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

[Karl Ekdahl]

Hi Eric

> 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.

I've seen 3086'es blow up before so i'd change it and see. I can't really tell from the schematic (ouch! hangover!) but if both oscillators aren behaving the same, look for any components shared by the two oscillators.

> 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? 

Def. sounds like a blown CMOS chip to me, the older CMOS seems to have been more picky and prone to failure so i'd replace it. Fault could just have been that something shorted the gate momentarily and that degraded the internal circuitry in the IC.

> 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.

What kind of keyboard is it? Sounds to me like it's a j-wire two-bus system? In that case, check for the wires under the affected keys and see if the wire going to the gate bus might have been bent.

Btw. for ease of debugging you might want to socket both the CA3086 and the CMOS chip (unless they already are)... 

Karl