Turntable hums thru amp even without t'table power?

I had the same issue with Pl Z93 humming, As someone mentioned above, I powered it with a 12V 1A DC transformer but when connected to amp, got a hum.

Opened up the turntable chassis and connected a cable on to the metal inside as the outside of the turntable is plastic and the screws on the outside aren't earthed to the metal inside.

Done this, then connected other end of cable to the earth terminal on the back of amp and works perfect.

=)