Moog 901-B Oscillator (Revised 91-079)

The Moog 901-B Oscillator was the initial VCO designed with a unijunction transistor ramp generator.  The 901-B Oscillator is controlled by the 901-A Oscillator controller which provides +12V for the ramp core circuit, control voltage, and the pulse reference voltage.  Module cabling is wired in parallel so that a single 901-A can control up to three 901-B modules.  The 901B was later revised with a 91-079 PCB where the additional hand-wired components were added to the PCB.  There were other circuit revisions including trimmers for scaling, Sine shape, and Sine Offset.  More information and scope photos of the original version are on my 901B Oscillator page.

This particular 901B came in pieces for me to restore so it offered a good view of the front and rear of the PCB.  I also built a Dotcom power adapter so he could run some vintage Moog modules in a modern Dotcom cabinet.


This module was built in 1970.


The front of the PCB is a very clean layout with two sockets for the Q10 unijunction and the Q7 hand-selected transistors.  You can see additional paralleled capacitors in the upper right where they padded values for the octave scaling.


The bottom of the PCB has additional capacitors for the octave scaling.  The octave switch was fairly accurate between 32' and 16' but was significantly flat for all the upper octaves.  I found that removing most of these capacitors from the back of the PCB restored the octave switch to very good performance.  It makes me wonder why they were added. 


The rebuild came out quite nice.  I believe this particular oscillator panel was part of a service or test fixture at Moog.  Note the "Property Of" legend on the panel.


I couldn't find the original style rotary switch so used a very nice Electroswitch C4D0206S-A switch.  I tried to duplicate the original wiring as best I could with the different switch configuration.  There is a photo of a 91-079 901B module at Studio Repair for comparison.