[sdiy] Prophet 5 rev2 conundrum

[g m montalbano]

I've been working on these for twenty years, and thought I'd seen pretty
much everything that could go wrong with them.
Now, I have one that works perfectly except for the fact that it will not
go through the tune procedure, either at powerup or when the Tune button is
pressed.

The confusing part is that the fault has been narrowed down to the voice
board: if I swap in a working voice board, the unit functions correctly; if
I install this voice board in another unit, the problem follows.

As far as I can see, the ONLY parts on the voice board that relate to
tuning are the two 4051 multiplexers (U423 and U428) and the LM311
comparator (and its associated resistors & caps). All of these components
have been replaced; traces and resistors have been checked &  measured. All
traces to the 60 pin connector between the CPU and Voice boards are good,
as is the 60 pin cable.
EVERYTHING is there, and everything works correctly except for Tune.

I've been hitting my head against this for a couple of weeks, and would
appreciate any suggestions as to where to look.

Thanks.
~GMM
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