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Philips 37PFL9604/H12 : backlight will switch off after some time on

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    #1

    Philips 37PFL9604/H12 : backlight will switch off after some time on

    Hi,

    My TV (Philips 37PFL9604, chassis Q549.2LE) has an annoying backlight issue since a few days.

    It will power up flawlessly but, after a certain amount of time, the backlight will switch off for a second, then switch on again. It will happen a few time until the screen goes black totally.

    When the backlights are on, I cannot see any dimming or artefact on the screen, thus I would not bet on a CCFL issue. The TV has been used (display on) for a bit more than 5000hours.

    I used a flashlight to check that the image was still present and it was the case and I also changed the 22µf and 47µf caps connected to the backlight.

    Might be worth mentionning, the ambilight will stay on when the backlight switch on and off.

    Is there some other components I should have changed or some other checks I should have done ? I tried to reduce the contrast and since then, the backlight issue didn't appear back (but the display is terrible !)

    I must say that I previously had another issue with the inverted/psu card (DPS-298CPA) some months ago: an annoying click could be heared after being on for a few minutes (without any backlight issue a that time). Changing the IC601 and all mosfets cured it.

    Thanks for your help
    Tags: None
    #2
    Re: Philips 37PFL9604/H12 : backlight will switch off after some time on

    turn the back lights down and see what happens. Other than that unplug the TV and check the output winding on each HV transformer on the inverter with your DMM in Ohms mode. The output windings have to be withing 3% of each other. Monitor the BLon voltage that's coming from the main board back to the PSU.

    Good Luck!

    Comment

      #3
      Re: Philips 37PFL9604/H12 : backlight will switch off after some time on

      thanks for your help.

      do you mean that the resistance measured should be approximately the save at each output ? what would be a "normal" absolute value ? as you may know, there are 2 different transformers on this PSU.
      also, why tracking the BL_ON, to identify if it is an issue from the main board ?

      Comment

        #4
        Re: Philips 37PFL9604/H12 : backlight will switch off after some time on

        .
        Last edited by CapLeaker; 09-17-2018, 10:05 AM.

        Comment

          #5
          Re: Philips 37PFL9604/H12 : backlight will switch off after some time on

          Originally posted by OobabeloO View Post
          thanks for your help.

          do you mean that the resistance measured should be approximately the save at each output ? what would be a "normal" absolute value ? as you may know, there are 2 different transformers on this PSU.
          also, why tracking the BL_ON, to identify if it is an issue from the main board ?
          yes, it's to see if the back light is getting a continuous on signal from the main IC. Upload some high resolution, straight shot pictures from your boards, so we can see what we are dealing with here.

          Comment

            #6
            Re: Philips 37PFL9604/H12 : backlight will switch off after some time on

            I have attached several picture of the visible side of the PSU.
            As for testing the HT transformer, I'm not sure about how I should proceed, What would be my reference point : one winding with another ?
            Also, in my initial post, I said that I replaced the ceramic caps on the HT side. I miswrote the values : those are 33 and 47 microfarads.
            Attached Files
            Last edited by OobabeloO; 09-17-2018, 04:07 PM.

            Comment

              #7
              Re: Philips 37PFL9604/H12 : backlight will switch off after some time on

              The HV transformer is on the second picture. Basically it is 2 transformers side by side. The output windings are the ones where the blue caps are on top. With the TV unplugged, put your DMM into Ohm mode. On the top of that HV transformer where the blue caps are there should be 4 legs. 2 legs for each transformer. Measure the outside leg to the inside leg on each side. Note down the resistance measured. If the resistance of these transformers are further apart then 3%, then it is bad.
              Since you say if you reduce the back light brightness, everything is fine, my bet is either the CCFL's or the HV transformer. Not the caps. On the LCD, is white still white, or does it start to have a pinkish color?

              Comment

                #8
                Re: Philips 37PFL9604/H12 : backlight will switch off after some time on

                There are 9 contacts on the HV transformers.

                I do not see the screen as yellowish when lighted, but the backlight will not stay long enough for me to use a white source to check.
                I'm only referring to the white inside the Philips logo.
                Attached Files
                Last edited by OobabeloO; 09-18-2018, 01:38 PM.

                Comment

                  #9
                  Re: Philips 37PFL9604/H12 : backlight will switch off after some time on

                  There are 9 contacts on the HV transformers. Upper ones, next to HV caps are numbered 5 to 6.
                  9 to 10 = 6 to 5 = 16 ohms
                  9 to 8 = 6 to 8 = 10 ohms

                  Regarding the BL_ON, there are about 26mV when the screen is lighted or when the issue happens.
                  Last edited by OobabeloO; 09-18-2018, 01:13 PM.

                  Comment

                    #10
                    Re: Philips 37PFL9604/H12 : backlight will switch off after some time on

                    I must add than since yesterday, the backlight will stay on for few seconds only.

                    Comment

                      #11
                      Re: Philips 37PFL9604/H12 : backlight will switch off after some time on

                      in post #6, first picture, there is a metal shield with 3 holes in it. It looks like there is a cap in there. Caps don't like getting backed like that. measure the ESR of that capacitor or change it. As far as your picture in post 8, I can't see enough I want. Make a high resolution, straight shot picture of the front and back of the whole board. It has to be high res, so we can zoom in. What kind of DMM do you have? At what scale is it set at? 16 Ohms is like way off with your reading its like a short. A low ohms reading is o.k. on the input of the transformer, but the output should be more like 1.3k Ohms. The backlights wouldn't come on at all with readings like you gave on the output side.
                      Last edited by CapLeaker; 09-18-2018, 07:00 PM.

                      Comment

                        #12
                        Re: Philips 37PFL9604/H12 : backlight will switch off after some time on

                        sorry, I forgot the multiplier:
                        9 to 10 = 6 to 5 = 16 ohms
                        9 to 8 = 6 to 8 = 10 to 8 = 8 to 5 = 10 Kohms

                        I have no ESR tool, but the cap mesaure 120uF and looks intact (no physical damage).

                        on the picture, you will see that:
                        - I changed the HV ceramic caps,
                        - I changed the mosfets
                        - I changed the IC601 (not a great job)
                        Attached Files

                        Comment

                          #13
                          Re: Philips 37PFL9604/H12 : backlight will switch off after some time on

                          I double check with the DMM that the contact are correct for IC601.
                          Attached Files

                          Comment

                            #14
                            Re: Philips 37PFL9604/H12 : backlight will switch off after some time on

                            The Fets and the IC wouldn't give that kind of symptom. Could be a cold solder joint, bad ccfls, bad transformer, because the backlight actually does come on at some point. What is the part number on that IC? Even 10k ohm doesn't sound right to me that would be bad. I had more like something like 1k or 1.3k ohm in my mind. The output winding has to be within 3%. So 1300 ohms on one side, the other side only could have 1339 ohms for example.
                            Last edited by CapLeaker; 09-19-2018, 04:41 PM.

                            Comment

                              #15
                              Re: Philips 37PFL9604/H12 : backlight will switch off after some time on

                              This is DDA002C. It drives the whole fets stage.

                              Comment

                                #16
                                Re: Philips 37PFL9604/H12 : backlight will switch off after some time on

                                I swapped the IC601 (DDA002C) with another one and the TV works without issue since 15'. Could it be that it was faulty or of very low quality ?

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