In addition to the suggestions above, test a source which you know has material for all the channels, i.e. test tones.
The response of the authorized repair facility is puzzling to me. The 7X5 series of NAD receivers has been out for what, maybe six months now? It sounds to me like this repair facility really isnÂ’t interested in working on your receiverÂ…could be for a number of reasons and none of which may have to do with you or your receiver. Also, I imagine NAD would be interested in hearing about your experience with this particular authorized repair facility.
I have a 751 which suddenly stopped working correctly about two years ago. After running a series of trouble shooting tests, I eliminated all the other possibilities and sent the receiver off to The Service Bench in Norwood, Mass. The problem was simple, the AC3 processing chip failed. From what I understand, it is not uncommon for these processing chips to fail after about six years, regardless of the brand name on the receiver. I had owned my receiver seven years at the time mine failed.