VIZIO E420i-A1 no backlight

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  • triplefour
    Badcaps Legend
    • May 2014
    • 1747
    • USA

    #1

    VIZIO E420i-A1 no backlight

    It's been a while since i tried to fix a TV and im feeling a bit rusty. bear with me. i didnt see this tv listed anywhere in the forum so i figured id start a thread for it.

    what it does:
    when you first plug in the power cord the white standby light lights up for a few seconds then fades away to off.
    pressing power brings the light back on full bright but there is no backlight and i cant see any image on the screen. the light fades a little but stays lit.
    after about 10 seconds of being on, pressing power again turns it off just fine, with the standby light glowing brighter for a second before turning off

    checking the voltage on the PSU, everything seems to be there in the ON state. 24v, 12v, 5v, are all there.

    looking at the LED driver board i see 42v on the two LED+ pins and 11v on all the others (i dont understand how this LED board works. used to seeing vled+ and vled- on other boards)
    but the board is getting the right power (24v) from the psu....

    i looked around and of course cant seem to find the service manual or schematic for this board.

    im leaning toward LED strip failure as a gut impulse, and i have LED tester but since im not sure how to use it here since i dont see vled+ and vled-
    just two vled+ and then fb1-9 for the other 9 pins.

    any help would be greatly appreciated.
    Attached Files
    Don't fear the repair...
  • budm
    Badcaps Legend
    • Feb 2010
    • 40746
    • USA

    #2
    Re: VIZIO E420i-A1 no backlight

    https://www.badcaps.net/forum/attach...0&d=1511742089
    You have 8 strings of LED thus 8 SMD PWM controlled MOSFETs
    All 8 strings of the LED Anode are connected to the VLED+
    FB1 ~12 are the Cathode return for for each LED string which is connected to the Drain pin of the MOSFET.
    For example FB2 is connected to the Drain pin of the Q801.
    The boost converter takes the 24V and boost it to higher Voltage, 42V boosted Voltage seems low to me, but since you have LED tester that will also show the Vf of the LED string then you should find out the reading will show on the tester.
    So remove the LED connected and probe between the VLED+ and each FB pin to see what you get.
    We also need the pictures of the whole backside of the TV showing how all the boards are connected together. We need to see where the connector CN804 of that LED driver board goes to.
    Last edited by budm; 11-26-2017, 09:01 PM.
    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

    • triplefour
      Badcaps Legend
      • May 2014
      • 1747
      • USA

      #3
      Re: VIZIO E420i-A1 no backlight

      thanks as always, budm for taking time to educate me
      i used my LED tester (pictured in attatchments)
      and put red probe on v+ and moved black probe from each fb1 thru fb9
      on fb9 i saw 90v and the LED's lit up
      on fb8-1 i saw around 36.5 on each

      not sure what to make of this but of course i have questions...
      what does FB stand for? and why are there 9 of them if it only has 8 strings? also why are there 2 v+ ? are they the same line actually? i got same result whether using one v+ or the other

      hope this helps
      thanks!
      Attached Files
      Don't fear the repair...

      Comment

      • budm
        Badcaps Legend
        • Feb 2010
        • 40746
        • USA

        #4
        Re: VIZIO E420i-A1 no backlight

        FB = FeedBack.
        There are only 8 SMD MOSFETs for 8 strings of LEDs.
        FB9 does not have any connection and no MOSFET Q808 is installed for FB9 as you can see where the trace for FB9 goes.
        The two V+ are tied together as you can verify by checking the resistance between the two pins.
        So the Vf of the LED string is about 36V, then I expect the boosted Voltage to be higher than 42V range.
        There are two lytics caps laying on its side on that LED driver board, what are the Voltage rating as printed on the body?
        At this point it does not look like you have LEDs problem.
        BTW, what is the LCD panel model number as shown on the sticker on the LCD panel?
        Last edited by budm; 11-27-2017, 04:18 PM.
        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

        • triplefour
          Badcaps Legend
          • May 2014
          • 1747
          • USA

          #5
          Re: VIZIO E420i-A1 no backlight

          hmm if its not connected to anything, then why when i tested fb9 i get 90v and the LED lights up?

          the 2 lytic caps on their side are glued down and impossible to read the side that shows the voltage and capacitance, but at the very bottom of the caps where the wrapper folds over i can just make out that one of them says 100v and the other one says 210uf.

          edit: i added pictures of the panel stickers:
          ID: X40G042099729A
          Attached Files
          Last edited by triplefour; 11-27-2017, 05:46 PM.
          Don't fear the repair...

