[wdfifteen]

I have a 3 year old LG flat screen tv and a 10 year old Onkyo receiver.
Ethernet from my router to the TV for input.
An optical cable from the TV to the receiver for sound out.

The sound stopped working a few days ago. It was working Friday night and Saturday morning it wasn't. I checked all the settings and figured there are only 3 components and the cheapest is the cable, so I got a new cable. It didn't help immediately, but we were watching TV (using the TV's internal speakers) a couple of nights ago and it spontaneously started working. The next day it was dead again. So it's down to the TV or the receiver. I don't know how to determine which it is. When I unplug the optical cable from the receiver I see a red light. I don't know if that's actually a sound signal or something else.
Anybody have any advice on how to get this thing working again?