[sdiy] Memorymoog acting strange

[ChristianH]

well, I know, they're almost all acting very strange...

Mine has developped something weird: it contiuously sees pot movements,
even when completely left alone. 
This is pretty nasty, since this will abort any calibration menu,
and the display will jump to the changed pot values instead. Most of the
time, I won't even get to the calibration menu at all, because I can't
complete the "C" and number button sequences without being struck by
some intervening pot change display.

At first I suspected the 4051s on the panel boards collecting the pot
voltages, but replacing those didn't change a thing. Same with the 393
comparator on the DMUX board. 
10 V pot reference voltage is perfectly stable within +/- 1 mV, both at
the DAC and after the buffer for pot supply.

The programmable threshold circuits on the DMUX board (IMHO not quite
correctly described as "hysteresis" in the technical manual, real
hysteresis is only achieved in conjunction with software operation)
appear to do their thing. If I manually activate them one by one, the
MUX BUS voltage is increased or decreased by some 50 mV, as intended.

If I completely remove the MUX BUS line, leaving the CR2/CR3 junction
before the 393 comparator open, there are no changes seen. 
However, connecting the bus line to a single pot or ground will show
continuous movement again (and that pretty much precludes the simplest
cause, i.e. a shaky pot).

Has anybody else ever seen behaviour like this?

regards
Christian

BTW, I can't remember having seen so much bad mechanical engineering in
one device (well, apart from my own wild'n'careless diy stuff I did at
the tender age of 16). The wiring is a mess, oodles of cables going from
everywhere to everywhere. Accessing the DMUX board (or even removing it...)
is a major pain in the you-know-where. Big fat ground wires soldered
closely in between two ICs only 100 mil apart, so you can hardly
unsolder them without burning the IC sockets. Add to that lousy board
silk screening where you mostly can't tell "6" from "8". Yuck.

If only that goddam thing wouldn't have that gorgeous sound ;-)