Make sure there is a ground wire from the turntable to the receiver chassis.
If you have an external phono preamp, you should definitely run a ground wire from the turntable to the preamp. The phono preamp should already be grounded to the amplifier through the interconnect cables. However, if the hum is still present, try running a second ground wire from the phono preamp chassis to the power amplifier or receiver chassis to possibly eliminate it.