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LG W1942S won't backlight, but LCD is OK

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    #1

    LG W1942S won't backlight, but LCD is OK

    LG W1942S won't backlight, but LCD is OK

    Hello,
    After few years of calm, another LCD screen failed on me. An LG W1942S monitor needs my attention and repair. But I need your help, once more.
    I hope the photo are acceptable.

    SUMMARY
    ======
    This is a LG W1942S made probably in 2008. The backlight won't power on, at all. The LCD matrix shows the (faint) computer vga signal alright. When I press the power button the power LED turns blue, as normal. The rest of the buttons and menus seem to work.

    THINGS TESTED
    ==========
    1) Measured steady 164V DC across main filter capacitor.
    2) With the power & logic board connected, the secondary voltages 5V, ON (3.3V) and M/S (3.3V) DC are present at connector location P201.
    3) Fuse FB101 & F101 on power board reads 0.1 ohms.
    4) All capacitors seem fine, apparently no bulge, cracks or fluid.
    5) Transistor U201 pins:1,2,3 measure 1-3 kOhm between them, seems OK
    6) High voltage transformer T301: primary winding gives 0.3 ohm, secondary1 1020 kOhm, secondary2 1020 kOhm, considered OK
    7) On the back of the power board, MOSFET U303 (type AP4525GEH) measures:
    S1-G1 OL
    S1-D1 2.5 MOhm falling
    G1-D1 3.5 MOhm falling
    S2-G2 32 ohm
    S2-D2 2.5 0.1 ohm
    G1-D1 32 ohm
    This doesn't seem normal to me.

    PROBLEM
    ======
    1) The backlight won't turn on, at all.
    2) The high voltage transformer buzzes like crazy when powered on.

    QUESTION
    =======
    Is MOSFET the primary suspect? Is there any conclusive test I should try?

    Thanks
    Attached Files
    Tags: None
    #2
    Re: LG W1942S won't backlight, but LCD is OK

    Ahh its either a failed transformer which is very common or failed CCFL lamp. Unplug the lams does the buzz continue then go away ? Also check mosfet out of circuit.
    Last edited by ReeceyBurger123; 10-30-2015, 01:08 PM.
    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

      #3
      Re: LG W1942S won't backlight, but LCD is OK

      Originally posted by amach View Post
      7) On the back of the power board, MOSFET U303 (type AP4525GEH) measures:
      S2-G2 32 ohm
      S2-D2 2.5 0.1 ohm
      G1-D1 32 ohm
      This doesn't seem normal to me.

      QUESTION
      =======
      Is MOSFET the primary suspect? Is there any conclusive test I should try?
      Yes. Desolder and measure the mosfet again to ensure it is shorted.
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        #4
        Re: LG W1942S won't backlight, but LCD is OK

        Just tried this. With CCFL lamps unpluged one at a time the buzz won't go away. I have noticed, it starts for a second, mutes for one more, and then restarts buzzing without end.

        I need some time to desolder the mosfet to check it out of circuit.
        Thanks for directing me on this.

        Comment

          #5
          Re: LG W1942S won't backlight, but LCD is OK

          BTW, there is some solder splash by Q302 (backside) that you should remove. It if comes free, it can cause a short.
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            #6
            Re: LG W1942S won't backlight, but LCD is OK

            Just in case you need service info ...

            http://www.jordansmanuals.com/Defaul...r&Model=W1942S

            Comment

              #7
              Re: LG W1942S won't backlight, but LCD is OK

              Sorry for the delay, I had been away for the weekend.
              @retiredcaps I cleared the the backsides' solder splash by Q302. See photo.
              @fzabkar thanks for the links.

              I 've just desolder mosfet U303 (4525GEH), with some difficulty, i.e. little burned finger while touching the neiboghring large ground pad! Not really skilled on this task.

              Off-circuit dmm measurements:
              S1-G1 OL
              S1-D1/2 OL
              G1-D1/2 OL
              S2-G2 31 ohms
              S2-D1/2 0.3 ohms
              G2-D1/2 31 ohms

              Does it mean it is dead?

              I 've also tried following this guide http://itrepairguide.com/how-to-test...th-multimeter/ however I cannot conclude which part of the chip pins is the N-Channel and which the P-Channel.

              How can I definitely test the functioning of the this fet?
              Attached Files

              Comment

                #8
                Re: LG W1942S won't backlight, but LCD is OK

                MOSFET #2 in the package is shorted.

