I have an old Toshiba SA-5000 receiver. It has two "Tape" inputs. "Tape1" is
having a problem. More often than not, it always loses the left channel when
the receiver is powered up. If I switch the selector knob back and forth
quickly a few times between "Tape1" and "Source", the left channel might
come back OK, but then after a while, it will lose the signal again.
Everything else seems to be working fine.

Could it be just a simple oxide-related "contact" problem at the selector
knob, or is it more likely to be caused by some failing transistor(s) and/or
capacitor(s)? Could you recommend a good brand of contact cleaner?

I have no experience in repairing amplifiers and receivers, but I do have
some working knowledge with electronics--mostly digital computer circuits. I
would appreciate any suggestion as to possible causes of this type of
circuit misbehavior to get me in the right troubleshooting direction.

Thanks
Al-U