About LED backlight…

Collapse
X
Collapse
+ More Options
Posts
 
  • Time
  • Show
Clear All
new posts
  • Behemot
    Badcaps Legend
    • Dec 2009
    • 4845
    • CZ

    #1

    About LED backlight…

    So I got this korean 27" wonder with LG display. No backlight. There are two backlight aluminum strips with 36 SMD diodes on each, in three channels (so 12 LEDs per channel). It is powered by 24V brick, I got 24 V on each channel or about 2 V on each diode. I disassembledthe panel to get the acklight off. Tested each diode, each one of them produced ligt being powered by multimeter. I measure those about 2 V on each single diode when it is plugged in.

    So why the hell I got no light even when there is voltage on the diodes?!
    Less jewellery, more gold into electrotech industry! Half of the computer problems is caused by bad contacts

    Exclusive caps, meters and more!
    Hardware Insights - power supply reviews and more!
  • stj
    Great Sage 齊天大聖
    • Dec 2009
    • 30953
    • Albion

    #2
    Re: About LED backlight…

    2v is hardly enough for a white led, they are usually 3.5-4v

    Comment

    • ReeceyBurger123
      Never Give Up !
      • May 2014
      • 7325
      • Britain

      #3
      Re: About LED backlight…

      2v isnt enough the booster is not running to drive the series circuit.
      Please Do Not PM My Page Asking For Help Badcaps Is The Place For Advise, Page Linked For Business Reasons Only. Anyone Doing So Will Be Banned Instantly !

      https://www.facebook.com/Telford-Tel...7894576335359/

      Comment

      • Behemot
        Badcaps Legend
        • Dec 2009
        • 4845
        • CZ

        #4
        Re: About LED backlight…

        It produces light with the multimeter, what voltage does it have?

        Is it because in series it does not reach the treshold voltage so the diodes do not "turn on"?
        Less jewellery, more gold into electrotech industry! Half of the computer problems is caused by bad contacts

        Exclusive caps, meters and more!
        Hardware Insights - power supply reviews and more!

        Comment

        • ReeceyBurger123
          Never Give Up !
          • May 2014
          • 7325
          • Britain

          #5
          Re: About LED backlight…

          Yes normally on edge lit they drive them in groups of 6 or so. Each Led needs 3-4v as Stj says. If you have 24v and nothing else and all the Leds light the booster is not woking. What Psu/Led driver does it have ?
          Please Do Not PM My Page Asking For Help Badcaps Is The Place For Advise, Page Linked For Business Reasons Only. Anyone Doing So Will Be Banned Instantly !

          https://www.facebook.com/Telford-Tel...7894576335359/

          Comment

          • Behemot
            Badcaps Legend
            • Dec 2009
            • 4845
            • CZ

            #6
            Re: About LED backlight…

            The power supply is some korean brick, I started with that but it was OK. Recapped it right away.

            There are two PWM drivers, one for each strip, BIT3251. Seems like they are not getting the enabling pin pulled high.

            The SOT23 transistor which is supposed to do that for both of them is MMBT3904, a PNP one. Its base is just about 0.65 V, collector and emitor are at zero. It is driven by the Weltrend WT61P8 MCU, pin 43 (GPIO E3 share with PWM3 output).

            The PWMs enable at 1.2 V so I guess I can jumper it to one of the 1.8V regulators to check the operation?

            ADD// yes, putting 1.8 V on the enable pin makes it shine like hell

            ADD2// I've checked the resistor in front of the MMBT3904 transistor, it is 4k7 as should be. The MCU outputs about 3.3 V so that is OK, on the other side of the resistor I only have 0.69 V. I think the transistor is shot…?
            Last edited by Behemot; 11-26-2016, 01:59 PM.
            Less jewellery, more gold into electrotech industry! Half of the computer problems is caused by bad contacts

            Exclusive caps, meters and more!
            Hardware Insights - power supply reviews and more!

            Comment

            • budm
              Badcaps Legend
              • Feb 2010
              • 40746
              • USA

              #7
              Re: About LED backlight…

              So are you getting the bl-on signal from the logic board?
              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

              • Behemot
                Badcaps Legend
                • Dec 2009
                • 4845
                • CZ

                #8
                Re: About LED backlight…

                There is 3.3 V on the resistor in front of the PNP base, which is also the supply voltage of the MCU, so I would guess yes. I won't get any more of the MCU if it's fed with 3.3 V.

                If the PNP is busted it draws more current on the base which means there is higher voltage drop on the 4k7 resistor, right?
                Last edited by Behemot; 11-26-2016, 02:28 PM.
                Less jewellery, more gold into electrotech industry! Half of the computer problems is caused by bad contacts

                Exclusive caps, meters and more!
                Hardware Insights - power supply reviews and more!

                Comment

                • Behemot
                  Badcaps Legend
                  • Dec 2009
                  • 4845
                  • CZ

                  #9
                  Re: About LED backlight…

                  Correction - the transistor is NPN of course. The PNP is the complementary part (MMBT3906).

                  Replaced with MMBT2222A did not help, obviously. Being NPN, the enabling pin is actually pulled high normally, when disabling, the MCU puts voltage on base which opens the transistor to ground and pulls the EA pin down. I tested this by removing the voltage from the base (grounding the other side of the 4k7 resistor closer to the base) which also turns the boost controllers on.

                  So everything behaves normally, just that the MCU is not supposed to put the voltage on the base when the backlight is to work.

