IC-7300 connectivity to an LDG AT-1000 ATU

Part 1 – Create a CW tuning carrier from the IC-7300.

There are a few designs available on the web, the following one from Hampedia.net


is the one I am using.


Background: Icom Autotuner Connector pinout:

Pin 1 is the TKEY pin, when the ‘TUNE’ button is pressed, it’s pulled low by the tuner to tell the radio to start transmitting.

Pin 2 is the TSTR pin to/from the microprocessor. Under normal circumstances the START pin is pulled high when a remote tuner is attached to tell the radio that the tuner is present. When you press ‘TUNE’ the radio pulls the pin low telling the tuner to start the matching sequence.

Pin 3 is POWER 13.8V, 1 amp max and switches on and off with the radio.

Pin 4 is ground.

A simple RC circuit built right on a Molex connector plugs into the AH-3 jack on the back of the IC7300 allows the IC7300 to provide a tuning carrier as it does for ICOMs own external antenna tuners.

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 on eBay and elsewhere.

ICOM-Tune-it 2The values I use are C=1000uF 16WV R=41K Ohms (2 x 82k in parallel) 1/4 Watt which result in about 10 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. If you wish to change the time the carrier stays on the values of R & C can be changed to suit.maxximu / minimum recomended values are:

2K<R<3.3M Ohms

Be sure to read the manual about the various Initial IC7300 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 IC7300, the radio sees TSTR is high during boot up and thinks there is a tuner connected.
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 10 seconds instead of a possible 30 seconds. One way to reset the timer is by powering the IC7300 off then on.

Part 2 . Triggering the ATU to auto-tune when the “tuner” button is pressed on the IC-7300.

I do not have the PROII, just the old LDG AT-1000. It has NO interface to it and has no RF triggering circuit. To “auto-tune” this ATU one has to press the “tune” button on the LDG tuner while a carrier is present. It is this action that I will need to automate.

I have brought the connections from the push switch out to the back of the LDG case and when I close these connections, it is the same as pressing the button on the front of the LDG ATU.

I wish however to push the “tuner” button on the IC-7300 and have that trigger a relay to close the contacts and so put the LDG into auto-tune mode. This action has to be a momentary action, possibly for 1 second, not longer. I believe this would be possible using a normally closed and a normally open relay with a capacitor across the coil of the normally open relay. I have however also found a relay board that can be triggered by a voltage from the Molex connector on the IC-7300, that will then hold its relay contact closed for a programmable period of time, after a programmable start delay. This unit would seem to do exactly what I require. After I press the “tuner” button on the IC-7300, because of the capacitor and resistor that I have connected to the MOLEX connector, the IC-7300 generates a 10w carrier. At the time this voltage trigger on the MOLEX connector comes, the relay module will (I hope) then wait for say 1 second and then close the LDG tune switch contacts for 1 second, which would kick the LDG AT1000 into tune mode.

Relay moduleMy plan is to build this module inside the LDG atu unit along with the capacitor and resistors from the Molex plug, so that I would only then need my Molex to DB15 connection cable between the IC7300 and the LDG ATU and keeping all electronics within the ATU case. As the IC7300 can supply 13.8v at 1A I could also power the ATU from this lead rather than having the current extra power lead.



Unfortunately upon receiving the relay board from China, I found it would not do what I had hoped, it required a trigger voltage the opposite way around than I was able to supply. I considered building a simple transistor based state invertor but in the end decided to stick with the “two button method” where I push “Tuner” on the IC-7300 followed by “Tune” on the LDG AT-1000.

When the auto-tuning failed on the LDG and their support said it would most likely be a processor fault – of which no more are available – I decided to replace the LDG AT-1000 with an MFJ 993B which, with a cable wired as per the manual, works from the “Tuner” button on the IC-7300 without any problems. The MFJ also has several other capabilities that I will be looking into using including support for connection of a single wire or a ladder line antenna lead. The MFJ ATU is both powered and controlled through the Molex connector.