Hey folks --
A friend of mine has an Adcom GFA-555 with a dead left channel. I'm pretty comfortable servicing tube gear and solid state not so much, but these Adcom amps aren't terribly complicated, so I'm taking a crack at diagnosing the problem and repairing it for him.
I had a few free minutes and took a cursory poke at it last night the the GFA-555 service manual and schematics at hand. I did an initial pass to verify that the power supply was working properly, and performed the warmup procedure in the service manual.
I proceeded to measure the behavior of the output finals of the (working) right channel to use as a comparison, and adjusted the idle current for 16mV across the emitter ballast resistors as documented in the service manual.
I then checked for any current flowing through the finals of the left channel, and there appears to be none -- no voltage dropped across the ballast resistors of either the top or bottom halves.
I measured the voltage at the emitter of Q622 and Q624 (right channel) and verified that they matched the 0.58V / -0.58V on the schematic, as well as the voltage at the base (1.1V / -1.1V). Q621 and Q623 (left channel) are not correct, however... I measure 0.5V on the base of those transistors, so I suspect something upstream is amiss... but this thing is DC-coupled the whole way, so I could be wrong about that.
The fuses appear to be intact from a quick visual inspection, but I know I should test them -- just didn't have time last night (they're a pain to get at).
In any case, I plan to take a more in-depth look at this tonight, but was wondering if anyone here has experience servicing these things and could give me a couple of pointers.
Thanks!
(Schematic attached for reference.)
A friend of mine has an Adcom GFA-555 with a dead left channel. I'm pretty comfortable servicing tube gear and solid state not so much, but these Adcom amps aren't terribly complicated, so I'm taking a crack at diagnosing the problem and repairing it for him.
I had a few free minutes and took a cursory poke at it last night the the GFA-555 service manual and schematics at hand. I did an initial pass to verify that the power supply was working properly, and performed the warmup procedure in the service manual.
I proceeded to measure the behavior of the output finals of the (working) right channel to use as a comparison, and adjusted the idle current for 16mV across the emitter ballast resistors as documented in the service manual.
I then checked for any current flowing through the finals of the left channel, and there appears to be none -- no voltage dropped across the ballast resistors of either the top or bottom halves.
I measured the voltage at the emitter of Q622 and Q624 (right channel) and verified that they matched the 0.58V / -0.58V on the schematic, as well as the voltage at the base (1.1V / -1.1V). Q621 and Q623 (left channel) are not correct, however... I measure 0.5V on the base of those transistors, so I suspect something upstream is amiss... but this thing is DC-coupled the whole way, so I could be wrong about that.
The fuses appear to be intact from a quick visual inspection, but I know I should test them -- just didn't have time last night (they're a pain to get at).
In any case, I plan to take a more in-depth look at this tonight, but was wondering if anyone here has experience servicing these things and could give me a couple of pointers.
Thanks!
(Schematic attached for reference.)
