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Benq T903 and FP937s lower inverter xfmr

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    Benq T903 and FP937s lower inverter xfmr

    Hi, long time reader, first time poster. Great site. I'm a EE trying to keep my skills up. I've done a lot of stereo repairs but recently came across some LCDs that needed fixing. The first NEC 19" was in my neighbour's garbage and an easy cap replacement. That went to my dad who's BENQ 19" was shutting down "when it was warm". That lead me to all the BENQ repairs here, but this one has me stumped. Here's the sum up:

    - BENQ FP937s worked fine then shut down when "warm"
    - so i took it, left on until it shut down and then there was a "pop" after the power cable was pulled. One of those component pops, not an AC arc pop.. plus it would have had to be a 1-10A AC pop, and this thing couldn't do that..
    - Now 2seconds to off when "cold". won't light backlight at all unless it "cools off" for 10-20 minutes. Super Cold can force this to do the 2s to off immediately. So a bad component somewhere has a bad connection?
    - visual inspection shows all caps in tact. Power supply is making good 3.3V, 5.0V, ~16V
    - diode checked all transistors: no shorts
    - touched up soldering on all inverter transformers and transistors
    - removed backlight panel. the CCFLs connected to the "upper" inverter transformer (farthest from the AC plug) always light up for the 2s. The "lower" CCFLs might flicker a little bit. I've swapped the CCFLs around, it is always the CCFLs connected to the lower inverter transformer.
    - on the FP937s board I noticed some arcing on an upper CCFL connector - fixed. same issue
    - on the FP937s I notice the lower inverter transformer has an open from the secondary pin (there's only one secondary pin?) to ground. All other xfmrs seem to have 200-400kohm. So I turned my focus to the T903 inverter board and did all the same above but the xfmrs check out fine. the PWM IC is getting 16V. Lower inverter still doesn't fire the CCFLs...

    So I broke some CCFLs moving them around. That caused me to go looking for another and I came across an identical T903 for $5 that had EXACTLY the same symptoms. So now I have two inverter boards with bad lower inverters!

    This has been a great learning experience on switch-mode (especially considering I am a rep for hf power supplies!), but it's starting to frustrate me. Me who is used to having it fixed by now. So I'm reaching out for some help. It's these single "Hight Voltage" xfmrs driving two CCFLs that are suspect to me. Everything I've read here has one xfmr per CCFL but I guess these xfmrs are so good they don't even have to spell right! Pics are of the T903 board.

    over to you experts,

    inline79
    PS. A schematic would be really helpful!
    Attached Files

    #2
    Re: Benq T903 and FP937s lower inverter xfmr

    1) What is the actual part number of the inverter transformer?

    2) Then take a look at

    http://www.lcdparts.net/T.aspx

    and see if it is there. If yes, click on "Buy Now" to see what the primary and secondary pin resistance measurements should be. Don't worry, "Buy Now" does not force you to get out the credit card or make any transaction. Consider it more like "More info".

    3) Report your secondary windings measurements.

    4) There should be two secondary winding pins for these transformers.

    5) Check the blue caps just below each connector as well. I believe if you do a resistance check across the pins of those caps, you should get open circuit. I learned this from budm as per

    https://www.badcaps.net/forum/showpo...5&postcount=22

    6) 2 seconds to black troubleshooting guide is at (rev 0.3 starting at post 19 is the latest revision)

    https://www.badcaps.net/forum/showthread.php?t=10419
    Last edited by retiredcaps; 05-29-2012, 12:31 AM.
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    Comment


      #3
      Re: Benq T903 and FP937s lower inverter xfmr

      Primary suspects would also be the 2 transisitors on either end of C824,and the timing cap itself.

      Comment


        #4
        Re: Benq T903 and FP937s lower inverter xfmr

        Not unusual but I am totally confused. Its never a good idea to work on two monitors at the same same unless they are exactly the same and only to borrow parts.

        What are we trying to fix here? T903? Do I just need read this bit and can we forget about the other set for the rest of this thread? If you want to work on that at the same time start a separate thread for it.

