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    iMac power supply short-circuit diode to ground

    Hi,
    I have an iMac with the power supply reference: ADP-200DFB.

    This power supply is not working and when trying to debug the problem with a multimeter, I found what I thought to be a short circuited diode (the short-circuit is to ground).

    With the continuity test it beeps both ways, so doesn't block current flow at all then reverse biased. But after removing the diode the beep continues and the diode seems healthy. It seems to exist a real short-circuit.

    As I have another ADP-200DFB power supply (faulty one as well), I have checked the same diode and it seems to be short-circuited as the first.

    I don't know if you agree with me, but there should be another component which is connected to this diode, which is faulty and creating this short-circuit to ground.

    Any ideas on how to debug this? Where should I start?

    Thanks in advance.
    Best regards,
    Pedro
    Attached Files

    #2
    Re: iMac power supply short-circuit diode to ground

    Ok
    Is the short on the secondary side of this switching power supply
    If so start removing parts one at a time until the short is gone

    If it is on the primary side of the switching power supply then you for a part with 5 or more pins that the data sheet says that it is a regular of some type and check it for short
    9 PC LCD Monitor
    6 LCD Flat Screen TV
    30 Desk Top Switching Power Supply
    10 Battery Charger Switching Power Supply for Power Tool
    6 18v Lithium Battery Power Boards for Tool Battery Packs
    1 XBox 360 Switching Power Supply and M Board
    25 Servo Drives 220/460 3 Phase
    6 De-soldering Station Switching Power Supply 1 Power Supply
    1 Dell Mother Board
    15 Computer Power Supply
    1 HP Printer Supply & Control Board * lighting finished it *


    These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10%

    1 Over Head Crane Current Sensing Board ( VFD Failure Five Years Later )
    2 Hem Saw Computer Stack Board

    All of these had CAPs POOF
    All of the mosfet that are taken out by bad caps

    Comment


      #3
      Re: iMac power supply short-circuit diode to ground

      Originally posted by sam_sam_sam View Post
      Ok
      Is the short on the secondary side of this switching power supply
      If so start removing parts one at a time until the short is gone

      If it is on the primary side of the switching power supply then you for a part with 5 or more pins that the data sheet says that it is a regular of some type and check it for short
      Hi,
      Thanks for your reply.
      This diode is on the primary side, but I could not really understand your last sentence. Could you, please, re-phrase it for me?

      Cheers,
      Pedro

      Comment


        #4
        Re: iMac power supply short-circuit diode to ground

        You should look and if you find an ic chip that the part description says that it is some type of online switcher or off line switcher or some type of PWM controller chip for switching power supply

        Here is an example “TOP246” or TOP247

        https://www.digchip.com/datasheets/p...TOP247-pdf.php
        Last edited by sam_sam_sam; 06-09-2020, 06:35 PM.
        9 PC LCD Monitor
        6 LCD Flat Screen TV
        30 Desk Top Switching Power Supply
        10 Battery Charger Switching Power Supply for Power Tool
        6 18v Lithium Battery Power Boards for Tool Battery Packs
        1 XBox 360 Switching Power Supply and M Board
        25 Servo Drives 220/460 3 Phase
        6 De-soldering Station Switching Power Supply 1 Power Supply
        1 Dell Mother Board
        15 Computer Power Supply
        1 HP Printer Supply & Control Board * lighting finished it *


        These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10%

        1 Over Head Crane Current Sensing Board ( VFD Failure Five Years Later )
        2 Hem Saw Computer Stack Board

        All of these had CAPs POOF
        All of the mosfet that are taken out by bad caps

        Comment


          #5
          Re: iMac power supply short-circuit diode to ground

          Originally posted by sam_sam_sam View Post
          You should look and if you find an ic chip that the part description says that it is some type of online switcher or off line switcher or some type of PWM controller chip for switching power supply

          Here is an example “TOP246” or TOP247

          https://www.digchip.com/datasheets/p...TOP247-pdf.php
          Hi,
          The ICs nearby the diode are the DAP017-AH, for which I cannot find a Datasheet and I don't even know what it is, and the DAP015-AD (some kind of oscillator/pwm generator).

          Could one of these be the culprits?

          Kind regards,
          Pedro

          Comment


            #6
            Re: iMac power supply short-circuit diode to ground

            Originally posted by pcardoso73 View Post
            Hi,
            The ICs nearby the diode are the DAP017-AH, for which I cannot find a Datasheet and I don't even know what it is, and the DAP015-AD (some kind of oscillator/pwm generator).

            Could one of these be the culprits?

            Kind regards,
            Pedro

            It look like the data sheets for this device has been pulled for some reason

            The part can be found but I do not know if it is becoming obsolete and that why you can find the data sheet

            Is where the short is coming from if so just replace it and see if the short is not there any more

            Can you please show a clear picture of this part so the information can be read

            Thanks
            Last edited by sam_sam_sam; 06-10-2020, 07:30 PM.
            9 PC LCD Monitor
            6 LCD Flat Screen TV
            30 Desk Top Switching Power Supply
            10 Battery Charger Switching Power Supply for Power Tool
            6 18v Lithium Battery Power Boards for Tool Battery Packs
            1 XBox 360 Switching Power Supply and M Board
            25 Servo Drives 220/460 3 Phase
            6 De-soldering Station Switching Power Supply 1 Power Supply
            1 Dell Mother Board
            15 Computer Power Supply
            1 HP Printer Supply & Control Board * lighting finished it *


            These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10%

            1 Over Head Crane Current Sensing Board ( VFD Failure Five Years Later )
            2 Hem Saw Computer Stack Board

            All of these had CAPs POOF
            All of the mosfet that are taken out by bad caps

            Comment


              #7
              Re: iMac power supply short-circuit diode to ground

              Originally posted by sam_sam_sam View Post

              Can you please show a clear picture of this part so the information can be read

              Thanks
              Please find the picture attached.
              As I have 2 PSUs with exactly the same problem, maybe this IC had a fundamental problem and was removed from the market.

