There was a computer you could switch between 50 and 60 Hz with a button press. This controlled the CPU clock too so you got a faster computer with 60 Hz. But I can't remember what computer it was? Can't have been the Amiga or Atari even though they also could do it, has to have been an older one?

Are you thinking of the Turbo button on PCs?

I've personally never seen the ability to switch from mains frequency. That's a switch on the back of the PSU that typical goes from 240v or 120v.

The turbo button and mains frequency are 2 totally different things.

No, not the turbo button, it was the 50/60 Hz software switch that made the computer faster. Hm, might just have been that the gfx chip was working less in 50 Hz come to think of it...

I do remember seeing several computers in the 80s having this switch, but they were all modded. They also required you to have a more expensive monitor that could handle the format change (PAL/NTSC and 50/60hz)