As you can see, this is pretty simple. The switch allows either VCO to sync to the other.
The .001 uF capacitor acts as a differentiator, ie it creates a spike on each transition of the square pulse output on pin 3 of the VCO's 555 timer IC. The positive spikes are selected by the diode and allowed to influence the 'trip' voltage of the other VCO's 555 timer. This kick in the pants causes the sync'd VCO to terminate it's current cycle and discharges it's capacitor. Then it begins a new cycle.
In combination with an LFO modulating the frequency of either VCO, or just by itself, oscillator sync can produce very interesting results.
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The SubHarmonic Generator produces a square wave signal, pitch locked to VCO 2 and one octave below it. A simple RC lowpass filter is provided to roll off harmonics as desired. A very near sine waveform output from it is possible. A level pot is provided to allow controlling the mix of the subharmonic and the output of the 1/2 vco mix control.
A 74L93 TTL binary counter is used to generate a subharmonic (one octave below) tone from the pulse stream output of the VCO 2 555 timer chip. The pulse output from IC16 is available on pin 3 as a -V referenced square pulse. This is incompatible with TTL logic so it is level converted by IC901A, clamped no lower than ground by D901 and then fed to the B input of the binary divide by 2 counter IC902. The B output of IC902 produces a symmetrical square wave, the frequency of which is exactly one octave below it's input. This is the raw subharmonic. The raw subharmonic, a square wave, is then sent to a simple lowpass filter to remove harmonics as desired, amplified by IC901B and mixed with the output of the VCO 1/2 MIX control via resistor R901. The combined output is then sent to the VCF.
As fat as the FatMan is, right out of the box, it is even fatter and thicker with the subharmonic generator. This mod has a rather profound effect on the harmonic structures available. It's almost like having a third VCO. The subharmonic generator also allows the FatMan to produce tones down to (approximately) 32 Hz!
2 words sum this mod up: thundering bass.