Denon AVR-3802
I have repaired many of these Denon receivers over the last few years.
Since the headphones work, that means if you dis-connect the speakers it will turn on and not go into protect (when power cuts off a few seconds after turning the unit on).
The most likely problem is a bad speaker. to find out which speaker is causing the problem, do this.
Start out with just one speaker connected and then turn on the unit. If it turns on and plays music, you know that speaker is good.
Now turn the unit off again and add another speaker and keep the first speaker connected. Once again, if the unit plays, that speaker is also good.
Keep doing this same procedure one speaker at a time. As soon as you connect a speaker and then try to turn on the unit and it goes into protect, the speaker you just connected is bad.
To confirm that this speaker is the bad speaker, dis-connect all speakers and only plug the suspected bad speaker into a speaker jack that worked with one of the other speakers.
If the unit still goes into protect you know then that you have found the bad speaker.
If the unit still goes into protect, that speaker is bad and you will need to take it in for repair or replace it.
make sure you use 8 ohm speakers. if i remember correctly these Denons can use 6 ohm speakers, but those are harder to find and most home speakers are 8 ohms.
If the unit does not go into protect and that suspected bad speaker plays in one of the other speaker jacks, what you really have is a bad channel in the amplifier, the channel that the suspected bad speaker was previously connected to.
If you find out you have a bad channel, you will need to take it in to an repair center. Preferably an authorized Denon service center. There are not very many of those around. I work at the only authorized Denon service center in the entire state of Michigan.
If I can be of more assistance, don't hesitate to ask. And a good rating is always appreciated. :)