I have a model STRK740P Sony Digital audio/video receiver. Of the 5.1 channels, the left for some reason stopped working, but all of the other speakers work fine. I swapped the speaker with another working channel, and it worked fine so that is not the problem. The cables connected the speakers themselves aalso work fine and are not causing the problem.. My conclusion is that the problem lies with the receiver; it is no longer outputting a left speaker signal. What is the problem?