Unknown Prototype VCO

These photos are courtesy of Tom Szakaly ( ELP tribute 'Noddy's Puncture). Tom contacted me about helping him with an unknown Moog prototype VCO.  The prototype had not been completely built and he had no additional information.  I had him photograph the front and back of the module and trace out a schematic and we worked exclusively by email.

The module was constructed on perfboard and had a single can style op-amp.  We were not able to determine any date codes.  There was only a single potentiometer control and no CV input jack.

In examining the partial schematic the circuit was a ramp core oscillator with a transistor reset.  The integrator for the ramp core was a topology called a Deboo integrator which is a Howland current source with a capacitive load.  Gordon J. Deboo worked for NASA and published a number of papers and received a number of patents.  Here are two relevant documents on the Deboo integrator.

Patent 3,470,495 is for a Feedback Integrator With A Grounded Capacitor

NASA Technical Brief 67-10135 is for Integrator Can Easily Be Set and Reset with an Electronic Switch


The Deboo integrator formed the core of the oscillator using a MC1439 op-amp and a rotary switch presumably for octave selection.  The integrator was followed by a 2N4303 JFET buffer providing the reset circuitry and the ramp output.  The triangle and pulse outputs were formed with circuits from the 901B oscillator.  The sine circuit, however, used the symmetrical JFET shaping circuit often used by Don Buchla.  The bias diodes were oriented for a P-Channel device.  I had him reverse the diodes so a more common N-channel JFET could be used.  There was no buffer after the JFET so I drew up a Moog-style 3 transistor transistor buffer to use.  All wave shapes are similar to the 901B.

My thought is this prototype must have followed the 901B design as they investigated moving away from unijunction transistors.  As such it would be somewhere after 1965 and probably a year or two before 1972 when the 921 VCO was released.  The Deboo documents are dated 1967 and 1969 so perhaps this prototype was built somewhere in the late 1966 - 1969 timeframe.

This unknown VCO prototype is certainly quite rare.  These photos of the prototype are as it was initially obtained.