Icom IC-740

Four crystal filters on 455 kHz

The IF unit can host only two filters on 455 kHz. One of them is mandatory (F1). It should be the SSB-width filter (such as FL-44A) as it is used also for SSB transmission. The other one is optional and is intended mostly for 500 Hz wide filter for CW.

The below modification enables to use 4 filters. There is an extra PCB that carries two more filters. It has the same size as the optional FM board and is placed instead of it on the chassis (yes, my IC-740 will never know FM mode).

The logic of switching of the filters for RX is as follows (considering also this modification):

Mode I.F. FILTER button
OFF ON
CW 9 MHz 500 Hz
FL-45		 270 Hz
FL-54
455 kHz 500 Hz
FL-52A		 250 Hz
FL-53A
SSB 9 MHz 2.3 kHz
FL-30
455 kHz 2.4 kHz
FL-44A		 1.8 kHz
FL-222

 


The extra board carries the narrower filters:
FL-222 (1.8 kHz)
FL-53A (250 Hz).
The extra board from the bottom.
The switching circuitry copies the original one in the IF unit but it is realized mostly by SMD parts.
Another small PCB works as an interface between the extra board and the board of the IF unit.
The interfacing PCB is mounted at the bottom of the optional filter on the board of IF unit.
Mind the 4 diodes (1N4148) soldered to the board of IF unit.
The filters are connected by 4 short RG-178 cables.

 

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