I bought a used SX-550 that seemed in pretty good condition from a thrift shop the other day for $50. It worked fine at the shop but now that I have it all hooked up there seems to be some problems playing into both speakers. Sometimes I get stereo sound through both L and R speaker and then it will just quit or get really muffled, typically in the R speaker. I tried it on both Speaker A and B set ups, as well as FM radio and different types of LPs... but the problem still persists. One thing that helps is if I press the Mono button. It seems to play fine through both speakers in Mono. Could it be the Phono jacks? The receiver is the only vintage piece of the equation and it all worked fine with my previous (newer) Sony receiver. It's my first vintage equipment in place, as I want to eventually find vintage speakers and finish refurbishing an older turntable I have. The other current components are just a pair of Sony SSC5 bookshelf speakers and a Kenwood KD-291R turntable (not sure if that info helps diagnose).

Please help.

I did not find the right solution from the internet.

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