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The analogue nature of the  921 VOLTAGE CONTROLLED OSCILLATOR module means it may require calibration before use to ensure the octave scaling is set to to an exact 1 V/oct. The procedure is as follows:

1. Power up the 921 module and allow it to warm up for 30 minutes.

2. Prepare the following control settings on the 921 module:

a. Set the SCALE toggle switch to the ±12 SEMI position.
b. Set the RANGE toggle switch to the AUDIO position.
c. Set the RANGE rotary switch to 2’.
d. Set the FREQUENCY control knob so that the sawtooth output is exactly 640 Hz as measured with an accurate frequency meter.
e. Make sure none of the FREQUENCY CONTROL INPUTS jacks are connected. 

3. Fine-tune the 921 oscillator’s low-frequency scaling via the following steps:

a. Apply exactly -2 V to a FREQUENCY CONTROL INPUTS input jack.
b. Trim the SCALE trimmer to set 160 Hz, then remove the -2 V input and readjust the FREQUENCY knob to 640 Hz.
c. Repeat this cycle until both 160 Hz and 640 Hz are accurate to ±1 Hz when the -2 V is plugged in and out of the FREQUENCY CONTROL INPUTS jack.

4. Fine-tune the 921 oscillator’s high-frequency scaling via the following steps:

a. Check the frequency is still set for 640 Hz output, then apply exactly +2 V to the FREQUENCY CONTROL INPUTS input.
b. Trim the HI ADJ trimmer to set the 921 sawtooth output to exactly 2.560 kHz.
c. Re-check that 640 Hz is still correct when the +2 V input is removed.
d. Repeat as required