Yamaha CS-20M -1979    Weight = 37 Lbs. Number mfg.= ? MRP=$1595

Circuit Overview:
     A subtractive analog synth similar in many ways to other CS series. Sine wave feed past filter to thicken up things, especially in Band or High pass mode. Capable of some great sounds. One LFO is a bit of a limitation of course. But a great CS synth. All 'yellow' knobs can be saved in 8 'snapshot' memory locations (ie. you can't edit the saves so write it down if you want to remember how you made the sound!) with typical knob scanning by CPU through analog switching. The white knobs (pitch controls, PWM mod amounts, initial level, glide and volume/HP volume) do not save to memory. This of course keeps pitch control perfectly smooth. Not sure why they didn't save PWM mod amounts :-)

Service Tips:
     Typical problems in CS series are power supply over voltages which blow tantalum capacitors and they short, or the caps just fail over the years. During this various chips may be taken out. I've seen LFO chip, VCA's and dac comparator fail so they might be most susceptable. In recent one the YM617 MUX chip failed out of the blue after running solidly for quite a while. Fortunately Yamaha still had some! (they don't anymore btw...don't ask. It's a chip that wouldn't be that hard to cross wire in a replacement I think if someone had to. ) ALSO CHECK THE FUSE RESISTORS that are inside the insulating jackets.
     I put up a post years ago about a machine (probably my first I think the one I still use) that had flickering Load light. This one had the same issue plus the other load light plus the save light. I noted that IC19 on the PGM board is common to two of those lights and is also having an input on the far left load LED's path to where it might have been leaking I thought to cause the issue also. I replaced it and the other two LED's stopped flickering but not that one. I cleaned the jack 'from tape' with no result. Ground looks good. The op amp had noise out on pin 7 (the other half of IC20 is just tied to V- and pin 1 is open btw). Replaced the 4558 and that one is good too!

I had a most peculiar manifestation shown in a video I did on CS5. The cutoff control would just cease to impact a highly filtered sound at the Ab second from top and below or like that. It would change a note during warmup. Turned out the VCF chip was the issue!