He said to only earth ground one piece of your audio equipment. If your equipment has a 3-prong power cord, get a 3-prong to 2-prong adapter. He said this helps to get rid of more ground hum.
If any of this ever makes a difference there is either a bug in the design or a defect in the equipment; in either case it should be returned to the manufacturer for repair! You should never use a 3-to-2 prong adapter except for testing, as they can cause shock or fire hazard with certain types of failure in the equipment!
Suteetat, without an LP on the platter. you should see if lowering the arm will induce the hum. Having the hum only show up when you lower the arm sounds like an intermittent wiring problem in the arm itself.