This is a simple analog VCO. Its “thing” is that the triangle output gets AC coupled back into the triangle core, thus self-frequency-modulating itself. New harmonic content results as the shape of the core becomes less definable. Since everything derives form the triangle core, all the wave outputs get distorted. The videos below demonstrate this.
Furthermore, the self mod control is sensitive to speed of knob rotation. If swept slowly there is no notable change in pitch. If swept not-slowly, the movement of that control will result is a momentary pitch swing. This can be “played” for effect and adds bellow or fluctuation to the pitch. If self-mod starts to near its maximum position, the VCO will go slightly flat. This can be noticed near the end of the sine video.
About the sine output. It’s nowhere near a perfect sine, but it’s pretty sine-ish. It is also inverted relative to the phase of the triangle and square outputs. If the sine is mixed with the other waveshapes (through external patching) this serves to cancel out the fundametal frequency in the other waveshapes, and only the harmonics are left. This has a highpass filtering effect and can make useful tones.
The frequency knob controls a useful audio range of about 10 – 10,000Hz. Through the use of CV that can be pushed to sub and supersonic levels. The circuit is not useful as an LFO, no voltage will result if the VCO is slowed (through high negative CV) to less than about 1Hz.
The circuit is well compensated against temperature related pitch drift, and has good 1V/oct tracking. Fine tracking adjustment can be done from the front panel via small flathead jewlers/glasses screwdriver. If overtracking turn left, if undertracking turn right. A blue trimpot on the circuit side of the module adjusts the initial frequency set.