[EADGBE]

Here's an update on the polarity of the Ariston turntable.

I removed the unit's base cover plate on the bottom, and
all of the wiring was plainly visible. The red wire was
attached to the centre contact of the voltage input jack,
and the black wire was connected to the sleeve contact.

I bought an AC adaptor with the correct female plug on
it and made sure that the positive contact was the centre.

Plugged it in, switched on the turntable, and SUCCESS!

The turntable works perfectly.

Thanks to everyone for your assistance with this! :-)