I saw the "Click2Tune" in the Feb 2018 QST (page 57), and being the consummate cheap bastard, decided that I could build a home-brew one. This is a push-button and circuit, that, when plugged into the "Remote Tuner" 4-pin Molex socket on the back of the radio, while pressed, will cause the radio to transmit about 10W of CW RF so that you can tune an external manual antenna tuner at low power without interfering with the internal tuner operation. First, I looked at the Owner's & Service Manual for the ICOM IC-7300 and for the AH-4 to see if the function of the two control signals is documented. I learned more on the Internet than from ICOM's manuals and did some testing with a temporary cable, a proto-board, and an oscilloscope. Here is what I discovered: The Molex connector has only four pins: 1. Key~ (the tilde ~ means that the signal is active-low) 2. Start~ 3. Switched 13.6V power. 4. Grd (chassis ground). These findings are specific to the IC-7300; I haven't tested other similar ICOM radios. If you have one, you will have to repeat the tests for your radio. With nothing connected to the Start~ pin, it sits low (about 50mV). In this state, the front-panel TUNER key, the TUNE and TX icons on the LCD screen work to control the Internal Antenna tuner as described in the manual. Furthermore, with Start~ floating (low), the radio ignores the Key~ pin. If Start~ is pulled to about 5V by an external voltage through a suitable resistor, then the IC-7300 detects this, and begins "listening" to the Key~ pin, which is important to the rest of this discussion. The Start~ pin is actually bi-directional (wired-or), meaning that when pulled-up by an external resistor, the radio can still momentary pulse the Start~ pin low to signal an external device. This is how the radio normally starts the tuning process if an AH-4 remote tuner is plugged into the radio. This happens when the TUNER key is tapped. This is as documented in the manual in the event the radio is used with the ICOM remote tuner. I am not using this feature of the interface in this design. The Key~ pin is pulled high (about 3.3V) by an internal-to-the- IC7300 pull-up. With radio having been told to listen to Key~ (by externally pulling-up Start~), then if you wait a while while the radio discovers this, then pull the Key~ pin to ground, then the radio responds by transmitting the 10W carrier, regardless of mode. This happens for as long as Key~ is low, and stops when Key~ is released. The radio generates a tone, and shows what is happening by animating the TUNE and TX screen icons. So, in summary, here is what I learned: In order to preserve the option of using the internal antenna tuner, you cannot permanently just tie Start~ high, because that disables the internal tuner. Rather, with the "Push2Tune" button, you have to first generate the Start~ pull-up signal, wait about 70ms for the radio to discover it, and then pull Key~ low for the tuning duration. Upon releasing the "Push2Tune" button, you simultaneously remove the pull-up from Start~ and allow Key~ to float, so the radio reverts to the mode where the internal tuner can be used. If the rising edge of Start~ is concurrent with the down-edge of Key~, the radio ignores them. With all of this in mind, I created my own version of the following "Push2Tune" circuit which is powered from and plugs into the Molex connector. It uses simple-to-get through-hole parts, all of which I had in my junk-box. I built mine on a chunk of Vector board. The circuit was simulated in LTSpice as shown in the attached schematic. Note the plot of trace V(start~) which shows the push-button being pressed at 0.01s, released at 0.41s, closed again at 0.81s. Note as shown by trace V(key~), this gives the IC7300 about 70ms (0.07s) to detect that the remote tuner interface is active before the Key~ line is switched low. This requirement is the take-away from this project! My "Push2Tune" works like a charm. With the "Push2Tune" permanently plugged into the IC-7300, this circuit lets you decide if you want the internal tuner on or not. When using the external tuner, you might want to disable the internal one first, although it is possible (if not useful) to run them in cascade.