Issue is either your tonearm or your RCA cables. Unfortunately, there is no easy way to determine which is the problem. I would suggest replacing the RCA cables first, as it is the least expensive. If it does not solve your problem, then the issue is with the tonearm.
You can also do this for now:
Remove your headshell/cartridge from the tonearm. Look inside the tonearm tube, you'll see 4 gold prongs. Make sure all 4 prongs are protruding, and are 'shiny'. If they are not, then the issue is with your tonearm. If they look good, then more than likely, the issue is with your RCA cables. Taking it to a qualified repair center will help in determining what is the issue, but they will need to open your unit to test, so be prepared to pay for the estimate right off the bat.
If your in the NY Metro area, look us up. We also offer nationwide service, if you cannot find anyone local:
http://www.tweakrepair.com
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TWEAK AV LAB