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    Asus pro p3540fa

    Hi, the laptop got a bit of liquid damage, under the thermal cam

    i found a shorted cap and RT8249CGQW getting hot.

    I have removed the bad cap and replace the RT8249CGQW ordered from here:

    https://www.aliexpress.com/item/3281...40d61802oFVwMv

    now nothing getting hot but MOBO will not charge and not start

    charger IC bq24780S
    VCC 18.99v
    ACDET 2.6v
    REGN 5.99v
    CMSRC 18.75v
    ACOK 5.93v
    ACDRV Not sure, multimetwr show 1
    How can i continue inspection ?
    Last edited by koby058; 05-22-2022, 12:48 PM.

    #2
    Re: Asus pro p3540fa

    Code:
    ACDRV Not sure, multimetwr show 1
    Hi. The '1' is the way your meter is reporting that the voltage scale is too low. Switch to the next higher scale on your meter and test again.

    Perhaps you are using the 20 volt scale?

    This is a good sign. Note that to turn ON the mosfet, the gate voltage should be about 6 volts higher than the voltage being passed through the mosfet between source & drain.

    The charger IC boosts the adapter voltage to drive the gate pin if all is well.

    Monitor the voltage on source. Then drain after the confirmation of the true gate voltage. Post your readings.

    Comment


      #3
      Re: Asus pro p3540fa

      Thanks for your help.

      It was actually less on the multi-meter as you suspected

      voltage is 24V.

      the question is on which mosfet's gate i should expect this voltage,

      I am attaching photos of the board
      Attached Files

      Comment


        #4
        Re: Asus pro p3540fa

        Can you post some pix of the charger IC area (bq24780S) ?

        The mosfet will be a 8 pin device (4 on each side; black in color) and will be near the DCIN connector for the adapter.

        This is the first mosfet on the logic board to receive the adapter voltage.

        One side will be DCIN adapter voltage; GATE = 24V (good voltage for ON); other side will be DCIN adapter (perhaps a bit less but about the same).

        The ANOTHER similar mosfet will follow.

        Check the voltage of the 1-2-3, GATE (4) and 5-6-7-8 pins of the SECOND mosfet.

        Often pin # 4 = GATE = same as the FIRST mosfet. Confirm the gate voltage and see if the mosfet is passing through the adapter voltage like mosfet # 1.

        Comment


          #5
          Re: Asus pro p3540fa

          well, i know what a mosfet is but thanks any way

          the charging IC is in the bottom of the back of the board

          the first mosfet is on the top right of the board S=19.2v D=19v G=24v

          the second mosfet is on the top back of the board and have 19 on S,D, and 24v on G.

          i would like to mention that i have 19v on the buck converters on the front bottom left of the board
          Attached Files

          Comment


            #6
            Re: Asus pro p3540fa

            well, i know what a mosfet is but thanks any way
            This is for the benefit of the future reader that may not.

            Check the voltage to ground of each coil on the logic board. Each coil is linked to a buck regulator. Something is not being powered or you have a corrupt bios - either main or with the KBC.

            Comment


              #7
              Re: Asus pro p3540fa

              I have 0 volt on all coils but one, that have 0.24v

              attached photo
              Attached Files

              Comment


                #8
                Re: Asus pro p3540fa

                Ok. Remove all power. Meter in resistance mode.

                Check the resistance to ground of each coil. Some of the coils are expected to be low as they are low voltage rails for the CPU / GPU, etc.

                Perhaps you have a shorted cap on one of these power rails and this is causing an excessive current draw that is forcing the buck regulator to shut off.

                As we do not know which coil is linked to which voltage, this will be a tough repair but start with the resistance checks. List each resistance value - for now, do not worry about their location (ie. Stevie Wonder Logic Repair services...)

                Comment


                  #9
                  Re: Asus pro p3540fa

                  10k
                  6.75k
                  0.54m
                  54 ohm
                  16.2 ohm
                  4.5 ohm
                  4.5 ohm
                  6.3 ohm
                  154 ohm

                  Comment


                    #10
                    Re: Asus pro p3540fa

                    0.54m
                    54 ohm
                    16.2 ohm
                    4.5 ohm
                    4.5 ohm
                    6.3 ohm
                    154 ohm
                    These are suspects.

                    0.54m
                    What scale and resistance is this reading ?

                    Comment


                      #11
                      Re: Asus pro p3540fa

                      scale = 20m

                      scales available : 200,2000,20k,200k,20m,200m

                      it take me long time to measure this one, its like keep growing slowly

                      Comment


                        #12
                        Re: Asus pro p3540fa

                        Ok. That is an ok reading.
                        16.2 ohm
                        4.5 ohm
                        4.5 ohm
                        6.3 ohm
                        154 ohm
                        At your discretion, consider to start with the top reading coil -> use your local power supply set to 0v8 (that is what we prefer to use) @ current limit 1A (for starters).

                        Then with no power to the logic board -> apply the RED power supply lead to the coil. Black power supply lead to ground. Then check for any part to see if it is getting warm. Increase the current only if required BUT not the voltage.

                        We like to use IPA (alcohol) poured over the caps to see if they start to mist the cap from heat evaporation.

                        We want to keep the voltage low just in case this rail under test is mating with the CPU rail which will kill the CPU if the rail is too high.

                        If you find a cap that is heating up / getting warm -> desolder it with flux and check the resistance to ground again to see if the 'short like' resistance is gone.

                        Comment


                          #13
                          Re: Asus pro p3540fa

                          16.2 ohm, this one is the smallest coil and it is the nearest to the CPU

                          I have a thermal cam

                          Comment


                            #14
                            Re: Asus pro p3540fa

                            Likely this is not a concern to test as the CPU will be a low resistance - for now you can skip this one. CPU rails are often with a low resistance.

                            Proceed to check the next coil. Often CPU voltage rails are very high current so they will use multiple & ganged in parallel voltage rails. Then each rail is cycled on / off to prevent any single rail from being too tasked for the high current.

                            Comment


                              #15
                              Re: Asus pro p3540fa

                              tried to inject voltage to coils, the board does not consuming current

                              could it be from the RT8249CGQW i have replaced?

                              maybe it is not working, can you help me debug this ?

                              Comment


                                #16
                                Re: Asus pro p3540fa

                                i have encountered a weird behavior, for a few minutes after disconnecting ac adapter, i have a short on this coil (after a few minutes short disappear ).

                                when injecting power the mosfet between the coil and the capacitor is getting hot.

                                i have tried to remove the brown capacitor short remains, the capacitor resistance was in mega ohms area

                                attached picture

                                any suggestion ?
                                Attached Files
                                Last edited by koby058; 05-24-2022, 01:32 PM.

                                Comment

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