The reciever is about 13 years old; it couldbe a capacitor problem.
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You can try resetting
1) unplug unit
2) press and hold tuner and video select on front panel
3) plug in while holding buttons
Unit display should flash until you let go of buttons.
Unit stays on you should be ok/ goes back to red- service
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There are 3 types of protection that will commonly occur on Denon receivers which are designed to protect the unit from permanent damage.
1) Thermal Protect = When the unit overheats this will occur, usually if there is not enough air space above and around the unit. There cannot be anything directly on top of the unit and there must be at least 4 inches of airspace above the unit.
If not hot this shouldn't be a problem.
2) Overload Protect = Most commonly occurs when a strand or more of copper speaker wire is not securely connected to the speaker terminal and is touching the chassis of the receiver. If the volume is turned higher than 85% this may also occur. If you need to turn the volume higher than this point to get the level you want, you need a more powerful amp.
You said you disconnected the speakers, but also turn your volume all the way down. see if it powers on and if so then add one speaker at a time back.
3) DC Protect = When an amplifier fails this will occur. This will protect dc current from damaging the speakers. When this happens the unit will need to be serviced.
This will be true if your power capicitor is shot/blown which can happen on older AVRs. This will probably mean a service call.
Denon is my brother.. to my Marantz. Worth getting fixed
Good Luck!
Forest Man.