cpenn :
I am having the same issue with a Samsung J6300 (just bought on 7/1/2015) and Comcast HD box (not X1 Platform). When I use the Comcast remote to turn on the TV and Cable Box, i get the "HDMI signal weak..." message. If I turn the cable box off and on, the picture comes in. It worked fine until June 17th. Is it possible that new firmware was pushed to the cable box at that time? Does it have anything to do with the timing of when the cable box is turned on in relation to the TV being switched on? The workaround it to leave the cable box on all the time, but I'd still like to find the cause of the issue. Any ideas?
I too have a Samsung J6300 (the 40" model from Best Buy) I purchased back in July 2015, about 4 months ago.
Today all the HDMI ports stopped working, the main one HDMI I use with a cable DVR box.
After trying different HDMI devices, cables, etc and no change I happened to try unplugging the TV power cord for a few seconds and plugging it back in.
After that the HDMI devices worked again but only until I turned the TV off (standby mode) using the TV's remote control.
I can only get the HDMI ports to work again after unplugging the TV's power cord again each time after turning off the TV!
At least thats working as a temporary work around.
I did just read thru internet searches on this issue, that its recommended to try a hard reboot on the TV by leaving its power cord unplugged for 10+minutes while all the HDMI ports are unplugged completely. I'm trying that right now and see if it solves it completely. Another recommended solution is a full factory reset via the TV's service menu, but I'll hold off on that as a last resort since I assume that would erase all my custom settings which would take a long time to set backup manually - I would have to first be sure to write down all my custom settings for white balance, color space, etc. I had to do for each of the inputs.