Vintage Synth Repair

Hi All
Is there anyone on this forum that has experience of the Elka E39 organ?  I'm having a problem with one!  On first visit the unit produced no sound from anywhere, I quickly ascertained that the pedal lamp was not illuminated, the lamp was fine but the -12V was missing.  This proved to be the LM317 defunct.  I changed this and the pedal lamp was now OK but I still had no sound.
I input a signal into the external amplifier jack and found the output was working very well.  I then found that the rhythm section was also working correctly, percussion was fine and I could do various sounds with this section, but still neither manual nor the pedals gave out any sound.
From the circuit I could see that the only section common to all keyboards is the microprocessor.  However as the rhythm section worked OK I find it unlikely that the microprocessor would fail all three key-sets and leave the rhythm section working.
I would be interested if anyone has any suggestions as to how to proceed.
Regards
Brian