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Toshiba 32W3433DL. Backlight shuts off in Auto

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    Toshiba 32W3433DL. Backlight shuts off in Auto

    Hi There.
    This is a weird problem with this TV set.

    User reports that image gets black (with sound) usually after 3-5 seconds after start up. Sometimes after 1h... Then he turns TV off, back on, and SOMETIMES, image comes back. He has to try several times

    I bring it home and test it. Play a USB drive movie for 2h, no issues
    Plug DVB antenna cable in and watch a TV channel for over 1h, no issue.

    BUT...Start changing channels up and while I keep rolling channels, the screen starts dimming off, until it suddenly turns completly off. I can tell it's the backlight, as I can use a flashlight and I see the image

    Manage to get back to work. Enter menu, change backlight from AUTO to LOW, and TV works fine. I can keep changing channels for a long time non stop and it works fine. Same with MID and HIGH backlight setting.

    I set it back to AUTO BACKLIGHT. TV shows fine but as soon as I press CH+ or CH- several times in a row, screen starts dimming down and gets to a point (unless I stop changing channel soon enough) when it gets totally dark and unrecoverable. I then discovered that the room light affects it, while I was recording the video. With the room light on, the screen would dim down to almost black but it'd stay there and wouldn't completly turn off, but as soon as I turned the room light off and kept changing channels, the BL would end up shutting off.

    This is the video I've got showing it:
    https://www.youtube.com/watch?v=046cqFus9d8

    Yes..I understand I can have the user keep the BL setting to LOW/MID/HIGH, but 1)It doesn't seem like a 100% valid solution 2)I don't know if it will work all the time in the future, cause there's got to be something faulty inside

    BTW I've checked for updates and it's already running the latest firmware version (3.6.3c)

    Thanks!!

    #2
    Re: Toshiba 32W3433DL. Backlight shuts off in Auto

    I just opened it up.
    With BL in AUTO, the output from thr PS to the LED panel starts going down from 70V to 0V slowly...When it reaches about 40V the LEDs shut off.
    This doesn't happen with the BL setting in LO/MID/HI
    I found the schematic, MP3394 IC controls the voltage to the LEDS. It has these BL_ON_OFF and PW_DIM pjns (2 & 3) which are supposed to manage the output voltage, and should be connected to the 12x2 socket to the Main Board, but I can't find this connection in this PS board. There's no continuity from the MP IC to the socket.
    Also, I can't find any LDR anywhere on this TV, how does it sense the outside light?
    Any help?

    Thanks

    Comment


      #3
      Re: Toshiba 32W3433DL. Backlight shuts off in Auto

      More info on this.
      Driver chip for backlight is a MP3394. PS is 17IP11 R4
      There's a pin in MP IC that drives the brightness of the backlight. It is pin 3 DBRT. It is PWM driven by yhe Main board
      When BL is set to HI I measure 99.9% duty cycle in DBRT
      When I set BL to MID I measure 70% in DBRT
      When I set BL to LO DBRT goes down to 46%
      When I set Backlight to AUTO, DBRT starts falling slowly until 5%. The light on the LED pane is still on but very dim
      Then when it switches off, I measure DBRT and it is still 5%, so this makes me think it's not the Main board but the PS who despite a 5% demand in brightness, it can't hold it. LED_ON-OFF is still HIGH
      If I momentariyl touch DBRT to +3.3v somethimes the backlight switches on and stays...
      Should I get a new PS?

      Comment


        #4
        Re: Toshiba 32W3433DL. Backlight shuts off in Auto

        Hello,
        It is very stragne, that it shuts off on the lowest duty cycle.
        Doesn't the IC or the FET get hot on this mode?
        Also, this might be the sign of failing LEDs in the backlight.

