[sdiy] Coarse and Fine Pots

[Ben Bradley]

It's been quite a while since I've touched an SEM, but yeah, I think
those are two separate knobs on a reduction system that goes to the
shaft of one pot. These were used on the tuning shafts of old radios
from many decades ago. It's usually a ball bearing reduction and they
appear to be rather expensive.
https://www.youtube.com/watch?v=wWe4Hc3DtZ4
https://www.orenelliottproducts.com/product-category/planetary-reduction-drives/

On Sat, 24 May 2025 at 15:57, Terry Bowman via Synth-diy
<synth-diy at synth-diy.org> wrote:
>
> I'm upgrading my existing Oscilloscope Artist:
>
> https://www.astarcloseup.com/2016/02/the-oscilloscope-artist.html
>
> I really need to add a fine tuning pot to two of the oscillators. I love the planetary mechanisms used for the VCO tuning on the Oberheim SEM but unfortunately they appear to be unobtainium right now. Are there any dual concentric pots with different resistances? It seems to me that a ratio of about 5:1 would be close to ideal so that would be 100kΩ/20kΩ.
>
> If not, can I just put a resistor in parallel with the fine pot? I know that the latter changes the curve and linear is preferable.
>
> A related question: how does the sync to oscillator C work?
>
>
> Terry Bowman, KA4HJH
> "The Mac Doctor"
>
> https://www.astarcloseup.com
>
> #1: There is no substitute for good tools;
>
> #2: Always use the right tool for the job.
>
> Linn Westcott
>
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