904-A Low Pass Filter Module

The 904-A is a voltage controlled low pass filter.  This is a later revision from the three cabinet Moog and the panel has the new logo and module number.

This module no longer has the separate edge finger PCB.  The two PCBs are back-to-back making repairs difficult.




There are calibration instructions on the schematic but they are incomplete.  There is no illustration to identify the trimmers and there are no directions for R17 and R25 other than set to center.  R17, R25, and R40 are all to balance differential pairs.  I've identified the trimmers by color code and the test points with the same colors.  Follow the directions for trimming as shown here but also adjust R17 and R25.  There may be a specific sequence that R17, R25, and R40 should be adjusted in but I found that simply iterating a few times  worked well.





There are scope images of the low pass filter operation on the three cabinet Moog 904A page.