LG 42LN5400 - No Backlight

[tekz]

Hi there, the backlight on my TV has failed (LG 42LN5400). Sound works but no image unless I shine a torch. The LED's don't seem to flash when turning on.

On standby the power LED is red like normal, but when turning on it flashes 3 times.

This started while using my chromecast to play music, looked up and the image was gone.

LED voltages below power on (not sure if my meter is cheap but doesn't stay on a fixed voltage when taking these, fluctuates around by 10 or 15v constantly):

Pin 1:
Peak ~ 100
10 secs roughly ~100

Pin 3:
Peak ~ 140
10 secs roughly ~140

Pin 5:
Peak ~ 140
10 secs roughly ~140

Pin 7:
Peak ~230
10 secs roughly ~ 230

They basically stay the same, although fluctuate around by 10 or 15v as I said. When I check them with the LED connector disconnected I only get a reading from pin 7 of 230/240v.

I am planning to inspect the LED's tonight, never done any of this before but from google vids looks like it will take a while.

Is this likely to be the LED's?

[neilc6]

Re: LG 42LN5400 - No Backlight

If the string has an open LED, the backlights wont flash.

https://techhometravel.com/lg-ln-ser...ed-backlights/

Without a LED tester and with no flash of the backlights, you can check every individual LED with a multimeter in diode test mode but need to scrape the solder mask.

[tekz]

Re: LG 42LN5400 - No Backlight

Thanks NeilC6, may end up trying that, it's an old TV so I want to try to pin down what's wrong before spending money on fixing.

Just measured voltages on the P201 port, below, do they look ok?:

STANDBY:
Top:
GND: 0
12V: 0
12V: 0
GND: 0
24V: 0
GND: 0
3.5V: 3.5V
3.5V: 3.5V
PWR-ON: 0

Below:
GND: 0
N.C. 0
12V: 0
GND: 0
24V: 0
GND: 0
N.C: 0
PDIM: 0
DRV-ON: 0

TURNED ON:
Top:
GND: 0
12V: 12V
12V: 12V
GND: 0
24V: 24V
GND: 0
3.5V: 3.5V
3.5V: 3.5V
POWER-ON: 3.2V

Below:
GND: 0
N.C.: 0
12V: 12V
GND: 0
24V: 24V
GND: 0
N.C. 0
PDIM: 1V
DRV-ON: 3V

[tekz]

Re: LG 42LN5400 - No Backlight

I've got the TV apart and exposed LED strips, any guides on how to test with a multimeter in diode mode? LED strips look fine, no burn marks.

[tekz]

Re: LG 42LN5400 - No Backlight

I think my multimeter diode mode is too lower wattage at 2.8V/1.5mA. Not sure where to go from here apart from ordering an LED tester. The LED's themselves don't seem to have anywhere to test individual LED's.

I have seen this on Amazon, would this work? https://www.amazon.co.uk/Digital-Com...14&sr=8-6&th=1

Added some pictures (not sure where the marks on the LED panel came from that look like some type of liquid, there are no marks anywhere on the TV apart from here and that area is totally sealed so I'm guessing must of been in the factory.

[neilc6]

Re: LG 42LN5400 - No Backlight

https://techhometravel.com/testing-l...klight-strips/

[tekz]

Re: LG 42LN5400 - No Backlight

Thanks neilC6, to confirm, I just scratch paint away either side of the LED's and test?

[neilc6]

Re: LG 42LN5400 - No Backlight

For the LN strips, scrape to the right of each LED and not on both sides. Little trickier to find an open LED since you have to isolate it.

[nomoresonys]

Re: LG 42LN5400 - No Backlight

Looks like you have 10 across so three 9 volts should do it or scrape them all: https://www.youtube.com/watch?v=1n-Nq8HLK20

[tekz]

Re: LG 42LN5400 - No Backlight

Hi all, I scraped and tested and I found one LED that refuses to light up, at the far end of one of the strips. Is it more practical to replace the whole strip as I don't have any equipment for changing LED's?
Any good sources for a LED strip quick delivery to UK?

