Hi David,
Thanks for the info and happy new year to you as well! This is actually the second time you've helped me, as I've been going by the CS-80 schematics and block diagrams from your website - thanks for uploading those!
I do have an oscilloscope, so will give it a whirl, and will try removing IC4 and see if pin 3 is still +15v. I'm also wondering what effect the TKC board has on the A, B, and C inputs of the 4051, and if that could be the culprit. One guy on Muffwiggler suggested a possible broken trace or wire going to the aftertouch PCB under the keyboard, and that if the -15v rail wasn't getting to this board, +15v at pin 3 could be the symptom. Another said to simply wiggle all the spider webs of wires going to the solder pads, as they can detach over time. So I have a few things to try. That thread can be found here: https://www.muffwiggler.com/forum/viewtopic.php?t=193847
For some reason, the last tech that serviced this board installed JRC 072BD's in place of the old 4558's. Am not sure if this is a common swap, but the design on their respective data sheets looked pretty close. Pin-outs are the same, just some subtle differences in the actual circuit.
Cheers,
Jonathan
