Vintage Synth Repair

Hello all, I'm working on a Moog Source and wondered if anyone here has experience with repairing these?
My Source powers up, all membrane switches register as does the data wheel, all parameter values change as expected but there is no sound. The power supply is fine with all supply voltages present. I have signals on the synth board eg VCOs and LFO but cannot adjust any values - I'm thinking there is a problem on the digital board but my experience with digital electronics is limited... therefore any help or suggestions would be greatly appreciated.
My Source is serial no:1788 and has version 2.2 eprom.
Many thanks in advance, James.