I just faced this problem and solved myself EASILY. I have 1 old and 1 new, and this only happens to the OLD one. This often happens when I insert a line-in plug. It works all ok when line-in. But when I pull out the plug, the R channel becomes either silent or noisy, randomly. When the problem happens, just PLUG IN AGAIN. Sometimes I try several times to solve it. *I believe alectronic is right. This problem happens after using the line-in plug intensely.