LCD backlight repair

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  1. pishta

    pishta I know I'm right....

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    Many new TVs use LED backlights in strips and these are not bulletproof as it may seem. OLED TVs do not have backlights as the O is organic, ie part of the display itself. Little tiny leds in the screen itself. Very slim LED tvs (Ie. LG) use a strip on the bottom that is a continuous row of rectangle LEDs on a aluminum bar shining across the panel. Anyway, back to the subject: usually the screen is LCD glass placed over 2 panels of thin white diffused plastic and then over the 2" deep light box made of reflective white plastic and rows of equally spaces bright white LEDs under round diffuser lens. These are usually on strips of white ribbon tape with a small gold test point flanking each LED. You can use a multimeter in continuity mode to probe these 2 points to make each led glow dimly (make sure polarity is correct as a LED is still a diode).
    . If your TV does not turn on, power light flashes or has only audio with images visible on the dark screen using a flashlight, your backlight may be bad. These are set up in series so just like a string of Christmas lights, if one led fails, it takes the whole stip out and can take the whole array down. My 43" example had 5 strips of 10 LEDs for 50 total with a total draw of about 37V per string, 3.7v per LED. My power supply had 3+2 37 volt feeds off to the strips. All my 12, 5 and 3.3v power levels on the power board checked out on the labeled plug twat points so I was certain the strips themselves were bad. You can bypass the bad leds with a lead across the 2 test points and probably won't even notice 1 or 2 out but I found 4 bad on 3 different strips. I'm not sure the power board will continue to feed 37v to a bad strip for safety sake and since there are only 2 circuits (3+2 on my sharp 43") both shut down. You can unsolder the bad leds and replace with super bright 3.7v white leds or the stock flat chip style but it will be a little harder to resolder the chip style as they are surface mount with the solder points under the chip. The plastic diffuser lens on each LED snaps off with a pair of pliers and will need to be glued back on with its 3 little feet as glue points as the LEDs do get warm and will not hold if glue is right on led face. Good luck. It's worth a looksie on a LED TV that has already failed and is heading for the next E-Waste event. Good luck!
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    • my5thmopar

      my5thmopar Life Long MOPAR Owner FABO Gold Member

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      So that's what's wrong with my Sharp 43". Been a good TV otherwise. It is on the floor near the door waiting to go to the e-waste. Craig
       
    • pishta

      pishta I know I'm right....

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      yeah, that pic is using an external LED strip power supply/tester to manually power up the arrays. The power board may not light these as it detects a voltage drop and goes into protection. Also some sites say a single LED will jack the whole strand...im thinking maybe the PS does that all by itself seeing these work with on external power with the 2 OOS like this...
       
    • MoparMike1974

      MoparMike1974 FABO Gold Member FABO Gold Member

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      I have had a couple tv's and computer monitors fail because of the back light. Otherwise worked fine. Seems to be the weak link in most of them along with the cheap capacitors they use.
       
    • 67Dart273

      67Dart273 FABO Gold Member FABO Gold Member

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      Pishta where do you find any documentation on this stuff?
       
    • pishta

      pishta I know I'm right....

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      Success! Replaced the 4 and we got light, and a functional TV again. This was saved from a customer that was tossing it and it was bigger and newer than the one I had in the kids room so blammo. One issue I found is that the LED;s are specific in shape to diffuse properly in the little diffuser lenses. I put an edge mount LED in place (all I had in the bin) and they are not the same shape so I have 4 brighter spots on the screen now. The LED's are 1W/3V 3 millimeter square surface mount, (3030?) no lens. They shoot up into an inverted cone in the diffuser lens and then radiate the light out the side, no direct illumination to the LCD panel.
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      They obviously are polarized so you need to make sure you got the right orientation: the anode/cathode pads are different sizes and that matches the solder mask so it should be easy to see its direction. There are 2 little test points outboard of each LED, a DMM in continuity will make the LED barely glow if its good. Be careful, the test points were opposite what the orientation of the LEDs are for some reason, I installed all 4 backwards the first time going off the test points as my LEDs didnt have the same solder pad shapes of the originals. The LED's themselves are bought in strips of 100 for about 8 bucks.
      @67Dart273 the info was found on youtube of all places. Seems a popular repair for the crafty folks, then looking up the replacement parts and finding the specs of the LED's themselves. The feeds were 37V so i figured the forward voltage of the LEDs were 3.7v as they are in series, strips of 10. You can buy them in continuous ribbons with cut points between LED's for a custom array. Hardest part was getting everything apart (30 bezel screws and snap tabs) and then aligning the 2 diffuser sheets onto the lugs under the very fragile LCD plate glass that's still hinged to the chassis by the 4-6 ribbon cables. Everything snaps together then you screw the bezel down to lock everything in place. You probably need a hot air station to resolder the SMD LEDs but someone laid the strip on a hot plate (they make such things!) and was able to melt the solder from the back as the ribbon is metal backed for heatsink duties. Seriously, if you get PLUTO stream (free), you can cut the cord as there are hundreds of channels on this stream, just use an outdoor antenna for local news if your in range. I was watching Africa Cup soccer last night then the at home college boy was streaming his youtube music playlist to it and hooking up the ol' PS3 to it for some midnight NHL15 action.
       
      Last edited: Mar 2, 2021
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