Tapping on electronics to get them working is often a sign of bad joints. You checked the balance and volume, but the problem could be in
the right channel amp circuit. Look for bad joints, bulging caps, cracked/blown transisors, burnt resistors...etc
You will see lots of talk about changing caps in these forums. Electo caps are good for about 15 years before they start to degrade
and a lot of items we own are now well past that time period.
If they are run hot, like in a reciever they will degrade much faster.
You can try changing the caps in the right channel and see if that helps....it shouldn't cost too much for high temp audio grade parts.
If that helps you can justify recapping the power section, the other channel and the rest of the electro caps.
DaMN!! That's a nice classic pickup, but it looks like it's a big ole' stack of discrete logic and one hell of a project you
have ahead of you.
I would start by cleaning everything, brush out and reseat all the interconnects (this might fix the missing collision logic)
and inspect the boards for blown transistors, burnt resistors, corroded traces, mouse droppings...etc
You should replace all the electrolitic caps since their lifespan is about 15-20 years.