LG 42LN540V LED Backlight Flickering After Replacement When Connecting The Screen

[RetroComputingGrotto]

This TV has been on the wall in our reception area for around 10 years and the other day the backlight failed. You could see a feint picture so we knew the screen was OK. I took the TV apart to get to the failed backlight and inject voltage in to each half of the LED strips to identify which ones had failed and 4 needed replacing. I ordered a set off of Amazon and they arrived the next day.

On fitting the new LED strips I plugged in the TV was blinded by the light!!




It was only when I partially put it all back together (including the screen) that I connected it via HDMI to a laptop and on the left hand side it was pulsating a bit. As you can see from the above photo they are all lit and there was no pulsatiing going on.

https://youtube.com/shorts/LLL_C6iLZ...YMS3lSBH_BAHrH

As you can see from this video, it's pulsating. Again if I disconnect the screen there is no pulsating going - any ideas what might be causing this and what I can check?

[Diah]

test it with out HDMI source... just the TV on and call the menu setting. check if there pulsing

[RetroComputingGrotto]

Without HDMI plugged in I get a spinning LG logo that doesnt quite go over to the left hand side where the pulsating occurs but I can't see any pulsating.

[EDIT]

I've just turned it on and got a menu up and yes I can see pulsating on the left hand side when nothing is connected to HDMI - I've also just tested it on the other HDMI port on the back rather than the side and sadly the pulsating is still there.

[Diah]

test with other source such USB with jpg photo or movie

[Diah]

Without HDMI plugged in I get a spinning LG logo that doesnt quite go over to the left hand side where the pulsating occurs but I can't see any pulsating.

[EDIT]

I've just turned it on and got a menu up and yes I can see pulsating on the left hand side when nothing is connected to HDMI - I've also just tested it on the other HDMI port on the back rather than the side and sadly the pulsating is still there.

okay, what happen if you reduce the BL level under the half ? if they are stable then yours replacement led fault

[RetroComputingGrotto]

Just found a USB memory stick and put a photo on it and it's still doing it but it's not quite as obvious:

[RetroComputingGrotto]

okay, what happen if you reduce the BL level under the half ? if they are stable then yours replacement led fault

Reception have lost the remote control so changing any levels is awkward with the limited buttons on the rear of the TV!

I haven't replaced all the LED strips, just 2 of them so I might remove all of the other original LG ones so they are all the same - do you think that might help?

[RetroComputingGrotto]

Reception have lost the remote control so changing any levels is awkward with the limited buttons on the rear of the TV!

I haven't replaced all the LED strips, just 2 of them so I might remove all of the other original LG ones so they are all the same - do you think that might help?

I've replaced all the remaining original LG LED strips with the ones I have and it's still doing it.

I have perservered with the buttons on the back of the TV and after lots of pressing the wrong buttons I managed to navigate the menu to the BL setting and even on 0 you could see it flickering.

What I shall have to do is put all the working original ones back on the TV and then the 2 faulty strips I'll have to try bodging a repair by scratching off the tracks and bypassing the faulty LEDs and see if there are any dark spots on the TV - it is perfectly possible that its been doing this fault before the backlight failed but no one notived as it's just used in Reception for a rolling powerpoint type of display saying welcome and where to go in a fire if you're a visitor etc.

Wish me luck!

[Diah]

show the old and the new strips led label... also show yours psu bl circuit from the downside.. you will see many sync SMT R

[EazyBone]

Did you only replace some strips? 2 strips look different color. I did read everything, but originally did you not replace them all with new ones and left old ones in?

I can see more closely you did. Never just change out a couple strips if you have all new ones. You aren't going to get more life out of it that way :/

[Davi.p]

I guess you cannot inspect much more, you need an osciloscope to grab the waveform of the pwm dim input of the led driver ic, next you can resolder completely the driver ic

[RetroComputingGrotto]

Did you only replace some strips? 2 strips look different color. I did read everything, but originally did you not replace them all with new ones and left old ones in?

