Need help on identifying amp defect Options

[soulsearchingsun]

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

[pdq]

The output stage of a solid state amplifier generally consists of a pair of transistors, operated in two different modes (one common emitter and one common collector, if that matters). They operate in what is referred to as "push-pull", i.e one drives the speaker output positive and the other negative.

If one of the transistors dies then you only get "pushing" or pulling". Without the current draw from its complement, the surviving transistor is starved for current and its gain is greatly reduced. More importantly, either the positive or negative lobes of the waves are practically cut off, making for a severely distorted waveform with lots of nasty overtones.

Back in the '60s I had a receiver that blew output transistors several times. I built it from a kit, so it was not difficult to replace them. But, since the problem was a bad design that made it prone to this, I eventually gave up and replaced it.

[soulsearchingsun]

If one of the transistors dies then you only get "pushing" or pulling". Without the current draw from its complement, the surviving transistor is starved for current and its gain is greatly reduced. More importantly, either the positive or negative lobes of the waves are practically cut off, making for a severely distorted waveform with lots of nasty overtones.

IMHO this is not the case with my amp. The recorded material shows a full sine wave. Plus it's not that much distorted. It's mainly added noise. I also thought I could hear a pitch difference between L & R, but this of course can't be true. Something must have tricked me. I suggest to listen to my file and comparing left to right by using the balance control.

Uploaded FFT images to this thread.

[Nessuno]

Try to repeatedly turn back and forth the balance and tone knobs, if there are, while playing or the volume one to full scale if is a coaxial one (this latter with no signal in, of course!) and see if anything changes: it could simply be oxidation on rotative contacts.

Just my two cents…