Q106 Calibration

While doing the tests and experiments for the MOTM-650 posts, I noticed that one of my Synthesizers.com Q106 VCOs was not scaling properly; high notes were playing sharp. This particular Q106 was one of my first modules; I've had it for five years and it hasn't been calibrated since it left the factory, so it was due. I have a second Q016 and I have calibrated that one (I bought that one used), so I know it can be a bit of a contest of wills, like most VCO calibrations are.

The Q016 has three adjustments, as pictured in the photo below:



The three blue pots right of center are the three basic tuning and scaling adjustments. The four at the upper left are associated with the Q106-CRS calibrated range switch; they allow you to trim scaling for each range (except the 2' and LFO ranges). The three pots at the right, from top to bottom, are the coarse tuning, scaling, and high-frequency compensation.

Dotcom's suggested calibration procedure has never worked for me, for some reason. Their procedure starts with setting the coarse tuning at C0 (32 Hz), going up one octave and adjusting the scaling there, and then going to high C and setting the high frequency compensation there. I always wind up in an endless loop of tuning the low C and then adjusting the scaling, and after I get done with that, it's still not in scale in higher octaves. Here's what I do instead:

1. Set the range switch to 32' and center the frequency knob.
2. Center the high frequency compensation pot.
3. Center the 32' pot on the CRS. (If you don't have a CRS, you can skip this step.)
   
4. Tune middle C, turning the coarse tune trimpot until it's in tune. If I hit the limits of the trim pot, I give the high frequency compensation pot about 1/4 turn clockwise.
   
5. Check low C, adjusting the scaling pot until it's in tune. If I hit the limit of the scale pot, I adjust the 32' pot on the CRS until I have some more leeway on the scale pot.
   
6. Re-check middle C and repeat steps 3 & 4 if necessary.
7. Check high C. If necessary, adjust the high frequency compensation pot and go back to step 4.
8. If you don't have a CRS, you're done. If you do, set the range switch to 16', play middle C, and adjust the CRS 16' pot until it is in tune.
9. Repeat for 8' and 4'. I'm not usually able to get 4' to be exactly in tune; it usually winds up a bit sharp. I don't mind because I rarely use the 4' or 2' settings.