Just to add a quick question for educational purposes, a stuck open LED means it failed in such a way that it opened up the circuit and now there's too much load on the rest of the LED strip circuit and the power supply is detecting excess voltage and turning off as a safety feature so the others don't blow?

[nomoresonys]

Re: LG 42LN5400 - No Backlight

With 7 years on those strips, probably best to replace all the strips, you could just replace one strip but usually when one led goes more follow close behind. If you didn't mind the job so much then you can save some money by just replacing one strip, be prepared to probably do it again tho, whatever you choose be sure to turn the backlights down to as low as you can tolerate to extend led life.

[tekz]

Re: LG 42LN5400 - No Backlight

That's a good point I had considered, I sourced one strip for £12, there were complete sets of 10 strips available on eBay for around £40. I'll just replace this strip, see if my repair is successful and consider replacing more or buying a new TV if more fail. I did just purchase a new TV 6 months ago so don't fancy buying a new one just yet but this has been a good learning experience so far which was the reason for half the effort (and to save money as the TV is mostly used as a YouTube player via Chromecast.

It's coming on Friday hopefully, will update with how it goes. Thanks for all the help all.

[neilc6]

Re: LG 42LN5400 - No Backlight

Just to add a quick question for educational purposes, a stuck open LED means it failed in such a way that it opened up the circuit and now there's too much load on the rest of the LED strip circuit and the power supply is detecting excess voltage and turning off as a safety feature so the others don't blow?

Yes almost every manufacturer (other than Samsung) have protection circuitry that shut down the backlights in case of shorted or open LEDs. The typical symptom is the backlights flash and the TV stays on but with no backlight. Sony and Panasonic will shut down completely and flash a diagnostic blink code.

Samsungs don't implement protection shutdown for the backlights so the LEDs will continue to fail until the screen gets too dim, a string goes open or the power supply gets damaged.

[tekz]

Re: LG 42LN5400 - No Backlight

That's interesting because while I was testing the LED connector at the power board a few times the lights did flash when testing PIN 7 (could see it light up through the hole the cables go through and off again). I suspect this was due to the multimeter lowering the resisting briefly, just enough for the circuit protection to not kick in before the rest of the bulbs lit up for a second, perhaps because this bulb was at the end of the a row near the bottom of the stack. I couldn't reproduce it however but happened a few times incidentally.

[Trevtron]

Re: LG 42LN5400 - No Backlight

If you want to check your repair before spending ££ you could just short out the open circuit LED thus allowing the current to flow once again to the rest. (With it being at the end of a strip you might not even notice much difference in the image). If that works then you at least know that is definitely the fault AND you have the use of the set until the new strip(s) arrive - if you don't mind opening it all up again.
Be VERY careful not to nip any edges when replacing the screen back into the frame!

[tekz]

Re: LG 42LN5400 - No Backlight

It's all still in parts on a table, as I understand it I can rip the LED off and solder the circuit closed? I do have a soldering iron but I've never used it before, purchased it to start to repair broken stuff but not got around to practising yet.

[neilc6]

Re: LG 42LN5400 - No Backlight

...you could just short out the open circuit LED thus allowing the current to flow once again to the rest. (With it being at the end of a strip you might not even notice much difference in the image)

You can not short the bad LEDs in models that have protection shutdown. This will cause undervoltage and the backlights to flash.

[Trevtron]

Re: LG 42LN5400 - No Backlight

It's all still in parts on a table, as I understand it I can rip the LED off and solder the circuit closed? I do have a soldering iron but I've never used it before, purchased it to start to repair broken stuff but not got around to practising yet.

Yes, this is correct. Remove the suspect LED and put a blob of solder across the contacts.
Bypassing one LED is unlikey to trip the protection circuit.

[tekz]

Re: LG 42LN5400 - No Backlight

Replaced the LED strip and TV is now working, thanks all, £12 fix

[neilc6]

Re: LG 42LN5400 - No Backlight

If you want to increase the chances of the other LEDs not shorting out as well, set your backlight level to 55 and turn off eco sensor. Do this for ALL inputs.