        EDIT:
        While you're at it, you should also probe the OVP (12) and ISENSE (13) pins of the led driver. It may get a (false) fault signal.
        I would also play around with the ISET (6) pin. It is tied to ground via 2 resistors in series. According to the datasheet, 2x6k8. This pin sets the driving current to the leds. You may want to try lower the current. Datasheet says, the value of the resistor is proportional to the driving current.
        Last edited by yohnsee; 06-15-2018, 09:10 AM.
        Cheers, Janos

        Comment


          #5
          Re: Toshiba 32W3433DL. Backlight shuts off in Auto

          I don't think it is realted to faulty LED's, as it only fails in Auto mode, that puts the BL in a very DIM brightness. If I set the BL to LO/MID/HIGH it works good for hours. Only in Auto, when it gets almost off, it eventually actually shuts off. Seems to me something more related to the MCP chip that for some reason it "disengages" while on very low brightness.
          I have the TV already assembled, next time I open it up I will check those protection signals into the driver chip

          Thanks!!

          Comment


            #6
            Re: Toshiba 32W3433DL. Backlight shuts off in Auto

            Re-reading the datasheet for the MP3394 IC you may absolutely be right. The open string or short string protections may be triggering it. It says when it detects them, the LED strips are powered off until next restart. Something I don't understand.

            In my schematic for this PS, (17IPS11 R4) the OVP comes from a 2/3 voltage divider. Voltage at the led was close to 40-60volts, while this OVP signal trips at 1.26 volts...How can this signal be in the range of 0..1 volts while coming from a 2/3 divider in a 60volt line? In the sample circuit in the MP3944 datasheet, the voltage divider is 1/36, which makes sense to get a ~1v signal out of 40-60volts

            Also, who are the LEDs arranged insode the TV set? All in series?
            Thanks

            Comment


              #7
              Re: Toshiba 32W3433DL. Backlight shuts off in Auto

              Can we at least see good clear pictures of the whole backside of the TV showing how all the boards are connected together, then good clear straight shot of the whole power supply board. Upload the schematic too since you have it.
              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


                #8
                Re: Toshiba 32W3433DL. Backlight shuts off in Auto

                Absolutely!

                Here's a link for the PS schematic and datasheet for the driver:
                LINK

                And here a couple photos I took inside. The thin black&red wire getting out of the PS board going all the way to the other end of the chassis is the LED output
                LINK

                The MP driver chips is soldered on the bottom side of the PS board

                Thanks!

                Comment


                  #9
                  Re: Toshiba 32W3433DL. Backlight shuts off in Auto

                  One thing to bear in mind: this is a Vestel PCB, we're talking about.
                  So if possible, check the OVP voltage divider routing, and resistor values, because they have a good tendency to change without noting.
                  Cheers, Janos

                  Comment


                    #10
                    Re: Toshiba 32W3433DL. Backlight shuts off in Auto

                    Ok. You mean this failure is common in this specific brand?
                    One thing I don'¡t understand, the schematic shows a 2/3 divider to get the OVP signal. The datasheet for the driver says OVP triggers at 1.23volt. The LED output seems in the range of 40-70v...Doesn't make sense to me!

                    Thanks

                    Comment


                      #11
                      Re: Toshiba 32W3433DL. Backlight shuts off in Auto

                      No, you got me wrong. (Altough this brand is a very low quality)
                      What I was saying, you must see with your own eyes, if they had a 2/3 divider there, or did they build in other values of resistors? So that it will work, but forgot to mark on the schematic.
                      Cheers, Janos

                      Comment


                        #12
                        Re: Toshiba 32W3433DL. Backlight shuts off in Auto

                        Gotcha now. I though you meant the resistors may have got their actual values modified along the time. Ok I will check that.
                        Anyway I think I will budget the owner for a new PS, the time to check and repair the board will probably double the cost of a spare PS
                        Thanks!

                        Comment


                          #13
                          Re: Toshiba 32W3433DL. Backlight shuts off in Auto

                          I talked to the client, he understands that repairing the board will end up being way more expensive than getting a spare board (in labor hours).

