LG 47LN5400 TV Backlight problem

[SurrealMustard]

covering bases

So just to recap, what kind of Leds are recommended for these sets? Also, can they be tested without a bench power supply?

[RobbieC]

Re: LG 47LN5400 TV Backlight problem

Have you worked your way along all of the strips confirming they all light up ? I had a similar issue and while many of the led's failed in shorted mode, one failed as open circuit with a noticable burn on the lens. As it was open it prevented the whole string from lighting. My quick resolution was to short it which meant the rest then illuminated.
Not a great fix and has since died again (yet to determine whether additional led's have failed or my short)
Also, thought these strips were 3535 leds although no-one seems to know full spec so will be very much trial and error

[NormL1]

Re: LG 47LN5400 TV Backlight problem

I have a LG 47LN5400 that I am attempting to repair. The Power supply board is PCB: EAX64905501(2.2). Along with many others I have found there is an image when you use a flash light and audio sound but the tv does not have back light. I Completed a careful disassembly using a youtube video https://www.youtube.com/watch?v=Bf5T2wwtydY. I found that some of the LED's were defective. I purchased some LED's off of Ebay by searching SMD 3528 ultra white light diode. I replaced the LED's that were shown to be faulty using the techniques in the youtube video. Placing 3v I could trigger on one LED chip. With a 9v battery I could test 3 LED's in series. and with 24v I could test a full strip. I put the tv together and tried it. A quick flash of backlight then black followed by audio. I retested the led strips and found more LED's on that sring were faulty. I replaced the faulty LED's but after retesting I only had more shorted LED's. I switched my attention to the Power supply. All primary voltages are good at P601-P0608. On the secondary side I have have replaced C802,C0801, C0805 and C0806. I tested D253 and D252. Replaced C257 and C258. I have replaced Led driver IC801 and Mosfets Q803 and Q806. Still my Back light issue persists. Does anyone have a schematic or know where I should look now for the fault?
Thanks

[jason123]

Re: LG 47LN5400 TV Backlight problem

New here and would appreciate the help. I am running into the same problem as 99% of the LG TV owner population.. Sound ok, but no picture. I believe one or more of the backlight LEDs are shorted. I would like to replace the LED(s), but doesn't anyone know the specs or part number of the actual LED part number? My TV is the LG 47LN5400.
I have seen sites that sell repaired strips for more than the TV is worth, so I'm trying to replace the LEDs myself.

[JBP]

Re: LG 47LN5400 TV Backlight problem

Ok thanks will check those next.

[budm]

Re: LG 47LN5400 TV Backlight problem

You have open circuit in the LED strings.

[JBP]

Re: LG 47LN5400 TV Backlight problem

Yep spikes to about 130 on 1 and 3 then goes back down to 78 . The second pin goes to about 70 DVC then back down to 21 . Can't send pics yet keeps timing out.

[budm]

Re: LG 47LN5400 TV Backlight problem

From left to right pin voltage is and this is based on 200 DC setting on multimeter 78.5 ,21.6,78.5,0

Put the meter on the connector, then hit the power switch to turn the TV ON, you should then the see the DCV Voltage jumps up high and then settle down to around 70VDC or so when the protection circuit kicks in.
It is more likely that you have bad LEDs.

[JBP]

Re: LG 47LN5400 TV Backlight problem

From left to right pin voltage is and this is based on 200 DC setting on multimeter 78.5 ,21.6,78.5,0

[sspoonmann]

Re: LG 47LN5400 TV Backlight problem

use a diode to take the voltage drop instead of just wire. that extra voltage needs to go somewhere other wise more voltage on remaining diodes will make them burn out faster.

[budm]

Re: LG 47LN5400 TV Backlight problem

Please up load good clear pictures of the power supply board (top and bottom sides) and the Voltage reading at the pins LED backlights connector (see post 4).

[JBP]

Re: LG 47LN5400 TV Backlight problem

I have a 47ln5700 LG that I am having backlight or board problems with . Ruled out bad caps on power board and output voltage seems to be 24v dc going into main board ok . Symptoms were screen went blank ( black ) turned it on and off would show LG logo then right back off . All LED lights seem to be working correctly when first turned on then right back off. T board seems to be working correctly . Have tried flashlight test , but nothing seems to be showing .Don't know whether to replace power board , which seems to be outputting correct voltage or replace CPU ( main board ) ?

[budm]

Re: LG 47LN5400 TV Backlight problem

"Measuring them one way the DMM reads .780 volts, measuring with the opposite polarity the DMM reads Open but the LED will light up." The LED has built-in protection diode connected in anti-parallel..

