In common with most Icom rigs, the "TUNE" button on the IC706 is designed for use with an external Auto-ATU (usually supplied by the Transceiver Manufacturer) and provides a 10 watt carrier to enable tuning. No Tune facility is intentionally provided for use with manual tuners. This feature seemed to disappear with Valve PA's and Pi-Tank couplers.I normally use a Z-Match tuner with balanced feeder, so need some RF output to enable me to tune the ATU for the best match.

Various circuits have been published for devices to "fool" the IC706 that it's connected to an Icom AH-4 tuner or similar and there are also at least 2 commercial gadgets available.  Most circuits I've seen have a high component count, using either a couple of transistors or even the 555 Timer IC.  I found an excellent circuit by Larry, WA2EHZ which is a real Engineer's solution - simple, elegant and does the job with 2 passive components and a $1.19 Molex Connector from Radio Shack.  

The schematic shows the simple Tune Device.   The device needs to be plugged into the IC706 before applying power, so that the powerup test detects a tuner is connected. When the "TUNE" button is pressed, the IC706 will output a 10 Watt carrier ( and CW sidetone) for the duration of the tune cycle.   I've built devices with differing tune cycles.  Using a 470uF 16v Capacitor and 100K resistor gives a tune cycle of approximately 8 seconds.  A 1000uF capacitor gives a tune cycle of around 15 seconds with the same resistor value.

1. If the 706 is allowed to finish the cycle, the TUNE light stays on - sometimes. This can be turned off if you hit the TUNE button soon after it finishes. If you wait too long, it will start a new cycle.
2. If you terminate the cycle early, to start a new cycle, you need to wait until the capacitor discharges.

NOTE: This tune device is not compatible with later Icom radios, such as the IC7000. On the later Icom rigs, pin 1 (TKEY) input is shared internally with the temperature control circuitry, so this pin MUST be left floating (not pulled high). If it isn't, the fan may not come on as required, which can lead to failure of the final transistors! To be safe, only use this device on an IC706.

Disclaimer: Although I have tested and used the above circuit on my own IC706mkIIG with no problems, I do not make any representation or guarantee that this circuit will function, nor do I accept any responsibility whatsoever for any damage or malfunction arising from the use of this circuit.