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    Dell 22" monitor backlight goes out right away.

    I have another monitor that I am trying to fix. This is a Dell 22" widescreen and the backlight goes out about 1 or 2 seconds after turning it on. Usually I see caps that need replaced right away, but this one has no bulging caps. I am stumped on where to look to repair this one. Any help about where to start would be great. I have pictures below.

    Thanks,
    Tony
    Attached Files

    #2
    Re: Dell 22" monitor backlight goes out right away.

    Originally posted by indyracer
    I have another monitor that I am trying to fix. This is a Dell 22" widescreen and the backlight goes out about 1 or 2 seconds after turning it on. Usually I see caps that need replaced right away, but this one has no bulging caps. I am stumped on where to look to repair this one. Any help about where to start would be great. I have pictures below.

    Thanks,
    Tony
    1. Measure the resistance across the secondaries of the inverter transformers (measure across the legs of the blue caps that are near the transformers). The resistances should be equal.

    2. Specifying the model number of the monitor is helpful to others.

    3. There is a 16 pin IC in the inverter area. What is the part number?

    4. If further troubleshooting is required, the picture of the bottom is inadequate. Remove the plastic shield, take the picture with ambient light, and get the dimensions as close to 2000 x 2000 as possible.

    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


      #3
      Re: Dell 22" monitor backlight goes out right away.

      Caps DO NOT HAVE TO BULGE to be bad. They can simply dry out, increasing ESR and decreasing capacitance. If you don't have a tester, go ahead and replace all of the electrolytics, just to be sure.

      Comment


        #4
        Re: Dell 22" monitor backlight goes out right away.

        Okay, I measured the resistance across the legs of the blue caps near the transformers. The resistance is 474 for both.

        Model number of the monitor is Dell E228WFPc

        IC has the following on it:
        83AREEM
        TL4941 G4

        I will attach a better picture shortly.

        Thanks for all the help.

        Tony

        Comment


          #5
          Re: Dell 22" monitor backlight goes out right away.

          You can't measure resistance of a cap with a DMM. You must have an ESR meter...

          Comment


            #6
            Re: Dell 22" monitor backlight goes out right away.

            Originally posted by el3ctroded
            You can't measure resistance of a cap with a DMM. You must have an ESR meter...
            He didn't. He measured the resistance of the transformer secondary. Even an ESR meter won't show much on a 22pF capacitor.

            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


              #7
              Re: Dell 22" monitor backlight goes out right away.

              Originally posted by indyracer
              Okay, I measured the resistance across the legs of the blue caps near the transformers. The resistance is 474 for both.

              Model number of the monitor is Dell E228WFPc

              IC has the following on it:
              83AREEM
              TL4941 G4

              I will attach a better picture shortly.

              Thanks for all the help.

              Tony
              That indicates the transformers are probably good. The IC is a TL494I, a SMPS controller, but not one specifically designed as a inverter controller. With a better picture we may be able to trace the circuit and figure out why the backlights turn off.

              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


                #8
                Re: Dell 22" monitor backlight goes out right away.

                I just read his post and the resistance didn't make sense: Blue cap. Resistance. Right after my post about reading ESR. Assumed blue electrolytic and made idiotic comment. That's what I get for assuming... I should learn LOL I do it a lot
                Last edited by el3ctroded; 07-12-2010, 07:29 PM.

                Comment


                  #9
                  Re: Dell 22" monitor backlight goes out right away.

                  Here is the updated picture.

                  Thanks,
                  Tony
                  Attached Files

                  Comment


                    #10
                    Re: Dell 22" monitor backlight goes out right away.

                    Originally posted by indyracer
                    Here is the updated picture.

                    Thanks,
                    Tony
                    This is rough. The detail is not quite as good as I had hoped, but I marked a pair of dual diodes with red dots. and labeled what I believe are the current sense lines from the four CCFLs with the letters A - D.

                    The quick background on this is the TL494 monitors the current through each CCFL. If the current goes too high it shuts down the backlights. There is also usually circuitry which will shut down the backlights if an open CCFL is detected, but I don't see any signs of it. AND there is usually circuitry that maintains the total current through the CCFLs at a certain level.

                    There are two approaches at this point. The easiest would be to take a spare CCFL and use it as a substitute for the original CCFLs one at a time. If the monitor works when a certain CCFL is replaced, we suspect that CCFL is bad, or a wire is broken, arcing to the metal, etc.

