I just picked up a used Yamaha R-V502 receiver. It appears to be in good working order except for one thing. When I connect all 4 speakers in the rear of the unit, the receiver will shut off. If I connect only 2 speakers, it stays on. The speakers are from the mid-70's (2 are from an old Sears stereo system, the other 2 were built from parts from Radio Shack). I had an old Denon receiver that accomodated these speakers for many years until it, too, started to shut down (it never powered back up). Does it sound like 2 of my speakers have just gone bad? Or did I get a bad receiver? Thanks!