Modifications for the Icom IC-746
(See list below)

[1] Increase mic. Gain of IC-746

[2] Add continious-on cooling fan

(More to follow..?)




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[1] -Increase the microphone gain of the IC-746 -

Look at the PHOTOS below for the pix that deal with these modifications

The modifications add more gain to the IC-746's mic preamp, and the second mod allow the cooling fan to run whenever the unit is turned ON, but at a very low rate.

The first mod addresses the lower-than-normal mic preamp gain that the non-Pro model of IC-746. In looking at the IC-746 and the IC-746-Pro schematics, I attempted to compute the GAIN of the respective mic preamps. In my attempts, I found that the IC-746 has a gain of 17, while the IC-746-Pro has a mic preamp gain of 22.

The modifications involves removal of the MAIN BOARD from the IC-746. Then, locate and remove the original 82K surface mount resistor at R-1129. (This is located on the bottom of the board...)

Install a 100K resistor as a replacement of the original R-1129 as shown in the photos. Note that there are two small holes at pins 21 and 22 of IC-1121. A potentiometer can be installed, if desired. (I wish I had done this!)

Carefully re-install the main board and all of the previously removed interconnects. Power it up after a careful look at your workmanship. In my case, I have noted a good differential in the mic. gain, but I would have liked a little more.

As ever, this mod is not endorsed by anyone but me, and if you do the mod, please be careful. I assume no responsibility for your care and workmanship!!

Above: This is a closeup of the surface mount resistor (R1129) that SETS the gain of the mic preamp. The two feedthrough holes that are circled are the same as pins 21 and 22 of the mic preamp IC.



Above: This is the same location of the MAIN PC board, but moved away slightly, to assist in LOCATING the area that needs to be addressed.



Above: The bottom of the main board, zoomed further back. Note the position of the COMP control, which normally is accessable from the rear panel of the IC-746.



Above: TOP view of the MAIN PC board.The mic preamp IC is circled.



Above: Once the original resistor has been removed, the new feedback resistor can be tacked-soldered to pins 21 and 22, as shown . (See the RED dots on the IC.)



Above: The completed modification; I added a 100K resistor, as shown. Please note that one could put in a potentiometer, but if this is attempted, I would suggest that a 56K resistor be added in series, just to insure that the feedback path does not got too LOW. (A 200 to 300K total value would be more than enough!)







[2] - Allow slow-speed CONTINIOUS operation
of the IC-746's fan -

Basically, I added a 100 Ohm resistor as shown, which allows the fan to turn at a low speed. I originally had tried an 82 Ohm, but decided to slow it a little more, as I feared having the fan cause too much noise in receive. As it turns out, the fan noise is totally silent, once the covers are back in place. (So, I'd suggest that an 82 or lower value resistor be used in place of the 100 Ohms as shown.)

As ever, this mod is not endorsed by anyone but me, and if you do the mod, please be careful. I assume no responsibility for your care and workmanship!!

Above: A view of the IC-746's schematic localized in the section that includes the cooling fan and it's associated control circuitry. The location of the added 100 Ohm resistor is shown in PENCIL.



Above: As you can see, I tack-soldered the resistor to the two lands indicated by the RED DOTS. I evaluated the fan's noise with the covers off; In retrospect, I would suggest a lower value, such as less than 82 Ohms. (When I completed the installation of the radio's covers, I found that I could not hear the fan!)



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