FYI, a txv is a thermostatic expansion valve, and it modulates the flow of freon to provide a given amount of super heat and should never freeze up though may close down if the system is too cold.
There is a bulb filled with a refrigerant that expands and contracts lifting or lowering a pin in its orfice to give a consistant cooling temperature. All residential units have gone to the txv to achieve the government's requirements of higher efficiency - 13.0 SEER ratings.
Auto Txvs also allow better operation and efficiency.
R12a is very flammable as it is mostly propane and isobutane, also illegal in the US.
Enjoy W
DY
https://en.wikipedia.org/wiki/HC-12a