Backlights still cutting out on LG 42LN5400 LCD after replacement

[SurrealMustard]

I'm working on this LG LCD with the typical Led backlight failure. After dismantling the panel, I tested each lamp and found one to be bad, so I ordered a replacement strip. Unfortunately, even with all functional Leds, it's still doing the same thing (middle three strips flash briefly). For the sake of thoroughness, I even went through and tested each and every Led on every single strip with the multimeter and they all still work properly (including the Leds on the replacement strip).

Any ideas as to why this could be? Is it possible that I've got a power supply issue or that something else is being overlooked somehow?

[dick_barton]

Re: Backlights still cutting out on LG 42LN5400 LCD after replacement

Could it be poor interconnections between strips?

[SurrealMustard]

He's on to something, but can it be verified?

Could it be poor interconnections between strips?

I considered that, but they all look and feel pretty solid. Would there be an easy way to test it?

[ReeceyBurger123]

Re: Backlights still cutting out on LG 42LN5400 LCD after replacement

Did you turn the Bl down ? Could be another on its way out. On Lgs I set energy to medium and go to expert setting and turn Bl to 35 and brightness to 60.

[dskall]

Re: Backlights still cutting out on LG 42LN5400 LCD after replacement

You can maybe switch strips around or connectors. Are you looking at them flashing directly or through the screen? Maybe check each strip with external supply and see what current they draw

[SurrealMustard]

Troubleshooting prospects

Did you turn the Bl down ? Could be another on its way out. On Lgs I set energy to medium and go to expert setting and turn Bl to 35 and brightness to 60.

I didn't get a chance to change the setting yet, as I have not reinstalled the panel. If one was on its way out, wouldn't it measure differently with the multimeter or not light up as brightly when tested the other way?

You can maybe switch strips around or connectors. Are you looking at them flashing directly or through the screen? Maybe check each strip with external supply and see what current they draw

I'm looking at them directly because the LCD panel has not been put back in yet. I considered the external power supply trick, but I don't have an external power supply with which to test it properly (unless there's a way to rig up a PC power supply or something). I'll try peeling some of the strips up and seeing if switching them around helps.

[dick_barton]

Re: Backlights still cutting out on LG 42LN5400 LCD after replacement

You could try poking all the connection points (connectors) and see if it has any affect on the leds.

[SurrealMustard]

Progress

After taking another look at it today, it appears that one of the other Led strips must somehow be bad, even though all the individual Leds test good. The good news is that I think I narrowed down which one that is.

As best as I can tell, the power supply seems to divide its output into two zones - one for the middle three strips and one for the top and bottom strip (correct me if I'm wrong). I switched the entire top strip with the entire middle one and was able to successfully invert the problem, getting the middle three strips to stay dark, but the very top and bottom ones to flicker on briefly.

From here, it was just a matter of finding out if it was the left or right half of the strip, so I just switched them back & forth and determined that it was in fact the left one (the module on the top row that I didn't replace). I can do an illustration if that's too confusing .

Given that this seems to have narrowed down the culprit, would it be wise to invest in another strip for this thing or is there anything else that I can test before plunking down the money?

[SurrealMustard]

Second backlight strip fixed it!

Well, I took the plunge and bought the L1 strip. It finally works! The backlights come on and stay on and there are no other issues. Problem solved!