(Some assembly required)

The ICOM IC-706 has a nifty feature built in to help the operator tune the HF antenna SWR. By pushing the TUNE button on the front panel, the radio switches to CW mode and transmits a 10 Watt carrier. This state last for approximately 10 seconds or the operator can terminate sooner by pushing the button again. This was designed to work with the Icom automatic antenna tuners. There have been a few circuits developed to fool the 706 into thinking there is a Icom tuner attached and produce the same tuning signal which can be used with a manual antenna tuner or many automatic tuners, some get quite complex.

Well, being the cheap guy I am, and being one to tinker, I felt there had to be a simple way to do this. I began reading the various postings on the Internet. I went back to an e-mail exchange I had with Ed, W1AAZ in early April 1999 on Vartel's ICOM 706 Discussion Group. Ed explained the functions of the TKEY and TSTR pins on the "AH-3" connector on the 706.

The outcome was a simple RC circuit which I have built right on a Molex connector which plugs into the AH-3 jack on the back of the IC706.

Construction:

The (+) side of C is connected to the 13.8VDC pin. The (-) side connected to TSTR and TKEY which are connected together. Resistor R is in parallel with C. Below is a diagram of the AH4 connector on the back of the IC706 where the < indicates the pointy end of the connector (Pin 1 = TKEY). You can buy the Molex connector at Radio Shack (#274-0224).

	< TKEY  TSTR  13.8V  GND ]
	   |_____|       |     
	      |    R     | 
	      +--\/\/\/--+ 
	      |      +   | 
	      +----)(----+ 
	           C 

Bottom view of connector
Note: I have also used this connector to tap power for "ClearSpeech" DSP speaker. The power cable shown is not part of the TUNE TRIGGER circuit.

The values I use are C=1000uF 16WV R=92K Ohms 1/8 Watt which result in 15 seconds of tune time. As you can see, these are tied in parallel. If you want to use a physically smaller capacitor such as 440 uF, you can adjust the time by raising R. The extreme values I found are:

		2K<R<3.3M Ohms
		10uF<C<4700uF.

Operation:

Be sure to read the manual about the various Initial Settings that can affect the operation with a tuner connected, otherwise you may go into the TUNE mode every time you change frequency or push the PTT.

Turn off the radio and plug in the unit. Make sure you have some sort of load on the HF antenna connector. Turn on the IC706, the radio sees TSTR is high during boot up and thinks there is a tuner connected. Be sure the meter is in the SWR mode. Push the TUNE button. The TUNE light should flash counting the seconds and stay lit when finished, as stated in the manual. You may interrupt the cycle by pressing TUNE again.

If you terminate the TUNE mode early by pressing the button again, you must wait for the remainder of the time-out period to re-start. If you don't then the cycle is started over again when you press TUNE and you still must wait, but longer. This is why I have opted for 15 seconds instead of a possible 30 seconds. One way to reset the timer is by powering the IC706 off then on.

I have noticed false triggering when the supply voltage AT THE IC706 POWER CONNECTOR drops during transmit and C has not yet totally discharged. This is caused by the TKEY voltage dropping through the trigger point because the supply voltage drops while C is discharging. This lead me to upgrade my power supply cable so there is less voltage drop when I transmit.

Alternate design:

You can eliminate the capacitor and the TKEY to TSTR connection if you want to manually trigger the IC706 into TUNE. You do need the pull-up resistor between TSTR and 13VDC to make the radio think there is a tuner present during power up. Then ground TKEY to trigger the TUNE mode as long as TKEY remains grounded.

Thanks to Ed, W1AAZ for the explanation of the connector.

Enjoy!