MRG 3340 VCO Tuning Guide
Theory of operation:
- Trimmers pre-tuning
- Square Wave Tuning
- 1CV/Octave Fine Tuning
Trimmers pre-tuning (optional)
This step is optional but highly recommended. The goal is to set the trimmers to value closer to their final value. They've been found through collecting the settings of the trimmers after tuning multiple modules.
Of course, no two circuits are the same in analog electronics, and fine tuning will always be required.
Step P1: Pre-tune the 100k SQUARE ADJ Trimmer
This trimmer potentiometer is used to control the duty cycle of the square wave when no PWM input is present.
This trimmer is connected to a complex resistor network, so its reading won't be a linear 0-100kOhm resistance once installed.
Connecting the probes as in the picture, turn the screw until the resistance read it's around 34 kOhm.
This will set a default square wave to something close than 50% duty cycle.
Step P2: Pre-tune the 10k SCALE ADJ Trimmer
This trimmer is used for adjusting the Scale Factor of the exponential converter for the 1V/O CV input.
The resistance between the side pins should be set around 7.1-7.3 kOhm.
Usually a SCALE ADJ trimmer set at this value has already a good enough 1V/Octave tracking for one or two octaves without fine tuning.
Square Wave Tuning
The SQR ADJ tuning controls the duty cycle of the default square wave (no PWM input).
Using an oscilloscope, adjust the SQR ADJ trimmer until the square wave output (SQUARE OUT) has a 50% duty cycle (equal high and low periods).
1CV/Octave Fine Tuning
Step T1: Shut down High Frequency Tracking.
The HFT ADJ trimmer is used for adjusting the high frequency tracking of the VCO: as the base frequency gets higher (15khz or more), even a perfectly tuned oscillator might not track perfectly on high CV values. This happens most of the time in non audible frequency, but – let's repeat that – no two analog circuits are the same.
This settings can disturb our main 1CV/Octave tracking circuit, so we have to shut it off before any fine tuning.
The resistance between the central pin and the pin closer to the edge of the board must be set to 0 Ohm.
Step T2: 1V/Oct CV tuning
Set the Coarse base frequency to a low audible frequency (e.g. 100Hz).
Connect a keyboard to the CV input of the module (most likely through a MIDI to CV module).
Keep going back and forth between the lowest C key of the keyboard and the next C one octave up, adjusting the SCALE ADJ trimmer until the output is tuned: i.e., the higher C has double the frequency of the lower. This can be done through an oscilloscope or by ear.
Repeat the previous step, this time starting from the previous higher C key and the next one. Adjust the trimmer – if needed – until frequency doubles.
Repeat this until the fourth or fifth octave. The higher you go, the less need for tuning will be necessary.
Step T3: High Frequency Tracking
Set the Coarse base frequency to circa 15khz.
Do as in step T2 but adjusting the HFT ADJ instead of the SCALE ADJ.
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