The Elecraft K3 can be remotely powered up by shorting pins 5 and 8 of the back ACC port momentarily. RemoteTx can do this by connecting a control relay to the RPi on pin 2(5V), pin 34(GND), and pin 36(GPIO 16). The KX2 is similar except one of the pins on the maintenance port must be shorted to ground.
With this relay installed, the K3/s can be turned on by clicking the ON button in the RemoteTx Radio tab.
After clicking the ON button, the relay will short pins 5 and 8 on the K3 ACC port for 1 second which will turn the radio on. The RemoteTx control interface will load within about 3 seconds. See figure below:
Clicking OFF will turn off the radio and close the K3 control interface.
Link to Double Head RPi jumpers on Amazon:
Audio, Control and Power control connections.
Power On Video:
Power Off Video: