Hi everybody. I recently bought a vintage Yamaha RA-200 from a  guy, he said its about early to mid 70's. This amp actually has 3 amplifiers. 1 for rotating speakers, and two for the top and bottom half of the 4x12s. Now the problem is that in one of the amps for the 4x12, one transistor (2SC783) started getting really hot and the sound started distorting. Now I have no idea what the problem is, and I want to get it fixed. I checked the resistors going to the base, and they are good, so I don't know what else could be the problem. Thanks for any help!

The transistor is labeled TR4 on the schematic.