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Need help on identifying amp defect
soulsearchingsun
post Mar 1 2012, 14:26
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I'm having issues with my mid-90s Sony solid state amp.
In particular the right channel is of lower volume (~-12dB in my measurements) and the sound is distorted in a way that there is noise added when I feed a signal to the amp. On silent parts, there is no additional noise. I ruled out the signal source and the speakers as the cause, it has to be the amp. I looked inside to see if I can find burned components, but I haven't had luck yet. I can't see the whole circuit board without disassembly (which I haven't done yet) though.

I uploaded a file here, with the following setup:
sound card output -> amp line level input; amp speaker out -> sound card line level input & speakers

Could anyone give me hints where I should look for the defect? Is this typical for some specific defect?
Thanks a lot!

edit:link now pointing to upload thread

This post has been edited by soulsearchingsun: Mar 1 2012, 14:32
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stephan_g
post Mar 4 2012, 20:53
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The first thing I'd associate with scratchy low-level output on a mid-'90s Sony unit is bad contact in the protection relay. (In which case you should try the other pair of outputs.) Next, a resistor near the output gone high, a bad transistor, or bad solder joints. Obtain a service manual if possible, and search for other people's problems with this model.
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soulsearchingsun
post Mar 5 2012, 20:16
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Thank you for your opinions. Seems I (sort of) solved the problem today. I wiggled the Input/Source selection PCB with it's electric source selection component and the noise was gone. The channels are 0,9 dB apart now. So it seems to be either mechanical (something inside source selection mechanism) or solder joint related. As soon as I have a free weekend, I'll pull said PCB out and resolder suspicious joints (if I can find any) and/or clean contacts.
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