I had the same problem when I had a Scout. I fashioned an auxiliary ground wire from some magnet wire and ran an additional ground from the terminal box to the ground terminal on my preamp.
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If it isn't a cable problem, check for vibration:
Lightly lift the arm. See if you feel any vibration.
If so, make sure the motor's powercord isn't touching the back left leg of the plinth. If you still have vibration, lightly feel around the top of your stand to see if the motor is transmitting vibration that way. You may have to further isolate the motor (I use sorbothane feet)
Hope this helps - frequently the ground wire on the connecting block is not connected at the underside of the turntable, so it acts as though there is no ground at all and hums. I've run into this problem a number of times with Scouts/Scoutmasters/SSM's.... try following the wire from the terminal block and make sure it is tightly connected to the nut on the bottom of the turntable. Good Luck - hope this helps.