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IC-746 Filter Modifications

By George P. Spillius, N9IRD

Below you will find the info for filter mods on the IC-746... 

Stock filter...F1711=Murata, sfpc455g...change to Murata cfzm455h

Stock filter...F1721=Murata, sfpc455e...change to Murata cfzm455F

Specifications: The cfzm455h= 6kHz at -6dB & 15kHz at -70dB

The cfzm455F= 12kHz at -6dB & 24kHz at -70dB 

These high-grade Murata filters are 9-pole devices with the performance of 11 to 15 poles. The input output impedances are the same 1500ohm as the sfpc series! As you can see from the specs, this is quite the improvement!  Download the Murata ceramic filter catalogue (pdf).

Why Icom would build a radio with such poor filters is beyond me! With these filters installed, the AM & FM reception is greatly improved! Now when you select the 9kHz filter you get 6kHz and 15kHz you get 12kHz...it should have been this way from the get-go! These filters are hard to get; you have to buy many hundreds from the wholesalers, but I found a way to get  them...call a sales rep and ask for some manufactures samples. It worked for me! Now these filters are small, but not of the surface-mount type. They are the leaded type. What I did is make a small circuit board from some double-sided material, using my Dremel tool, mounted the filter to the board and made 3 little pads with old pins from an IC and soldered it in place of the sfpc filters. The cfzm filters then stand on end, but they fit just fine.

As for changing out the FL65 ssb filter, it just seemed to me that I could improve the passband with a better, MECHANICAL filter. I found some KOKUSAI 2.3kHz mechanical filters, and their passband is great...better than INRAD or ICOM. The specs on the KOKUSAI filter are...2.3kHz at -6dB & less than 4kHz at -60 dB. 

Again.....NOT BAD! This mod was a little more involved because I had to remove the FL65, and that was not easy! then I had to make another board to mount the KOKUSAI filter to, so I could match the solder holes of the old FL65 filter. I did it and it WORKS GREAT! This mod is not for the faint of heart, as it's easy to destroy things in the process. It's very tough to get that FL65 out of the circuit board! For the casual radio operator, I would not recommend this mod, as you can just plug a 455 kHz filter (e.g. Icom FL-44A or Inrad 109) into the open slot. I wanted to save the open slot for another filter, and thatís why I undertook this change. 

Copyright © 2002, George P. Spillius, N9IRD

 A. Farson VA7OJ/AB4OJ edited George's original text and created this page.

Last updated: 09/20/2002

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