Service Manual ProTone
Page 9 of 20
© 1996-2001 Spectral Audio
5 Adjustment of the ProTone
5.1 Digital PCB (MIDI to CV converter)
The slope of the control voltage CV is set with trimmer R10. Octave jumps which are initiated via
MIDI-IN, must result in a 1.00 V change.
For this purpose, an accurate voltmeter must be connected to the CV socket J3 and the reference
note (default: 36=C1) initiated and the voltage value noted (some mV). Now initiate the MIDI note
four octaves higher (C4) and adjust R10 so that the voltage is 4.00 V higher. Check the reference
note again and repeat the adjustment if necessary.
5.2 Analog PCB (VCO1 and VCO 2)
In order to tune the VCOs, the analog PCB must first be separated from the front panel (6 screws
with spacers). The analog PCB must then be reconnected to the digital PCB and started.
After disassembly, the Tune VCO1 and Tune VCO2 knobs must be replaced and positioned exactly
in the centre (Tune setting = 0).
Before adjustment, the unit should be in operation for approximately one hour. The adjustment
should be made at room temperature.
Before adjusting the VCO, check the CV slope (chapter above). The setting of R10 in the MIDI to
CV converter is valid for both VCO of course.
5.2.1 VCO1
The frequency of the oscillator can be taken from pin 8 of U11 or at the unit output, whereby the
filter must be open and set to Lowpass. Only VCO1 must be audible (set the mixer correctly). Make
sure that no LFO modulation take place.
The offset for VCO1 is set with R21 and the slope with R157. Nonlinearity in the upper frequency
range is adjusted with R29.
Proceed as follows to make the adjustment:
1. Set trimmers R21, R157 and R29 to the base settings shown below.
2. Set the ProTone
MIDI reference note
to C1 by pressing the learn-key.
3. Press key A3 and set the VCO1 frequency with R21 to 220.00 Hz, measured with an exactly
frequency meter.
4. Press key A1 and set the frequency to 55.00 Hz with R157.
5. Repeat steps 3 and 4 until frequency values are stable.
6. Press the key A5 and set the frequency to 880.0 Hz with R29.
7. Repeat steps 3 to 6 and re-adjust if necessary.
5.2.2 VCO2
The offset for VCO2 is set with R131 and the slope with R156. Nonlinearity in the upper frequency
range is adjusted with R132.
Proceed as follows to make the adjustment:
8. Set trimmers R131, R156 and R132 to the base settings shown below.
9. Set the ProTone
MIDI reference note
to C1 by pressing the learn-key if not done yet.
