Calibration
•4K7 Octave (Control PCB): Set the octave switch to it’s highest octave position (2’) and
adjust trimmer until you get a reading of 7.5V at the Test on the octave rotary switch
daughter board.
•10K Scale (VCO):This trimmer sets the 1V/oct tracking of the VCO, and it’s really worth
spending some time to get it right. On first power up, the VCO should already be pretty close
to 1V/oct with the trimmer in it’s mid position, as I carefully chose associated resistor values
to make it so. To set it closer, play two notes an octave apart and adjust until it sounds spot
on. Then play two notes two octaves apart, and adjust still further until it sounds spot on. If
you have a frequency counter, still better!
•10K Duty (VCO): This sets the duty cycle of the square wave, should be just about spot on
with the trimmer in the mid position, as I carefully chose associated resistor values to make
it so. Set the Pulse Width knob to 50 (hard left), and twiddle the trimmer until you hear the
easily recognisable hollow sounding square wave.
•100K Offset (VCO): Sets up the initial frequency of the VCO. Set Fine Tune to it’s centre
position, play an A on your keyboard. Adjust the Offset trimmer until you can read a
frequency of 110 Hz, 220 Hz, 440 Hz, 880 Hz etc when playing octaves of A.
•100K Bias (VCA): set both VCA switches to the up position to send a nice snappy ADSR into
CV3 input. Adjust Bias trimmer to sweet spot, ie there is no DC thump. The chances are that
that the sweet spot is around the mid position.
•10K 1V/Oct (VCF): turn Resonance all the way to self oscillation. Patch a 1V/oct source into
CV input 1, with the attenuator fully clockwise. Play octaves and adjust the V/Oct trimmer
until they are spot on.
•100K Offset (VCF): you want to tweak this so that the filter is fully open when the Cutoff pot
is fully clockwise.
RDH 04/01/20
https://www.frequencycentral.co.uk/
