First just reseat it, it may be loose or the contacts are dirty. Also make sure the antenna wires from the handrails have a good connection and the handrails are going into plastic bushings in the pilot deck. Sometimes the bushings are lost and the antenna gets shorted out.
If all that checks out then you have to swap out the radio board. If you have another TMCC engine use that one first before spending money on a replacement.
What does the headlight do when you first apply power? If it blinks continuously then its a signal issue.
Pete