        So I turned my focus to the T903 inverter board and did all the same above but the xfmrs check out fine. the PWM IC is getting 16V. Lower inverter still doesn't fire the CCFLs...

        So I broke some CCFLs moving them around. That caused me to go looking for another and I came across an identical T903 for $5 that had EXACTLY the same symptoms. So now I have two inverter boards with bad lower inverters!

        This has been a great learning experience on switch-mode (especially considering I am a rep for hf power supplies!), but it's starting to frustrate me. Me who is used to having it fixed by now. So I'm reaching out for some help. It's these single "Hight Voltage" xfmrs driving two CCFLs that are suspect to me. Everything I've read here has one xfmr per CCFL but I guess these xfmrs are so good they don't even have to spell right! Pics are of the T903 board.
        Please upload pictures using attachment function when ask for help on the repair
        http://www.badcaps.net/forum/showthread.php?t=39740

        Comment


          #5
          Re: Benq T903 and FP937s lower inverter xfmr

          Thanks for the tips so far.

          Sorry, I should clarify that the FP937s PS/Inverter board is PN 48.L0G02.A00 and the T903 is 48.L0G02.A02. They're identical and have the same symptoms and I did all the same tests on both boards.

          I'm going to pull the suspect transformers and test them next, maybe do some xfmr swapping around using the .A00 as the donor.

          The main difference between the 2004 FP937S and the 2005 T903 is that the T903 has a totally different graphics board which has one big micro (probably cheaper).

          Comment


            #6
            Re: Benq T903 and FP937s lower inverter xfmr

            Hi - thanks that makes it a bit clearer. Have you got the part numbers off any of the transformers - Have you decided they test bad and are just taking off to check?
            Please upload pictures using attachment function when ask for help on the repair
            http://www.badcaps.net/forum/showthread.php?t=39740

            Comment


              #7
              Re: Benq T903 and FP937s lower inverter xfmr

              SUCCESS!

              I pulled the suspect transformer and checked the resistances. It was very high resistance across the secondary (ground is on the primary side??), and there was leakage into the primary windings... this was probably the "POP". The attached image shows the transformer, a 19.26043.001 with 7 pins on one side and 4 pins on the other side, though 3 of those 4 pins are not connected to anything!

              Then off the parts monitor I pulled the "good" transformer and it was ~450k on the secondary. Dropped it in and it has been staying on long enough for me to write this!

              Thanks to all for being a forum for me to post my thoughts and get some opinions!
              Attached Files

              Comment


                #8
                Re: Benq T903 and FP937s lower inverter xfmr [FIXED]

                I also want to add that I think this is an inherent design flaw in this inverter. The transformers seem to be way undersized as they are driving two 19" monitor CCFLs with no heat sinking. Then again, if the transformers were OK sized, the C5707s would probably have blown!

                Comment


                  #9
                  Re: Benq T903 and FP937s lower inverter xfmr

                  I have read and re-read and re-read,and still cannot work out a reading of 450K on the secondary !! the biggest I have seen is probably about 1.2K.!!!

                  Comment


                    #10
                    Re: Benq T903 and FP937s lower inverter xfmr

                    I know, the 450k measurement is a head scratcher. I can't find any info about this transformer and I've never seen one xfmr run two lamps before so all I can say is it WORKS and I'm not going to ask any more questions until something else blows up!

                    Comment


                      #11
                      Re: Benq T903 and FP937s lower inverter xfmr

                      I wonder if he is getting his body resistance reading (fingers touching both probes)
                      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


                        #12
                        Re: Benq T903 and FP937s lower inverter xfmr [FIXED]

                        Originally posted by inline79 View Post
                        I also want to add that I think this is an inherent design flaw in this inverter. The transformers seem to be way undersized as they are driving two 19" monitor CCFLs with no heat sinking. Then again, if the transformers were OK sized, the C5707s would probably have blown!
                        I don't you're going to see any LCD monitors with heatsinked inverter transformers.

                        Comment

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