Yamaha CS80

Today I had a CS80 in for a service. Basically most of the voices were working, but a couple were dead and there was the usual problem with unequal volume and filter cutoff. Nothing too drastic then. I found two dead oscillator chips, but have some spares, so that fixed that. So, after several hours work, all of panel 1 was sounding pretty decent. 

Then the audio just quit. No output. Nothing. I checked the fuses and power supplies, all appear good. Taking an oscillator card, I can see a key voltage on Pin 2 of the VCO chip which changes as I play the keyboard, yet no waveform output. Take another card, same thing. 

So, how about the white noise? It comes into the M card, but doesn’t leave the filters. 

Next thing – I find that the ground on the M card is referenced to GND2. If I meter between the GND1 and GND2, I see around 2k ohms. Confused?   So am I. So if I measure the 15V rail at the VCO chip, I see around +11V if I take the negative lead of the meter to the metal chassis. 

Where to go from here? Any ideas welcome. 

MTIA

Lucid.