3. Add Deglitching Capacitors to Comparators
I came across this tidbit regarding the MOTM-120 a few years ago:
When it came to testing this module, I was a little perturbed that it
didn't work as I expected. in sub mode, turning up each of the four sub
octave pots produced glitches in the audio if I had signal on both the A
IN and B IN jacks. This couldn't be right. ... This time I happened to
put the scope probe on the non-inverting input of the input comparator,
and lo, the glitching stopped. Aha.. the capacitance of the scope probe
had stopped the clock jitter. Solution: I happened to have a couple of
18pF ceramic caps handy, so I put these across the non-inverting input
and ground (Pins 2 & 1) of both of the 311 comparators. Problem solved.
The module now appears to work as intended....
I made the same changes to my MOTM-120 modules (and so have a number of other people over the past few years) and I can confirm that this simple addition of two capacitors cures all kinds of ills with the MOTM-120 (and the modified MOTM-120R). The exact value of the capacitor isn't all that important, anything from 15 pF to 33 pF should work.
The best place to add these two capacitors is on the underside of the PCB, directly between pins 2 and 1 of both comparators. Yes, it's a bit of work to remove the mounting bracket to reach the underside, but there isn't really any good place to add these capacitors on the topside. See the picture below for an example.