I would look for a screw on the motor and attach a wire to that and run it to either the ground lug on your pre-amp/receiver/Phono pre-amp if one of them has one has one. Or if not try using the center screw on the wall plate if you have 3 prong recepticles.
How to ground old turntable with no grounding wire
I have an old Marantz turntable that I want to start using again. I have a terrible hum through my speakers when it is turned on--worse when the tone arm is on its rest, better when it's over or on a record. I'm guessing it needs to be grounded (although it was never grounded when I used it years ago). But, it has no grounding screw or wire that I can locate. Suggestions? Any ideas for other causes of this new hum? The other thing I'm wondering about is electrical polarity, because it has an old (non-polarized) plug, plugged into a polarized extension cord. Any and all suggestions welcome!