The FT1000D comes equipped with additions including I.F. Filters for the 455khz. c.f. The narrowest of these has a 500hz. bandwidth, but no matching filter for the 250hz. bandwidth for the 455khz. I.F. This means that if you select the 250hz. bandwidth filter in the second I.F., it is paired with the 2.4khz. filter in the third I.F. It is a simple modification to have the control section select the 500hz. filter in the third I.F. instead.
This modification requires the addition of a switching diode (1N4148, 1N918 etc.) between the select line on the 250hz filter spot and the 500hz. filter.
First access the filters on the I.F. board, and clip the diode as indicated for the installation of the 250hz. filter in the manual. Then remove the 500hz. filter, note that the I.F. filter position for the 500hz. filter is the second from the front of the radio.
Then solder the diode between the cathode end of the 250hz. select diode, and the cathode end of the diode under the 500hz. filter. This diode should be oriented with the cathode toward the 250hz. select diode's cathode end.
Now when the 250hz. third I.F. control pulls the select line low, the new diode will pull the 500hz. filter select line low, selecting that filter in place of the 2.4khz. filter.
When you finish this modification you will note that the 500hz. filter reduces the wideband noise, and the "Width" control works when the 250hz. filter is selected.
Because of the mismatch in the filter bandwidths, the first few detents on the width control act as an I.F. shift, when the 500hz. filter is offset enough to "catch up" with the bandwidth of the 250hz. filter, the action changes to a bandwidth control.
On my radio, the first three detents act as an I.F. shift, and the next three reduce the bandwidth to under 100hz.
Important Addendum to KD5VC 250Hz Filter Mod
In KD5VC's mod, the references to connecting a new diode between the cathodes of the 250Hz and 500Hz select diodes means to connect to the PCB solder pads that the cut diodes' cathodes were formerly connected to, not to the cut flying leads on the actual diodes themselves! The 250Hz select diode is D2045 and the 500Hz select diode is D2046. Both are clearly marked on the PCB.
This mod also appeared in QST, Jun 96, p.70 (with the same ambiguous narrative). I have installed it and it works very well. An FT-1000D does not ship with the 250Hz 3rd IF filter- it is an option. This circuit trick can save you the cost of a crystal filter- with a 5 cent diode- while still greatly improving CW performance at 250Hz BW over the factory stock configuration. For FT-1000 owners: note that the mod assumes that the 250Hz 2nd IF filter is already installed. The purpose here is to cascade the 500Hz 3rd If filter with the 250Hz 2nd IF filter.