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    Backlight problem on viewSonic VP171b

    Hello all,

    I've got a LCD from ViewSonic. The model is VP171b. It shows the right image for 1 second when I turn it on and then goes black. Powering it off and on again makes the same.

    If I direct a flash light on it, I can see what I should see. It's very dark though. From what I've read, it seems to be an inverter problem, am I wrong ?

    and reading the posts, it seems that this is usually related to a dead caps.
    The caps of my LCD are (according to another post in badcaps)
    PSU:
    Li-Shin AREX 94V-0
    2 Panasonic 16V 1000uF 105oC HFA
    1 Panasonic (unknown value because epoxy covered that side, but it is about 1inhc in round, and what I can see says Ce GH)
    1 Rubycon XYG 50V 22uF

    Backlight Inverter:
    Emax 44.61101.001
    2 HERMEI 25V 100uF
    1 SSA 50V 2.2uF (M)P1 CE 105oC
    4 3kV 22J Ceramic caps
    2 250V PMS124J Ceramic Caps

    Video Board:
    Coretronic Corp
    Genesis gm5120 chip
    13 Taicon 16V 180uF 0304(M) 105oC (VT)
    2 Taicon 10V 1000uF 0304(M) 105oC (HF)
    2 Taicon 25V 10uF 0302(M) 105oC (PW)
    1 Taicon 16V 2200uF 0238(M) 105oC (PW)
    1 Taicon 16V 470uF 0246(M) 105oC (VZ)

    I'de like to know what would be the most pragmatic solution for trying to identify bad caps and replacing them as I've never tried to repair a LCD. I put some picture of the LCD boards in attachments.

    Can you help me ? Thanks a lot in advance, I'd really like to make this screen works again

    sebastien
    Attached Files

    #2
    Re: Backlight problem on viewSonic VP171b

    Yes your inverter is shot.

    You are lucky enough to have a separate inverter module (the little board with the purple probably Hermei crap caps on it).

    I would check the SMT driver transistors near the blue caps for shorts with a DMM. If they check out good.

    Replace the 2 Hermeiphrodite caps with good Low ESR caps and try to monitor again also check for any cracked solder joints.

    KC stated in another post that the yellow adhesive used on the torroid inductors on that board breaks down and eats at the traces so check that too. Remove the inductors remove all of that yellow crap and repair any broken traces.

    Check out this thread:

    https://www.badcaps.net/forum/showthread.php?t=5043

    In a worse case scenario you could always just buy a new inverter module just replace any bad caps on it off the bat so the your new inverter has a long life.
    Last edited by Krankshaft; 10-22-2008, 09:45 AM.
    Elements of the past and the future combining to make something not quite as good as either.

    Comment


      #3
      Re: Backlight problem on viewSonic VP171b

      ok. Thanks a lot for your reply. I will check the transistors and the adhesive on the torroids. Then if the inverter always fails, I will replace both the Hermei caps.

      Thanks again!

      sebastien

      Comment


        #4
        Re: Backlight problem on viewSonic VP171b

        I also have the VP171b, but my inverter is the "44.58402.002" model.

        The caps are Hermei brand, 180 microfarad, 25 volt. I can not find a matching cap at the local stores, including Fry's.
        I haven't looked online yet, but am about to. If I can just replace the caps, that's great.

        If I can't find the same spec caps, can I substitute something else?



        Last edited by Starlite528; 12-28-2008, 05:32 PM.

        Comment


          #5
          Re: Backlight problem on viewSonic VP171b

          I replaced the two caps shown with Panasonic/ECG "FC" caps (Digikey # P11217-ND). Problem still remains. The screen works for a while, then dies until it cools off for ten minutes or so. I do hear a faint high pitch buzzing before it shuts the monitor off.

          I don't know what else to do, other than replace the whole inverter, which I can't seem to find.

          Edit: I found the proper model number on the bottom of the inverter;
          PLCD2417414
          Last edited by Starlite528; 01-05-2009, 03:48 PM.

          Comment


            #6
            Re: Backlight problem on viewSonic VP171b

            You may also have bad tubes.
            Try the inverter with different tubes, or the tubes with a different inverter.

            A little buzzing is normal.
            The windings in the Torrids and transformers can vibrate a little.
            It's louder because you have it out of the case.

            .
            Mann-Made Global Warming.
            - We should be more concerned about the Intellectual Climate.

            -
            Be who you are and say what you feel, because those who mind don't matter and those who matter don't mind.

            - Dr Seuss
            -
            You can teach a man to fish and feed him for life, but if he can't handle sushi you must also teach him to cook.
            -

            Comment


              #7
              Re: Backlight problem on viewSonic VP171b

              Originally posted by PCBONEZ
              You may also have bad tubes.
              Try the inverter with different tubes, or the tubes with a different inverter.

              A little buzzing is normal.
              The windings in the Torrids and transformers can vibrate a little.
              It's louder because you have it out of the case.

              .
              I've not taken it apart that far. I would think that it would only knock out half the lamps if they were bad, then again I have no idea how the inverter is programmed to react.

              I don't know what kind of lamps or where to get new ones.

