In any case, refurbing that thing will involve replacing the dozen or so electro caps in it. They're over fifty years old, if original. The resistors, non-electro caps, and semiconductors don't have the same wear-out mechanism that electro caps do. Replacing electros, remelting all the solder joints, and cleaning/replacing all mechanical switch contacts will cure the vast majority of old effects.
Even if that's not all of it, once you get there, you can apply the techniques of "What to do when it doesn't work" and "When good opamps go bad".
Come on, there are only a dozen electros in there, all 1uF and/or 10uF as I (dimly...) remember. How bad could it be?