Hi all,

I have an old Japanese Companion A-200G 20W solid state amp (also sold under different brand names; Shin-Ei, Univox U-150R, Teisco) from the late 70s/early 80s that stopped working after my friend yelled a bit too hard through it with a normal voice microphone.

After switching it on it works (clean tone, full volume) for a few seconds and then fades down to a low output which is heavily distorted. Switching it off and back on does not help, ie. it is distorted from the second I turn it on. However, if I leave it unpowered for a longer time, it works for a few seconds after switching on (cold start).

I have some experience with electronics but I'd really appreciate if you could share some ideas where and how to start troubleshooting. Here's a schematic I found on the internet for a newer model with different power transistors and some other minor differences

http://www.aijaa.com/v.php?i=6727635.jpg
http://www.aijaa.com/v.php?i=6727633.jpg


The amp has a single clean channel meaning that this has nothing to do with channel switching problems.