          Comment

          • budm
            Badcaps Legend
            • Feb 2010
            • 40746
            • USA

            #6
            Re: VIZIO E420i-A1 no backlight

            Originally posted by triplefour
            hmm if its not connected to anything, then why when i tested fb9 i get 90v and the LED lights up?

            the 2 lytic caps on their side are glued down and impossible to read the side that shows the voltage and capacitance, but at the very bottom of the caps where the wrapper folds over i can just make out that one of them says 100v and the other one says 210uf.
            Just have to trace that FB9 to see how it is connected to the LED strip some how, since it shows only 90V that means not all the strips are lit up.
            The bad part right now is that we know that the boost converter is working which is being turned on by the BL-ON, but we do not know if the dimming command is working of not. The small connector on that LED driver board should go to the T-CON board (I cannot tell from the pictures)? You can try disconnecting that connector to see if the backlights will come on at all or not.
            Last edited by budm; 11-27-2017, 05:46 PM.
            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

            • budm
              Badcaps Legend
              • Feb 2010
              • 40746
              • USA

              #7
              Re: VIZIO E420i-A1 no backlight

              I found the info about your LEDs strips.
              http://www.shopjimmy.com/tpv-tpt420h...ps-1-board.htm
              LED strips for your LCD panel TPT420H2-HVN01
              The panel has 8 LONG LED strips, 8 SHORT LED strips.
              LONG strip has 7 LED + SHORT strip has 7 LEDs so one LED string has 13 LEDs.
              So it has 8 strings of LEDs.
              13 LEDs x 3V = 39V fully driven (300~400Ma typical) (you are getting 36V because the LEDs are driven at may be only 50mA so the Vf is lower than 39V).
              My thinking now is that 42V at VLED+ is too low.
              The complete boards set is about $50~60, the LED driver board is $19.
              http://www.shopjimmy.com/catalogsear...&q=VIZIO+E420i
              Last edited by budm; 11-27-2017, 06:02 PM.
              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

              • triplefour
                Badcaps Legend
                • May 2014
                • 1747
                • USA

                #8
                Re: VIZIO E420i-A1 no backlight

                im confused. if the strips want 39v why does my LED tester stop at 36? not entirely sure how my LED tester works
                i tried disconnecting the the small connector from the led board, and it didnt change anything. that connector goes to the mainboard. there is only one cable going to the TCON and its from the mainboard, and i tried disconnecting that as well, still no change
                Don't fear the repair...

                Comment

                • budm
                  Badcaps Legend
                  • Feb 2010
                  • 40746
                  • USA

                  #9
                  Re: VIZIO E420i-A1 no backlight

                  The Vf of the LED will be less than 3V at 50mA of current flowing through the LED using the LED tester. The LEDs in the TV is driven at 200~300mA or more, lots more than you tester.
                  If you look at the LED spec Vf VS If chart you will see why that is.
                  https://cdn.badcaps-static.com/pdfs/...7b96d6549d.pdf
                  Attached Files
                  Last edited by budm; 11-27-2017, 06:27 PM.
                  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

                  • triplefour
                    Badcaps Legend
                    • May 2014
                    • 1747
                    • USA

                    #10
                    Re: VIZIO E420i-A1 no backlight

                    so basically my tester is designed to just barely power the LED's but not fully power them? ...for safety?
                    Don't fear the repair...

                    Comment

                    • budm
                      Badcaps Legend
                      • Feb 2010
                      • 40746
                      • USA

                      #11
                      Re: VIZIO E420i-A1 no backlight

                      Originally posted by triplefour
                      so basically my tester is designed to just barely power the LED's but not fully power them? ...for safety?
                      That is correct, if they try to make tester that can drive 30~50-LED string at 300mA, the power supply will be big to be able to deliver that much power.
                      50 LEDs x 3V @300mA If = 45 Watts!
                      The LED for TV is rated at 1W.
                      Last edited by budm; 11-27-2017, 06:33 PM.
                      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

                      • triplefour
                        Badcaps Legend
                        • May 2014
                        • 1747
                        • USA

                        #12
                        Re: VIZIO E420i-A1 no backlight

                        so is this line of diagnostic thinking correct?:

                        1. use led tester to test about what each string requires to light up
                        2. power on the tv with all boards connected and measure the voltage with a voltmeter between the Vled+ and VLED- (or FB in this case)
                        3. if the voltage is lower than what the LED tester showed was required, then you have LED driver board problem?


                        testing voltage with voltmeter in my case here i find that between v+ and chassis screw (or fb9) i get 42v, but between v+ and any of the other FB's im getting between 1.5 and 2.5v
                        testing voltage between any of the FB1~8 and chassis screw i get around 11v each
                        i thought the voltmeter tests the difference in voltage between two points...i would have expected a more like 30v there between vled+ and the fb's
                        (42-11)
                        Last edited by triplefour; 11-27-2017, 06:41 PM.
                        Don't fear the repair...

