No, the issue is either with the RCA cables or the tonearm. You might also want to check your cartridge and headshell connection. Make sure the contacts are nice and clean, and securely on the headshell. Have you tried swapping out your turntables? Take the left turntable, and put it on the right channel's input, and vice-versa? This will tell help isolate the issue. If the problem just moves, then we know it's not your mixer. If the problem remains on the same side, then you know there is something wrong with your mixer.
If the problem moves, then you can safely say it's your turntable with the issue. The STR8-30's aren't the greatest quality, so it's also possible that you have a faulty tonearm or RCA cables.
Hope this helps.
DJPROAUDIO
TWEAK AV LAB