Abstract: For the purpose of maintaining verticality during erection of antenna masts nd the like, opposing guy control cables having corresponding ends secured to the top mast section are directed onto a pair of capstans which in turn are coupled to the input elements of a pair of Weston brakes, arranged in back-to-back relation on a common support, and whose output elements are coupled to the sun gears of an intervening planetary gear differential. The other ends of the two guy control cables wrapped on the capstans are wound and stored on two cable reels arranged above the capstans on the common support, under control of a reel turning means employed to reel in the guy control cables during mast lowering or retraction.