Because there is such are such a varity of turntable options. First you must identify if you turntable is sending a Phono signal or a Line level signal.
Phono/analog set up
If your turntables are set to Phono:
>Connect the r.c.a cables to the inputs marked "phono" on your mixer.
Line level set up
If your turntables have a built in pre-amp like the T.80,T.120 or the St.150. There a an option to send an amplified signal level to your mixer.
>Connect the r.c.a cable to the inputs marked
"Line" on your mixer.