An analog synthesizer is a musical instrument. and all musical instruments have to be properly tuned...
One of the weakest module in an analog synthesizer is the power supply. Sometimes, due to unwanted shocks during transport, the miniature " trimpots" loose their settings : this means ,that the power supply does not deliver the proper voltage needed by the various synthesizer modules. One module in particular, is very sensitive to voltage inaccuracy: the VCO.
CHECKING THE POWER SUPPLY VOLTAGES
First, check if the power supply delivers the proper voltages. Let's assume that your synthesizer needs a symmetrical power supply delivering +15 Volt/-15Volt.(30Volt peak-to-peak). Take an analog or digital Multimeter and set it to 20V DC Voltage range. Connect a BLACK testing wire to the COM input and a RED testing wire to the Multimeter's Volt/ohm input . Now, connect the Black wire to the GROUND of the Power supply and the RED Wire to the +15Volt Pin. If the voltage is correct, repeat the same procedure for the -15 Volt Pin.
Note : this procedure should NOT be done immediately after Powering up : power supplies need at least 5 minutes of operation to warm up and stabilize.
If both voltages are incorrect, you will have to recalibrate the + Volt and the -Volt "trimpots" on the power supply
Locate the (+) trimpot : take a small screwdriver and make the neccessary adjustments: the voltage should be exactly +15.00 Volt.
If necessary, repeat the same procedure for the (-) trimpot : the voltage should be exactly -15.00Volt. (the negative voltage has to be exceptionally accurate for it affects the Volt/oct calibration of the VCO directly ).
Next, borrow your girlfriend 'Nail Enamel' and put a 'blob' of enamel between the center part of the trimpot and the body itself : the trimpot 'Wiper' should be sealed in place for some time.
Now that the power supply has been checked for accuracy, we have to recalibrate the Volt/octave relationship on all VCO's.: the tuning procedure is simple, but you need a good hear and a lot of patience ...
There are two calibration trimpots per VCO : the first one (A) is the 1Volt/octave HIGH range calibration trimpot (it is used to calibrate the VCO to the voltage delivered by the highest key (Upper range) of the keyboard), the second one (B) is the 1 Volt/Octave LOW range calibration trimpot (it is used to fine tune the VCO to the voltage delivered by the lowest key of the keyboard.
Note : Under any circonstances should you touch all the other trimpots, especially the one responsible for temperature control.!!!
Follow this procedure :
1) Let us assume, for simplification, that your keyboard range is 5 octaves (from C0 to C5).
2) If you have an electronic tuning generator, tune it to the frequency of the last key of your keyboard i.e C5. Send the sound to your audio mixer on the LEFT channel only.
3)Center the Master Frequency potemtiometer of your VCO. :If you have a modular synthesizer, send your VCO1 Sawtooth output direct to your VCA input (full VCA)without passing through the VCF. Send the sound to your audio mixer on the RIGHT channel only. Depress key C5 on your keyboard and hold the key down.
4)Place yourself in the middle of the stereo field between L & R Loudspeakers and listen to the two frequencies.
5)Now fine tune HI calibration pot(A) for zero beat (sync) : do not touch the Master frequency potemtiometer of your VCO1.
6)Tune the electronic tuning generator to C0. Depress key C0 on your keyboard and hold it depressed, fine tune LOW calibration pot B for zero beat (Sync): do not touch the master frequency pot of your VCO1.
7) depress C5 and C0 repeatedly to make sure that the Volt/octave ratio between the lower and higher keys is perfect. In case of slow beats, repeat the procedure from the beginning.
8) Repeat the same procedure for all the other individual VCO's in your synth. (for mono synths only).
9) That's it! Your synth should be well calibrated to 1Volt/octave.
10) As a rule, and before performing an important gig, you should always check the voltage delivered by your power supply and, if necessary, tune all your VCO's properly .
After all, your analog synthesizer is a musical instrument... :-)