20 9720 VCO
010321
During calibration one
oscillator will be used as a
fixed reference while the
other oscillator is adjusted.
Use the Triangle outputs
(
) of OscA and OscB
and arrange for both
to be heard at the same
time at approximately equal
amplitudes. If a PAiA 9710
Triple VCA is available it's
L and R inputs can be used
to mix the two to a single
output. Lacking a mixer, the
L and R inputs of a Stereo Amp switched to Mono can be used as shown in the illustration.
8) Begin calibration with OscA. Apply a 1V/octave controller, such as a midi2cv8, to the P2
input of OscA and cause the controller to output 4V (typically by pressing the 5th C on a
keyboard). Adjust OscA Pitch control until the pitch of OscA is 2 octaves above that of OscB.
Cause the controller to output 0V (typically by pressing lowest C) and adjust Scale A trimmer
(R27) until the pitch of OscA is 2 octaves below the reference.
Once again cause the controller to output 4V and again adjust OscA Pitch for 2 octaves above
reference, then 0V again while adjusting Scale A for 2 octaves below the reference.
Iterate the above procedure until satisfied with both the high and low end.
OscB is calibrated in the same manner. Disconnect the controller from OscA P2 and reconnect
to OscB P2 as shown in grey in the illustration. With 4V. output from the controller adjust OscB
pitch for 2 octaves above OscA then with 0V output adjust Scale B trimmer (R52) for a pitch 2
octaves below OscA. Iterate this procedure until satisfied with the tuning at high and low end.
Disconnect the controller from OscB P2 input and reconnect to OscA P1 so that both Oscilla-
tors track the controller and observe tracking as pitch CV varies. Some further tweaking of the
calibration of one or both VCOs may be necessary for best tracking.
It's unlikely that the VCOs
will pass previous tests and then not calibrate properly. If it happens, most likely cause will be
a resistor with an incorrect value. Double check values of resistors associated with the CV
summing/current sources as outlined in step 4.
9) Test OscA Sync input by reconnecting the controller to OscA P2 input and using a patch
cord to connect OscB Pulse output to OscA Sync as shown on the facing page. Set both Pitch
and Pulse Width controls of OscB fully CW. While listening to the Sine output of OscA only,
press the 3rd C on the controller for a 2V. CV output. As you rotate the OscA Pitch control you
should hear the output switch between a number of timbres that are considerably more
complex than a simple sine wave. As pitch is varied by playing up and down scale notes on the
controller the timbre should stay the same.
If there are problems here check the wiring of J4
and C17 and R9 on the circuit board.
(10) Switch the Modulator from Cycle to
Summary of Contents for 9720
Page 27: ...9720 VCO 27 010321...