                  So, I have a bad MCU (or the program in it)?
                  Last edited by Behemot; 11-27-2016, 04:24 AM.
                  Less jewellery, more gold into electrotech industry! Half of the computer problems is caused by bad contacts

                  Exclusive caps, meters and more!
                  Hardware Insights - power supply reviews and more!

                  Comment

                  • Behemot
                    Badcaps Legend
                    • Dec 2009
                    • 4845
                    • CZ

                    #10
                    Re: About LED backlight…

                    BTW the display in question is the this wonder.

                    This thing seems to be quite problematic even though how simple it is. The ITE IT6251FN signal converter overheats quite much as it is discolored beneath it. I am not sure if it is the reason but I've been told this board has already been baked. I fail to see the reason other than people who call themselves electronics repairman are stupid pricks. They even made a video so first thing every fool does now is to bake it https://www.youtube.com/watch?v=By7hrUXJCPk

                    I think I don't have to state it explicitely, but there is NO BGA chip on this board. So if you think there is a problem with cold joint, just take micro soldering pen and go ahead!
                    Last edited by Behemot; 11-27-2016, 06:32 AM.
                    Less jewellery, more gold into electrotech industry! Half of the computer problems is caused by bad contacts

                    Exclusive caps, meters and more!
                    Hardware Insights - power supply reviews and more!

                    Comment

                    • Behemot
                      Badcaps Legend
                      • Dec 2009
                      • 4845
                      • CZ

                      #11
                      Re: About LED backlight…

                      I've jumpwired it to stay ON for testing, besides brief flash of some color stripes upon plugging in the power, it stays black than. I don't have dual-link nothing and as it has no scaler (cheaped out) I can not run it at any other resolution. But I think the MCU or the converter may be shot, especially when it has already been baked once.

                      Than I am sure if it already was (as these were sold with cheap defective panels), or I damaged it, but the last difusor foil beneath the LCD itself has defect and I think the panel is also scratched. Fortunatelly 27" displays with this resolution went down in price and they have scaler, multiple inputs and all.
                      Less jewellery, more gold into electrotech industry! Half of the computer problems is caused by bad contacts

                      Exclusive caps, meters and more!
                      Hardware Insights - power supply reviews and more!

                      Comment

                      Related Topics

                      Collapse

                      • unimatrix93
                        TV backlight zener diode
                        by unimatrix93
                        Hello everyone, I'm repairing an LG TV backlight and I noticed some strange behavior when I touching the backlight strip with a missing Zener diode.

                        So I replaced the leds on the strips and they seemingly working fine, but soon as I touch the strip which has a missing Zener diode the whole backlight turns off for a few seconds and then turns back on.

                        Is this caused by the missing diode? If so what kind of zener diode should I use to replace it?.
                        ​​​​
                        The led strips has 9 6V leds.


                        ​​​
                        02-10-2025, 12:13 PM
                      • triplefour
                        UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt fix!
                        by triplefour
                        this tv is throwing me for a loop. do samsung tvs no longer have the backlight self test feature built into the power supply?
                        i tested the backlight with my LED tester and it lights up both strips. one of them reads 100v and the other reads 63.7v.
                        unplugged mainboard from power supply but left backlight plugged in, and then plugged psu wall power in. expected to see backlights light up but see nothing.
                        according to how ive diagnosed in the past, this would almost always be a bad power supply. replacement power supply was cheap like 20$ from ebay. but still i get not even...
                        09-13-2023, 10:43 PM
                      • dj8452000
                        How To Test a P-channel Trench Mosfet Using A Multimeter In Diode Mode
                        by dj8452000
                        I am new to electronics and I'm understanding most of what I'm reading but I have a mosfet on my board that I've identified as a P-channel Trench Mosfet. I haven't a clue how to test this and what I should expect to see with the multimeter probes. I am hoping someone with years more experience than myself can help me understand how to use a multimeter to read this. Please look at the drawing I have attached to this and can you try to explain to me what I should be doing with my multimeter in diode mode and what results I should expect from a good mosfet?...
                        02-18-2025, 08:28 AM
                      • eccerr0r
                        old X58 triple channel board... memtest86 memory speed?
                        by eccerr0r
                        Anyone run memory speed tests with an old X58-based triple channel 1st gen or any other >2 channel board? The one I have is a Gigabyte I think.

                        Anyway I had four 4GB DDR3-1333 modules in the 6 memory slots and got around 11GB/sec memory speeds assuming that it went dual channel since it didn't have enough modules for triple channel. Now I stuck in two more 4GB modules, but they are DDR3-1600. I'm still getting 11GB/sec speeds according to memtest86...

                        Just wondering, anyone do memory speed analysis on these triple (and higher) channel memory boards? Are there any...
                        06-28-2025, 09:06 AM
                      • spleenharvester
                        Reverse engineering of 18650 4-channel tester
                        by spleenharvester
                        Hi friends

                        I have reverse engineered this tester, schematic is attached and is complete except for capacitor values. These testers annoyingly have no model number but they are the ones you find on the first page of just about any website if you type in "18650 4-channel tester". There is also a 2-channel variant and a newer 8-channel variant. They are very cheap (about £15) and can measure voltage, current, internal resistance and capacity. They have a multicolour display for all these statistics. They also have terminal blocks so that if you want to use a breakout board...
                        05-04-2025, 02:43 AM
                      • Loading...
                      • No more items.
                      Working...