Recent Stereo problems?

I have a 1980's Technics SA-360 receiver hooked up with a 1970's Sony receiver via a simple audio cable. They are both powering 4 speakers, so the whole system is powering 8 speakers. I have an audio to MP3 cable going from the Technics receiver to my laptop, so I have all 8 speakers playing out of my laptop. Recently, when both receivers have A+B speakers ON (main and remote speakers) it will only play the left A+B speakers, as if I have my balance all the way to the left. When I switch the A speakers on (main speakers only) it plays BOTH main speakers. And when I switch the B speakers on (remote speakers only) it plays only the left B speaker. Is a fuse blown? I know they are speakers are phased correctly, but i have flipped the audio cables back and forth from all the inputs and i get nothing. Just a low "hum". Both receivers are showing the same pattern Any suggestions?