Power Supply Unit
Power supply units are used to power radio sets and similar equipment from a different power source then batteries. The older hollow state sets (wit hvalve technology) usually need a heaters voltage (to heat the filaments of the valves) and a plate voltage (which is applied to the anodes of the valves).
The power supply units provide the necessary voltages.
The can be powered from vehicle or aircraft batteries (usually a vibrator or rotary converter is used to generate alternating current which can be transformed to the necessary high tension) or they can be operated on the mains („mains power supply“). In Switzerland, the 220 V AC mains is common, in contrast to the 110 V AC in the U.S., in earlier years, there were several other mains voltages used in Switzerland that's why older sets usually have a voltage selector with many different positions. DC mains was never popular in Switzerland, that's why AC/DC sets are really uncommon.