Pioneer SX-950 1976 to 1977 vintage

[kidd]

Hello, trying to get some information as to the problem with this stereo, turns on plays music on the AM side and then when I go to the FM it plays music and if I try to increase the audio level it activates the protect relay and shuts down. If I reduce the volume and switch off then back on it will play again, then the same thing happens, the FM light as well as the Stereo light are not coming on neither, but if I switch between Stereo and Mono there is a definite difference in sound, FM more spacious then Mono, where should I start to look, also need the schematic if anybody has one that I could get a copy of. I have heard that it might be the relay itself and that it should be cleaned the contacts that's I have found out so far, thanks.