Late 70's Acoustic Model 220 Distorting
I got a late 70's Acoustic Model 220 head at a flea market for $40 yesterday, and it sounded great and everything until today. The longer I had it on and was playing through it the more distorted it started getting. I was using a 77' Fender Jazz during one test, and an 82' Rickenbacker 4003 so none of the hot active basses. I tried it in both the 0 db and -10 db inputs, and it appeared to get worse on the -10 db actually. When I shut it off though it played non distorted for just a second while it was cooling down to power off though, so possibly a heating issue?
Its sort of beat up, but still sounded great at first so I'd love to fix this thing if possible!
If its a major thing what do you think the repair bill would look like? (I'm hardly an amp engineer, as instruments are more my specialty).