Thank you! It worked!
I have had the same problem with my older AVR-688 on and off for several years now. This morning when it was not working again, I decided to purchase a new receiver. I had a new Onkyo receiver (will not buy a Denon again) in my cart ready to finalize and decided to search again for possible solutions. I found your reply and went out to my great room to give it a try. The first time I blew into the headphone jack, it clicked but did not stay on. Then founds some headphones to plug in and it worked immediately. Unbelievable! Thank you so much!