Since Samsung sell a broken product which they also doesnt fix, here is a workaround which temporarily fixes the issue.
I did find a way to reset the soundbar reliably (for me at least). Pull all the cables from the bar besides power (turn the bar off before doing this), start the bar again and change the input to "d.in". Then at the same time press the + & - buttons on the bar and hold, after a few seconds the text "init" will be shown and the bar will turn off. This should reset it. After this plug, the cables in again and it should work.
Ive done this thrice and it worked each time. Note that since you resetted the bar, stuff like smart things will need to be set up again.
Thank you, userg7N4fxaJgs! The Samsung information did not help me at all, but your fix did work. What a pain to reset it all but at least it's working again. I've had my sound bar for 6 mo's and not had any trouble until now but looks like this is a known problem. Why can't this be fixed with a firmware update? Anyway, much appreciate your post.
I was hoping this would fix my issue, and it did bring the INIT message, and I also removed all cables. But after starting the problem persisted. I made many tries.
I have learned a hard lesson about samsung. No matter how big a discount samsung is giving on its products it is never a good deal.
Funny thing is my TV is also samsung MU6300 and for one day the two (TV and soundbar) worked together and I could control soundbar with samsung tv remote even change the modes "standard ... etc". Now the TV does not even recognize anything. And this is not even the first big problem I have dealt with with this soundbar within barely 3 days of ownership.
update: I had to reset both TV and Soundbar and then I could ARC back. Not something I'd like to do often.
I have just encountered the same issue where the Q90R sound bar no longer display option "TV ARC" and it stays in "D.IN" . It was working fine and TV audio output was working with option "TV ARC" since I got it 2months back until now.
Tried reset both TV(Samsung Model ua55js7200k) and at the Q90R Soundbar , no luck getting it back and I can only connect it using WiFi option
Current firmware version (with issue)
Samsung soundbar Q90R: HW-Q90RWWB-1010.5
Samsung TV (ua55js7200k): v1550
& notice both TV and soundbar have auto update turn on , not sure if there was auto update recently leading this problem.