LG 50LN5100-UB LED Back light Out / Repair

[Doc38343]

LG 50LN5100-UB LED Back light Out / Repair

I noticed a few threads for same issues on this model. I decided to repair this one for the experience. It is another trash rescue.. (bring me your garbage LOL) but I'll be glad to keep it out of the ground water. Thanks for any input to help me keep things moving along faster.

OK voltages are steady and perfect. 78.5v whether back light is connected or not. Note to the strange.. I have a cheap little LED back light tester that when connected to the led it lighted, both side one at a time... After I connected the leds back to the power supply it came on and worked beautifully for about 30 min then it blacked out again.
I now suspect a component failure on the led control/ connector strip or a bad solder joint on an led itself. The voltage pull did not change at all, ready steady 78.5v.
I will photo document this one for future guests....

QUESTION: On one of the previous threads the fellow said he removed the back panel rather than disassembling the screen panels from the front. If someone could advise me on this method I would appreciate it.

Thanks!

[dskall]

Re: LG 50LN5100-UB LED Back light Out / Repair

Your led tester does not test under a high load. Only about 30ma, but when driven from the tv they are driven a lot harder. You could connect tester and move connections to see if bad strip connection. Answer to your question. There are numerous ways to remove lcd panel but anyway you do it the panel has to come off. Whether you lay it face down, face up or on it's edge

[budm]

Re: LG 50LN5100-UB LED Back light Out / Repair

That 78V reading on the LED connector is reading you are getting after the LED Voltage boost converter went into shutdown mode so you are only seeing the feeding Voltage to the boost converter.
If you put black probe on the chassis and red probe on the LED+, keep the probes there then turn on the TV then you should see the Voltage jumps up to over 120Vdc for seconds and then drops back down to 78VDC.
The drive Voltage for the LED with load is 110VDc as printed on the board.

[Charleyd]

Re: LG 50LN5100-UB LED Back light Out / Repair

I'm the person who posted about getting access to the LEDs by removing the back of the TV instead of disassembling from the front. I have only found this to be possible with LGs thus far. The beauty of this approach is that you avoid the risk of breaking the LCD panel and the fragile flexible connections to the panel. After removing the back plastic cover from the TV, it makes the work easier if you remove the PS, Main, and T-Con boards from the TV, but as a minimum you need to remove the speakers and disconnect the two ribbon cables between the T-Con board and the narrow circuit boards along the bottom of the TV.

Lay the TV face-down and remove all of the small black screws around the perimeter. A few of the screws are partially covered by frame plastic clips -- it makes it easier to get the screws out and back in if you shave a bit off of the edge of the plastic clips with a small utility knife.

Then stick toothpicks between the plastic frame and the metal to disengage the clips holding the frame onto the metal panel and to hold these clips open. This includes the clips all along the bottom edge of the TV. You can see where the clips are located by looking along the edge of the metal panel.

With the clips all released, starting at any corner gently lift the metal panel. If you have trouble getting hold of the metal panel, grab along the edge with a pair of pliers. This part is easier if you have a helper. Otherwise, whenever you let go of a corner, it will drop back into the plastic frame (after the toothpicks have fallen out) and get locked back in. As the metal panel comes free, lift it away, turn it over, and set it aside to repair/replace the LEDS. Replacing with new is the best approach as once the LEDs start dying, more will be ready to fail as they all have the same amount of time on them.

With the LEDs taken care of, turn the metal panel back over and drop it into the front assembly, pressing down to re-affirm all of the plastic clips. Install the small perimeter screws and you're ready to replace the boards, speakers, and the rear plastic cover.

LG recommends that you install the latest TV software to minimize the risk of repeat LED failure. I also go through all of the TV's Picture settings and turn down the Backlight and warn the owner about changing this setting.

[Doc38343]

Re: LG 50LN5100-UB LED Back light Out / Repair

Hey thanks guys. Familiar with the term FUBAR? LOL I'm glad this was a trash pic. Here's how I screwed it up good. I took off the screen down to the LED's. Soldered the LED back and got it working. HOWEVER.... in the process of taking the screen apart I stupidly (hope others learn from my screw up) took the screen apart in two pieces, removing the narrow circuit boards from the frame and then the screen. I then removed the remaining panels together. Apparently in this process I screwed up the screen.
2 issues I didn't realize:
1 I should have just un-snapped the secondary plastic frame that had the thin circuit strips and the screen in it and then removed it as a whole unit.
2 I think i may have let the screen bend too much in the whole process.
Either way i have a great back light and a streaked up jacked up screen. (to make me sicker it had a perfect picture when it worked for the short time i mentioned earlier.

I wish I had gotten these posts earlier or had waited but hey I was in a hurry...
I now see how Charleyd method could have worked just fine and i would now had a sweet picture instead of just audio and streaks.. sorry to be a disappointment to you guys but live and learn right? I may try some things or just wait for another one like it to show up.

[budm]

Re: LG 50LN5100-UB LED Back light Out / Repair

Too bad! one thing for sure is that using LED tester cannot be rely on 100% due to the fact that the LEDs are driven by very low current from the LED tester.

[Doc38343]

Re: LG 50LN5100-UB LED Back light Out / Repair

Too bad! one thing for sure is that using LED tester cannot be rely on 100% due to the fact that the LEDs are driven by very low current from the LED tester.

