Pinout of the Icom Ic706 tuner socket
Pin 1 is the KEY pin, it's pulled low by the tuner to tell the radio to start transmitting.
Pin 2 is the START 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.A 10k resistor, connected to the 3 pin is enough tell the ic 706 that a fake tuner is present
Pin 3 is POWER rated for 1 amp max and switches on and off with the radio.
Pin 4 is ground.