Yamaha CS80

Hello, I hate joining a group just to get a question answered or problem solved, so I'll try to stick around and be as helpful as I can be.

In addition to the other old synths and effetcs I've accumulated, I have a CS-80 here that I'm repairing.

It's got a weird problem and I wonder if anyone has seen this before.  It plays normally as long as I don't pres any keys in the topmost octave.

I popped the hood and traced it down to the YM26700 D/A converter on the KAS board.  Well, at least I think that's the problem.  Here's what I measured:

pins 19-30 are getting the correct voltages for each key in the octave,
pins 13-18 have the correct voltages for the octaves (0.125 through 4),
pins 5-8 are reacting correcting for each key in the octave,
pins 9-11 are reacting correctly to indicate the octaves,

pins 31-38 (channels 1-8) output the correct voltages up and down the keyboard until I press keys in the upper octave.  If I press a key up there, the voltage reading drops into the negative (i.e. -2.5v, 3.5v, etc).

Could there be anything outside the 26700 causing this behavior?

If not, are there any sources for a YM26700?

If not, who wants to buy a CS50 or 60 with me for parts?

Oh - I do happen to have a handful of lever switches, keys (white and black), some panel buttons w. bulbs, and the entire 4-bank 'memory' board that fits under the flip top panel, so I'm not coming here completely empty handed...

thanks,

Tom