Hum coming out of phono channels DJM500 Mixer?

[Final Boss]

Hey guys,
So i played out my first vinyl set last night with my DJM500 mixer and two technics 1200MK2s(the radio station's). When I plugged the technics into the phono inputs and put them on the master output, i was getting that classic hum you hear when you're plugging audio(aux usually) in. Is this an electricity loop? I think all the gear was plugged into the same strip of outlets. If i change the outlets that the gear is plugged into, do you think the hum will go away? I bought the mixer second-hand, is it possible that it's just busted?

Any help would be much appreciated. Also, if anybody has any tips on working with the technics (like what is the silver button on the bottom of the deck that sticks up with a light on it?) or vinyl in general I would love to hear them.

[Huts]

sounds like they weren't grounded. the silver button with the light is to help you see the record (grooves) in the dark