Robertson Forty Ten noise problem

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I have a robertson forty ten amplifier that I am having a noise problem through the left channel. The strange thing is the noise is loudest with no RCA cables plugged in but once I do plug them in most of the noise goes away (although not completely). I opened up the amp and see no burned components, checked for any loose grounds but found none. I took a few electronics classes years back in school but don't have much experience, any ideas of what to look for would be greatly appreciated.
 
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