Vintage Synth Repair

Hi folks. I've been part of this list for a couple of years now but I 
don't think I've ever posted here. I'm not a pro technician, just the 
kind of guy who tries to fix everything by himself before eventually 
paying up a professional.

Now to the prob: My Roland D-50 will only play notes and change patches 
if externally controlled by midi. All keys and PC buttons answer to test 
mode but won't serve their normal functions. In other words, they work 
but they lost their ability to control the D-50, which otherwise 
functions fine as a module.

Any hints? I've taken the whole thing apart, in hopes that something 
just needed to be plugged back on, but found nothing too suspicious. I'm 
afraid it might be motherboard-related.

I've got its service manual in PDF format, in case anyone would like to 
take a look.

Thanks
Dan