TS-450 / TS-690 Hints
When using the KT-100 with the TS-450 or TS-690 radios, the radio will need to be configured to use an external tuner. To do this, hold down the ENT key when turning on the radio, then turn the M.CH / VFO CH knob to select menu number 01 on the display. Use UP/DOWN to select “On” on the display. Press the CLR key. This initializes the TS-450/690 to use an external antenna tuner, and disables the internal tuner. When tuning, be sure the CAR control is not rotated fully counterclockwise.

So I imagine the KT-100 would work fine. If there’s a button you push on the 450 to cause the internal tuner to go into a tuning cycle, it sounds like this would cause the KT-100 to do the same in the same way you use the internal one now. The external tuner has a wider range then most internal tuners though I can’t tune 160 on my G5RV even with the KT-100.

I hope that helps a bit.

According to http://www.ldgelectronics.com/c/252/products/1/42/2 the LDG site says:
Works with any Kenwood radio that is AT-300 compatible.
Partial radio list: TS-50, TS-480, TS-570, TS-690, TS-850, TS-870, TS-2000.

Is the 680S AT-300 compatible? If so, then it should work. If not, I think the LDG AT-100 Pro would b an option as that should work with any radio.

73,
K2DSL – David

Thanks for posting. I don’t know if this is what you want to do, but you can completely turn off the antenna tuner by quickly pressing the AT button on the front panel. You should see the AT indicator just above the button go off. If you press and hold the AT button, it will go into a tuning cycle. I downloaded the manual (I’m at work) and it’s on page 72 of the PDF I downloaded from the Kenwood site in a section labeled Automatic Antenna Tuner.

73,
K2DSL – David

KC2URT