              Comment


                #8
                Re: Backlight problem on viewSonic VP171b

                I just joined this forum so I could post this reply. I figured it might help out
                others who end up in a similar predicament.

                With a factory assembly line partially down because of a failed Planar
                touchscreen backlight, and no other staff or spare touchscreens available
                since everyone was off for the holidays, I was forced to dig into
                troubleshooting the EMAX LCD backlight driver board. The board on this bad
                monitor is identical to the one cited in this thread.

                Thanks to some initial leads from this thread, I was able to eliminate the caps
                and also get confirmation on the key symptom of this board's particular failure
                mode: The monitor does light up for about 1 second after power up, and
                then it will go dark. This turned out to be key to verifying the root cause.
                From other searches around the web, it would seem this is a common failure
                mode for these Emax LCD backlight driver boards that have two or more
                outputs. Here's why.

                As it turns out, U1 (BA9741) is a dual switch-mode power supply chip that
                drives the primary sides of two high-voltage step-up transformers. The
                datasheet is readily accessible online. It has two outputs, each that drive
                the two separate high voltage supplies on opposite sides of the board. In
                short, the reason why the monitor is lighting up for 1 second is because one
                stage is working, but the other stage is not. Because the dead stage has
                zero volts at its ultimate output, the short circuit detection feature of the
                BA9741 shuts down the entire chip (both stages) in about 1 second. You
                may notice when the monitor backlight comes on, it's only at about half
                brightness anyway.

                I dug deeper and found that the root cause had nothing to do with the
                capacitors. Having dual, identical circuits to compare against each obviously
                made what could have been a difficult job much easier. The problem was due
                to a failed U8 (SDM9435A), the main P-channel MOSFET in an SO-8 package
                that is used to switch 12V to the primary of one of the high voltage
                transformers. I ordered up some replacements from Mouser for $0.80 each
                and will receive them tomorrow. Should be a piece of cake from here, but I
                will post another reply confirming that the fix worked.

                Comment


                  #9
                  Re: Backlight problem on viewSonic VP171b

                  Did you figure out what made the MOSFET blow?
                  Mann-Made Global Warming.
                  - We should be more concerned about the Intellectual Climate.

                  -
                  Be who you are and say what you feel, because those who mind don't matter and those who matter don't mind.

                  - Dr Seuss
                  -
                  You can teach a man to fish and feed him for life, but if he can't handle sushi you must also teach him to cook.
                  -

                  Comment


                    #10
                    Re: Backlight problem on viewSonic VP171b

                    That is a good question since replacing the bad MOSFET might result in
                    another failure. I checked the resistance across the 4 backlight lamps and
                    also their resistance to ground. All seemed normal there, or at least
                    consistent (infinite resistance everywhere).

                    Update and declaration of success: I received the replacement MOSFETs on
                    time via FedEx, soldered them in, and reassembled the monitor. It is up and
                    running and functioning properly.

                    My conclusion is that perhaps the design of the circuit is a bit marginal. The
                    MOSFETs only have a maximum VDS of -30Vdc and the body/flyback diode
                    can only handle 1.7A vs. the 5A that the FET can conduct. It's possible that
                    over time the diode will fail because of this under rating, and then leave the
                    FET unprotected resulting in its ultimate destruction. Perhaps adding an
                    external flyback diode might help. I have 5 more new MOSFETS and about a
                    dozen more Planar monitors of identical model type throughout my factory.
                    I'm ready if and when the next one does the same thing, which won't surprise
                    me.

                    Comment


                      #11
                      Re: Backlight problem on viewSonic VP171b

                      Originally posted by matteo_g
                      That is a good question since replacing the bad MOSFET might result in
                      another failure. I checked the resistance across the 4 backlight lamps and
                      also their resistance to ground. All seemed normal there, or at least
                      consistent (infinite resistance everywhere).

                      Update and declaration of success: I received the replacement MOSFETs on
                      time via FedEx, soldered them in, and reassembled the monitor. It is up and
                      running and functioning properly.

                      My conclusion is that perhaps the design of the circuit is a bit marginal. The
                      MOSFETs only have a maximum VDS of -30Vdc and the body/flyback diode
                      can only handle 1.7A vs. the 5A that the FET can conduct. It's possible that
                      over time the diode will fail because of this under rating, and then leave the
                      FET unprotected resulting in its ultimate destruction. Perhaps adding an
                      external flyback diode might help. I have 5 more new MOSFETS and about a
                      dozen more Planar monitors of identical model type throughout my factory.
                      I'm ready if and when the next one does the same thing, which won't surprise
                      me.
                      I can't speak for this particular model, but one common cause is bad solder joints on the inverter transformers.

                      PlainBill
                      For a number of reasons, both health and personal, I will no longer be active on this board. Any PMs asking for assistance will be ignored.

                      Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic.

                      Comment


                        #12
                        Re: Backlight problem on viewSonic VP171b

                        Hi Matteo_G, i have the same problem on my LCD screen. The part that i have to replace is not the same. My U8 is a EEPROM and not a main P-channel MOSFET. Can you help me with that ASAP to clarify things?

                        Thanks!

