Hi,
It looks like this was originally a kit, so the manual gives lots of testing and adjusting help.
I noted that it looks like all the IC's are using sockets. I would carefully remove each one and then re-seat it in its socket as the contacts can become oxidized over time. I would also add some deoxit to any mechanical switches used.
The kit didn't look to be using any tantalum caps which would for sure be suspect, but you could check the electrolytic ones used either by sight for leaking, or with an ESR meter.
73