The encoder is a key part of servo system in CINRAD/SA radar, which contacts dig ital control unit with mechanism of the pedestal. While failure happens in azimu th loop, it will cause ananeose echo in products, and while fault happens in ele vation loop, it will cause INOP about radar system. Five faults about the encode r were analyzed. It discovered that the faults mainly come from the magnetizatio n voltage. When the temperature and humidity are too high in the antenna dome, partial parts will be degenerated. The problem will be throughout resolved after some measures are taken.