TURNTABLE HUM:
IT IS THE GROUND WIRE

• The ground wire goes to SIGNAL GND on the mixer, not to any convenient screw. It is the spade terminal on the deck's own thin wire, separate from the RCA pair.
• Tighten it. Technics are explicit that loose is as bad as absent, for the SL-1200MK2: "Be sure to connect the ground terminal firmly to the amplifier or receiver. If this connection is not made or is loose, a power source "HUM" will result."
• One deck at a time. If two decks share a mixer, connect one earth, listen, then the other. That tells you which side the fault is on in ten seconds.
• Check the lead is even there. Technics supply an earth lead with the SL-1200MK7 and publish a part number for it, K4EY1YY00189, which matters on a secondhand deck where the previous owner kept it.

This is the second cause and both makers document it. A phono cable carries a signal small enough that a mains cable or a lighting circuit running alongside it gets picked up and amplified with the music.

Technics, SL-1200MK7: "Are there other appliances or their AC power supply cord near the stereo connection cable? Separate the appliances and their AC power supply cord from this unit."

AlphaTheta, PLX-1000: keep the connection cord away from an electric device such as a fluorescent lamp or its power cord near the connection cord, which the manual says to place as far away as possible.

That is why a hum can appear when the venue turns the house lights on, or when somebody moves the deck four inches. It is wiring geometry, not a fault in the turntable.

AlphaTheta document the whole problem for this deck in a single entry, and it is the clearest statement of it on this site.

A low-pitched hum or buzz under playback on a PLX-CRSS12 has two documented causes and neither is inside the deck. The first is what the connection cords are lying next to: keep them as far as possible from fluorescent lamps and from the power cords of other equipment. The second is the earth path: check the ground wire of the unit is connected correctly, which on a DJ mixer means the SIGNAL GND terminal rather than any convenient screw. A phono cartridge puts out a tiny signal that a phono stage then lifts enormously, so anything the cable picks up on the way gets lifted with it. That is why a hum that appears when you move the deck, or when the venue turns the lights on, is a wiring geometry problem rather than a fault.

Technics publish this one for the SL-1200MK2 and it is worth knowing on any deck with a removable headshell, because it explains a hum that comes and goes rather than sitting there steadily.

"Dust and dirt at the headshell terminals may result in increased "HUM" noise or intermittent sound. Use a soft dry cloth to clean the headshell terminals."

A headshell that has been swapped a hundred times, or lived in a bag, collects exactly that. It reads as a failing cartridge and it is four contacts and a dry cloth.

There are 8 turntables in this catalogue. Four of them have hum guidance published by their maker, and those four are the ones quoted above: the PLX-1000, the PLX-CRSS12, the SL-1200MK7 and the SL-1200MK2.

The other four have none that we have found. The physics does not change between decks, and neither does the ground wire, so the order above is still the order to work through. But this site will not put words in a manufacturer's mouth, so read the sections above as what AlphaTheta and Technics state for their own decks.