Using Crystal Filters in FT-897D
This rig enables to use as much as
two optional mechanical (Collins) filters on the second i.f.
stage (455 kHz). Considering what filters are currently available
on the market it makes sense to employ one or two of the
following three: 2 kHz (SSB), 500 Hz, 300 Hz (CW).
Installing these filters is a big improvement in the quality of the rig, however, the small mechanical filters can't match the "old good" crystal ones. Crystal filters provide much better shape factor (ratio between width at 6 dB and 60 dB) and much better attenuation outside the passband. Besides this there is a large selection of widths in crystal filters, such as 125 Hz, 250 Hz, 400 Hz, 500 Hz, 1000 Hz, 1.8 kHz, 2.1 kHz, 2.4 kHz, 2.8 kHz, etc (source: Inrad web page).
Making space for one filter (fig  on the picture) was relatively easy. All you need is to push the ferrite toroid on the thick power cables down into the hole that is behind it. It was necessary to slighly shorten screws of the filter (some filters don't have screws).
I use the embedded power source therefore I can't use internal batteries. Therefore, to make space for another filter (fig  on the picture), I removed interfacing PCB that is below the huge battery switch on the top cover, as well as the switch itself. This makes space for another crystal filter. It was necessary to shorten one of the screws of the filter.
The filters are connected in the "wire-in" style. I used sub-boards of Inrad filters after the filters themselves were soldered out. The used coaxial cable is RG 174/U.
I use filters Icom FL-222 (1.8 kHz/455 kHz) for SSB and Icom FL-53A (250 Hz/455 kHz) for CW. What a radio!
 - Position of optional
mechanical filters. Filters were replaced with thin coax cables
on the sub-boards.
 - Four coax cables RG 174 are tacked by stripped screws at three spots.
 - Crystal filter Icom FL-222 (1.8 kHz / 455 kHz) for SSB.
 - Crystal filter Icom FL-53A (250 Hz / 455 kHz) for CW.
Petr OK1FIG, January 2008
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