9720 VCO 19
010321
5) Listen to the Pulse output (
) of each
oscillator. They should be the same pitch as
the Ramp outputs and approximately as loud.
The two Pulse outputs will probably be set to
different pulse widths so timbres may not be
identical.
OscA Pulse problems point to
IC6:C and the resistors associated with it.
The OscB Pulse output is shaped by IC6:B
and it's associated parts.
While listening to the OscB Pulse output rotate
the Pulse Width control from fully CCW to fully
CW and observe the characteristic "flanging"
timbre of varying duty factors.
Check the wiring
of R206 if there is no change or if the output
goes quiet at either end of the control rotation.
Set the Pulse Width control to the CCW limit and adjust the Modulator's Attack and Decay
control so the output LED shows a cycle time of a second or so. While listening to OscB pulse
output increase Modulator output level and observe PWM effect. Observe that rotation of the
Modulator Output control CW beyond the 3 o'clock cause the output to be silent for a part of
each Modulator cycle.
Use a Patch cord to connect the Modulator out to OscA PW input and listen to observe PWM
effect. Notice that the output is silent for a part of each cycle when the Output control is CW
beyond the 12 o'clock position.
Problems with these tests of the Pulse outputs will likely trace
to resistor value or soldering problems in the components associated with the Pulse Width
shapers.
Switch OscA to LFO and observe 50 Hz at Pitch max decreasing to less than a cycle/minute at
min.
Problems here focus on wiring of S2.
6) Listen to Triangle outputs (
). Adjust sym A (R87) and sym B (R5) for most mellow tone
which should happen about mid range.
If there is no "most mellow" point between the ex-
tremes of the trimmer's rotations check A- IC8:B B-IC3:C
7) Listen to the Sine output of OscA (
) for a mellow sine sound.
If not check IC8:A and
polarity of diodes D11-D13,D17,D18.
We're now ready to calibrate the oscillators so that a 1V change in Control Voltage produces a
1 octave change in pitch. The calibration procedure is written as if no equipment such as
Oscilloscope, Frequency Counter or digital voltmeter is available. If you have this kind of
equipment it may be used and there are some tips and pointers at http://paia.com/
9720test.htm
.
But the results produced using this equipment will be no more accurate the
equipment-free procedure that follows.
Summary of Contents for 9720
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