[sdiy] [AH] SEM LFO square wave mod...

[Tim Parkhurst]

The 3080 in the original SEM is used as a tri to sine shaper. Not
quite sure what could be accomplished by tweaking the supply voltage
to that chip. You should be able to tap a square wave off pin 6 of
A25, and just install a switch to select sine or square. Check the
schematics at: http://www.synthfool.com/docs/Oberheim/Oberheim_SEM1A/SEM_1A_Schematics.pdf

Remember that these schematics for the original SEM. The reissue
appears to use 13700 OTAs instead. I don't know if there is one used
as a waveshaper for the LFO, but a little careful poking around with a
scope should point you to the square wave already available in the LFO
circuit.


Tim (hip to be square wave) Servo
-- 
"Sire, the church of God is an anvil that has worn out many hammers."
- H.L. Hastings


On Fri, May 27, 2011 at 1:57 PM, James Coplin <james at ticalun.net> wrote:
> I've been told that you can modify the SEM LFO waveshape by applying
> negative voltage to pin 5 of A23 (a CA3080).  However, I'm not sure how to
> implement this exactly.  Looking at the schematic that pin is a 15v supply
> for the chip.  I know how to invert the VC to get a negative voltage using
> a transistor but what is the right way to sum it in with the +15v supply?
> Anyone done this and know what the max "safe" negative voltage to blend in
> is or am I not looking/seeing something right? Thanks as always!
>
> James R. Coplin
>