This is the procedure for adjusting the VCO-2131 module to get accurate 1-volt-per-octave tracking and a reasonable tuning range. It’s a bit fiddly and takes a few minutes but I was able to get excellent pitch tracking over 9 octaves.

In addition to the tuning controls on the front panel, there are three adjustable trimmers on the back of the module: Pitch, scaling, and high-frequency trim. The scaling and high-frequency trim controls adjust the module’s volt-per-octave response so that all notes up & down the keyboard are in tune. The pitch trimmer adjusts the overall tuning so that the front panel knobs cover a reasonable range.

Required Equipment

Either:

• Precision voltage source, adjustable from 0V to +10V. An accurate 1V/octave keyboard or MIDI-CV converter can be used.
• Precision frequency counter or music tuner.

Or:

• Stable (but non-precision) voltage source, e.g. from a voltage regulator. +5V and +10V (approximately) are required.
• Precision voltmeter.
• Precision frequency counter. (Not a music tuner – the frequencies will not necessarily correspond to musical note pitches.)

Calibration Parameters

Three appropriate pitch control voltages and corresponding frequencies must be selected:

• Low note with 0V CV input, e.g. C0 (16.35 Hz) or A0 (27.5 Hz).
• Middle note with +5V CV input, e.g. C5 or A5. If the +5V is not precise (i.e. it’s from a voltage regulator rather than a proper pitch CV source), calculate the frequency corresponding to the actual measured voltage.
• High note with +10 CV input, e.g. C10 or A10. If the +10V is not precise, calculate the frequency corresponding to the actual measured voltage.

Other appropriate voltages may be used instead of 5V and 10V.

Here is a page listing the frequencies of musical notes.

Examples

Preparation

1. Connect a waveform output (sawtooth, triangle, or pulse) to the frequency counter or tuner.
2. Do not connect the voltage source to the CV input yet.
3. Leave the other inputs & outputs unconnected.
4. Set the Range switch to the low (bass) setting, the Coarse Tuning knob fully counter-clockwise, and Fine Tuning to the centre.
5. Turn the HF TRIM trimmer on the module’s circuit board fully counter-clockwise.

Trimming the VCO

Adjust the trimmer potentiometers on the module’s circuit board as follows:

1. Adjust low note: With the controls set as above and no voltage applied to the CV input, adjust the PITCH trimmer (RV11) so that the output frequency equals the chosen low note frequency.
2. Adjust scaling: Set the voltage source to +5 V and connect it to the 1V/Octave CV input. Adjust the SCALE trimmer (RV9) so that the output frequency equals the chosen middle note frequency.
3. Repeat the above steps until both the low and middle frequencies are correct.
4. Adjust high frequency compensation: Set the voltage source to +10 V and adjust the HF TRIM trimmer (RV10) so that the output frequency equals the chosen high note frequency.
5. Repeat the above steps until all three frequencies are correct.