                    The second is a little harder. Try to measure the voltage at pints A, B, C, and D in the time the backlights are on. Repeat it several times for each point. We are looking for a point that is either lower or higher than the rest.

                    PlainBill
                    Attached Files
                    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


                      #11
                      Re: Dell 22" monitor backlight goes out right away.

                      To emphasize one point el3ctroded made, replacing the electrolytic capacitors may solve the problem. If no voltage differences are found at the 4 points I specified, or if a known good CCFL does not help; replacing the caps would be the next order of business (unless they test good on an ESR tester).

                      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: Dell 22" monitor backlight goes out right away.

                        Well, I tried to measure the voltage at the points suggested. I was measuring on the 20V DC scale for all of these. Point A climbed to .20 before the light went out and it settled at .15. Actually all points settled at .15 after the lights went out. Point B climbed to .20 several times and .18 a couple times. Points C & D went to .30 to .35.

                        My guess is it isn't getting to the voltage it should before the lights go out. So I guess the next step is to replace some caps?

                        Thanks,
                        Tony

                        Comment


                          #13
                          Re: Dell 22" monitor backlight goes out right away.

                          Originally posted by indyracer View Post
                          Well, I tried to measure the voltage at the points suggested. I was measuring on the 20V DC scale for all of these. Point A climbed to .20 before the light went out and it settled at .15. Actually all points settled at .15 after the lights went out. Point B climbed to .20 several times and .18 a couple times. Points C & D went to .30 to .35.

                          My guess is it isn't getting to the voltage it should before the lights go out. So I guess the next step is to replace some caps?

                          Thanks,
                          Tony
                          These reading indicate a problem. I would expect each point to reach about the same voltage. Those results indicate two of the lamps are drawing more current. And I don't have a good idea why. Are the leads to the CCFLS long enough you can reverse the connections? I'd like to see the readings with the CCFLs from the left side hooked to the connectors on the right side and vice versa.

                          As far as replacing some caps, what brand are the original caps?

                          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: Dell 22" monitor backlight goes out right away.

                            The caps say Elite on them.

                            I think I will try to find a meter to test the caps. That sounds like something useful to have anyway if you do many of these monitors.

                            I will try switching the lights and see what happens.

                            Thanks,
                            Tony

                            Comment


                              #15
                              Re: Dell 22" monitor backlight goes out right away.

                              Originally posted by indyracer View Post
                              The caps say Elite on them.

                              I think I will try to find a meter to test the caps.
                              Most multimeters do not have a capacitance range high enough for most caps used in LCD monitors. Most multimeters only go to 200uF. The Vichy VC99 measures up to 2000uF, but Dave Jones from eevblog hates it. His 53 minute review is at

                              http://www.youtube.com/watch?v=ZoeUgMFLyAw

                              Additionally, multimeters will not read the ESR of the cap. For that you need an ESR meter.
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                                #16
                                Re: Dell 22" monitor backlight goes out right away.

                                Yep, I am going to look for an ESR meter. That is the one that sounds like would be helpful with this.

                                Tony

                                Comment


                                  #17
                                  Re: Dell 22" monitor backlight goes out right away.

                                  I switched the lights and got the same results. Then I got some lights from a monitor that has a broken screen and plugged those in and still the same results.

                                  Comment


                                    #18
                                    Re: Dell 22" monitor backlight goes out right away.

                                    If you are going to replace caps as the next step, may I suggest you replace just two first and then retest? The two to replace are the ones closest to each inverter transformer.

                                    I'm just curious if those 2 are responsible for this problem.
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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.

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                                      #19
                                      Re: Dell 22" monitor backlight goes out right away.

                                      I am fighting on of these monitors, rather than boring everyone with a bunch of voltage measurements, an checks I've done I hoping someone could point me to a datasheet as how the TL4941 would work in a CCFL circuit. It is different than I am accustomed to seeing.
                                      The problem I am having is similar to a 2 sec. to black problem, except its taking about one hour to occur, and the signal and power supply has to be covered to retain heat for the problem to occur.
                                      I know about Appling heat to various spots on the boards to accelerate the problem, even just a little understanding what the TL4941 in a CCFL circuit is doing would help.
                                      Whatever I do, I consider it a success, if in the end I am breathing, seeing, feeling and hearing!

                                      Comment


                                        #20
                                        Re: Dell 22" monitor backlight goes out right away.

                                        Alexanna, I had one do that when it had a bad ccfl solder joint... I also had one do that with a bad transformer. If I cooled it the screen would stay on.

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