This one actually automatically cycles from low to maximum current draw. If you leave it on too long it will eventually burn out the led because it dosen't stop increasing current until it peaks. Voltage starts at 300v very low current. As the current increases the voltage decreases. Works a charm. I don't like to power a full array with it but it has no problems powering 3 rows of LEDs to maximum bright. A single stick of leds on that TV is 28.2 V two joined in middle is 55.4 v the bad row was off a little I forgot the exact but it was around 62 I think. Then I spotted the burnt out LED. It was a broke solder joint under it but i fried it trying to clean the corrosion. I just jumpered the gap on the strip since i am going to use this one my self in my shop.
UPDATE ON SCREEN:
I took the plastic molding front back off and then the bottom metal strip so I could get at the ribbon boards. I simply ran my finger along each ribbon where it joins the board and VIOLA when i tested I had my beautiful picture back. So I shut it back down and put some scotch tape over each one.
Wow what a learning experience this is turning out to be. LOL
Will post pics if it works when reassembled.

[Doc38343]

Re: LG 50LN5100-UB LED Back light Out / Repair

OK final update... SUCCESS!
OK what it turned out to be was as follows. I put the tape on and then put the metal strip back and tested.. it was screwed up again. I took the metal strip back off and it was still messed up so i tried touching each strip to see if any changes. Then I pushed in on the clip where the ribbon cable goes in on the left side one facing back and it changed so I fiddled with it some and it kicked in good but then messed up when i let go. Now I had already cleaned and examined the ribbon cable so i took a magnifier and looked down into the clip jack it self and there was a speck of foil or a tiny piece of solder. I blew the jack out and re-seated the ribbon cable and it works fine now.
I did include pictures. I am very happy to have this success, I couldn't sleep last night for thinking about how i ruined a sorta good 50 inch TV... LOL
It was stubborn but I am more stubborn.
Included a few more pics. I hope all this helps someone else in the future.
Doc

[budm]

Re: LG 50LN5100-UB LED Back light Out / Repair

This one actually automatically cycles from low to maximum current draw. If you leave it on too long it will eventually burn out the led because it dosen't stop increasing current until it peaks. Voltage starts at 300v very low current. As the current increases the voltage decreases. Works a charm. I don't like to power a full array with it but it has no problems powering 3 rows of LEDs to maximum bright. A single stick of leds on that TV is 28.2 V two joined in middle is 55.4 v the bad row was off a little I forgot the exact but it was around 62 I think. Then I spotted the burnt out LED. It was a broke solder joint under it but i fried it trying to clean the corrosion. I just jumpered the gap on the strip since i am going to use this one my self in my shop.
UPDATE ON SCREEN:
I took the plastic molding front back off and then the bottom metal strip so I could get at the ribbon boards. I simply ran my finger along each ribbon where it joins the board and VIOLA when i tested I had my beautiful picture back. So I shut it back down and put some scotch tape over each one.
Wow what a learning experience this is turning out to be. LOL
Will post pics if it works when reassembled.

The max current that LED tester can put out is only around 30~50mA range (the typical one sold on EBAY and at shopjimmy), the TV runs them at 400mA full tilt so they burn up the LED fast.

[Pukehead]

Re: LG 50LN5100-UB LED Back light Out / Repair

Hi everyone. New on here. I have the same TV and issue. I originally replaced the single LED and had to use jump wires to complete the strip because it was so burnt. It was the last led on (R1). TV worked for 5 Mins. I had the menu up and then it went black. I then went and bought a brand new strip (R1). Turned on TV. It just flashed and didn't even come on. Any idea?

[budm]

Re: LG 50LN5100-UB LED Back light Out / Repair

Do the exact test as shown on post 3 but add the test on the LED - pins to the test too, so 4 tests total.

[Pukehead]

Re: LG 50LN5100-UB LED Back light Out / Repair

Ok Thanks,
I will try it tonight.

[Andrew F. Ali]

Re: LG 50LN5100-UB LED Back light Out / Repair

Just change out ALL the strips. Every single pair. I have tried 'used' strips before as I could not get them 'new' and I've had no complaints thus far. Like two years and counting.

[Pukehead]

Re: LG 50LN5100-UB LED Back light Out / Repair

Ok. So i got it up and running. After testing each LED on every strip, I found there were a couple other (3) leds that where no good also. I didnt have enough R/L strips to cover all of them. I replace the actual LED and then glued the lens back on. Only problem is i now have one hot spot on the screen.

Lens back on wrong or its the only new led in that are. Im not sure. I hate to keep tearing this thing apart. afraid the screen is going to break . especially those ribbon strips.

[DXseekerMO]

Re: LG 50LN5100-UB LED Back light Out / Repair

If the hot spot is a concentrated dot on the screen the diffuser lens probably fell off. Hear anything rolling around in the bottom of the screen/display area?

I did backlights in a 47ln5600 once where I glued the diffuser lens down with Krazy Glue, put it back together and had a hot spot appear over the next 30 minutes. Turns out, with the legs of the diffuser being close to the center the KG was outgassing in to the center of the diffuser and clouding it up, causing the hot spot. I now use epoxy for this reason.

After re-gluing diffuser lenses I place the diffusion sheets back in the TV and turn it on. Just make sure the FFC cables are not installed in the T-CON, if they touch chassis ground it's bad. This way you can get everything re-adjusted and check to make sure its right before you put the display panel back in.

[Pukehead]

Re: LG 50LN5100-UB LED Back light Out / Repair

I would say its a cloudy whiter spot maybe 4-5 inches in Dia. I dont hear anything rattling around. I need to see them without the screen i guess. I wasn't sure how to test with the screen off. Last time i couldnt see them. The led's would not come on unless i plugged some wires in. I thought the T-con had to be plugged into the screen for some crazy reason. Anyhow thanks for the tip. I might disassemble it one more time. LOL.