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.