                AP4525GEH, Advanced Power Electronics Corp, N AND P-CHANNEL ENHANCEMENT MODE POWER MOSFET, 40V, 15A and -40V, -12A:

                Comment

                  #9
                  Re: LG W1942S won't backlight, but LCD is OK

                  Unfortunately I can't source this smd mosfet, locally. Greek suppliers have a policy of minimum order of 10 pieces at 6 euros each!
                  On the other hand, ordering from abroad is blocked due to current capitol control on Greek banks. Buying on Ebay (& paypal) is impossible, as money cannot be transferred abroad, even involving few euros.

                  I've hit a wall here.
                  Is it possible to use common through-hole transistors/diodes combination to replace this item?

                  Comment

                    #10
                    Re: LG W1942S won't backlight, but LCD is OK

                    Will these people only sell 10? I can put 1 in my basket and they are in athens
                    http://www.markidis.gr/product_info....ducts_id=45425
                    Please upload pictures using attachment function when ask for help on the repair
                    http://www.badcaps.net/forum/showthread.php?t=39740

                    Comment

                      #11
                      Re: LG W1942S won't backlight, but LCD is OK

                      You are right, I have already tried that. But talking to him on the phone reveals that, actually, there is no stocked item. Minimum order from abroad applies here, too, similar to any other (inland) supplier I have come to contact with.
                      Thanks for your interest, though.

                      Comment

                        #12
                        Re: LG W1942S won't backlight, but LCD is OK

                        you people need to organise a revolution or an OPEN election.

                        Comment

                          #13
                          Re: LG W1942S won't backlight, but LCD is OK

                          Originally posted by amach View Post
                          On the other hand, ordering from abroad is blocked due to current capitol control on Greek banks. Buying on Ebay (& paypal) is impossible, as money cannot be transferred abroad, even involving few euros.
                          Get a friend outside Greece to buy and pay for it and then mail it to you. You can then send them paper money in the mail.

                          Alternatively, if someone has an Amazon account, they can buy it and send it to you as a gift. I know I can buy something on amazon.com (USA) and have it shipped to friends in the USA even though I live in Canada. As long as I pay for it, amazon will ship it.
                          Last edited by retiredcaps; 11-08-2015, 10:50 PM.
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                          If you are new to this forum, we can help a lot more if you please post clear focused pictures (max resolution 2000x2000 and 2MB) of your boards using the manage attachments button so they are hosted here. Information and picture clarity compositions should look like this post.

                          We respectfully ask that you make some time and effort to read some of the guides available for basic troubleshooting. After you have read through them, then ask clarification questions or report your findings.

                          Please do not post inline and offsite as they slow down the loading of pages.

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                          Comment

                            #14
                            Re: LG W1942S won't backlight, but LCD is OK

                            I finally managed to secure an AP4525GEH from spain on ebay. I'll post when I receive, test and solder it.
                            A friend working in a bank tipped me that some categories of electronic parts are exempted from capital controls. There is no consistency though, and in my experience on ebay, it is a matter of trial-and-error.
                            The revolution will have to wait.

                            Comment

                              #15
                              Re: LG W1942S won't backlight, but LCD is OK

                              Finally, I replaced the faulty mosfet with a new one bought on ebay. The P-channel tests fine now.
                              Not being skilled on soldering, I post a few close-ups for you to comment on.

                              The backlight now comes on and the screens works as seen on the last photo. No buzzing transformer as well.
                              Congratulations!

                              Thanks to all the forumers who contributed to this thread, especially selldoor who offered to help sourcing the mosfet.
                              Attached Files

                              Comment

                                #16
                                Re: LG W1942S won't backlight, but LCD is OK

                                Originally posted by amach View Post
                                Not being skilled on soldering, I post a few close-ups for you to comment on.
                                It isn't the greatest, but as long as it works, it is not a big deal.

                                There are lots of soldering tutorials online (including eevblog 3 part one) and with more practice you will get better.

                                https://www.youtube.com/watch?v=J5Sb21qbpEQ
                                --- begin sig file ---

                                If you are new to this forum, we can help a lot more if you please post clear focused pictures (max resolution 2000x2000 and 2MB) of your boards using the manage attachments button so they are hosted here. Information and picture clarity compositions should look like this post.

                                We respectfully ask that you make some time and effort to read some of the guides available for basic troubleshooting. After you have read through them, then ask clarification questions or report your findings.

                                Please do not post inline and offsite as they slow down the loading of pages.

                                --- end sig file ---

                                Comment

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