Hans Remeeus PA0Q - Icom IC-775DSP Roofing Filter installation instruction

 Icom IC-775DSP

Inrad Roofing Filter installation instruction


Special thanks to George and Georgia from Inrad (International Radio Corp)

for developing and merchandising this excellent mod!


The text of the page is from the original installation instructions

(by courtesy of George from Inrad)

together with photo's and some advice from Hans PA0Q



Icom IC-775DSP before installation of the Inrad Roofing Filter.


The IC-775 Roofing Filter Mod consists of a 6 pole, 5 kHz wide filter

followed by a high dynamic range, feedback amplifier.

The amplifier provides enough gain to override the filter insertion loss plus a few dB.


The upper side of the board of the Inrad Roofing Filter.


The under side of the Inrad Roofing Filter.


The plot above shows the sweep frequency response of the RF board in an IC-775 radio.

The wider curve is the original Icom response

and the narrow curve is with the Inrad Roofing Filter Mod added.


The improvement in first IF selectivity is dramatic!

The result of this bandwidth improvement is the reduction of close-in

intermodulation from multiple signals.


As you get into crowded band situations you will notice there are more open spaces

in the SSB-band and less burps and beeps on CW from strong signals.


With the main receiver as the overall widest bandwidth of the receiver

will be determined by the Roofing Filter, which is about 5 kHz.

This may degrade the audio bandwidth for AM and FM reception somewhat.

These modes will still be available for reception with the sub receiver.


The wires, coax cable and standoffs, supplied from Inrad together with the Roofing Filter Mod.


WARNING: modern radios contain components which may be damaged by static discharge.

Precautions must be taken to eliminate any static electricity  buildup

between the operator and the radio before any of the internal circuits are touched.

If you are not familiar with the proper techniques for this,

consult the ARRL's Radio Amateurs Handbook.


Remove the AC line cord from the transceiver;

remove the bottom cover;

remove the top cover.


Place the radio bottom side up and locate the IF-board; it has the cyrstal filters on it.

Locate J8 in the rear corner near the center of the back panel.

Remove the coax cable from J8.


Remove the 6 screws holding the RF-board in place.

The RF-board is the smallest of the three boards on the bottom side of the radio.

Remove the screw near to the RX ANT IN connector on the rear of the chassis.

Lift the metal shield can off of the RF-board.


While keeping a sketch of where these cables go, unplug the following coax cables:

- from J1 and J2 on the PLL-board;

- from J6 and J8 on the RF-board;

- from the RX ANT IN on the rear of the chassis.


Remove the ribbon cable from the end of the RF-board.

And gently lift the RF-board out of the chassis.


Strip 1/8 inch insulation from the end of the red, yellow and black wire supplied by Inrad.

Tin each end of the three wires.

The wires will be tack soldered onto the ribbon connector on the RF-board,

as shown on the photograph above.


Return the RF-board to it's mounting place and return the cables to the proper connectors,

except for the short coax which was in J8 on the IF-board.


PA1HR's advise:

IMHO it is not necessary to remove the RF-board!

I found it quite easy to solder the three wires onto the ribbon connector,

while keeping the RF-board on its place.

The only thing you have to do first is to disconnect the ribbon cable from it's socket

at both sides (from the RF-board and from the IF-board), as shown on the photograph above.


Gently bent the three colored wires and tighten them with the cable clip which is mounted on the chassis.


Feed the coax that was in J8 through the chassis hole nearby.

Also, feed the three colored wires through this hole.


Turn the radio top side up with the front panel away from you.

Remove two screws from the loudspeaker and set it aside.


The PA-unit has as series of fins sticking out the back.

Remove three screws holding it in place; one on the back and two on the front of the PA-unit.

Do not move the PA-unit at this time!


The Regulator-unit (Reg-unit or Power Supply) is to the right and has the AC receptacle on it.

Remove the four screws holding it in place; two are on the back and two are up front.

Do not move the Reg-unit at this time!


The shield plate has the grounding wing nut on it.

Remove the two screws holding the shield plate in place.

Do not try to remove the shield plate at this time!


Now be very careful!

You will find a small ribon cable between the PA-unit and the Ctrl-unit,

as shown on the photograph below.

During installation of the IC-775 Roofing Rilter it is possible to disconnect this ribbon cable,
without realizing it at the time. The result is the radio will not transmit.


There is no stress relief on the ribbon cable inside of the PA-unit.

When you move the PA-unit for about 1/8 inch,

this ribbon cable can become disconnected from it's socket (connector) inside the PA-unit!


To prevent the ribbon cable disconnected from the connector inside the PA-unit

you should first remove the two little tapes, securing the ribbon cable to the top of the Tuner-unit,

as shown on the photographs above.


Then disconnect the ribbon cable from it's connector on the Ctrl-unit,

as shown on the photograph above.


After you did this, there will be no stress on the ribbon cable

when you move the PA-unit a bit.


Now gently push the front end of the Reg-unit towards the center of the radio

and pull it's rear end away from the center of the radio about 1/8 inch.

This will free up one end of the shield plate.


Then lift the rear end of the PA-unit high enough (about 3/8 inch) to remove the shield plate.



Remove the lower right screw from the fan


Prepare the Roofing Filter Mod by inserting the four plastic standoffs.

Do not remove the safety papers yet!


Plug the coax cable from the RF-board into J1 on the Roofing Filter Mod board;

plug the Inrad supplied coax cable into J2 of the Roofing Filter Mod board.

Be sure the two coax cables are seated well,

as it will become difficult to insert them after the next few steps.


Prepare the ends of the three colored wires by cutting to length

and stripping 1/4 inch of insulation. Tin the ends of the three wires.


Insert the three wires into the terminal on the Roofing Filter Mod board,

as shown on the photograph below and tighten the screws.


I advise you to tighten the three screws before you fix the board on the PA-unit!



Feed the Inrad supplied coax cable through the chassis hole to the bottom side.


Remove the safety paper of the four standoffs and install the board exactly as shown on the photograph above.


Re-assemble the PA-unit, the shield plate and the Reg-unit.

Then reconnect the ribbon cable into the connector of the Ctrl-unit.

And replace the two pieces of tape, securing the ribbon cable on the top of the PA-unit.

Tighten the screw of the fan.


Turn the radio bottom side up.


Insert the Inrad coax cable into J8 on the IF-board.


Tighten the coax cable and the three colored wires with cable ties.


Replace the top and bottom covers of the radio.


Quite a different Icom IC-775DSP with the Inrad Roofing Filter Mod inside...

Enjoy and 73! Hans PA0Q.