The MEOPTA No. 87328 was an optional power supply designed for the 16mm sound film projector named Meoclub Automatic 16, also made by MEOPTA. Because the projector itself was powered from a 220V mains and used a 220V halogen lamp too, this enabled the usage of 75V/375W and 110-120V/1000W projector lamps, and also made it possible to power the projector from a 127V mains. In addition, an analog illuminated AC voltmeter and a huge 10-positional rotary switch have been implemented to manually compensate for incidental mains voltage drops and to ensure the correct voltage is applied to the lamp, so it’s additionally some kind of a manual stabilization supply.
Except for the transformer and all the mechanical selectors, there are no other electronic parts in the cage – not even a fuse. 🙂
If there’s 220 volts AC supplied (or 127V AC, if switched properly) and the switch is at the “minimum” setting, all output voltages are nominal, i.e. same as the transformer itself is rated for. For verification, the AC voltmeter is connected to the 220 volt secondary and has a red marker on its scale, indicating nominal voltages on all outputs.
Should the input mains voltage be lower than nominal, the voltmeter goes below the red “220V” marker and the rotary switch is used to short-circuit the tapped primary by a few tens of turns, basically, yielding an increase of all secondary voltages as a compensation. Doing such tapping on a today’s 230-240V mains supply – if stable – can lead to destruction, as it will oversaturate the transformer and is going to cause, along with a violent growl, some unpleasant smell; so it mustn’t be done unless there’s some mains dropout.
Since the primary had been constructed for a 220V input which was used back in communist Czechoslovakia and neighboring countries, operating this supply nowadays on 230/240 volts is going to yield slightly higher voltages than the rated nominals. Because it does not allow dimming (going lower than the nominal input) I’d have to rewind the primary to solve this 🙂
Overall, it weighs around 13 kg, with at least 6 kg being the copper windings in the transformer. (The good old days when electronic things were actually built to work properly…) Finally, some reverse-engineered schematic of the guts, for those interested. I’ve originally done it in Slovak, but hopefully it shouldn’t be that hard to understand.