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IC-756 455 kHz SSB Filter Upgrade

by Johnny Siu VR2XMC

Click image to view filter passband curves.


The following is a procedure for replacing the stock FL-96 455kHz IF filter (2.8kHz @ 6dB) with an FL-44A (2.4kHz @ 6dB) in the "original" IC-756. The  filter upgrade is worthwhile, as it improves the receiver's adjacent-channel selectivity and  "tightens up" the Twin PBT, whilst maintaining reasonable receive audio quality. The FL-80/FL-44A combination is the standard SSB filter pair in the IC-751A, the IC-761, the IC-781 "NATO variant" and the IC-M700. It is a very effective and good all-round SSB filter set.

The ICOM FL-222 (455 kHz, 1.8kHz @ 6dB), when used in combination with the FL-223  (9 MHz, 1.9 kHz @ -6 dB) will greatly improve the selectivity. However, the receive audio with this filter pair is quite  harsh. In 6-metre DX hunting, the signal is always weak (often S2 - S3), and using the  FL-222/FL-223 pair can cause you to lose your RX signal entirely. 

Here is the filter upgrade procedure: 

  1. With the LCD front panel facing you and the bottom cover facing upwards, remove the  bottom cover. 

  2. Remove a total of 6 screws holding the Main Unit: 4 at the corners, 1 near  the middle and the remaining 1 near the voice synthesizer socket. 

  3. Remove the screw on rear panel in between ALC & SEND RCA sockets. 

  4. Remove heatsink clip holding transistor to chassis (left hand  side of the main unit PCB). 

  5. Remove all the flat ribbon cables (white in colour) by holding the  cables (not the socket) and carefully pull them up. Do not bend or fold  them!! Avoid using excessive force; the cables are quite delicate. 

  6. Watch out the for small flat cable at the lower left corner towards the  front panel. gently pull it out towards the right. Be careful; this small flat  cable is very delicate, and is held in place by a piece of adhesive tape. Remove  the adhesive tape first before pulling it out. 

  7. Remove the RF Unit metal shield and loosen 2 (or 3 as appropriate)  shielded cables to allow the Main Unit PCB to fold out to a workable  position. 

  8. Fold out the Main Unit PCB; remove the two hex nuts & washers holding  the FL-96. 

  9. Desolder the filter pins of the FL-96 using a solder sucker and a 20 to 25 watt  soldering iron temperature-controlled if possible). 

  10. Remove FL-96 by wiggling it gently to free pins. You may need to  re-heat pins quickly to release filter. 

  11. Replace with FL-44A; tighten down studs with nuts and washers (do not  screw too tight). 

  12. Re-solder pins of FL-44A; clean off excess flux and make sure that solder  joints are sound. 

  13. If you have already installed a FL-223 (9 Mhz narrow IF filter), you may  wish to take this opportunity to solder the retaining lugs of the metal case to the PCB.  This will hold the filter to the PCB firmly better contact and hence  less noise. Clearly, you should only do this if you do not want to remove  the FL-223 in the near future. 

  14. Re-install the board and all cables unplugged in previous steps. When  re-installing the flat ribbon cables, make sure you use the minimum force, and re-insert  cables only when the pin positions are correct. Put back the adhesive tapes and  heat sink clips removed earlier. 

  15. Test the radio and replace covers. If the FL-223 and FL-222 are already in the  optional filter slots, you will now have 4 IF filters: FL-80 (original stock) + FL-223 at 9  MHz, and FL-44A + FL-222 at 455 kHz a great variety of choices, which you can "mix and  match" using the filter menu. 

  16. As an alternative to the FL-44A, you may consider using the INRAD # 109 (455 kHz,  2.4 kHz @ -6dB), which will yield a slightly better shape factor. 

Contents copyright 2002 Johnny Siu VR2XMC. All rights reserved.

A. Farson VA7OJ/AB4OJ edited Johnny's original text, and created this page.
Last updated: 08/16/2002

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