Unit often has no output what's that about?

I have had this unit since new. It has worked well since then. I added a Harmon Kardon Blue Tooth receiver to play iPhone through two years ago which is really the only thing different.

When I turned it on it rarely has any output. I play with input knob or change the radio station and that sometimes does it it seems. Turning it off and on sometimes does it.

Any ideas?