The TUNE feature on the Icom 7000 transceiver is intended to to be used with an Icom AH3 or AH4 antenna tuner and a wire or conventional shortened vertical antenna. When connected to an AH3 or AH4, pressing the TUNE button generates a 30W carrier and keys the transmitter while the tuner searches for the best match between the load and the transmitter. But unless the antenna tuner is attached, the TUNE button does nothing.
IcoTune is an accessory that tricks the '7000 into thinking that an AH3 or AH4 is connected. Pressing the TUNE button on the '7000 to causes the transceiver to generate a carrier at reduced power. This is handy when the '7000 is used with one of the very popular screwdriver antennas and an external SWR indicator or when using a conventional antenna tuner. The length of time that the transmitter is keyed is user-adjustable from approximately 15 seconds to 45 seconds.
The IcoTune input circuit is very similar to the interface to the AH4 tuner. A high-value pullup resistor between pins 2 and 3 is recognized by the IC-7000 as an antenna tuner. An open-collector transistor, controlled by an analog timer, pulls pin 1 to ground to key the transceiver when a tune cycle is initiated. This is identical to the keying arrangement found in the AH4. This does not affect the thermal sensing arrangement in the IC7000. The SWR indication is disabled in the IC7000 during the tune cycle, but returns to normal indication when the tune cycle is completed.