Icom to MFJ Antenna Tuner Cable

While at Belton Hamfest a year ago, I purchased an MFJ 994 Auto Antenna Tuner which is 600 Watts SSB and 600 CW and can be used with all HF equipment. There are two main ways to use this tuner, manual and auto. The manual way of using the antenna tuner can be a little bit of a pain but works just fine once you get used to how it works. The auto tuner side works real well if you use a cable between your radio and tuner.

From the manual – “The MFJ-5124I interface cable provides power and control signals between an most radios and the MFJ automatic tuner. For use with the Icom Radios, Push and hold the radio’s [TUNER] button for two seconds to start the tuning process. Push [TUNER] quickly to bypass the tuner.



Below shows the parts and tools that I used to create this simple cable to allow the Icom IC-756Pro to control the MFJ-994. I was able to find the connector that I needed at Radio Shack (2740224) –  4-Position – Male Interlocking Connector. I had a 3.5 mm male stereo connector and female power connector at home. I brought out the meter to do ohm testing of the wires which came in handy.

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So I started with figuring out which wires need to go where and the manual shows which pins are used for which purpose.

Pin 1 (Key) connects to the Tip of 3.5 mm Stereo Phone Plug.
Pin 2 (Start) connects to the Ring of 3.5 mm Stereo Phone Plug.
Pin 3 (+13.8V) connects to the Center Pin of Power Plug.
Pin 4 (Ground) connects to the Sleeves of both 3.5 mm Stereo Phone Plug and Power Plug.

I stripped the two sets of wires and used the meter to find which ends went to which part of the plugs. The 4 pin plug came with four pins which can be crimped on the ends of the wires making the install much easier that I thought.



The first set of wires that I added were for the power connection. Pin 3 – Center power plug and Pin 4 – Sleeve of power plug. All you need to do is crimp the pins to the wire and push them into the holes. You should feel a slight snap once the pins are in correctly. Once the power leads were in place, I added the Pin 1 – Connects to 3.5mm Tip and Pin 2 – Connects to 3.5mm Ring to the top two holes of the 4 pin connector. So from the arrow on top, the holes go in order from 1, 2, 3 to 4.

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Once the cable was built, it was time to test it out. Make sure that the Radio and the Auto Tuner are powered off. The black power cable plugs into the 12V power connector on the Auto Antenna Tuner and the grey cable plugs into the Radio interface Connector of the Auto Antenna Tuner. The 4 pin connector that we built plugs into the female connector on the back of the Icom Radio. Now, use the following to power up and use the new cabled Auto Tuner.

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The following came from the MFJ AutoTuner manual:

1. Insert the 3.5 mm stereo phone plug into the tuner’s RADIO INTERFACE jack.
2. Insert the 2.1 x 5.5 mm coaxial power plug into the tuner’s POWER jack.
3. Important: Make sure the DC power to the radio has been turned off. The radio does not fuse the DC power to the tuner, and damage to the radio can occur if the interface’s power connection comes in contact with ground.
4. Connect the 4-pin molex connector to the mating 4-pin molex connector, marked as TUNER, on the back of the Icom radio.
5. Push the [POWER] button on the tuner to the in position and then power on the radio.
6. Some Icom radios, such as the IC-706, automatically check to see if an external antenna tuner is connected during power on. For other Icom radios, such as the IC-718, the Tuner Type menu in the radio menu system must be enabled. Refer to your radio’s operating manual for accessing the menu system and preparing the AH-4 antenna tuner operation. Use IC-718 as an example:
a. Push [PWR] for 1 second to turn power off.
b. While pushing and holding [SET], push [PWR] to turn power on.
c. Push [UP] or [DN] one or more times to select TUNER.
d. Rotate the Main Dial to select “4” for AH-4 antenna tuner.
e. Push [PWR] for one second to turn power off.
f. Push [PWR] to turn power on again.
The operation of the MFJ-5124I radio interface is similar to the operation of the AH-3 or AH-4 described in the Icom radio’s operating manual.
1. Push and hold the [TUNER] button on the radio for one or two seconds to initiate automatic tuning process. The radio will automatically switch to CW mode, transmit a 10-watt carrier, and start the tuning process. When the tuning process is completed, the radio will stop transmitting, return to its previous mode and power setting.
2. Push the [TUNER] button on the radio quickly to bypass the tuner or to cancel tuning in progress. Note the [TUNE] button on the tuner will also key the Icom radio interface.

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