I have limited experience with repairing these boards but capacitors degrade over time and replacing them could help. Its a shotgun approach but you may get lucky- the replacement is pretty cheap and not too difficult. If you find someone would can debug down to the component level that would be ideal.
I would start with a thorough cleaning of the board(s). Mine were covered in years of oil and ?. Being able to do a detailed inspection will be much easier to see an open trace or a bad solder joint, or burnt component. Using a meter in suspect areas will confirm continuity.
My approach, as I've mentioned before, was to get rid of all the electronics and start over because I have very little knowledge of electronics repair.
Either way its a big job but I found it easier to work with new stuff that I knew would work than old stuff that will always be suspect.