[sdiy] Oberheim 4-voice issues (SEM, keyboard)

[Eric Frampton]

Hi y'all -

I'm finally digging into my Obie 4-voice! It's an exciting and frustrating time. The mixer is working and 3 of the 4 SEM's are good. I've pulled out the programmer (for the time being) and am driving the SEM's straight off of the keyboard circuit. But (surprise!) I'm having a few problems...

SEM:

So, the one dead SEM doesn't make any sound; well, not much. I can put the filter into oscillation and verify that it's working, and that it talks to Envelope 2. And then I can change the envelope for the VCA and verify that that's working. But the 2 oscillators themselves don't work. If I set them both to Pulse output, then switch VCO mod to LFO on both of 'em, then turn up the amount on the PW Mod side all the way, I can get some crackling out of the VCO's, but that's it.

I've verified that the voltages out of each initial 741 are changing at (roughly) 1V per octave. Where it gets weird is the 3086: both sides of it (i.e. both VCO's) are outputting a really high frequency sawtooth. If I compare this to a working SEM, the outputs from the working SEM's chip are sort of a modified sawtooth, a saw with a curve in the middle, and at a lower frequency. I've tried swapping 741's on the front end and the 3086; no change.

Keyboard circuit:

I'm new to troubleshooting CMOS logic, so any rules of thumb would be appreciated here. Basically what's happening (so far) is two things:

1. The gate for output #1 (SEM #1) seems to open and close, rather fade in and out, in a very slow and slightly random oscillation; i.e., the keyboard circuit is opening up the gate for #1 on it's own whether a key is pressed or not. I can swap SEM's to that first position and they all do the same thing, so I know it's not the SEM. One particular IC is in charge of the gate output for both voices on that card; on the working side the output swings from 0-10V, but the bad one only swings from 0-4V or thereabouts. Does this sound like the actual chip failing, or something else pulling it down? 

2. The keyboard is only sending the C whole note scale; the C# scale sends gate and pitch (on all voices, as it should) but won't keep the gate open; it sort of snaps open but won't stay there.

Programmer:

I think I'll hold off on that until I get the rest of this thing stable...yikes.

Thanks in advance for whatever help you have time to offer.

Eric