Product detector for 75A-2 / 75A-3

Glen Zook gzook at YAHOO.COM
Tue Mar 31 21:37:33 EDT 2009

Several people have asked if I would provide them with a built version of the solid-state product detector for the Collins 75A-2 / 75A-3 receivers which would plug into the NBFM socket.  Therefore, I have fabricated the circuit into a sealed unit.  To use the product detector (which requires no DC or AC voltage to operate) you have to change a jumper on the mode switch and re-route the BFO signal to the NBFM adapter socket.  A marked up section of the schematic is provided.

When the modifications are made, the product detector functions in both the CW and NBFM positions of the mode switch.  When in the CW position the AVC is turned off and when it the NBFM position the AVC is turned on.

The unit utilizes Schottky diodes and is isolated from any DC voltages that are present on the NBFM switch.  Other than the 2 connections there are no other changes required in the receiver.  Those modifications can be restored with very little effort to remove the circuit and return the receiver to its original condition.

Details of the product detector can be found at

I am working on a "generic" version of the product detector that can be used in many "boat anchor" receivers that will require hard wiring or some sort of additional socket.

Glen, K9STH



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