If they haven't changed much since the late 80's I'd guess that the issue with some of the Grado cartridges is shielding, and NOT grounding. The Grado cartridges back then were unshielded by choice...I forget the logic, but seem to recall that Joe Grado had some reason for doing so. I had (and still have, though not in use) the original AR Turntable from the mid-80's, and it had the same issue with the Grado cartridges. One company, I think it may have been Merril, even marketed a lead 'shield' that mounted between the arm and plinth to shield the cartridge from the motor. It had limited effectiveness. I had both a Blue Point Special, as well as a Grado Signature (don't know which, but it was their $600 model in the mid-80's). The Blue Point was a bargain and not a bad cartridge at all, but the Grado was really wonderful, warm and detailed as I recall. The hum was a small price to pay for the bang-for-the-buck! Don't know how bad it may, or may not be on the MMF-7. If shielding is the issue, grounding won't help. Hope that can assist you figure out the issue.
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