Loud 60hz hum from all channels expect subwoofer

My SONY STR-SE501 receiver has developed a loud 60hz hum on all channels except the subwoofer in all listening modes. the hum can be stopped by turning the volume all the way down or muting. Other then that, the volume has no control over the hum. It remains at the same level anytime the volume is on. I've managed to get my hands on a service manual and will start taking a look in the next few days, but I was just wondering if anybody has seen this before and might know off the top of your head what may be causing this. Thanks.

Reply