Icom IC-740

Three filters at 9 MHz IF

No doubt this is the most brutal, violent and damnable modification I ever made to IC-740. However, it works "as expected" while nothing else went wrong.

By the original design, on the second I.F. (9 MHz) there are three crystal filters there:

I completely removed the 15 kHz wide filter including the surrounding coils and replaced with FL-54. For TX the only change is that the non-compressed SSB mode signal also goes through FL-30. This can only improve the stopbands and is a positive change.

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

Mode I.F. FILTER button
CW 9 MHz 500 Hz
270 Hz
455 kHz 500 Hz
250 Hz
SSB 9 MHz 2.3 kHz
455 kHz 2.4 kHz
1.8 kHz

Schematics of the modification.

Changes of the PCB.


View of the FILTER board. The FL-54 is the filter that was added this way.

Bottom view of the FILTER board. A few new holes had to be drilled.
Most of the additional parts are solder from this side. Some of them are SMD, some are through-hole parts.


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