Abstract: A step-transformer drum formed with throughgoing apertures each centered on a respective axis has a contact assembly in each aperture having an insulating washer lining the aperture and having an inside part lying on an inside surface of the drum and an outside part lying on an outside surface of the drum. A stud has an enlarged head bearing outward on the inside washer part and a shaft projecting through the washer past the outside washer part and formed outside the outside washer part with a transversely throughgoing hole. A pin extends transversely through the hole and bears inwardly on an outwardly directed ramp so that rotation of the stud and pin in one direction relative to the structure makes the pin ride up on the ramp and tightens the stud in the aperture and opposite relative rotation loosens the stud.