The 904-B is a voltage controlled high 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.

Operation

This scope image shows the high pass filtering of a square wave.

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

This scope XY plog shows the frequency response of the high pass filter.

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