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    VX 724 black screen green light flashing

    HI Guys

    My Viewsonic VX724 monitor died a few weeks ago, following a few hours where it smelt of burning, it now gives the black screen, green on/off button flashes. (im not an electronics bod but i know which end of the soldering iron to hold!)

    I have replaced 5 domed caps on the power board, the green board caps look ok. The button still flashes green.

    following previous threads and a bit more troubleshooting, there is 12.3v on the connector that joins the 2 boards with power lead connected.

    There are two small connectors at top left and top right of power board ( not sure what these do)

    unplug both top right connectors screen flashes
    unplug both top left connectors screen does not flash

    when i connected up the mains just now there was a bit burning smell and smoke coming from power board although i couldnt see which bit as i turned it off quick.

    hope there is enough clues here for you to help me

    any help gratefully recieved

    steve
    Attached Files
    Last edited by stevepw; 12-12-2010, 12:49 PM.

    #2
    Re: VX 724 black screen green light flashing

    Try checking these transistors for shorts. What's the part number?
    Attached Files

    Comment


      #3
      Re: VX 724 black screen green light flashing

      HI
      The part number of the board is AS02B012D75
      all 4 resisitors have DK QR on them
      to test for shorts am i just checking the 3 small connections on the right of each transistor or the fat one on the left too

      Comment


        #4
        Re: VX 724 black screen green light flashing

        hi jetadm guessing its just the three small connectors, i changed the multimeter setting for3and4 as they appeared open on 200ohm

        testing from Left to right with pin 1 at top
        MMset at 200ohm MM set at 2k ohm
        PINS No1 No2 No3 No4
        1-2 37.8 38.3 0.500 0.500
        1-3 36.7 38.3 0.563 0.564
        2-3 1.1 1.4 0.559 0.559

        Comment


          #5
          Re: VX 724 black screen green light flashing

          Looking at the Board from the top,and having had something to do with these boards,I suspect the top right transformer, has failed,if you unsolder it you will probably find it is burnt on the underside.

          Comment


            #6
            Re: VX 724 black screen green light flashing

            Originally posted by stevepw View Post
            testing from Left to right with pin 1 at top
            MMset at 200ohm MM set at 2k ohm
            PINS No1 No2 No3 No4
            1-2 37.8 38.3 0.500 0.500
            1-3 36.7 38.3 0.563 0.564
            2-3 1.1 1.4 0.559 0.559
            The above readings suggest that transistors No1 and No2 are shorted. PlainBill identifies them as Rohm transistors in this post below ...

            https://www.badcaps.net/forum/showpo...2&postcount=10
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              #7
              Re: VX 724 black screen green light flashing

              Originally posted by Rtech View Post
              Looking at the Board from the top,and having had something to do with these boards,I suspect the top right transformer, has failed,if you unsolder it you will probably find it is burnt on the underside.

              Hi Rtech

              thanks for that,having a closer look with eyeglass the underside of the transformer is burnt just where the Q6 transistor is, Q6 also seems to be pitted on top.

              so transistors Q6 and Q7 need replacing and a new transformer it has EPC19 stamped on it , would this be the part number hopefully

              thanks retiredcaps ill have a look for the transistors

              Comment


                #8
                Re: VX 724 black screen green light flashing

                Originally posted by stevepw View Post
                a new transformer it has EPC19 stamped on it , would this be the part number hopefully
                This might be a match

                http://cgi.ebay.com/CCFL-transformer...item563715b969

                If yes, you might be able to find it cheaper.
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                  #9
                  Re: VX 724 black screen green light flashing

                  Originally posted by retiredcaps View Post
                  This might be a match

                  http://cgi.ebay.com/CCFL-transformer...item563715b969

                  If yes, you might be able to find it cheaper.

                  Thanks it looks like it could work
                  there are some other numbers on the side SPI 8T J00029
                  0523 1 FC

                  i will have a look at some Uk suppliers

                  Comment


                    #10
                    Re: VX 724 black screen green light flashing

                    Originally posted by stevepw View Post
                    SPI 8T J00029
                    0523 1 FC
                    I forgot to mention that since you have 2 transformers, you can measure both and see if there are any differences.

                    Try the test below with both transformers unsoldered. It won't catch a shorted turn, but maybe the transformer is good and you may not have to order a replacement?

                    See #4 (transformer) at

                    https://www.badcaps.net/forum/showpo...1&postcount=14
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                      #11
                      Re: VX 724 black screen green light flashing

                      Good work troubleshooting. Again, this is the same Royer oscillator circuit we see on Benq, (Dell, HP), etc monitors. Also, I've seen cases where the actual failure was caused by the timing capacitor (the brown cap near the transformer).

