Vintage Synth Repair

Hi

I have 3 SCI instruments with problems. Mayby somebody have some 
suggestions for help.

Prophet VS. 
Some minutes after switching on and sometimes 30..60 min later a 
voice stop iimmediate after pressing a key. playing the same key 
again every thing is normal. 
Question does anybody know where to look for and is there a way to 
switch off the voices

Multitrak
Startup problem:
When removing the analog board everything is ok. 
With the analog board installed most of the time it doesn't start up 
(the display is off)or strange signs are displayed.
With the analog board "half" installed startup seems to be ok. With 
half i mean that the analog board is connected to the PSU and the 
connector near the PSU is also connected to the cpu bard. The 
connector at the other site isn't connected.
Does anybody know where the problem could be. 

ProFX
This unit has an expansion box. The cpu gives an error for module 4,5 
and up in the expansion box. I have replaced the ic for the bus in 
the expansion box. The mainsystem is working ok.
Does anybody seen this problem before?

thanks

Bert