Vintage Synth Repair

Hello,

I bought some time ago this synth with a lot of problem that I've been
fixing. But now I don't know how to solve the remaining problem.

I found the problem in cutoff voltage control from D/A converter. It
looks in some of the 8 stages the voltage is right (1st, 2nd,3th and
5th) but in other is not possible to adjust the Voltage in the SSM
filters. I also swap two chips just in case and afortunately the
SSM2044 are ok. I change the circuit multiplexer and the IC's that
provides de D/A conversion throght the R-Array. Some of the wrong
stages are just impossible to adjust, other are noisy. My thinking is
that the switching time is delayed between CPU and multiplexer but as
I cannot chceck with osciloscope -I don't get one- I'm just sttoped in
that situation. 
The system (apart from SSM chips) is quite similar to JUNO's. Does
anayone get a similar problem? Or more important does anyone know thow
fix it?

Many thanks in advance.

Joan M.
www.soundclick.com/nolabelavailable