The 904-A is a voltage controlled low pass filter.  It is pretty simple in operation and controls.

This module has the separate edge finger PCB.  The two PCBs are back-to-back making repairs difficult.  This is an early version with a minimal number of trimmers.  Resistor values do not match the earliest schematics dated July 25, 1967.

Operation

This scope image show the low pass filtering of a square wave.

This scope image show the low pass filtering of a square wave with a small amount of resonance.

This scope image show the low pass filtering of a square wave with a large amount of resonance.

This scope image shows the nice sine wave output of self-resonance oscillation.

This scope image shows the frequency response of the low pass filter.

This scope XY plot shows the frequency response of the low pass filter.

back