As long as you use a cap that has twice the peak to peak rating of the
voltage at the secondary, you are fine. Since this is a 9V regulated supply
with a 3-pin regulator, I would expect at least 12VAC at the transformer
output. Multiply that by 2.828, double it again, and you have the required
voltage for the cap to be within its safe operating area. A 100V cap is fine
in this case.
As this poster mentioned, the primary voltage isn't the determining factor,
but the secondary voltage is the one you want to look at. Make sure you use
either a ceramic or film cap. A non-polarized electrolytic cap wouldn't be
suitable.

Pete

"bw" wrote in message
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wrote in message
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BTW that "The 1uF 100V non polarized cap"
that you mention - my set is a RP2100 from China with 240v AC mains,
does that alter the value?


No, in power supplies the transformer secondary is the low voltage side.