A drive assembly for a closed loop cable system in which a flexible driver member drives a door or the like back and forth has the flexible drive member wrapped around and driven by a drive pulley. The drive assembly includes a motor unit having a motor shaft that rotates about a drive axis for rotating the drive pulley about a pulley axis, and a tensioning mechanism for adjusting the position of the pulley axis with respect to the drive axis responsive to tension in the flexible drive member.