I wouldn't waste too much time with examining the backplane. True, it's possible that a wire wrap connection may have broken, but I've never seen that happen on a 601. Then again, who knows what may have happened in shipping? That was one of the reasons for my last suggestion with reversing the ring voltage vs: 10 volt signaling.
I doubt that it's contacts either. Those card contacts are huge, so there's little likelihood that they aren't at least making partial contact. True, a relay contact could be bad. Since it's a matrix of relays, all it takes is one to mess up everything.
I guess it's best to go ahead and wait for the unit that Sam is sending. I hate to give up since I've never seen one of these things fail, but there is a first time for everything. Maybe that's why it was sold on e-bay!
