Pioneer A-757 tone control not working

Hi people,

I have got this heavy integrated amp from Pioneer on the bench. It was found in a garden shed, where the owner left it as he moved. First I wanted to recycle it for the parts, but after I opened it, I couldnt do this to the amp.
So I connected some "test-speakers" and powered it up. Ok, no smoke, no flames, but a weird somehow mechanical noise. First I thought of some kind of protection-mode relay, but, after asking google, it seemed to be a well known failure, which made the loudspeaker output relay "flatter".
Source of this problem is a tiny 3,3µF/50V electrolytic which dies pretty often of heat, because of the not very well placed power supply pcb.
Thanks to the www this was not a big problem. I replaced it with a 3,3µF/100V and the loudspeaker relay pulls as it should.
I powered the amp up again, and it played really well, as long as in "direct"-mode. When switching off "direct", there is music coming, but only really silent, with the volume knob turnd right to the max. .
Again I asked google, and found a video on YT which was about "tone control not working". Sadly there was only this one video on this topic, and it is spoken in pashtu (I think). But with the help of the pictures it was clear to see, there was a problem with the 7824/7924 voltage controllers. I measured these voltage controllers and they are working (+/-24V at the output). I think there wouldn't be any music if they weren't ok.
Any tipps/experiences you could share with me? Where should I start?


  • 3,3µF-50V.jpg
    287.8 KB · Views: 76
  • 3,3µF-100V.jpg
    257.4 KB · Views: 72
  • A-757 bench.JPG
    A-757 bench.JPG
    372.9 KB · Views: 91
  • 06 power supply II.JPG
    06 power supply II.JPG
    216.1 KB · Views: 75
  • tone amp - input selector.JPG
    tone amp - input selector.JPG
    351.3 KB · Views: 81