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Hey guys,
Working on an Adcom GFA-555 power amplifier. It cut out, and after opening it up i found a burnt R21. I replaced that, and then checked around for shorts. I ended up replacing Q17-Q20, with MJ21194 in place of the 2SD424. Also replaced resistors R24-R27, and the driver Q12. D2 diode tests fine. Powering it back up i;m getting some incorrect voltages. My +/- supply is 75/-75vdc. Once i get to D2 however, i'm getting -73vdc on the other side of it, instead of the -55mV that i'm getting from the working channel of the amplifier. It would seem to me that Q12 is at fault, but it appears to test okay. Any ideas of where i should be looking?
Adcom-GFA-555-Service-Manual.pdf
Working on an Adcom GFA-555 power amplifier. It cut out, and after opening it up i found a burnt R21. I replaced that, and then checked around for shorts. I ended up replacing Q17-Q20, with MJ21194 in place of the 2SD424. Also replaced resistors R24-R27, and the driver Q12. D2 diode tests fine. Powering it back up i;m getting some incorrect voltages. My +/- supply is 75/-75vdc. Once i get to D2 however, i'm getting -73vdc on the other side of it, instead of the -55mV that i'm getting from the working channel of the amplifier. It would seem to me that Q12 is at fault, but it appears to test okay. Any ideas of where i should be looking?
Adcom-GFA-555-Service-Manual.pdf