[INRAD] ICOM IC-756 roofing filter mod.

[Charlie Mazoch Jr.]

I installed an INRAD  IC-765 performance roofing filter mod in my 
IC-756. It is wired into the RF board between L201 and L84. This is a 
low signal level receive only path and is on the dual watch combiner ( 
L201 ) output.  I cut the original  circuit trace using a sharp xacto 
knife and used RG-178 B/U teflon coax which is 0.074 diameter for the 
two tie points. Clean some of the ground plane area for soldering the 
coax shields. Keep the exposed center conductor short because this is a 
64 Mhz signal! I believe the teflon coax jumpers INRAD uses are also ok 
to use. They are about 0.100 inch diameter. Before removing the RF board 
mark all the original coaxial cables with different symbols for ease of 
reinstallation. The added two coaxial cables are routed out of the RF 
board enclosure about 2.5 to 3 inches and soldered into the INRAD 
circuit board. The input coax to INRADS board at J1 is ac coupled with 
an added 0.1 uF capacitor. This capacitor is necessary! The negative 
feedback resistor R6 on INRAD's amplifier has been changed to 62 ohms. I 
did a lot of substitution of this resistor value from 39 to 150 ohms and 
settled on 62 ohms. At 39 ohms gain through the amplifier is within one 
dbm of the bypass mode. The reason 62 ohms was picked is that it 
provides  4 dbm of gain over the OEM circuit and does not add any noise. 
The original value of 220 ohms added  more gain and a considerable 
amount of quiescent or internal generated noise. I installed a miniature 
toggle switch into one of the rear air slots of the power amplifier 
enclosure. I picked up a switched 12 volts from the ribbon cable 
connector to the power amplifier board and added a 7806 voltage 
regulator to lower the switching voltage to 6 volts. The 2 leads were 
twisted for shielding and brought from INRAD's board in the lower 
enclosure into and under the power amplifier to the rear mounted toggle. 
The air slot had to be enlarged with a round file to mount the toggle 
switch. Clean out the filings! One may even consider another place to 
mount the toggle switch. Probably not necessary but a 0.030 small 
aluminum plate the size of INRAD's board was attached with 4-40 
fasteners and standoffs, ( 8-32 nuts ) to the trace side of the board. 
This was an added idea for shielding. Then the whole assembly was double 
wrapped with Scotch 33+ electrical tape for insulation and lays in an 
open area on top of the second IF board. The rigs bottom cover keeps it 
in place. You may have to move the board somewhat to find a spot where 
the lower cover doesn't push hard against the INRAD board. Before 
replacing the cover on the rf enclosure I repeaked L84 after the 
modification in the bypass mode. MDS for a 3 dbm rise in audio level 
over noise is  -130 dbm with preamplifier #2 in and the INRAD roofing 
filter in or out of the circuit. Icom IC-756 specifications list 
sensitivity @ 0.16 uV or -123 dbm. I would perform  2 tone tests but but 
don't have two high purity crystal controlled oscillators offset in 
frequency. This filter is not a cure all for close in signals because it 
does have a width of approximately 5 Khz at 3 dbm down. No roofing 
filter can be if the interfering signal is within it's pass band. But it 
does help over the OEM filters, two 2 pole at 15 Khz width and was well 
worth the effort and cost. It does present an improvement while 
listening to signals on the bands with strong adjacent signals.  
Listening to wide mode AM while switching the filter in or out really 
demonstrates it's capability. Installing the INRAD filter in a receive 
only circuit allows one to bypass it for FM or AM mode as it isn't used 
in the transmit path. This is a modification for the experienced 
technician as care has to exercised. INRAD has been sent a set of 
photo's and my e-mail is good in QRZ or here. I can bear no 
responsibility for someone botching their rig.   Charlie  W5VIN    
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