Orig Model D oscillator drifts up when attached to CV

Tips and techniques for Minimoog Analog Synthesizers
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Spodzone
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Joined: Thu Nov 17, 2022 4:45 pm

Orig Model D oscillator drifts up when attached to CV

Post by Spodzone » Thu Nov 17, 2022 4:50 pm

Hi,

Possibly a bad day for a question but is it a known issue for original Model ads to have their oscillator raise in pitch slowly when attached to CV? Mine is pretty solid tuning wise when not CVed but as soon as I connect it, it slowly just pitched up out of tune constantly.

Any info on this?

Cheers!

Synth.builder
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Location: Cumbria, UK
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Re: Orig Model D oscillator drifts up when attached to CV

Post by Synth.builder » Sun Nov 20, 2022 11:51 am

Spodzone wrote: Thu Nov 17, 2022 4:50 pmis it a known issue for original Model Ds to have their oscillator raise in pitch slowly when attached to CV?
Yes. This is because the external CV adds to the voltage coming out of the keyboard circuitry. The keyboard voltage is stored in a simple capacitor the moment you stop pressing the key down on the keyboard. This then drifts slowly upward or downward due to leakage currents in the circuitry. The drift is slow enough for you not to notice when you are playing it from the keyboard. But leave the keyboard alone for a while and the pitch will shift significantly from where it was.

What you need to do is hold the bottom key down which fixes the pitch. Sadly this keeps the synth internally triggered so it may not be useful to you.

The usual way is to modify the CV input to accept standard 1V/octave input without needing the key to be pressed down. You can't normally use the glide circuit though when you do this.

ProspectHillMusic
Posts: 3
Joined: Mon Jan 09, 2023 4:47 pm

Re: Orig Model D oscillator drifts up when attached to CV

Post by ProspectHillMusic » Fri Jan 20, 2023 7:09 pm

I solved the keyboard drift problem by replacing the entire keyboard circuit with a digitally scanned CMOS based circuit which I designed and built. It has 3 narrow PCBs under the keyboard itself which is the scanning section and a small PCB where the original keyboard circuit was, that board contains the scanning logic and a 12 bit DAC. When playing this Minimoog it plays exactly like the original design with lowest note priority and glide, but whichever note is played last will hold without drifting as long as the Minimoog is powered up which in theory could be indefinitely.

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