Moog Music Forum

I'm sure I know where that 901B updated design came from. In late 1970 Jim Scott and Bill Hemsath were designing the first two Minimoog prototypes. The first two designs used surplus or junked circuit boards including 901A's and 901Bs that were laying around at the Moog factory. The design team knew that the 901 oscillator was not nearly good enough for the proposed Minimoog and that is when Bob Moog contributed a totally new oscillator design to the project. Most technically minded people know that the roughly 12,000 Minimoogs that were manufactured used either one of two oscillator boards but there were actually THREE oscillator board designs. The first oscillator board was only used in the first 100 or so Minimoogs that were built in Trumansburg, although it was conceptionally the same design as the later two oscillator boards and the 921 series it was implemented quite differently. Try to find R.A. Moog drawing #1441 dated 12/21/70 and you will see what I mean. That drawing, titled "Triple Oscillator", shows a PNP transistor pair as the exponential converter which sourced current to a sawtooth oscillator that had a rising ramp, the 901B also had a rising sawtooth ramp but used diode strings as an expo converter which wasn't nearly as accurate as a transistor pair. Although this oscillator had numerous op-amps they were all done with discrete transistors. Because no monolithic op-amps were used the overall performance wasn't the greatest by today's standards but it was still a huge improvement over the 901 diode based design in terms of tracking accuracy and temperature stability. I'm guessing that a variation of that early Minimoog oscillator design is what was put into Keith Emerson's 901s, if nothing else the timeframe is about right. The later Minimoog oscillator board that used a CA3046 transistor array and a falling sawtooth waveform wasn't released until April 1972 and the board with the ua726 based expo converter was released 6 years after that so I don't see how those designs could have been used for the 901 upgrade.

About the Minimoog VCOs:
There is a scans of a schematic using a SG3821 chip for the exponential transistor dates 10th April 1972. Drawing number 08-001. Hand drawn.

And of cause, there is the well known CA3046 based design shown in the service manual. This is also hand drawn.

According to a service manual version, the ua726 was built into serial numbers 10175 and above. This schematic looks more modern and uses black labels for the trimmers, pods and switches.

Yes, the 08-001 VCO drawing and the later design using the ua726 are well documented and well known.

Many years ago I ran the service department at a large music store that was also a Moog dealer and I accumulated a large number of drawings from Moog, ARP, and others, all of which I still have. Some of the drawings that I got from Moog such as 08-013 921B oscillator, 08-009 921A oscillator controller and a few others, were sent to me as actual blueprints, most likely printed out directly from their inventory of drawings.

I also have some of what I think might be rare drawings for the ARP 2600, they look like the original circuit design drawings with title blocks and design revisions. The title block show the name of the company as Tonus, Inc. which was the original name of the company when Alan R. Pearlman first set it up. I've been thinking of scanning those drawings and posting them in case anyone would like to see them if they are not already available somewhere.

Really enjoyed the KE solo of Tarkus on your moog modular. You sound great and you Modular looks cool as well. My drummer friend Tony Pia played drums for KE before KE passed. Looking forward to hearing more of your playing and pics of your cool modular if you release any more YouTube videos.

Here is Jim Scott explaining how the Emerson 1Ca was an evolutionary step towards what became the minimoog:

Courtesy of EMEAPP

https://emeapp.org/2021/10/19/jim-scott ... thesizers/