                      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: VX 724 black screen green light flashing

                        Have also had cases where the Brown cap(as in Bill's note above)has failed,but similar to an electrolytic, it has swollen a great deal.Have also seen similar PSU/Inverter PCBs in models VX710,910 and 924,and I suspect some others as well.As the poster is in the UK,eqivalents for the transistors are available via RS,and I may be able to help him with the Tfmr,so hopefully all the help people have given may result in another 'saved' Monitor.

                        Comment


                          #13
                          Re: VX 724 black screen green light flashing

                          Originally posted by retiredcaps View Post
                          I forgot to mention that since you have 2 transformers, you can measure both and see if there are any differences.

                          Try the test below with both transformers unsoldered. It won't catch a shorted turn, but maybe the transformer is good and you may not have to order a replacement?

                          See #4 (transformer) at

                          https://www.badcaps.net/forum/showpo...1&postcount=14
                          hi
                          Yes tested both transformers
                          T1 next to shorted transistors all readings between 0.7-1.6ohms
                          T2 shows
                          1-2 746ohms
                          1-3 746ohms
                          1-4 746ohms
                          1-5 746ohms
                          1-6 0
                          2-3 0
                          2-4 0
                          2-5 0
                          2-6 556ohms
                          3-4 0
                          3-5 0
                          3-6 559ohms
                          4-5 0
                          4-6 559ohms
                          5-6 559 ohms

                          guess T1 is a goner!

                          Comment


                            #14
                            Re: VX 724 black screen green light flashing

                            Rtech and Plainbill

                            I was wondering what bit if any started the chain of events leading to failure to work,

                            transformer getting hot making transistor fail or vice versa

                            electrolytics going domed and causing above to fail

                            or the other caps... brown , blue , yellow even

                            i read on another thread asjemb? who seemed to replace everything just to be sure

                            if the transistorsand transformer are replaced would another duff cap just make them fail again?

                            Comment


                              #15
                              Re: VX 724 black screen green light flashing

                              Originally posted by stevepw View Post
                              Rtech and Plainbill

                              I was wondering what bit if any started the chain of events leading to failure to work,

                              transformer getting hot making transistor fail or vice versa

                              electrolytics going domed and causing above to fail

                              or the other caps... brown , blue , yellow even

                              i read on another thread asjemb? who seemed to replace everything just to be sure

                              if the transistorsand transformer are replaced would another duff cap just make them fail again?
                              Without detailed analysis by an expert, it would be impossible to say. A Royer oscillator uses the pulse generated by one transistor to turn the other off. The frequency is limited by the film capacitor. If the capacitor goes open the frequency and the current drawn will rise until the transistors short. I one of the pins on the transformer primary opens (cracked solder joint) the transistor will stay on, overheat, and short. There is a (zener?) diode in the circuit that has been known to short, resulting (I believe) in shorted transistors. I've got a Viewsonic monitor sitting on the shelf right now that will short the transistors in seconds. The transformer passes ring tests, the capacitor checks out, the diode isn't shorted. My next plan is to use a current limited variable power supply to gently power it up and see if I can identify the faulty component.

                              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: VX 724 black screen green light flashing

                                Whenever i see one of these,the faults are caps,being the lytics or the blue/brown cap near to the tfmr,the output transisitors on the bottom of the board and the tfmr,or a combination of these.In each case it is ONLY the side near to the Heatsink and associated Schottkys that do fail,and so I think the overall problem is simply caused by the heat generated in that area.

                                Comment


                                  #17
                                  Re: VX 724 black screen green light flashing

                                  I have just removed the two brown radial polypropylene caps near the transformers
                                  Rtech you mentioned these on another thread thyre H154 J , with my meter set on 2UF
                                  i get 0.141 on the good side and 0 on the one nearest burnt out transformer
                                  I have somne transistors ordered from RS , shoukd be in in next 2 days
                                  It may be worth checking the shottkey diode MBR101100CT, as this has been highlighted as an issue on threads does this need to be removed from board to do it properly,

                                  Comment


                                    #18
                                    Re: VX 724 black screen green light flashing

                                    You can check the schottkys in situ...outside pins are common to each other.and simple resistance measurements between the centre and the extremes should soon tell you if there is a problem...they normally go short cct when they fail.Tfmr posted today,if you still have probs possibly with caps after you have put the tfmr and transistors in then let us know.

                                    Comment


                                      #19
                                      Re: VX 724 black screen green light flashing

                                      Many thanks Rtech
                                      looks ok, outsides common , from centre pin to either outside pins gives 198ohms and swopping leads gives open reading both ways.
                                      for the brown H154 J there are some with 150nF is this the same as 0.15UF both rated at 250V these are blue too , does it matter , seebelow
                                      http://uk.rs-online.com/web/search/s...duct&R=6689961

                                      Comment


                                        #20
                                        Re: VX 724 black screen green light flashing

                                        Your cap values are fine BUT the tolerance has to be + or - 5%(usually code J)as you have noted.

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