Adcom GFA-2535: DC on output
I have a Adcom GFA-2535 amp. It is basically a pair of two channel amps (about 60 watt / channel). That is, there are two seperate power supplies for each "stereo amp".
Well, rather suddenly amplifier A did not work - both channels went bad. The voltages rails on amp B (the side that works) measure +/- 47 volts at the fuses. On amp A (the bad side), the rails measure +/- 49 volts.
Then I looked at the outputs for DC. On the good side (amp b) the two channels measure about 15 - 25 mV DC. However, on the bad side (amp A), the speaker outputs measure 4.5 and 30 Volts DC. Fortunately, no speakers were harmed.
Does any one have a service manual (pdf) for the Adcom GFA-2535. I had thought about checking the likely culprits by measuring on the good amp and comparing them to the bad amp. But that could take forever.
Hopefully someone can provide you a copy of the service manual, if not you can purchase one directly from Adcom for a nominal fee. The amp more than likely has some shorted outputs and possible driver stage problems. The 2 volt difference in the power supply is not a big difference. The high Dc on the one channel is though. The first thing I would do is inspect the outputs, emitter/ballast resistors and drivers. Also very closely inspect all the parts of any signs of damage (burnt, discolored). The need for basic test equipment is a must and just assuming that a handful of parts will get you back up and running would be a lesion in futility without testing. The question is “why did it fail” driving a low impedance load very hard, shorted speaker of shorted speaker wiring? These amps usually are rock solid unless something like this happens. Parts can fail also if exposed to repeated power /current surged also as any electrical device can. Let us know what test equipment you have to trouble shoot with also.
My visual inspection did not reveal anything and I was not terribly worried about difference in rail voltages.
Why it happened is interesting and I have no clue at this point. Nothing was disconnected or reconnected, so I doubt the outputs were shorted by clumsy fingers. It happened in both channels, that is why I initially assumed that it was "early" in the circuit. so I checked the rail vlotages. The heat sinks were not overly warm. I did not want to guess on where the emmitter resistors were until I had a schematic.
I will continue...
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