hifi engine

Marantz 2238b channels clipping out/ fuzz

So I've finally gotten out my Dad's old Marantz receiver. It has always had this issue according to him. And he got it new back in the day. A nicely thought out tap on the side tends to fix it temporarily. Anyone have a permanent solution? I just got the service manual and I'm somewhat knowledgeable in tinkering.
Thanks, Jake