Having repaired several of these, apart from PSU problems (low value caps, o/c diodes etc), most problems are with the really bad IC sockets, which in worst case you will need to replace. Other than that the op-amps can be just weird, many faults can be cured by replacements. I had one that had uncurable glitches on the trace and added 0.1uF caps across the power rails right on some of the logic chip power pins, can't remember which, sorry. One had a collector transformer breaking down, eventually burned out, not fun. That was fine until collector volts turned up...........then arcing.
