[cmjones01]

Yes, as MrBungle says, if it's a red slide switch, it's the voltage selector. It's wired so that it changes the input arrangement from a bridge rectifier with both reservoir capacitors in series on 230V, to a voltage doubler on 115V. That way the half-bridge primary side of the power supply always has a 300V-ish supply with a centre tap (for high frequency switching purposes) at the junction of the two reservoir capacitors. I've always thought it quite a neat arrangement. I'm assuming that it's been made obsolete by the advent of power-factor-correcting input stages which don't work that way.

Chris