[Icom] IC756 filter modification

[Siu Johnny]

Hi

The original IC756 is still a very good performer though the Pro 2 is now
more sought after.  In the past, there were messages about replacing the
455kc IF filter FL96 (2.8khz -6db) with a FL44A (2.4khz -6db) in this
reflector.  It is a very worthwhile exercise because you will be able to
both improve the selectivity of the RX and maintain a reasonable sound
quality.

Although the ICOM FL222 (1.8khz -6db), when using with FL223, will greatly
improve the selectivity, the sound is too harsh.  In the 6 m DX hunting,
signal is always weak (often 42 or 53), using FL222 will make you lose your
RX signal entirely.

I understand that some of the hams simply like myself - not a radioman by
profession- find difficulties in doing the modification.  Therefore, I would
take this chance to detail the steps:

1. With the LCD front panel facing yourself and bottom facing upwards (the
carrying handle is now at your right hand side, this will be the referencing
position in the following steps), remove the bottom cover;
2. Remove total 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 heating sinking 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!!
Also no excessive force;
6. Watch out the small flat cable at the lower left corner towards the front
panel; gently pull it out towards right.  Be careful, this small flat cable
is very delicate and is fixed by a piece of adhesive tape.  Remove the
adhesive tape first before pull 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, loosen the two hexa nuts & washers holding
the FL96;
9. Desolder the filter pins of FL96 using solder sucker and a 20 to 25 watt
soldering iron;
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 FL44A; tighten down studs with nuts and washers (do not
screw too tight);
12. Re-solder pins of FL-44A; clean excess flux and make sure good solder
join;
13. If you have already installed a FL223 (9mhz narrow IF filter), you may
wish to take this chance to solder the studs 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 that if you do not want to remove the
FL223 in the nearest future.
14. Re-install the board and all cables unplugged in previous steps.  When
re-install the flat ribbon cables, make sure you use the minimum force and
only when the pin positions are correct.  Re-instate the adhesive tapes,
heat sink clips removed earlier;
15. Test the radio and replace covers.  I now have 4 IF filters namely FL80
(original stock) and FL223; FL44A and FL222 - a great varieties.
16. If you do not like FL44A, you may consider using INRAD # 109, which will
give you a slightly better shape factor.

Clearly, any modification to the radio is at your own risks and I will not
take up any responsibility on the above.

Before I did this exercise, I had consulted both Adam AB4OJ and Frank W3UHF
who had been very kind to show me the way.  Thanks Adam and Frank.

Johnny Siu VR2XMC