                        Comment

                        • budm
                          Badcaps Legend
                          • Feb 2010
                          • 40746
                          • USA

                          #13
                          Re: VIZIO E420i-A1 no backlight

                          1) You will test the Boost Voltage (this case, its the VLED + (or LED+)) feeding the Anode of the LED string when the TV is first turned on to see how high it goes up to and then see how low it goes down to after 5 seconds or so (you will have to leave the meter's probes in place. If the LED string has open circuit, the Voltage will go way up much higher than the Vf of the LED string. Most board has output Voltage with load printed on the board. When there is no load the Voltage will go up too high and will trip the Over Voltage protection circuit.

                          2) If you have more than one LED string (you have to be able to tell by looking and tracing out the LED driver output section to know that), then you will leave the meter probe between GND and the LED - (in your case its the FBx) which is the Cathode return, when you turn on the TV you should see the Voltage at the LED - pin, typical is 10~20VDC range (the is the Vdrops of the MOSFET and the current sensing resistor), if the LED string has open circuit then you will not see that Voltage on the LED-.

                          3) Most helpful info is the spec the LCD panel, it will tell you the Vf of the LED string and the current requirement.
                          If you have spare white LED for the backlights, the you can use your LED tester to test and see what Vf it will show on that built-in meter of the tester, beware that the meter may not be calibrated, so you can use that as a guide to find out what the total LEDs you have on the string.

                          In your case, we know that you have 13 LED in the LED string so the Vf at that 50 mA is about 36V /13 LEDs =2.8V per LED which is about right for that low current, some tester may do only 30mA.
                          Tester is good to find out if you have open circuit or not but not good for telling the condition of the LED since the LED may light up OK at 50mA but at full 300mA the LED may breakdown. So I do not trust LED tester 100% to tell me if the LED is in good condition or not.
                          It is the same as your meter when testing the Diode or Transistor, it may look OK using the meter but the diode will breakdown when used in the circuit due to exposing to the real working Voltage that your meter cannot put out.
                          Last edited by budm; 11-27-2017, 07:02 PM.
                          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

                          • triplefour
                            Badcaps Legend
                            • May 2014
                            • 1747
                            • USA

                            #14
                            Re: VIZIO E420i-A1 no backlight

                            ok that makes sense...but kind of frustrating. how can i tell if the LED's are ok if the meter wont put out enough to fully test them?
                            Don't fear the repair...

                            Comment

                            • budm
                              Badcaps Legend
                              • Feb 2010
                              • 40746
                              • USA

                              #15
                              Re: VIZIO E420i-A1 no backlight

                              That means you have to have a bench power supply with current limiter as the power supply source to drive the LED at high current.
                              0~60V up to 1A adjustable Voltage and current is nice to have if you are going to work on lots of TV with LED backlights.
                              Or you can buy boost converter board (that will take in 12V) that the output Voltage can be adjusted and the DC Amp meter built-in so you can at least monitor the load current.
                              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

                              • triplefour
                                Badcaps Legend
                                • May 2014
                                • 1747
                                • USA

                                #16
                                Re: VIZIO E420i-A1 no backlight

                                ok thanks...i learned a lot today. but now the tv is doing crazy stuff. the backlights come on now when i turn on the tv, but i see no image on the screen. after about 10 seconds the lights go out, and a few times after that, they flash like theyre trying to turn on but cant. measuring between ground and the FB's i get 1-2v when the lights are on, and 11v when they are off. i am not seeing the voltage on vled+ jump up to anything over 44v at any time. also i was able to see some panel damage when the lights were on that looks like water got in the screen somehow it would probably still be watchable if it would actually show an image but for sure there would be some discoloration. not worth it to fix this tv now
                                Last edited by triplefour; 11-27-2017, 08:00 PM.
                                Don't fear the repair...

                                Comment

                                • triplefour
                                  Badcaps Legend
                                  • May 2014
                                  • 1747
                                  • USA

                                  #17
                                  Re: VIZIO E420i-A1 no backlight

                                  i do have a bench power supply but its only 30v5amp ... guess its time to upgrade!
                                  Don't fear the repair...

                                  Comment

                                  • budm
                                    Badcaps Legend
                                    • Feb 2010
                                    • 40746
                                    • USA

                                    #18
                                    Re: VIZIO E420i-A1 no backlight

                                    So it is good to know that it is only 44V boosted Voltage and reading of 1~2V on the FB is about right when it working which means those MOSFETs are turned on, and when it does not work the MOSFET's are not on so you get 11V on FB which means the Vdrops ont he LED string is 44V -11V = 28V which we know that we need about 39V for the LED string to be fully on.
                                    So something starts running and turns on those MOSFET's.
                                    Last edited by budm; 11-28-2017, 03:19 PM.
                                    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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