[ICOM] New Inrad Roofing Filter in 756Pro3

[Ron NØAT]

I installed the Inrad roofing filter in my PRO3.   It took about 2 
hours.   Without the proper SMD equipment it would have been very 
difficult, if not impossible to complete the installation.

My initial impression is the radio is slightly quieter now.    It is 
difficult to determine if/when the filter is improving the reception. 
I operated the ARRL DX CW contest and did not hear any  spurious 
signals.

I use the following procedure to compare the two roofing filters inside 
the PRO3:
   Turn the BAL control full CW
   Press and hold the DUAL WATCH button until both VFOs are on the same 
frequency
   Toggle between the A and B filters by pushing the DUAL WATCH button.

I have another (unmodified) PRO3, so I'll be making some side by side 
comparisons as soon as I get time.

73 de Ron NØAT



----- Original Message ----- 
From: "Ekki Plicht (DF4OR)" <ep at plicht.de>
To: "ICOM Reflector" <icom at mailman.qth.net>
Sent: Saturday, February 16, 2008 17:25
Subject: [ICOM] New Inrad Roofing Filter in 756Pro3


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
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