Tune button won't do anything unless you have an ATAS, I do believe. If you use an interface cable, I believe you must push tune button on the tuner itself. Use the mic method to throw a 10 watt carrier.
Several online video show the 939Y with the 891 and just a Tune button on radio to key it as it goes thru the 8 pin DIN on the 891. You set radio to external and it functions like the FC30.