[sdiy] SH-3a VCO - how does it work
steve at bendentech.co.uk
Mon Apr 20 12:14:19 CEST 2020
On 20/04/2020 10:20, René Schmitz wrote:
> On 20.04.2020 10:01, Florian Anwander wrote:
>> 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
>> Page 9
> 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.
It's a temperature-compensated Zener diode built out of discrete
components (and they do it again with Tr107 and Tr108).
The forward-biassed B-E junction has a negative temperature coefficient.
The breakdown voltage of the reverse-biassed B-E junction has a positive
temperature coefficient. They cancel each other out to some extent - not
totally, but the result is better than a Zener alone.
You can buy TC Zeners as a single device now but perhaps you couldn't
back in the day. Or perhaps the transistor version was slightly cheaper.
I would speculate that the temperature stability was more important to
Roland than the exact Zener voltage, because that wouldn't have been
very well controlled.
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