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

[Stewart Pye]

Hi James,

According to the schematic that Tim linked to the triangle wave would be 
on pin 6 of A22. Square wave on pin 6 of A25 as Tim remarked. You may 
have the IC's mixed up. Take a look at pin 6 of the other op amps...

Once you find it you'll have to use a resistive voltage divider if you 
want a 10V p-p square wave.

Regards,
Stewart.

James Coplin wrote:
> 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
>>>
>>>       
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