This is a very cool VCO it puts out sine, triangle, ramp, and pulse waveforms and tracks 1 volt/octave over about 8 octaves or so quite nicely even though it is not temperature compensated.
If you have any TEL-LABS Q81s laying about you can directly replace R10 (2K) with one. What I would suggest if you do is that you epoxy it to the CA3046 case and run short leads from it to the PC board mounting holes. The circuit provides sync in and out and you can control the pulse wave's width from about 10% to 90%. All of the parts are very obtainable and not extremely expensive.
IC1A and IC1B and associated components comprise a voltage to current convertor that has a logarithmic relationship. As voltage applied at R3,R5 and R6 goes from 0 to 1 volt the current flowing into pin 5 of the CA3046 goes from approximately 1uA to twice that level (for example 1 uA to 2uA) when the voltage goes from 1 volt to 2 volts the current doubles again (2uA to 4uA). This doubling continues as the voltage continues to increase by 1 volt increments. (for example 3V=8uA,4V=16uA,5V=32uA,6V=64uA,...10V=1024uA). If the current starts at .9uA or 2uA it really doesn't matter what is important is that the current doubles for each increase of one volt in control voltage. The reason the voltage to current relationship is important is that it directly relates to the frequency produced by the oscillator controlled by the current. Your keyboard needs to produce 1/12 of a volt per half step or 83 mV. The V to I convertor produces the logarithmically scaled current. This can be accomplished with a voltage divider or with my MIDI to CV Convertor. I certainly did not invent this method, it is quite ingenious and has been around for some time now.
The heart of the oscillator is formed by IC2A and B. IC2A is an integrator whose output ramps up at a rate that is directly proportional to the current flowing out of its inverting input. IC2B is used as a comparator that causes Q1 to reset the voltage at IC2A's output back to zero. So now we have our control voltage to current convertor and our directly proportional ramp oscillator. The remaining components are used to rectify the ramp (which oscillates about ground) which results in a triangle wave, take the triangle and distort it into a sine wave (using transconductance op amp IC5, and run the triangle through a comparator to square it up (and to provide pulsewidth modulation control).
The sync input just lets you reset the integrator using an external pulse source and the linear modulation allows you to modulate the oscillator frequency with linear response instead of the log response you get at the CV inputs. Syncing two oscillators and then detuning them produces some very cool timbres.
http://www.musicfromouterspace.com/analogsynth/vco.html
