For microphones made in central Europe (AKG, Beyer, Microtech Gefell, Neumann, Sennheiser, Schoeps) most manufacturers have national distribution and service centers which can fix common electrical and mechanical problems. (Microtech Gefell doesn't have representation in North America at the moment and is handling all repairs directly, but they probably will have some kind of representation again here soon.)
For any procedures that involve detailed testing of acoustical performance (e.g. frequency response), the microphone typically needs to go back to the manufacturer's facilities in Europe. Repair stations in the U.S. generally don't have setups for acoustic testing, which would cost more than the whole rest of the test bench.
--best regards