This is a simple but yet reliable VCO. It has Sawtooth and Square wave outputs, even if it would be easy to add wave-shapers for Triangle or Sine wave outputs. It is originally based on Terry Mikulic's and Bernie Hutchins' designs for the late 70'ies, adopted by Gene Stopp and and Magnus Danielsson for the ASM-1 modular synthesizer. This version of the schematic was created by Laurie Biddulph at Elby Designs, however it is changed for +/-12V by me (with helps from Magnus Danielsson).
Unfortunately there are no building instructions for this module, but at least you'll find the schematic, some photos, and some comments.
Resistor R8 might need to be considered. R8 and P2 sets the volt/octave range, too high values and your will have more than one octave frequency change for a volt change in CV, too small values and you'll not reach an octave with an volt. You want to have P2 small (i.e. 10k) for fine adjustment, but depending on other component tolerances R8 must not be too big or too small. For me in the first VCO R8 was 89k, in the second VCO R8 was 95k.
The finished module.
Here's a demo of the VCO square wave output. After a while pulse width modulation is added. There are some filter tweaks and envelope generator setting changes. Sub-octave is added. The offset of the sequencer is changed to test different frequency registers.
That's it! Have fun and make nice sounds!