The mast whip and two large nylon gears are common problems. The gears go when the antenna sits for a while and the mast is forced up when the telescopic segments stick together. The lobe on one of the gears breaks off easily.
Ebay sells "Generic" Hirshmann electric replacement antennas for about $40 and the mast/whip assembly for a bit less. Replacement parts can also be purchased from
www.antennaworld.com.