I had a problem with my FT-847 in the same way. It was the microphone – an older model 8-pin model with the UP, DOWN, and FAST buttons – but the FAST did not work. When I used the MH-31B8 Microphone (that was supplied originally with the radio), all worked fine. Some earlier model Yaesu microphones work fine for audio and PTT activation, and even UP and DOWN – but they may have some problems in things like that FAST control.
Well now that the UHF transmit problem was resolved (thank you again to everyone), my next issue is the FAST function does not work from the microphone or by shorting the appropriate pins on the mic jack.
When I press the FAST button on the front panel the icon shows up and tuning increases as expected, so it is working from the panel. I'd like to resolve this as I need to align the SHIFT control and I can't get in to the adjustment menu without the FAST button working on the mic.