Thoughts to think about:
There is a menu feature in the radio that may be causing the problem?
The DTMF micrphone for the FT-100 is designed to function ONLY when the radio is placed in the FM mode. Therefore, If a DTMF key was shorted out or in a constant state of "on", it would only be noticed when the radio mode switch is placed in FM mode. Also the FT-100 does not give any audio feedback to the user when pressing DTMF tones, so you may be completely unaware it is occuring.
Pressing the DTMF pad while operating HF or other modes will have no effect.
Perhaps a setting in the crossband feature of the radio is causing the radio to transmit? What happens when you adjust the squelch, RF gain or other reciever settings.. does the carrier drop?
Some previous internal radio mods have shorted out causing the radio to TX?
The FT100 has a known UHF/VHF thermal sensor mod that affects V/U receive mode fans. Has the thermal sensor mod failed or shorted out with the circuit board causing it to TX?
See photos of Mod here: - http://www.k0lee.com/thermalmod.htm
ATAS100 autotuner feature:
The FT100 and FT847 has autotuning functionality. Menu item 61 is has several settings for controlling the way the ATAS100 antenna works including VHF/UHF operation. Is the radio stuck in TX mode because it is looking for the ATAS antenna to tune up?
Is the autotune feature enabled for VHF/UHF bands only?
Any other ideas out there?
73 Charles - KC8VWM