There exists antenna oscillation in PPI scan in Yangjiang CINRAD/SA radar, which may cause data loss in the azimuth sector, leading to limited functions. In order to find the cause of the malfunction, the signal flow of the servo system of the Yangjiang CINRAD/SA radar is analyzed. Combining with the analysis software of base data, the running track of the antenna is examined. Through measuring and comparing the key parameters in the signal flow, it is discovered that the faults are mainly resulted from the azimuth encoder. The analysis methods and experiences in handling are summarized, which is of reference significance for the station level technological support of radar.