An improved design of positioning mechanism, mountable to an upright, allows smooth, readily accomplished adjustment of the angular, rotational and translational positions of a boom with respect to the upright. A hub is mounted to and rotatable with respect to a base. A friction plate between the base and the hub applies frictional forces that are established by the mechanism, by urging the hub and base together. A pressure sleeve partially contained in a matching opening in the hub controls the rotational and longitudinal movement of the boom with respect to the hub. The design of the frictional engagement surfaces allows adjustment of the resistance to movement of the boom in the angular, rotational and translational directions to extend smoothly and predictably from a condition of virtually no resistance to a completely locked and rigid condition.