DISCONTINUED: As of 9/25/17 we have exhausted our supply of daughter boards for Collins 75S-3B/C radios for the now out-of-production 455 kHz mechanical filters. Information below is for legacy purposes. The board layout, schematic and parts list for the discontinued board is downloadable below.
Inrad has designed an adapter board to allow the use of the newer Collins mechanical filters in the 75S-3B/C radios. The filters available are listed below. All filters have a center frequency of 455 kHz.
Two specially designed networks are included on the adapter board to match the input and output impedance of the new series filters to the radios. The original filters had a very high impedance whereas the new series are 2000 ohms. The resulting signal level through the adapter board is very close to that of the original filters.
The adapter is constructed on a double sided FR4 pc board with plated through holes. Surface mount components are used in the production units. The board contains pads for through hole components which can be supplied for special uses. Gold plated pins are swaged into the board so it may be plugged directly into the 75S3B/C filter sockets.
Installation consists of plugging the adapter, assembled with the filter of choice, into the radio. The resonating capacitors in the radio can be set to fully CW, then backed off 1/4 turn.
Full installation instructions are available here:
Schematic, parts list, and board layout for the discontinued circuit board: