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

René Schmitz synth at schmitzbits.de
Mon Apr 20 11:20:17 CEST 2020

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.


synth at schmitzbits.de

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