Fender Frontman 212R distortion problem

[goldtop5]

I have a Fender Frontman 212R amp that's having a problem with distortion on the clean channel. When the amp is turned on, it would be as clear as a bell until it heats up. It's also distorting when I plug a guitar into the poweramp in jack. I've replaced 2 output transistors (2sc3263, 2sc1294), but the problem still persists. I've noticed resistors r144 and r145 were getting extremely hot. Not sure if that could be the probable cause. Any ideas?

[diydidi]

Sounds like a problem with the biasing circuitry. Do you have a scope to put on the output?