              Pedro
              Attached Files

              Comment


                #8
                Re: iMac power supply short-circuit diode to ground

                Can we see good clear straight shot pictures of the whole top and bottom side of the board, pictures must be clear enough to see the traces?
                That diode may be snubber Diode that is connected in parallel with the primary driving winding of the power transformer so that is why it shows low resistance reading, we need pictures to verify since it can also be connected in parallel with low Ohm resistor.
                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


                  #9
                  Re: iMac power supply short-circuit diode to ground

                  Originally posted by budm View Post
                  Can we see good clear straight shot pictures of the whole top and bottom side of the board, pictures must be clear enough to see the traces?
                  That diode may be snubber Diode that is connected in parallel with the primary driving winding of the power transformer so that is why it shows low resistance reading, we need pictures to verify since it can also be connected in parallel with low Ohm resistor.
                  Hi,
                  Thanks for your reply. I've taken some pictures and I hope they are good enough.

                  Cheers,
                  Pedro
                  Attached Files

                  Comment


                    #10
                    Re: iMac power supply short-circuit diode to ground

                    Forgot this picture.
                    Attached Files

                    Comment


                      #11
                      Re: iMac power supply short-circuit diode to ground

                      I look at the bottom side of the board but unable to trace out the diode connections due to the black solder mask, I cannot see the traces.
                      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: iMac power supply short-circuit diode to ground

                        Hi,
                        I am sorry for not replying.
                        I bought another power supply, which works. As I have 2 power supplies from DELTA, this time I used another brand (ACBEL) and it works fine.

                        However, I really want to fix the other 2 or at least one of them.
                        I can try to clean it, so you can see the tracks, but even looking at it in my hand I cannot see much.
                        Maybe this PCB is multi-layer, since there are few tracks visible.

                        I am feeling tempted to sacrifice one of them, removing all the components, and try to draw a schematic from there, so I can fix the other.
                        I would like to take the components off the sacrificed board and design a PSU myself. In principle, I should have good quality components there.
                        Cheers,
                        Pedro

                        Comment


                          #13
                          Re: iMac power supply short-circuit diode to ground

                          This will be something that will be very time consuming
                          At the same time it might be a good learning experience for you

                          The two power supply are they the exactly the same or are they little different
                          Last edited by sam_sam_sam; 07-29-2020, 07:08 PM.
                          9 PC LCD Monitor
                          6 LCD Flat Screen TV
                          30 Desk Top Switching Power Supply
                          10 Battery Charger Switching Power Supply for Power Tool
                          6 18v Lithium Battery Power Boards for Tool Battery Packs
                          1 XBox 360 Switching Power Supply and M Board
                          25 Servo Drives 220/460 3 Phase
                          6 De-soldering Station Switching Power Supply 1 Power Supply
                          1 Dell Mother Board
                          15 Computer Power Supply
                          1 HP Printer Supply & Control Board * lighting finished it *


                          These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10%

                          1 Over Head Crane Current Sensing Board ( VFD Failure Five Years Later )
                          2 Hem Saw Computer Stack Board

                          All of these had CAPs POOF
                          All of the mosfet that are taken out by bad caps

                          Comment


                            #14
                            Re: iMac power supply short-circuit diode to ground

                            Hi
                            The power supplies are exactly the same.
                            Pedro

                            Comment


                              #15
                              Re: iMac power supply short-circuit diode to ground

                              Originally posted by pcardoso73 View Post
                              I can try to clean it, so you can see the tracks, but even looking at it in my hand I cannot see much.
                              Maybe this PCB is multi-layer, since there are few tracks visible.
                              Most likely it is just a dual layer PCB, the problem is the black solder mask.
                              Try to get sunlight through the PCB, or a really strong artificial light.
                              That may reveal the tracks...
                              Example:
                              https://www.overclock.net/photopost/...4744-Crop.jpeg
                              https://www.overclock.net/photopost/...4741-Crop.jpeg
                              "The one who says it cannot be done should never interrupt the one who is doing it."

                              Comment


                                #16
                                Re: iMac power supply short-circuit diode to ground

                                Hi,
                                I tried both sun and a lamp, but I can't see anything.
                                The board is very populated, so it is difficult for the light to pass. I think it will be possible to do that if I start desoldering heatsinks, transformers, inductors and large components.
                                As I have 2 identical power supplies maybe I will do that to one of them.
                                However I think it is a multi-layer PCB and I will tell you why. Maybe I am wrong.

                                When I test the diode, as referred in the beginning of this thread, the multimeter shows a short circuit. After removing the diode, the diode is ok and the short circuit remains.

                                When I put the test probes in the hole where the diode was, the multimeter beeps but I am touching the inside of the hole.

                                You see, my soldering iron is not very good. It is a 15w one, which has no thermal capacity to unsolder thick solder balls. To desolder the diode I had to keep the iron there for a long time and apply lots of pressure. With that, the pads lifted from the pcb. So, now the only tracks I have are possibly tracks inside that hole. I don't know it is just a thought.

                                Cheers,
                                Pedro

                                Comment

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