[sdiy] prophet 600 -- tough problem

[greg montalbano]

Hi--
I'm hoping to hear from anyone who has specific, detailed knowledge of the 
workings of the digital portion of this synth.
I have several control voltages that are completely screwed;  but the 
problem is definitely upstream from the four 4051 demultiplexers*.  Looking 
at the outputs of those four ICs in order (on schematic "Board 4, page 1 of 
4"), the bad CVs form a pattern: 00X00XOO XOOXOOXO etc etc, where O= a good 
CV and X= a bad one.  This leads me to suspect a problem with the data or 
address lines controlling the DAC and/or demultiplexers;  can anyone tell 
me exactly what to look for and where to look?
Everything else on the synth is functioning correctly & consistently.

Thanks.

~GMM

*This was determined by removing them & testing them out of circuit;  while 
they were removed, I also tested each of the voice S&Hs  with a known CV 
and found them to respond correctly.