The stock FL-96 (455 kHz SSB: 2.8 kHz wide @ -6 dB) can be replaced with an FL-44A or an Inrad 109 (2.4 kHz wide @ -6 dB) to improve adjacent-channel selectivity and sharpen Twin PBT operation. In addition, the stock FL-102 (9 MHz, AM) can be replaced with an FL-223 (9 MHz SSB: 1.9 kHz wide @ -6 dB) to provide an SSB-NARROW bandwidth setting.
I would suggest that you purchase or download an IC-781 service manual for guidance, if you do not already have one. The FL-44A has become rare The Inrad 109 is an excellent alternative, and is electrically and mechanically compatible with the FL-44A.
Remove the top and bottom case covers.
Lay the radio upside down on a towel, with the front panel facing you.
Using a magnetic-tip manual screwdriver with a #2 Phillips tip having rounded blade edges (or a #2 JIS tip), remove all eleven retaining screws from the IF board (the board with all the filters).
Unplug the multi-conductor header cables from the board, to allow the board to be moved aside.
Grab the inner edge of the board, and swing the board away from the chassis. Do not stress coax cables on the board. Rest the board on a stand (e.g. 2 phone books) next to the radio.
Locate the FL-96. Using a solder-sucker or wick, and a temperature-controlled pencil iron, desolder the 4 filter pins.
Remove all nuts & washers from the filter mounting studs. (As I recall, the nuts are M3 or M4.)
Remove the FL-96, wiggling it gently to free its pins. You may need to re-heat the pins quickly to release the filter. The board is G-10, and can take reasonable heat.
Place the FL-44A (or 109) on the board; tighten down the studs with nuts & washers.
Re-solder the FL-44A (or 109) pins; clean off excess flux with nylon brush & isopropyl alcohol. Also ensure that there are no solder blobs or splashes.
Re-install the board and all cables unplugged in Step 4.
Pull the FL-102; insert the FL-223, and flip the 2 dip switches (per user manual) from AM to SSB.
Test the radio, then replace the case covers.
Good luck! The whole job should take about 30 minutes. Note that the FL-223 is selected in USB or LSB mode by pressing the [WIDE] button.
This is an updated version of my article originally published in 1999 in mods.dk
Last revised: May 29, 2016