LG 49SK8000 intermittent dim backlight zones

[zward]

I have a 2018 LG 49SK8000 with a bit of an odd problem that I haven't seen discussed elsewhere. It's an edge-lit (bottom) LED UHD TV with local-dimming (6-zones I believe). About 5-10% of the time when I turn the TV on after it hasn't been used for a couple hours, one or more backlight zones will not power on normally.

Some observations:
- If I power off (standby) and back on, it almost always works normally and continues to do so indefinitely.
- If I switch inputs, the zone will briefly light up and then go dim again.
- If I change the video preset (e.g. cinema to HDR effect), it will also briefly light up and then go dim again.
- A zone may come on normal, dim, or totally dark.
- Local dimming settings, including off, have no impact
- The dim zone seems to vary each time, although maybe the ones on the sides are more commonly problematic.
- This happened very rarely when the TV was newer, but has become somewhat more frequent.

The backlight is usually at 100, which I've now learned is not the best thing for longevity.

I'm ready to open it up and do some testing, but since I can't reproduce the problem reliably, I'm afraid it will be a waste of time. This seems to potentially be a power supply (includes the LED driver) vs LED strip problem, but that's just a guess. Maybe something that is impacted by a cold start and is resolved after a brief warm up?

I have attached a few pictures of the screen displaying a white test pattern, as well as pictures of the PSU from Shop Jimmy. 3rd pic is it looking normal.

[nomoresonys]

Re: LG 49SK8000 intermittent dim backlight zones

make sure the firmware is up to date.

[zward]

Re: LG 49SK8000 intermittent dim backlight zones

make sure the firmware is up to date.

Thank you for the suggestion. Firmware is up to date using the built-in updater. I had long hoped that would do the trick, but no dice.

[nomoresonys]

Re: LG 49SK8000 intermittent dim backlight zones

Some turn all the eco, ambient and such off to keep it from being so annoying but then you probably lose some of the benefits of the local-dimming, wonder if its maybe a resistor that isn't doing what it should anymore?

[zward]

Re: LG 49SK8000 intermittent dim backlight zones

Some turn all the eco, ambient and such off to keep it from being so annoying but then you probably lose some of the benefits of the local-dimming, wonder if its maybe a resistor that isn't doing what it should anymore?

The eco mode and ambient light sensor is turned off. This phenomenon happens with local dimming off altogether. Local dimming isn't that useful on this TV because the dimming zones are so huge (6 vertical sections basically).

My guess is a bad solder joint, which could be anywhere, or something with the driver section of the PSU.

[nomoresonys]

Re: LG 49SK8000 intermittent dim backlight zones

Indeed, local dimming is pretty much an oxymoron on an edge lit tv.

[Storyteller12]

Re: LG 49SK8000 intermittent dim backlight zones

Have you tested the backlights to make sure their not going bad? Try turning the backlights down to like 50 and see if the problem persist or if it fix the problem.

[zward]

Re: LG 49SK8000 intermittent dim backlight zones

Have you tested the backlights to make sure their not going bad? Try turning the backlights down to like 50 and see if the problem persist or if it fix the problem.

Ok I'll give that a try. My initial thought was that the backlight itself is ok since when it powers up ok, it continues to work fine indefinitely.

I feel like it very rarely did this when it was relatively new also - should have replaced it or addressed it back then while it was under warranty. But that also makes my think it's not just a worn out backlight.