                        Comment


                          #13
                          Re: Backlight problem on viewSonic VP171b

                          Originally posted by jowel80
                          Hi Matteo_G, i have the same problem on my LCD screen. The part that i have to replace is not the same. My U8 is a EEPROM and not a main P-channel MOSFET. Can you help me with that ASAP to clarify things?

                          Thanks!
                          Pictures and a part number will help clarifying the issue.

                          PlainBill
                          For a number of reasons, both health and personal, I will no longer be active on this board. Any PMs asking for assistance will be ignored.

                          Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic.

                          Comment


                            #14
                            Re: Backlight problem on viewSonic VP171b

                            I checked on the board and the part that you are talking about seem to be Q8 and not U8. U8 is a EEPROM and Q8 is the part number that you are talking about. So I'll go with Q8

                            Comment


                              #15
                              Re: Backlight problem on viewSonic VP171b

                              Originally posted by jowel80
                              I checked on the board and the part that you are talking about seem to be Q8 and not U8. U8 is a EEPROM and Q8 is the part number that you are talking about. So I'll go with Q8
                              Hold it!!!! There are a number of reasons for the 'two seconds to black' problem, a bad mosfet would be one of them. Before replacing anything check Q7 and Q8. Each of them is as likely to fail. Also, it could be a bad transformer, bad solder joint, frayed wire, or bad CCFL.

                              PlainBill
                              For a number of reasons, both health and personal, I will no longer be active on this board. Any PMs asking for assistance will be ignored.

                              Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic.

                              Comment


                                #16
                                Re: Backlight problem on viewSonic VP171b

                                Originally posted by jowel80
                                Hi Matteo_G, i have the same problem on my LCD screen. The part that i have to replace is not the same. My U8 is a EEPROM and not a main P-channel MOSFET. Can you help me with that ASAP to clarify things?

                                Thanks!
                                ASAP? This is kind of a free/spare time forum ....
                                There are 10 kind of people in this world: those that understand binary, and those who don't.
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                                  2 x Acer KA240H + 1 Vewsonic VP2130 21 (a cap replacement job )

                                Comment


                                  #17
                                  Re: Backlight problem on viewSonic VP171b

                                  Sorry, i was waiting to order bad parts. And it was a typo error.
                                  Sorry again.

                                  thanks!

                                  Comment


                                    #18
                                    Re: Backlight problem on viewSonic VP171b

                                    You are correct, Jowel80. In my haste and since the MOSFET was in an
                                    SO-8 package, thinking it was an IC I called it "U8" instead of "Q8" as it
                                    really is.

                                    PlainBill is also correct in stating that there could be multiple causes of the
                                    2-seconds-then-blackout symptom. U1 (BA9741), the switcher chip, will
                                    shutdown in the same amount time if either of its two power supply outputs
                                    have zero voltage (which it interprets as a short circuit, thus invoking its
                                    short-circuit protection shutdown). There is an LC circuit in line with the
                                    MOSFET, and other devices that ultimately feedback the output voltage to
                                    U1. I did not totally reverse engineer the entire circuit, so I am not sure if
                                    the feedback is actually coming from primary (low voltage) or the secondary
                                    (high voltage) side of the transformer. It would be best to check if there is
                                    output voltage on the primary side at the output of the MOSFET (for each
                                    power supply circuit) and go from there. It will be there and be measurable
                                    for the 2 seconds before U1 shutsdown. If there is voltage all the way to
                                    the primary of the transformers, then something may be wrong with the
                                    transformers themselves and/or their soldering, or with the feedback circuits
                                    thereafter (if that is where they are located). Since you can't effectively
                                    measure the secondary voltage without an attenuating/high-voltage probe,
                                    you'll have to check for problems there using continuity and ohm readings
                                    when the power is off. If voltage does not exist at one or both of the
                                    primaries, you need to probe on the gate side of the MOSFETs and
                                    trace back from there. There's not much more than a resistor or two before
                                    you reach U1, so if the gate signal is dead, it could be due to a faulty
                                    switcher chip.

                                    The good news is that since there are two identical circuits, you can go back
                                    and forth in order to determine where the difference lies.

                                    Comment


                                      #19
                                      Re: Backlight problem on viewSonic VP171b

                                      sorry, don't know if this is the right place for this question, can anyone tell me what is the procedure to remove the cover of this VP171b monitor to get to the boards?

                                      Comment


                                        #20
                                        Re: Backlight problem on viewSonic VP171b

                                        Originally posted by joehill3 View Post
                                        sorry, don't know if this is the right place for this question, can anyone tell me what is the procedure to remove the cover of this VP171b monitor to get to the boards?
                                        Viewsonics have some of the more difficult covers to remove. In general, remove all the screws attaching the mount. There may be four more at the corners, remove them too. Usually there is a seam that runs around the monitor between the front and the back.

                                        Since you saw fit to double post, at this point you can search for videos on Youtube.

                                        PlainBill
                                        Last edited by PlainBill; 10-15-2011, 07:19 AM. Reason: remove instructions
                                        For a number of reasons, both health and personal, I will no longer be active on this board. Any PMs asking for assistance will be ignored.

                                        Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic.

                                        Comment

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