Disconnect everything from the mixer... except your amp/speakers. Keep volume off (so you don't damage your speakers), and one at a time, re-connect a device back onto your mixer. After each connection, check for the ground hum. When you add the device and hear the hum, you've found the unit that's giving you difficulty.
You can then start looking at ways to isolate it once you've discovered the actual unit. Could be a bad ground on the turntable, or a ground loop, or even simply a bad cable.
Long process to troubleshoot but there's really no other way....
Bruce