                          So I will order a spare board.

                          Can I just purchase ANY VESTEL 17IPS11 board? Or do I have to pay attention to more details on the board?

                          Thank you

                          Comment


                            #14
                            Re: Toshiba 32W3433DL. Backlight shuts off in Auto

                            To be absolutely sure, you need to match the type (17IPS11) and if there is a suffix, like -R4 or something. And you also need to match the numbers seen on the barcode.
                            Cheers, Janos

                            Comment


                              #15
                              Re: Toshiba 32W3433DL. Backlight shuts off in Auto

                              I've found a vendor that has the 17IPS11 but doesn't show any more details...The pic looks exactly the same, but there's no resolution enough to zoom in and see the codes...I will probably go ahead and order it

                              Comment


                                #16
                                Re: Toshiba 32W3433DL. Backlight shuts off in Auto

                                You better be asking the seller before ordering for the numbers.
                                But, if you are brave enough, and it looks the same (the connectors are at the same position, same size, etc) then order it.
                                Be extra careful with the connector to the main board, they come in 2x6pin and 2x10pin variants.

                                P.s.: can you put the link here, the one you found the new board?
                                Last edited by yohnsee; 06-19-2018, 12:14 PM.
                                Cheers, Janos

                                Comment


                                  #17
                                  Re: Toshiba 32W3433DL. Backlight shuts off in Auto

                                  Yes I realized of those 2 connector sizes. Mine comes with the 2x10, like the one for sale
                                  I asked the guy for details, he said there's no need to add anything for these PS as they are all the same. I told him I was afraid the LED output could be different if it was used in a TV with more or fewer LEDs or in a different arrangement and he said "don't worry they are all the same..."
                                  Physically, yes, it does look exactly the same, component by component.
                                  BUT, before ordering it, I was planning to tell the owner (cause I don't want to take the risk of 1)Get a PS that doesn't really fit and 2)There's still a chance that some leds are faulty and they are putting the board in self protection mode, but checking that takes too much time)..I was saying, planning to tell the owner thet I found a way to get this TV working, setting the BL to LOW and it does work for hours...
                                  I have to talk to him

                                  Comment


                                    #18
                                    Re: Toshiba 32W3433DL. Backlight shuts off in Auto

                                    Bad news...I went ahead and bought a new PS. Put it in place and keep having the same issue...Should I now suspect the LEDs themselves are the issue?
                                    Can I test these leds without removing the panel? I've seen videos explaining how to replace a broken led and this is not something I feel I can do, removing the LCD screen layer, etc...
                                    But how can I test them from the led socket? I don't know the voltage/current they are expected to use


                                    Thanks!

                                    Comment


                                      #19
                                      Re: Toshiba 32W3433DL. Backlight shuts off in Auto

                                      In the auto mode does the pin 3 (DBRT) of the MP3394 have a DC voltage between 0.2 - 1.2V to give 0% to 100% dimming supplied by the main board.
                                      Willing to help but I'm no expert.

                                      Comment


                                        #20
                                        Re: Toshiba 32W3433DL. Backlight shuts off in Auto

                                        Good point. This driver chip, according to the datasheet, works in 2 modes to set the brightness:
                                        1.- DC. The one you mean, supplying a 0.2-1.2v to this pin and placing a proper Capacitor o BOSC to drive the local oscillator
                                        2.-PWM, supplying a variable 0-100% duty cycle to this pin, while you have a 100K resistor between BOSC and GND. According to the schematic for this PS, this is the method used in this case.

                                        I did measure the signal in this pin, and it did have a true variable 10-100% duty cycle PWM signal from the main board, depending on the setting for BL in the menu.

                                        For some reason, now (but before I swapped the PS, and also after) it's not even lighting the LEDs on at all on any BL setting (AUTO, LOW, MID, HIGH), not sure if the LEDS are getting bad or what....

                                        Comment

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