WB8BFS
Icom 706 MkIIG
HF Amplifier Keying Modification

If you have an Icom 706 MkIIG and you want to key your HF amplifier without having to use that cumbersome 13 pin ACC Socket connector along with an external relay or solid state switching box, then read on.  This modification requires NO HOLES to be drilled, and will take about an hour.  An RCA female connector is installed on the back of the Icom 706 MkIIG where your HF amplifier key line cable gets plugged in.  The only requirement is that your amplifier's keying voltage be positive DC when not keyed, and be less than 120 volts DC.  You can test for proper voltage and polarity by using a voltmeter on the keying line cable coming out of your HF amplifier.  Unplug the key line cable from the radio, leaving the cable plugged into the HF amplifier.  With the amplifier AC power turned on and the amplifier mode switch in the operate position, measure the voltage between the pin and shield of the RCA male connector on the key line cable.
Note that the 13 pin ACC Socket can still be used for other purposes with this modification.

This WB8BFS modification is presented to those hams who enjoy working on equipment to make improvements.  The parts are not too difficult to locate.  If you perform this modification to your radio, please email me to let me know if this was of any value to you.  Your comments are my inspiration for this and other modifications.

Note that I am in no way responsible for any damage caused by this modification or voiding of the warranty.


Components Required:

The rear panel of the 706 has a small plate with two screws securing it in place.  Remove the screws and plate, and save these in case you decide to remove the modification in the future.  The following pictures show the installation details.  Click on the image to make it larger.

Figure 1 Rear Panel RCA connector

Figure 2 RCA Lug and Nut position

Remove the top cover of the 706 by removing the 3 screws on the top closest to the back, and the 2 screws on the sides at the rear.  Disconnect the speaker connector and set the top cover aside.  Remove the 6 screws securing the top board at the rear of radio.  Gently lift the board when installing the RCA connector (Figure 1 and 2) to avoid damage to the board connections.  Make sure the ground lug is bent as shown in Figure 2.  You may need a small wrench to tighten the connector nut.

Figure 3 TIP127 solder connections

Figure 4 TIP127 solder connections

Clip the base lead of the TIP127 to about 1/8" and solder the wire wrap connection to the base.  The base has the red wire wrap connection as seen in Figure 3 and 4.  Position the TIP127 leads so that the collector can be soldered to the ground lug of the RCA connector, and the emitter to the center conductor of the RCA connector.  It does not matter if the TIP127 metal tab (collector) touches the chassis because it is at ground potential.

Figure 5 Board Wire Connection

Route the free end of the wire soldered to the TIP127 base through the Filter Board (Service Manual Dwg 8-12, Filter Board bottom view) using the existing hole and solder to the pin 4 connection (junction of R42 & L35) as shown in Figure 5.  Carefully put the board back in place and secure with the 6 screws removed earlier.  Install the speaker connection on the top cover and secure the cover with the 5 screws
removed earlier.  The modification is now complete.

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