[ICOM] New Inrad Roofing Filter in 756Pro3

[Ekki Plicht (DF4OR)]

Hello all,

today I installed the Inrad roofing filter in my 756Pro3, with the help of a 
friend (Stefan, DL2OCB) who has good measuring equipment and good eyes, hi.

The procedure went well, it took about 1.5 hrs. It's not really that difficult 
if you can work with SMD components, but if you can't it's impossible :-) The 
installation requires the removal of one tiny SMD resistor and then soldering 
two traditional components to the solder pads where the resistor used to be. 
That's something I can't do anymore, but Stefan could.

The rest is not that difficult and I did that myself (removing the RF-A board, 
removing a shielding case, drilling holes in that shield, installing the 
small coaxial cables etc.).

Soldering was a little bit strange at first due to the new 'RoHS' leadfree 
solder. You need much higher temperatures to get a good flow, we used two 
soldering irons to heat the shielding case.

Before the installation we measured the required input level to show an 
arbitrarily selected s-meter value on three bands (80, 20 and 10m). The same 
measurement was repeated after installation. We saw that the S-meter now 
shows about half an S-level more than before, at same input levels. So the 
amplifier on the filter board very slightly increases sensitivity, but 
without discernible increase in noise level.


So what is the effect now?

As expected, it's hard to tell. The built-in roofing filter has a bandwidth of 
15kHz, the new one 5kHz. To compare between the two roofing filters I 
set 'Dual Watch' to on, on the same frequency (no split). With this setting I 
can use the balance control to fade between both receivers, Main (balance 
full counter-clockwise, new 5kHz filter) and Sub (balance full clockwise, old 
15kHz filter).

To see if it has any effect at all I first listened to an AM broadcast 
station, with 9kHz IF filter. There was a distinct difference between 
Main/Sub, now I can be really sure that the filter works at all :-)

It's much harder to hear a difference in SSB or CW. There is a CW contest 
going on right now, the 40m band is fairly crowded with strong signals. Do I 
hear a difference? Well, a definitive maybe. 
I have the impression as if the Main receiver is slightly quieter, slightly 
less mush at the bottom, slightly clearer signals than on the Sub receiver at 
same frequency. 
But this is only a very subjective impression after only one hour of testing, 
and certainly influenced by wishful thinking after spending US$200 on the new 
filter. Only time will tell, like in the next contest. 
I think this modification will show it's effect in the rare 3% of a DXers 
life, when really the last dB in IMD and BDR counts. For the remaining 97% of 
my QSOs I probably could do without the mod. Still I love to have it 
installed.

Due to lack of equipment we did not do a proper IMD or BDR measurement 
(calculation), both figures should improve significantly with a narrower 
roofing filter, Inrad claims up to 15dB better IMD. I will try to round up the 
needed equipment (and expertise) somewhen in the future.

73,
Ekki, DF4OR