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

[James Coplin]

Poking around I found the triangle wave on A25 pin 6 that is the raw LFO
wave before running through the shaper to make the sine wave that
eventually gets output.  I'm not finding a square anywhere and I don't see
how there is one in looking at the schematics.  However, I'm little better
than a semi-trained monkey at these things so I could be entirely
mistaken.  I guess I'm at a point of either forgetting about the square
entirely, figuring out what Kevin was talking about by playing with the
bias on the 3080, or building some type of comparator circuit to make a
square.  Ideas?

James R. Coplin

> -----Original Message-----
> From: Tim Parkhurst [mailto:tim.parkhurst at gmail.com]
> Sent: Friday, May 27, 2011 6:05 PM
> To: James Coplin
> Cc: synth-diy at dropmix.xs4all.nl; analogue at hyperreal.org
> Subject: Re: [AH] SEM LFO square wave mod...
>
> 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
> >