On some of the tracks, when I turn the lows knob, the process of turning produces this scrappy static-sounding noise. Really weird.

I have DJM-600 and Technics MKII TT's, my carts are Shures M44-7. My equipment is properly grounded.

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you need to clean the knobs.. take off the knobs and get some compressed air in there.

how old is the mixer? if the compressed air don't work you can usually get the knobs replaced, but 9/10 times this problem is caused by guff in the knob.

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MUGGETS