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

Eric Frampton eric at ericframpton.com
Thu Jun 7 19:28:12 CEST 2012

So, after a year's worth of life happening (I'm now married and a step-dad, etc.) I've pulled the 4-voice out of the closet to take another shot at getting it working. In all this time I've managed to fix exactly two issues from my list. :-(

Today's topic: back to the wonky SEM, specifically both oscillators. VCF/VCA/LFO/both envelopes seem to be fine.

I've re-capped the electrolytics in the whole machine. I'm leaving all the tantalums in place, since (as best I can tell) none of them are in the power circuit. Haven't replaced any ceramics…should I go there?

I've verified that the oscillators are oscillating, but they're up in the ultra-high KHz/low MHz range. I've figured out that the high frequency whine I'm hearing is just the beating between the two of them. They do track the keyboard and respond to LFO mod.

I've tried swapping the 3086 with spares I have, with no change. Also tried swapping out the 1000pF polystyrene timing caps, also no change.

On both oscillators: the sawtooth is horribly misshapen, and the pulse output isn't even happening.

Since they're both misbehaving identically, one would naturally gravitate toward common components. But all I can see that's common is the 3086 (which I've swapped), power (which is OK), and ground (duh).




my original post (from April 2011):

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.


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

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