Thank you Salmon Trout. That is the right answer.
During assembly the switch was mounted upside down.
Later, it was wired correctly as seen from the inside of the PSU.
It would have taken me hours to find the diagrams. Or to take photos of how the switch looks when it is upside down. You would hardly notice the difference. As seen from the diagram. the switch just closes one circuit, not two. The PSU operates from 220 volts if the switch is not coded. Close the switch and you have a classic voltage doubler, used in TV sets may, many years ago when vacuum tubes needed voltages over 150 volts to perform economically.
The use of the simple analog voltage couple provides a simple way to switch from 220 to 110 operation. The 300 volt DC output goes staring to the converter, where it becomes pules reacting near 1000 volts and drives the primary of a *high-frequency transformer that isolates the power from direct connection to the mains. Secondary windings provide the requires low-voltages needed by the motherboard and other devices.
* Actually low-frequency, but well above audio.
Some newer power supplies use a different technique.
