Welcome aboard! There are several possibilities that could be the cause from power supply, protection circuit, speaker relay, output transistors etc. These old amps can be great sounding pieces and good bargains on the used market IF you can do the work.
Vintage recievers are ticking time bombs and sooner or later they will need repair, and if you have to send them to a tech the expenses can rack up pretty quickly. Having said that if you come across one in very good condition cosmetically that you may want to condiser spending the money To have it completely rebuilt including the boards, caps and solder joints etc.
i did that with a marantz 2285b and its been problem free for 8 years.
Best of luck
Vintage recievers are ticking time bombs and sooner or later they will need repair, and if you have to send them to a tech the expenses can rack up pretty quickly. Having said that if you come across one in very good condition cosmetically that you may want to condiser spending the money To have it completely rebuilt including the boards, caps and solder joints etc.
i did that with a marantz 2285b and its been problem free for 8 years.
Best of luck