Calibration
You should use a proper trimmer tool for the adjusting of all four multiturn trimmers. Vishay,
Bourns and others make trimmer adjusters for less than a pound.
Before you calibrate you should make sure that the HFT trimmer is adjusted so that the HFT
circuitry is turned off. To do this you must disconnect the module from the power supply and
then with a resistance meter measure between the right hand end of R45 and pin 3 of U6. Turn
the HFT trimmer until this reading drops to zero, or near zero, ohms.
Now power up the module and make sure it has been powered up for at least twenty minutes
prior to calibration. Also, it is a good idea to have the room temperature close to what it
would normally be when playing your modular.
SCL:
This is the scale trimmer. Use this to generate a perfect 1V/octave scaling. This trimmer
will need to be adjusted along with the tune or frequency pot on the front panel. You will need
a digital frequency counter, or my favourite, a guitar/chromatic tuner or tuner plug-in. Some
people use another keyboard or a calibrated VCO and listen to the beats but that can take
some practice to do it right.
Plug your midi-CV convertor or 1V/octave keyboard into the 1V/OCT input of the SVCO. If
you have an SVCO-B module then use the INC/DEC switch to set the module to the 8”
setting. Play a lowish note on the keyboard, then play two octaves higher. Adjust SCL until
the interval is
exactly
two octaves. I normally try to work between the two As of 220Hz and
880Hz.
However, please note we are only setting the interval and not the actual frequency. It does not
have to be a perfect A when A is being pressed on the keyboard. It could be an F or whatever.
The important thing is that we are setting the musical gap between the notes. If you do need
to alter the pitch of the VCO use the front panel control only. Leave the TUN trimmer until
later.
For any interval, if you find the higher note is flat, then turn the SCL trimmer to make it flatter
still. This actually reduces the range between the two notes. Conversely, if you find your
interval is greater than an octave, turn the trimmer to make the top note even higher. I always
adjust SCL on the high note of any interval, and only adjust the front panel Tune pot on the
lower.
This will probably require some patience and plenty of twiddling of the front panel tune
control as well. But you will get there. Once you get the hang of it, its easy. I can do it in
about one minute but I’ve had a lot of practice.
You should be able to get it as accurate as +/-1 cent. But very low notes will be out by a little
more than this.
Now leave it on for a further 20 minutes and then check the scaling again. Adjust if necessary.
8