You're really asking an electronics questions, not a camera one. If there's an electronics repair subreddit this question would be better suited there. Old electronics just sometimes conk out, but they're usually able to be repaired, and not turning on at all is often a relatively easy problem to fix, especially if it still worked a few years ago.
But you're going to need at least some familiarity with electronics. I found this book really helpful (this is an Amazon link but I got it from my local library).
Maybe this can help https://www.amazon.com/How-Diagnose-Everything-Electronic-Second-ebook/dp/B016EEU3S4
How to Diagnose and Fix Everything Electronic
I don't have an answer, but perhaps these ideas could help.
First, review the schematics (there may be clearer ones elsewhere--this one is a bit rough). How does what you're experiencing compare to what's on the block diagram? Are you able to rule out any particular sections? Left or right only, or both? Does it happen when only headphones and no speakers are attached (no speaker wires connected at all)? That should help get you into the right zone to test components.
If you have an oscilloscope you can feed a signal into the amp and trace it (don't just grab a scope and do this arbitrarily--there are things inside that can really zap you or the scope!). I recently scoped a Kenwood KR-3060 that was making a buzzing sound only on the right speaker after about 1/4 volume. Things sounded fine up to that point with speakers, and no issues at all when only using headphones. Perhaps the speaker impedance caused a bigger pull through the circuit compared to phones-only? This narrowed the issue down to the first even-numbered (right channel) transistor in the power amp stage. (I'm still new-ish to debugging and haven't actually tested the transistor yet.)
If you want to get more into electronics debugging, How to Diagnose and Fix Everything Electronic could be useful. It's not a perfect book, and it's long, but it's a pretty easy read and you could skip the sections on video, cameras, MP3 players, etc.
did you know that through your own adventures or learn like I did from the book https://www.amazon.com/dp/B016EEU3S4/ref=cm_sw_r_cp_apa_glt_J1ZE0QZPFQF58X0WHAME?_encoding=UTF8&psc=1