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

[Steve Lenham]

On 20/04/2020 10:20, René Schmitz wrote:

> 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
> 
> 
> 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.

Cheers,

Steve L.