The Moog 901-A Oscillator Controller provides the  +12V for the ramp core circuit, control voltage, and the pulse reference voltage for the 901B Oscillator.  It contains a control voltage summer and the exponential converter.  It has a precision 1V octave switch and the pulse width control.  Module cabling is wired in parallel so that a single 901A can control up to three 901B modules.  

The 901-A Oscillator Controller is a 1 MU wide module.  The single turn trimmers have been replaced with multi-turn trimmers.  The unmarked trimmer in the lower left is the Mid-Hi which only appeared on the early version of the 901-A.

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