[hendricks026]

Re: LG 47LN5400 TV Backlight problem

Hey Abduct,

I've got two of these 47s with the same issue. Im not sure if you took off every diffuser but since I had one good LED string I took apart the bad string and substituted one strip of the good string for one from the bad. (I peeled the LED strips off the back shell and pulled them apart into the individual strips that each had 5 LEDs) With the good string complete with 1 strip from the bad string I would turn the TV on and see if the LEDs flashed, if they did I'd consider the strip from the bad string to be okay and then I would move on to the next one.
This allowed me to narrow it down to one of 5 LEDs instead of 1 of 30.

I was going to try and replace the LED for a new LED but am somewhat confused when I take readings of the LED. I tried using the 'diode test' setting on my DMM and tried the leads both ways (since I dont know which side of the LED is positive and which is negative). Measuring them one way the DMM reads .780 volts, measuring with the opposite polarity the DMM reads Open but the LED will light up. Is it safe to say that the forward voltage on one of these LEDs is .780 volts or is there something going on that I dont get?

[abduct]

Re: LG 47LN5400 TV Backlight problem

Thanks for all the help guys.

It was easier than I thought to strip the TV down to the LED backlighting and I used my DSLR camera with extended exposure to figure out which LEDS were broken. I had 2 LEDS shorted closed, and 1 LED shorted open. The open led caused 1 entire LED channel to fail causing the PSU to shutdown.

Instead of buying new parts I simply scratched the solder mask off the PCB and soldered in a piece of wire to complete the circuit bypassing the broken LED's and using epoxy to secure the connection/insulate it. With the diffuser in place you can hardly tell there are broken LEDs anyways unless you are watching something with allot of white or black, then again it just looks like normal light bleed anyways and doesn't bother anyone.

TV works like new, and I am going to apply similar methods to the TV's that have the same problem.

Thanks again.

Edit:: As a final question, is there an easier way to tell which LED is causing an open in the circuit? I had to pop open a handful of the diffuser lenses before I found one. None of the LED's looked burnt or damaged. Is it just luck of the draw I guess if you start popping them open at random?

[newtothis]

Re: LG 47LN5400 TV Backlight problem

https://www.badcaps.net/forum/showth...t=diodes+5+led

[abduct]

Re: LG 47LN5400 TV Backlight problem

55" may give out different voltages for the LED's as there is probably more in the 55". So what would shut down the 47" PSU may not shut down the 55" PSU.

That makes sense, but why would the normal 47" PSU be failing. These were all returned to the store, but I can't see all the TV's having a bad PSU causing similar or the same symptoms.

Is there a certain process I should take to confirm all the output voltages from the PSU as well as specific parts? I have checked the fuses and visually inspected for damage, but beyond that I haven't checked many of the parts.

What confuses me is if a LED did short or become open, which is causing the 47" PSU to shut down due to safety settings, why is the 55" PSU not doing the same.

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Edit::

I just measured the resistance of the LED strips, since LED's are diodes and they are in series (I would believe) One direction would have extremely high resistance, the other direction extremely low. With knowing this LED CH1 has 50k ohms in one direction, and 1m ohms in the other, and LED CH2 has 30k ohms in one direction, and 50-60k ohms in the other. This would mean that LED CH2 probably has a short somewhere (I would imagine). Now is this 55" PSU smart enough to detect this and shut that channel down to keep operations going, or what.
Can I place a diode+resistor in place of the shorted LED strip to trick the 47" PSU into thinking it's operational?

I am not looking to resell or sell these TV's they are just for my personal use so any hacks to get them working are welcome.

[newtothis]

Re: LG 47LN5400 TV Backlight problem

55" may give out different voltages for the LED's as there is probably more in the 55". So what would shut down the 47" PSU may not shut down the 55" PSU.

[abduct]

Re: LG 47LN5400 TV Backlight problem

you have a bad led string.

Is there a way to bypass this? Why does one power supply successfully make the TV work, and the original one does not?

Is it possible to cut the traces leading to the broken LED strip and solder a resistor onto the PCB tricking the IC into believing it is functioning properly?

I've gotten around to my other LG 47" which had similar symptoms and placing the 55" PSU in it as well fixed the problem, so that makes 2 TV's.

Is there steps to test all the outputs to the mainboard + LED strings? What kind of voltages should I be seeing on them. Can I test the power supplies without the main board plugged in, or does it need to be plugged in?

[CapLeaker]

Re: LG 47LN5400 TV Backlight problem

you have a bad led string.