Defective Replacement LED Strips?

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  • packrat79
    Member
    • Mar 2017
    • 17
    • Canada

    #1

    Defective Replacement LED Strips?

    A few weeks ago I picked up an LG 55LN5700, the backlight would stay on just long enough to display the LG logo and then shut off. It seemed like a pretty obvious backlight failure, and I've serviced a number of similar LG sets with good results, so I went ahead and ordered a new set of backlight strips (from China of course, and had to wait almost a month). I installed them all and ran the set for several hours, and all seemed good. I sold the set to a customer and a few days later he told me the picture was randomly shutting off. So he brought it back and it started doing the same thing for me - sometimes after several minutes, sometimes within a second or two of turning the set on. Thinking that it couldn't possibly be the backlights again, I spent a good deal of time trying to bypass the overvoltage / current protection pins on the LED controller chip... thinking that perhaps the new LED's may be using slightly more or less current and triggering shutdown. I had seen something similar many years ago with a Zenith CRT set; an aftermarket flyback transformer was just far enough out of spec to trigger the HV shutdown, and the circuit had to be modified to get around this. Anyway, after much experimenting, the backlights began to shut off more and more frequently and finally failed altogether - just flashing for a fraction of a second on startup. So I took the TV apart again and found at least one LED burned and another opened. Every time I tried to jump a defective one and power the circuit (with a DC power supply adjusted to slightly less than normal operating voltage), another one would instantly pop. I soon realized that all the strips in this circuit (6 strips in all) were toast and would have to be replaced.
    Fortunately the customer hadn't paid me yet so I didn't have to refund his money (I do give credit to a certain few trusted customers), however he really liked this TV so I want to get it working.... otherwise I would huck it out into the snowbank. I ordered a new set of LED's from a different (Chinese) vendor, they weren't terribly expensive so I'll end up losing more time than money on this one. But I thought it was worth getting some advice before I install more new LED's. Has anyone else had brand new strips that were defective? I can't see this being a power supply issue since most BL inverters have ample overvoltage and current protection.
  • comptek
    Badcaps Veteran
    • Aug 2013
    • 606
    • uk

    #2
    Re: Defective Replacement LED Strips?

    Could of been just a bad batch, most of the time they are not fake as such,
    For example Sony or LG would get companies to make the led strips for them and after spending money for jigs supplies etc they get shafted and they go else where in the mean time the machines and supplies are surplus so they produce them without the original markings
    Check voltages as well as they may have sent 3v instead of 6v

    Comment

    • neilc6
      Badcaps Legend
      • Nov 2015
      • 1550
      • Canada

      #3
      Re: Defective Replacement LED Strips?

      After backlight replacement, I always turn off eco sensor/energy savings mode, turn down the backlight level down to 55% on all inputs and apps and instruct the new owner to be aware of how bright the screen is and check it if it looks bright.

      Comment

      • budm
        Badcaps Legend
        • Feb 2010
        • 40746
        • USA

        #4
        Re: Defective Replacement LED Strips?

        Did you look at the P/N printed on the old strips and the replacement strips?
        Never stop learning
        Basic LCD TV and Monitor troubleshooting guides.
        http://www.badcaps.net/forum/showthr...956#post305956

        Voltage Regulator (LDO) testing:
        http://www.badcaps.net/forum/showthr...999#post300999

        Inverter testing using old CFL:
        http://www.badcaps.net/forum/showthr...er+testing+cfl

        Tear down pictures : Hit the ">" Show Albums and stories" on the left side
        http://s807.photobucket.com/user/budm/library/

        TV Factory reset codes listing:
        http://www.badcaps.net/forum/showthread.php?t=24809

        Comment

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