A heavy power supply. I remember how these beasts were clattering when the bulb striked and the amp meter was jumping between 20 and 150 amperes until the arc stabilized. I talked with a projectionist who worked here when these projectors were installed. It turned out that the Russian cinema technicians used aluminum wires instead of copper between the power supply and lamphouse! 100 amps running through thin aluminum wire? Wohooo!!! These wires were changed out immediately. I heard that the technicians were drunk most of the time. Back then the pure alcohol was given to all photographers, projectionists and technicians to keep the lenses clean. Pretty often the alcohol "was used to clean up the optical axis".
April 2009 photo from the Olev Mihkelmaa collection.