IC740 Three filters at 9 MHz

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:

9M15A - only small 2-pole resonator with width about 15 kHz. In use when:
- RX: If shift is on, and always when FM mode
- TX: non-compressed SSB, and always when FM mode
9M22D2 - standard SSB crystal filter FL-30, 2.3 kHz wide, fixed on the PCB, in use when:
- RX: PBT is on and SSB or RTTY mode
- TX: compressed SSB, and CW mode
Optional filter, intended for CW, either FL-45 (500Hz) or FL-54 (270Hz), in use when:
- RX: CW mode and filter button is depressed

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
		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

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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