[sdiy] SH-3a VCO - how does it work

Adam Inglis (sdiy) synthdiy at adambaby.com
Mon Apr 20 12:07:03 CEST 2020

I had this question a year ago or so, when I was looking at external control for my SH-2000. Not exactly the same, but again, based around a UJT.

Then I discovered people have been asking this question on this list for a very long time!
Below is a comment by Juergen Haible from 1997. (and I think Rene contributed to that discussion as well!)

Haible Juergen 
Mon Aug 11 14:02:24 CEST 1997

Most Korg VCOs actually use SCR circuits !

(MS-20, Polysix, Trident, to name just a few.)

Only that they don't use an SCR *component* - they
build their own SCR circuit from two bipolar transistors.
But it works exactly the same way: It starts to conduct
when a certain gk voltage is reached, and it stops when
the current thru the device reaches zero (i.e. when the
capacitor is discharged).
The first Moog VCOs used unijunction transistors - not
the same, but similar in operation.
An actual SCR component I have only seen in Roland 
System 700 docs. Not in the VCO's, but in the envelopes,
if memory serves.


PS: The Korg circuit looks very elegant, as you don't need
any voltage comparator (it's buit in), nor hysterestic switch
or monoflop (the end condition is "current == 0").
Does this circuit also have some drawbacks ?
Tolerances of trigger voltage ?
Temperature dependance ?
Any idea why it wasn't used by everybody?

The answer to his last question that many people will give you is - they are no good over a large number of octaves, hence the divide down switching.
Personally, I don’t rate that quality very highly, especially given that many orchestral instruments are only used (in any one piece of music) over a fairly restricted octave range.
The issue of temperature stability is also raised - which doesn’t explain why my Korg 700 and Roland SH-2000 never need tuning!


> On 20 Apr 2020, at 7:20 pm, René Schmitz <synth at schmitzbits.de> wrote:
> Hi Florian and all,
> On 20.04.2020 10:01, Florian Anwander wrote:
>> Hello,
>> I may turn out now to be dumb and blind, but can someone please explain me how the core VCO of the SH-3a works.
>> The service manual is here
>> http://www.synfo.nl/servicemanuals/Roland/ROLAND_SH-3A_SERVICE_NOTES_5th.pdf Page 9
> It is based on a programmable unijunction transistor N13T2.
> Tr134 and 132 form a loop to stabilize the reference current, for the exponentiator Tr 129.
> C141 gets linearly charged up, until the PUT fires. The firing point is determined by a voltage divider at the Gate terminal of the device. (R137, and the 680 on the Pack#8)
> When the PUT fires it engages an additional 2sc373 transistor to help discharge the cap.
> The cap voltage is buffered by the 2sk30a and 2SC373.
> Most odd is the arrangement of Tr130 and Tr131, which appears to either protect the capacitor from overvoltage, or is used as some very crude voltage reference.
> All in all a very idiosyncratic circuit.
> Best,
> René
> --
> synth at schmitzbits.de
> http://schmitzbits.de
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