Motorboating audio

motorboating is caused by three main things
the controls, contacts switches in the unit are dirty, no longer making contact?
a transistor is blown?
you have the unit hooked up wrong?

But you have to decide if an obsolete unit is worth putting money into. I would charge $50 at least to fix it in my shop, and there are NO fisher parts available for it.
Maybe it's time to change over to CDs?