This is cut and paste from a post on the Synth DIY Facebook group:
This is for the Paia users out there, specifically if you've got the 9720 VCO and would like to improve the pitch tracking. I've been working on months for this and found some solutions...if you set the pitch tracking/scaling as called for in the instructions, it'll sound okay if used by itself. But if you compare the tracking to a digital source which is more accurate, you'll find that the pitch goes sharp in an arc, so to speak, between the notes that were used to set the scaling. It'll go sharp, then dramatically flat above the set point.
The single best thing you can do is install a 1 meg trimmer on the back of the circuit board across R21 and R43. On an early version of the schematic, these are labeled "high track". The 1.5M resistor in the original design is just way too large. Set your (new) trimmer to maximum resistance and set your scaling the normal way. Then start testing between, say, middle C on your controller and two octaves above, advancing the "high trim" as I would call it. Keep going back and forth resetting the pitch on the front panel to be in tune with an accurate external source, then play two octaves up and advance the high trim until it's in tune. Keep going for an eternity (it seems like) until middle C and the C two octaves up are in tune. Then readjust the original Scale pot on the PCB. Back and forth like this, you can eventually get the tracking much, much better than stock. Now: advancing the high trim will raise the pitch of the VCO overall, quite drastically. If you want it back where it started (approximately), add a .01uf cap on the back of the circuit board across C11 and C16, the 4700pf timing caps. (Note: for someone as anal retentive as I am a 25 turn trimmer for the high trim is the only way to go.)
Going further, if you want better temperature stability, replace the exponential transistors, (Q2-Q3 and Q8-Q9 pairs) with a THAT 320 PNP array. The attached figures show how to wire one of the chips (the drawing is from THAT corp website, which I annotated) and where those leads connect to where the expo transistors would be on the PCB.
Now: even with these mods the VCOs aren't perfect and they aren't going to be. Rather than get frustrated I've learned to live within the limitations, and in each pair of VCOs, the first one is optimized for lower pitches (16'-8' pitch range) and the second one is for higher pitches (4'-2'), because that's the way I like to combine sounds. Doing it this way, you can create lots of off-unison partials which will play in tune satisfactorily, and I can't get enough of that!
Free-for-all discussion about PAiA products and the kinds of audio you produce with them.
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