Funkyd, can you post on your latest findings?
FWIW, I had similar issues. A very old receiver without HDMI that needs to be connected to the TV. The TV manual shows an "optical" cable hooked to the digital audio output but I'm surprised it's round (unlike my rectangular/square optical toslinks sockets in the old receiver). Googling a bit, it seems that there are now round toslink cables, so I thought it might be it.
But then, my receiver had a similarly colored socket (orange), so I'd thought, maybe the manual is misprinted and it does take a coaxial? I tried a 75 ohms* cable (specs for the orange coaxial) which worked.
Now I'm wondering if that socket also works optically. Can anyone that uses it optically opine?
Pardon the semi-nubeness; it seems I'd missed 10 years of electronics innovations, which my purchase of the LC60E69U is giving me an opportunity to redress.
* I don't have an orange RCA coax at home and used a yellow (video) one, since it's rated at 75 ohms. The red or white coaxes does not work (different resistance).