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).
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.
Audio, Control and Power control connections.
Power On Video: (Sorry for choice of finger. My arms are to short!)
Power Off Video: