Is there any reason why a turntable should produce a 60-cycle hum? (Actually, it’s more of a buzz, but it’s similar to the 60-cycle ‘hum’ from single-coil guitar pickups - but much louder.

I’ve plugged the earth cable into the GND socket on the amp.

I’ve plugged an iPod into the Phono socket, but it didn’t hum without the turntable.

The turntable hasn’t been used for at least a decade and I wonder if the sockets have corroded.

Welcome richardsmallfield!

Yes, unfortunately common to get hum from TT-phono sources.
What TT & cartridge?


PS. Normally we get 50Hz hum here

DarkLantern blog - darklanternforowen.wordpress.com

Bound to be a connection issue after all this time.
I suggest you remove clean and reinstall all connections from the cartridge pins to the RCA plugs on the amp/preamp.
If you just wiggle/twist the connections you may find the offending joint.
Any corrosion on these connection surfaces will almost certainly result in a hum or buzz.
Cheers
Graham