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Polivoks VCG pitch random modulation/wandering issue?
MUFF WIGGLER Forum Index -> The Harvestman  
Author Polivoks VCG pitch random modulation/wandering issue?
Hi everyone, I've just bought a brand new Polivoks VCG oscillator and the pitch is random modulating itself by +/- 0.5 semitone but then over time it will drift up or down by several semitones (this is with no external pitch CV).

I've tried it in two different cases and still the same issue.

Also the pulse width only seems to work between 2 & 5 o'clock.

Sounds to me as if there is something fundamentally wrong with it?
I don't know the Harvestman VCG Oscillator but the Soviet transistors used in the Erica synths VCO are light sensitive - including infrared! The diy build requires that you cover the components with a copper box to cut out the light woah

Maybe it's just this and it's how it is, but possibly not, it doesn't sound right, I would contact Scott directly for confirmation.

Hope it works out okay!
Analog VCO. Probably requires tuning/calibration.

Tuning VCG

"A quick and easy way to tune it is to send it a fast arpeggio of just octaves from your MIDI/CV converter, and adjust the "SCALE" trimmer until it sounds like they are in tune. This will get you in the ballpark for any fine adjustments that need to be made. You might have to re-calibrate the octave switch trimmer after this. Note that the VCG circuit was originally not designed for modular use, so it will probably need to be recalibrated if the module moves to a different case."

and a later response as I was confused by the other:

"The pitch will offset no matter what when turning that trimmer, so you will need to re-adjust the base pitch using the front panel freq knobs afterwards. So, to get the octaves in tune, you can't use a numerical tuner or compare against another oscillator- just listen to the vcg playing fast octaves and it so the octaves sound in tune, by ear. Then, to fine tune it once it sounds "right" to you, remove the CV Input, tune it to one of your other oscillators using the front panel control, plug the arpeggio cv into both oscs, then adjust the trimmer to correct for any error you hear. After each trimmer adjustment remove the CV and adjust both oscs to the same base pitch using the front panel knobs only. A trick has to do with the direction you are turning the trimmer: if the octaves are too far apart, you turn the trimmer so the pitch goes sharp. If the octaves are too close together, turn it so it goes flat. This seems to make no sense but it works!"
Thanks to both of you for your input - I plugged it back in last night and tried to shield the back from any light in the case but it was still randomly fluctuating in pitch. It's really quite odd, I've got in touch with the retailer to see what they think.
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