"TJ Hertz" wrote ...
I've got a Japanese-bought AKAI S1000 sampler that says
"100v AC" on the back. I live in the UK, where the power
supply is 240V or thereabouts. Can I use a 110V transformer,
or does it need to be strictly 100V?

I wouldn't risk 110V. So get a 10-12V transformer and hook it up
to "buck" part of the 110V to reduce it to ~100V.