LG 42LD520 TV rapid power off/on cycling

Need some help with this old thing.

It started power cycling today out of nowhere, and the speed has increased to the point where remotes and manual operation are impossible.

At first it behaved like IR(?) interference, then it seemed like a bad remote, then it seemed like the powered OTA antenna, then it behaved like a Fire Cube issue. I’ve tried removing each item/category from the situation, 1 by 1, but the issue keeps coming back. I’ve also had EVERYTHING removed and power reset on all, and it comes back. That includes all fresh batteries for the remotes. I even tried to Fonz it. No matter what order I reconnect in, it comes back, and at different points. And when it comes back, it jumps right back into the fastest cycling possible.

LG 42LD520 TV (manual)

https://www.lg.com/us/products/documents...

The only inputs I use are:

aux:

Monoprice Active Curved HD5 HDTV Antenna, 60 Mile Range (https://www.monoprice.com/product?p_id=1...)

hdmi1:

Fire Cube (g1)

The original LG remote died long ago so I’m using sane model replacement (not universal).

Googling said there is a known issue with Fire Cube and some LG TVs power cycling, with the primary fix being verbal commands to Alexa to turn the connected [LG] TV on and off a couple times. I think I tried this correctly with no success before the cycling sped up too fast to do anything. The secondary possible fixes are in the Fire Cube settings, but I can’t keep the TV on long enough to get into the Fire TV settings. I want to point out that Fire Cube may *NOT* be the issue at all, and the problem persists even when Fire Cube is off and literally unplugged, including pulling the batteries out of the Fire remote..

I’m really at a loss.

Update 2 & 3:

The original time, I had to do a mass disconnect and power down (including batteries from remotes) for a day or more, then reconnect everything and it worked normally again. But now it happened again at about a month, out of nowhere. I can’t find any pattern that would point a finger at which item is to blame.

Update 4:

(note: added manual link above, also)

Mass powerdown overnight didn’t work this time. I think the problem may be worse, too. When I removed the HDMI cable this time (for Fire, but already long powered down and disconnected on Fire end) it seemed to stop the cycling, but I still went with the power down overnight. When I went back to start everything up, the TV was fine, but no inputs connected yet, besides power. When I screwed the AUX cable on (first thing reconnected) it started cycling (faster?) again. So I took the AUX back off, but it kept cycling with nothing connected.

I noticed the standby/power light on the TV was doing something new now. Normally it does a long red or blue glow as I turn the TV on and off. I honestly don’t remember the color meanings. The manual says NOTHING about color meanings either. This time it was doing a short RED flash with each cycle, which I swear I have never seen before. Manual still says nothing about color flashes, including the troubleshooting section.

Update 5:

Tried a power down one more time. Reconnected AUX antenna before TV power. Magic! I’ve got OTA back! Tried the same with HDMI and now HDMI is worse. TV won’t even recognize the active cable in ANY of the 3 HDMI ports on the TV (It was recognizing fine until this afternoon). I tried selecting correct ports manually on TV settings and still not finding HDMI input any more. I want to try an alternate HDMI cable just to rule it out, but none handy. If that fails, I’m thinking the input hardware in the TV has a problem, but then I can’t explain the AUX being fine now.

Update 6:

Insanity. Got my old BluRay player out, pulled the HDMI out of the Fire Cube and put it into the BluRay player AND IT WORKS FINE. Tv saw it right away. I want to go back to blaming the Fire Cube, but that won’t explain the power cycling when there was no HDMI attached and I was just screwing the AUX antenna onto the tv. I swear it’s like Fire Cube herpes or something and it gave it to the tv.

Update 7:

Shifted the TV a little to avoid a reflection and now I have power cycling again (went back to slow cycling at least). Then I notice the Fire Cube is still on (not by cable, but still has IR), so I can’t 100% blame the physical shift. New HDMI cable is in the mail, so I’ll test that next. For the last 4 days though, HAD normal TV with BluRay player and antenna.

Update 8:

New HDMI cable arrived. Old one fully removed. TV cycles so fast now it never fully turns on. If no inputs, it turns on fine and stays on. I guess the issue is somewhere *in* the tv, along the input path. I popped it open and no obvious problems visible anywhere. I checked the back of the input board as well. The main cable between input board and main board looks fine also. Fully connected. Air dusted it, but not that dusty to begin with. Pictures added.