DENON DRA-685 in 'protection mode' will not play

Does anyone have a clue to why our venerable Denon DRA-685 receiver is stuck in 'protection mode'? Everything fires up as normal: tuner, phono, cd, etc. but upon system turn-on it immediately locks into protective mode. No clue on our end, receiver has always worked fine. Any ideas as to problem would be much appreciated as well as any LINKS to a SERVICE manual, I cannot find one anywhere on the WEB!Thanks much, Martinluke!
I cannot find one anywhere on the WEB!

You didnt look very hard.....
usually it means you're speakers are clipping--i.e. the speaker wires are touching somewhere. try rewiring your speakers and make sure the = and - aren't making any contact.
Clipping does not mean shorted speaker connections. Clipping refers to the cutting off of the tops of the output signal waveform (as viewed on an oscilloscope) and resultant distortion when an amp is driven into overload.

Martinluke, if the problem is not shorted speaker wires, then the receiver could be going into protection mode due to some internal problem. Unless you have troubleshooting and repair experience, IMO a service manual is going to be of little or no help to you.