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Suggestions for repairing MLT666B PSU board for 32" Powerpoint LCD TV (Pics)

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    #1

    Suggestions for repairing MLT666B PSU board for 32" Powerpoint LCD TV (Pics)

    Hi!
    I was wondering if I could get a bit of advice. My Powerpoint 32" LCD TV has given up the ghost. When turned on, after ~5sec the backlit attempts to power up. Instead it "fizzles" for about 1sec, then cuts out after a brief flash. Using a flashlight, an image can still be seen on the screen.

    So the backlight isnt getting powered up. I opened up the TV and found a MLT666B power supply board. This was powering the backlight inverter circuit with 24v, according to the markings on the board (see pic). Measuring across this using a multimeter, i noticed that there was 24v on this rail, but after the 5sec after power up, then the tv attempted to power up the inverter, the 24v rail becomes unstable and dies to 0v within a second.

    Two of the CrapXon capacitors on the secondary stage of the supply board were bulging on the top. Even though these were on the 5v rail, i decided to replace all the capacitors on the secondary stage of power supply with some panasonic low ESRs caps I found on Ebay.

    The problem is, this didnt work. The 24v rail was still unstable. I connected up my tabletop power supply and fed 24v @ 3amp to the backlight inverter and found that the TV powered up fine after the 5seconds. It displayed an image on the screen perfectly with little or no noise from the backlight inverter. I also connected a load of two car 12v headlamps in series with the 24 rail of the board and found that they briefly flashed as soon as the tv was powered, but died after 1seconds, not even attempting to light on the 5seconds mark. This confirms that the 24v rail on the PSU isnt working.

    So as I am at a bit of a loss, I was wondering if anyone has any other suggestions of what else I could test on the power supply board? If so, how would I go about doing it? Thanks!!



    For anyone interested, or for future reference for anyone using this board, I have also included the BOM for the secondary capacitors and a circuit diagram/schematic of the MLT666 board.
    Attached Files
    Last edited by SchrodingersCat; 09-19-2012, 04:16 AM.
    Tags: None
    #2
    Re: Suggestions for repairing MLT666B PSU board for 32" Powerpoint LCD TV (Pics)

    change all the small lytics too.
    they are baked dry.

    Comment

      #3
      Re: Suggestions for repairing MLT666B PSU board for 32" Powerpoint LCD TV (Pics)

      Originally posted by SchrodingersCat View Post
      i decided to replace all the capacitors on the secondary stage of power supply with some panasonic low ESRs caps I found on Ebay.
      List the auction and buyer and show pictures of the top (vent), bottom (bung) of the caps purchased on ebay. They may be fakes.
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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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      Comment

        #4
        Re: Suggestions for repairing MLT666B PSU board for 32" Powerpoint LCD TV (Pics)

        Originally posted by retiredcaps View Post
        List the auction and buyer and show pictures of the top (vent), bottom (bung) of the caps purchased on ebay. They may be fakes.
        Thanks for the responses! @kc8adu, there are one or two small electrolytics before the secondary change. I will try and replace them so when I find some suitable ones. @ retiredcaps, these are the caps that I bought:
        These are the caps that I bought:
        http://www.ebay.co.uk/itm/2510100314...84.m1497.l2649

        http://www.ebay.co.uk/itm/1806821224...84.m1497.l2649

        http://www.ebay.co.uk/itm/3903236220...84.m1497.l2649


        Do you both still reckon its the electrolytic caps that are at fault?

        Comment

          #5
          Re: Suggestions for repairing MLT666B PSU board for 32" Powerpoint LCD TV (Pics)

          The ebay pics are inconclusive because I can't clearly see the top or bottom of the caps. Regardless, some sellers do the "bait and switch". They show pictures of the real caps and send fakes. Ideally, we want to see pictures of what you actually received.

          What country do you live in? If you are in the USA, there is no reason to go to ebay to buy caps.
          --- 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

            #6
            Re: Suggestions for repairing MLT666B PSU board for 32" Powerpoint LCD TV (Pics)

            Thanks for the advice retiredcaps. I have taken photos of the three main batches of Caps that I bought. The reason I had to go with Ebay is because im in Ireland and so couldnt get them cheap from anywhere else that I could find. From first looks, it looks that maybe one of the batches may actually be fakes. The color of the label is more brown on navy than gold on navy compared to the other two batches. They have flat rubber for the bung, and a more common cross shape for the vent. However, im a bit sceptical that this may be the issue, as I have replaced the cap with a authentic spare one from another brand that I had on hand, and found that the 24v supply still dies to 0v when under load. What do you think?
            Attached Files
            Last edited by SchrodingersCat; 09-24-2012, 07:28 AM.

            Comment

              #7
              Re: Suggestions for repairing MLT666B PSU board for 32" Powerpoint LCD TV (Pics)

              Those 470uF 35V are definitely fake Panasonics. It just goes to show that 99.8% feedback for goodbuy7888 means nothing.
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                #8
                Re: Suggestions for repairing MLT666B PSU board for 32" Powerpoint LCD TV (Pics)

                Originally posted by retiredcaps View Post
                Those 470uF 35V are definitely fake Panasonics. It just goes to show that 99.8% feedback for goodbuy7888 means nothing.
                Agreed. Feedback usually reflects shipping and packaging, not long term quality (the feedback is only as credible as the author). Ebay caps are usually fake; here are a few good sellers over there but I don't remember who they are off the top of my head.

                Badcaps.net sells caps overseas... not sure if shipping would be cost prohibitive or not.

                Otherwise, If I were in your place, I'd try farnell next time.
                sigpic

                (Insert witty quote here)

                Comment

                  #9
                  Re: Suggestions for repairing MLT666B PSU board for 32" Powerpoint LCD TV (Pics)

                  Thanks for the responses retiredcaps and ratdude747. I will message "goodbuy"7888 with the images and inform him. Farnell and Radionics would be the main electronic distributers here, but they require a minimum of 25 euro per order (about $30). At that money I could get a replacement PSU board off ebay (35 euro).

                  I will try and salvage suitable caps from some old items that I can find. However, as I mentioned earlier, even though the fake cap was stabilising the 24v line that I was having the issues with, im still a bit sceptical that its this thats causing the problem as I replaced the cap with another that I had around (I think it was luxon) but it still gave problems. As the psu briefly flashes the two 12v car bulbs when i connect the supply, but does nothing after 5 sec when you would expect the psu to enable the 24v for the inverter, i suspect that it may be a signalling issue. I will spend a bit of time trying to permanently enable the 24v output of the psu using the circuit diagram from my first post.

                  Comment

                    #10
                    Re: Suggestions for repairing MLT666B PSU board for 32" Powerpoint LCD TV (Pics)

                    Did you ever find a sollution to not having 24V? The power supply (MLT666 Rev 2.5) on one of my TVs has lost its 24v. Replaced all caps with reliable new replacements (most were bulging) and got back the STB-5v leg but still have no 24v on any of the legs it should be on.
                    Any help on this issue will be appreciated greatly!

                    Thank you in advance, JR

                    Comment

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