Ray 54 not transmitting

[Tmevers]

I bought a 1999 Pursuit 2870CC and it had a Raymarine Ray54 VHF radio in it. The radio receives fine, even out 25 miles from shore. Upon trying to do a radio check, the radio would kick down from HI power to Low power and no transmission would go through. After speaking to an electronics person they told me it could possibly be a low or weak battery. I put in a new 12volt battery and a land charging system for the 3 boat batteries. I still get the problem. I also changed the antenna splitter and got a seperate antenna for the UHF radio, and put the VHF on it's own antenna. I still get no transmission, however, it no longer kicks down to Low power from Hi power after these changes. Is it possible I have a short in the microphone?