ONKYO receiver shuts off after EXACTLY 5 minutes
I have an ONKYO receiver TX-NR801. I have disconnected all speakers and audio/video inputs. When I turn on the receiver, it works great for EXACTLY 5 minutes (5 minutes and 5 seconds, to be exact), and then it turns off and goes into STANDBY mode. When I turn it back on, again it works for exactly 5 minutes and 5 seconds and then turns off again. It doesn't matter how many times I turn it on, it will turn off after exactly 5 minutes and 5 seconds. It happens the first time it's turned on after being off or if it's been turned on an off several times. It happens if the volume is all the way down or all the way up. I thought it might be an auto-STANDBY feature of the receiver, but it happens regardless of whether it's playing audio. I'm totally stumped. Any ideas of what could be causing this? I'm ready to take it to a repairman (since it's out of warranty), but I'd like to know what I'm getting into beforehand.