, what is a generic sub for both NPN and PNPs?