Having just returned from a 6-week Europe/Russia tour with Rosetta, this seems relevant. I like to take my customized TSL-100 on tour overseas, but it doesn’t have a voltage selector switch. Fortunately, the power transformer has a dual primary, so you can wire it in either series (220/230/240VAC) or parallel (110/120VAC).
The primary side of the power transformer has four wires coming out of it — two red, one blue, and one brown. The two red wires are the insides of the dual primaries, and where you plug them in determines whether the primaries are in series or parallel.
For 110/120VAC in North America, set the primaries to parallel by running the red wire closest to the blue wire to W6 on the mains input PCB, and the red wire closest to the brown wire to W1. For 220/230/240VAC in Europe and elsewhere, wire the primaries in series by running one red wire to W4 and the other to W5.
Don’t forget to change the mains fuse value to an appropriate one — 4A for 110/120VAC, and 2A for 220/230/240VAC.