Mooney M20E io360 low voltage problem

Hello, I own a 65 m20e with a io360. Recently been experiencing noise on com1 and have read about low voltage being a problem with nav/com's. I have a king ki 165 for both com1 and com2. I've tried to diagnose the problem and it appers that my alternator isn't charging the batter correctly nor is it bringing the system bus up to 14volts. I've noticed this via a volt meter to the 12 volt(car type) adatper(although i don't know if this is the correct procedure) and I've also made notes of the bus voltage before starting and after shutdown on a long flight and it went from 13.5 volts to 12.2 volts(enging off). Does this seem like it could be a low voltage noise from a alternator not charging correctly. If so has anyone had experience repairing alternators on m20e. I would imagine a new one would cost around 1K. But I don't know.

Thanks

Mike