Basically I got a cs10 that is near mint that has worked perfectly for 6 years. Last time I used it was 6 months ago and I was sending a dx7 through its external input and controlling the audio with the cs10s filter.
Anyways yesterday I turned it on and come to find out I have one pitch across all keys and no tracking, my pitch slider seems to behave more like a brilliance slider instead of its original full octave up/down and my footing selector is all wrong. Lowest footing setting is quite high and any higher settings go into non audible range.
Sounds like parts replacement time. Mint condition means nothing on a 40+ year old machine. The capacitors all time expire from the corrosive electrolyte inside. Open it up look to see if any caps are leaking, burnt or exploded. If there is nothing obvious, take it to a tech. Your local music store should know of one.
First of all: all Yamaha CS-series synth use ICs, which are very difficult / expensive to replace or kind of “unobtainium”. So any wrong action might cause extremly expensive repair.
The first thing to try is to insert a cable in the “Key Volt. In” socket.
If the pitch falls down, then it is quite likely that the key scanning circuit is broken. You may check whether the Opamps IC5,6 or 7 are dead. But most likely the large digital keyboard scanner might be broken. A original spare part is assumingly not available. Scott Rider aka old crow developped a replacement for the same chip in for the CS15 on with a modern ATMega368. The pcb would not replace the original pcb, but from the functionality the scanning circuit is compatible (btw: the MIDI in Scotts replacement board does not work, but the keyboard scanning works).
We solved the problem. It was the main VCO IC and a replacement has been sourced