Check the filter caps and plate resistors (although if they are metal film resistors, they should be fine). In all my old Fenders, the plate resistors start going and I get all kinds of crackling, hissing, etc. Anyone who has a lot of experience with AB765 Fenders should be able to get it back in shape, but Ed's advice is right on. The Mothership has all the high quality parts with which to do the repairs. You don't want cheap components being thrown in there.