I have a little follow on with this. There's also 10 volts used and that regulator is not in the power supply but on board R71 instead, that regulator was not working due to an open 5.6 volt zener diode, I replace that and got the 10 volts back. Still, nothing on the output. I checked around the SUB (sub oscillator) board and the VCO's check out good and output waves in all the variations just fine. I also injected the PRE (preamp) board and the output preamplifier, headphone driver separately and through the ring modulator circuit on the board and everything checks out good.
My problem is that there are so many VCO's and VCA's in this system I loose track of the signal path when tracing things through, that plus, I don't really understand how this thing works at the component level, and at the users level as well. This coupled with proprietary Yamaha chips, especially the keyboard decoder and polyphonic processor all over the thing makes the going a bit tough.
Really guys? none of you have any words or wisdom, ridicule or even pity?