For any Icom radio with the 4 pin Molex TUNER connector, and any external tuner that offers an Icom Molex TUNER port adapter cable:??
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The Icom transceiver has weak pull-up resistors to 13.8 volts on the _START and _KEY pins.? The tuner _KEY pin has a stronger pull-up to +5 volts.? ?When the Icom transceiver starts up and detects +5 volts on the _Key pin,? the Icom transceiver assumes an AH-4 compatible tuner is connected.? ?When the transceiver front panel TUNER button is pressed, the Icom transceiver pulls down the _START pin and waits for the the tuner to pull down the _KEY pin, at which point the Icom transceiver
automatically transmits a continuous carrier at a reduced power of 10 watts.? When the tuning process completes (or times out) the Icom transceiver automatically resumes the previous mode and power setting.??
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Refer to pages 6 and 7 to see how the _START and _Key signals operate on the LDG and Icom autotuners and ATU- 100
N0YWB