Realistic MPA-20

Here's something:
https://www.circuitlab.com/circuit/func75/mpa-20-realistic-audio-amp/

And it seems to use some chipamp, so track its datasheet, the Realistic should be real close to the typical example shown.

Anyway it's more probable that you have a pot/jack/switch/connector, so clean/check them all, for that you don't need a schematic.

There's a reasonable possibility you'll save it.