I can see more closely you did. Never just change out a couple strips if you have all new ones. You aren't going to get more life out of it that way :/

The strips I got were not LG branded just some generic brand that happened to be in stock on Amazon for next day delivery - my thinking was that LG branded ones already in the TV would be better so kept the working ones in. I've since put all the ones I got from Amazon in the TV and it still pulsates when I connect the screen. No idea if it's been doing this previous to the LEDs failing as it's used for digital signage and they may not have noticed.

[Davi.p]

Amazon+next day delivery+not original parts=bad choice.. you don't know which led specs have the original ones so..

PS: i don't know this injury of buying in 1 day.. modern times

[RetroComputingGrotto]

Amazon+next day delivery+not original parts=bad choice.. you don't know which led specs have the original ones so..

PS: i don't know this injury of buying in 1 day.. modern times

Limited Space + TV In Bits + Delicate Screen = Need To Sort Something Out Quick before it gets damaged!

Working in Education there is no money for training so on-the-job training like this is how we have to learn new skills and gain experience.

The TV was going to go in to WEEE/eWaste and the LED strips that had the exact model number listed in the listing cost £24 and I have learnt how a modern TV works (the last time I repaired a TV was at College 20+ years ago and they were CRTs)! I've learnt that a backlight repair is a relatively easy repair and that you should try and get hold of genuine LG LED strips rather than a random brand 😎 I've also learnt that at least one of the PCBs (the one that provides the power) is not unique to this set and can be found in a lot of other TVs so I now know what parts to keep as spares.

[Davi.p]

Because also colour temperature is slight different... anyway my advice on resodering IC ? what do you think? Don't remove the chip, add an amount of solder, remove the solder with braid, put new solder.. seems a stupid procedure but it's not so.. trust me..

[EazyBone]

The strips I got were not LG branded just some generic brand that happened to be in stock on Amazon for next day delivery - my thinking was that LG branded ones already in the TV would be better so kept the working ones in. I've since put all the ones I got from Amazon in the TV and it still pulsates when I connect the screen. No idea if it's been doing this previous to the LEDs failing as it's used for digital signage and they may not have noticed.

Got it, always replace all the lights my opinion.

This has happened to me before also, on Vizio. I have updated the TV and the factory reset it and it has gone away. This was also after led change. Maybe give that a try.

[RetroComputingGrotto]

Because also colour temperature is slight different... anyway my advice on resodering IC ? what do you think? Don't remove the chip, add an amount of solder, remove the solder with braid, put new solder.. seems a stupid procedure but it's not so.. trust me..

100% going to give that I try - I've reflowed a few IC's in the past that has solved issues - it certainly wont make it any worse and if it does fix it then that's another piece of knowledge in the bank!

Got it, always replace all the lights my opinion.

This has happened to me before also, on Vizio. I have updated the TV and the factory reset it and it has gone away. This was also after led change. Maybe give that a try.

I wish they hadnt lost the remote control as navigating the menus with the buttons on the back is a pain!! I will give that a go though.


Constructive critism is always welcomed as how else can someone increase their knowledge and experience?! Thank you all who have replied, much appreciated. I'll update the thread with what I do next but I've got a couple of pairs of speakers I need to fix first - they have been on for the last 10 years making and are now making a loud hum so I'm hoping it's just some warn out capactrors...

[RetroComputingGrotto]

RIP LG TV

My repair journey with this TV is now at an end and I will be stripping it for parts and putting the rest in the WEEE/eWaste.

I did however manager to get one of the two pairs of speakers working that I also had come in from a classroom so 33% success rate today!

[howardc64]

RIP LG TV

My repair journey with this TV is now at an end and I will be stripping it for parts and putting the rest in the WEEE/eWaste.

I did however manager to get one of the two pairs of speakers working that I also had come in from a classroom so 33% success rate today!

Before you give up, I see signage use for 10 years which likely mean a lot of hours on the backlight. Do you have a LED tester? And besides testing on the contacts on the LED strips, try testing it from the other side of the connector on the vertical strip and maybe all the way back to the 2 serial circuit. If you see variable/different voltage, then there is resistance across those connections. I recently fixed a randomly flickering backlight with this problem ( link ) First time I've seen that.

[Davi.p]

Can you explain a bit? have you fractured the panel?