Hi!

I'm a happy owner of 30 year old GF-9191XR which my dad bought back in old good days. It's in a good cosmetic shape but unfortunately it has a problem with sound reproduction. At the beginning there was loud crackling sound from right speaker, even when volume sliders were at 0. https://www.youtube.com/watch?v=t42w0Ct2hsw

Then I removed rec/pb switch and cleaned it thoroughly. Though this unpleasant noise disappeared there was still some noise coming from the right speaker. https://www.youtube.com/watch?v=8Ur1nUpjKb0

I tried to clean switches with Kontakt 60 to no avail. Then I decided to isolate problem. I've disconnected audio amplifier inputs by desoldering pin 8 of both left and right channels. When I connect speakers back I hear same LOUD crackling noise from right speaker but left speaker is silent. If I ground input pin of the right channel amplifier noise is gone. I'm lost at this point. What should I look for?

Thanks!

I mean at first when I'd disconnected right channel input high noise from only this channel suggested there was a problem with amplifier circuit. But when I ground input noise is completely gone and it means amplifier is OK? I'm confused

Well uhmm... Do you have a balanced stereo sound out of both speakers now?

usually if i clean the switches will take care of that, but this seems to be a more complicated issue. 

Yes, even at 15-20% volume this noise gets masked by music. But it's just annoying to know that there's something wrong.

a) Yes, even in mono mode I can hear noise from right speaker only.

b) Yes.

I'm not sure about voltages though. I've ordered ESR meter and will check caps as soon as I get it. Also I'll try to swap inputs to amplifier to see if the noise is persistent in one amplifier or it's some other circuitry.

I don't think it's switch at this stage. Swapping input channels would probably prove it's amplifier problem.

Slow down with the desoldering and swapping inputs. You just need a digital multimeter to check voltages as per the circuit diagram you have. I am pretty certain that will tell you if there is a problem in the preamp or amplifier stages. You could also do with a signal tracer, this will allow you to check where you are getting this weird noise and where you aren't and therefore isolating the problem area.

Unfortunately checking voltages at IC pins revealed nothing. I just reverted to swapping inputs and as I would expected this proved problem is at amplifier section. Connecting speaker directly to IC output also gave noisy output. I don't think anything wrong with IC so most probably one of the caps do indeed suck.

Try to clean headphones socket.

Like docs says. You need to trace the signal with a oscilloscope and see where it goes wrong. This is the easiest way to find the wrong component. Much better than guessing which cap could be wrong etc. 

Hi JT Techno,

Yes, I'm referring to the long bar behind cassette mechanism. Crackling sound is practically gone but there's still noticeably noise coming from one of the speakers.