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Adcom GTP-500 II problems

klamsoss

klamsoss

Be willing to fail and try again.
I've been away from the repair bench for a while but I'm finally back at it, clearing out some pieces I have neglected for a long time. First up is the adcom preamp mentioned above. I have the service manual, but before I dig in too deep, I figured I'd stop here and see if anyone has experience with these.

Symptoms:
both channels have static when the volume knob is rotated (I've already hit the pot with deoxit, no change)
left channel barely passes any signal to the main outputs. like, when right passes 4mV, left passes 0.1mV with 1khz test tone
both channels pass signal to the headphone jack
tuner seems to work just fine but there is background hiss
the balance knob causes static, also doesn't appear to affect the right channel (based on headphones). All the way left, I get very faint signal in both channels, all the way to the right, and through headphones I get the same volume in both channels as when the balance pot is centered. There isn't enough main output to left channel to determine if the balance pot has any effect on output to the speakers.

At it's age, I figure it's probably going to need some electrolytic capacitors replaced but before I start replacing parts willy-nilly, I want to figure out if there is something else going on.

Any guidance is greatly appreciated.
 
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klamsoss said:
experience
Click to expand...

Not enough to count for much. I think you are going to need to do some signal tracing, and the one thing I would suggest is to start with the TC9152P chip that handles input switching - i.e., make sure that whatever input is selected shows up at the chip's outputs.

The headphone issue sounds to me like the headphone jack is failing to make the ground connection.

I wouldn't worry about blanket replacement of electrolytics until/unless you resolve the other problems. Having said that, though, it is very likely that there will be some electrolytics that are installed with glue, which will have gotten corrosive by now - so, replacing those caps and eradicating the glue will probably be on your to-do list eventually.
 
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