Archive of the former Yahoo!Groups mailing list: MOTM

Hi, I don't know where to direct my MOTM-2.0 build questions, since nobody seems to do that here, nor at e-m forums, muff, etc. Clearly Paul's excellent instructions must result in few gaffes, but I've managed to bungle one of mine at least. Let me know if you'd recommend asking elsewhere.

I'm trying to troubleshoot VCA "A" from my MOTM-190. "B" has worked great for a while, and I've just been neglecting bothering to fix "A".

In VCA mode, the Blend Pot works more like a mix of the 2 sources:
CCW = Audio In, CW = CV In.

In Ring mode, the Blend Pot works as follows:
CCW = Audio In
12:00 = CV In
CW = Sum of Audio In + CV In

Additional weirdness: Neither TP1 nor TP2 do anything of consequence. (True to form, TP3 did trim out thumping as expected.)

I've checked all the dumb stuff, as far as I can tell, or most of it. Components seem correct, with correct orientations, continuity checks throughout the circuit, +/-15v is at all the right IC pins, and the Ring/VCA switch switches between +5v and -0.2v. TP2 is adjusting the +/-15v to R21. Obviously I've missed something dumb, either that or a component is bad.

I've read through Paul's description of the circuit, which is excellent and detailed and still partially written in Greek as far as I can tell. (Sorry, I'm still struggling to grasp the theory behind a lot of the things I'm building.) In my limited understanding it seems like something around the 3280 must be going wrong since "dry" audio works fine and it seems like CV just comes directly out of pin 13.

If anyone with a better read on the workings of this module has some tips of components or areas of the circuit to check, I'd really appreciate it.

Cheers,
Jim