Speakers work at low volume.
Decibels correspond to the output voltage being sent to the speakers. In your case, the voltage is reached at -40dB is the threshold that causes whatever is happening to operate the receiver's protection circuit. You might have a bad speaker, with a voice coil that shorts at the higher voltage. Below that level it's okay, but at the magic point the insulation breaks down, and that's when the receiver sees trouble.
You might try connecting only one speaker at a time using a short length of wire other than your current ones. If the trouble occurs on one speaker only, you've found the culprit. If the receiver operates normally on each speaker, try reconnecting one speaker at a time to determine if one of the speaker wires may have a short somewhere. There might be a weak spot in the insulation, not causing any trouble until the voltage is high enough.
If the receiver still goes into protection with just one speaker, there's an internal problem. Some part in the amplifier may be breaking down at that voltage. Or a part value has changed, causing the protection circuit to kick in even though there's really nothing wrong. Bad solder connections may have developed. Unfortunately, this kind of problem usually needs the services of a shop with the equipment and circuit information to figure out what's happening. If you can find a Yamaha-authorized warranty servicer, they might already know some typical causes if they've seen units with the same trouble.
Try the speaker test first. Then let a shop tackle the receiver if it looks like that's where the fault is. Good luck and thanks for using Fixya!
