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Lenovo ThinkPad L13 18834-1M strange fault

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    Lenovo ThinkPad L13 18834-1M strange fault

    Hi People,

    i have a really strange fault on a laptop, which i am reparing. It's a Lenovo Thinkpad L13 18834-1M.

    Visually I could see that PU4501 (TPS51393RJER-GP) was burned so I removed the IC and ordered a spare part. Looking into Schematics showed, that the IC is responsible for the 3.3V rail. I noticed, that after removing the IC there is still a short on that bus (0.4 Ohm). Furthermore PU5201 & PU4551 are also short. Voltage injection showed a huge current drawn of up to 2.5 A but with only a slight temperature flickering at U4004 (TPS22976DPUR-GP) using ethanol. I removed that IC too and the short is still present. IC appears to be fine after testing.

    Although taking the great amount of current at 3 V I could not find any component getting hot. Also CPU is not hot at all.


    Using the USB-C charger there is no input voltage present when battery is disconnected. With battery attached, I get 12V at Vin. Vin seems to not be short. EC seems to not function, since the charging LED does not turn on. However, I can confirm 12V also being present at the battery + side. So I guess it's charging. When battery is attached alone, it's less than 1 V so indicating, that the battery is entirely empty.


    Does anyone have an idea to identify the fault?


    Thank you very much in advance!

    All the best

    Cool-Oase
    Last edited by cool-oase; 08-08-2022, 09:13 AM.

    #2
    Re: Lenovo ThinkPad L13 18834-1M strange fault

    See attached.

    Remove all power. No adapter and no battery.

    Meter in resistance mode.

    Check and post the resistance of the following:

    Source (1-2-3) and drain (5-6-7-8)
    Source and gate (4)
    Gate (4) and drain (5-6-7-8)


    Review the above checks on each of the DCIN mosfets.

    Post their resistance values and the scale used on your meter for these measurements.
    Attached Files

    Comment


      #3
      Re: Lenovo ThinkPad L13 18834-1M strange fault

      Check the PCH for heating up when injecting. With 2 - 3A you should feel something.
      FairRepair on YouTube

      Comment


        #4
        Re: Lenovo ThinkPad L13 18834-1M strange fault

        Originally posted by Sephir0th View Post
        Check the PCH for heating up when injecting. With 2 - 3A you should feel something.
        @Sephir0th
        Yes, I can confirm that the PCH is heating up. Did not see it at
        the beginning.

        Comment


          #5
          Re: Lenovo ThinkPad L13 18834-1M strange fault

          Originally posted by mon2 View Post
          See attached.

          Remove all power. No adapter and no battery.

          Meter in resistance mode.

          Check and post the resistance of the following:

          Source (1-2-3) and drain (5-6-7-8)
          Source and gate (4)
          Gate (4) and drain (5-6-7-8)


          Review the above checks on each of the DCIN mosfets.

          Post their resistance values and the scale used on your meter for these measurements.

          @mon2

          Q4201
          Source & Drain 15.22 MOhm (Meter set to 200KOhm)
          Source & Gate 100.9 kOhm (Meter set to 20MOhm )
          Drain & Gate 15.10 MOhm (Meter set to 200KOhm)


          Q4202
          Source & Drain 15.85 MOhm (Meter set to 200KOhm)
          Source & Gate 100.7 kOhm (Meter set to 20MOhm )
          Drain & Gate 15.40 MOhm (Meter set to 200KOhm)


          Q4206 and Q4205 have similar readings, i think that's why
          I rushed over it

          Thank's people!

          Comment


            #6
            Re: Lenovo ThinkPad L13 18834-1M strange fault

            Can you confirm the meter scale? The readings are higher than the meter scale as posted.

            The ~100k ohms reading is considered low for the mosfet world.

            Remove the 2nd mosfet (q4202) and see if the gate-source reading changes for the first dc-in mosfet.

            Measure the removed mosfet while it is out of the circuit. Often, the 2nd mosfet gets killed in such charger designs.

            Comment


              #7
              Re: Lenovo ThinkPad L13 18834-1M strange fault

              Originally posted by mon2 View Post
              Can you confirm the meter scale? The readings are higher than the meter scale as posted.

              The ~100k ohms reading is considered low for the mosfet world.

              Remove the 2nd mosfet (q4202) and see if the gate-source reading changes for the first dc-in mosfet.

              Measure the removed mosfet while it is out of the circuit. Often, the 2nd mosfet gets killed in such charger designs.

              Hi there,
              the given values were correct. I simply messud up the given scale in the breckets. My Voltmeter directly outputs the correct unit.

              So I removed the Mosfet(s) and both mosfets seem to be fine. I either read nothing between gate to source & drain or shortly sth. in the great MOhms.

              Also I do not get any reading on the pads of the board, so I guess that's not the issue.

              What brings a resistance of about 0.4 Ohm?

              Comment


                #8
                Re: Lenovo ThinkPad L13 18834-1M strange fault

                What brings a resistance of about 0.4 Ohm?
                That is a short. Connect the 2 meter probes together when in resistance mode and you should see probably the same value.

                What voltage do you see @ F4201 (fuse) - check both sides with the power adapter ?

                Comment


                  #9
                  Re: Lenovo ThinkPad L13 18834-1M strange fault

                  F4201 brings me 20.1 V. And the fuse is good!

                  F5501 is reading the 12 V and it is supposed to be 19V for the backlight.

                  True the resistance of the Voltmeter is also around 0.3 Ohm.
                  Last edited by cool-oase; 08-12-2022, 10:49 AM.

                  Comment


                    #10
                    Re: Lenovo ThinkPad L13 18834-1M strange fault

                    Review the details of PU401 = charger IC.

                    Where exactly are you measuring your battery voltage flow ?

                    Use only the names / labels from the schematic otherwise it is impossible to guess and follow.

                    Do you mean 20V_VINT_IN is VIN ??

                    Comment


                      #11
                      Re: Lenovo ThinkPad L13 18834-1M strange fault

                      Originally posted by mon2 View Post
                      Review the details of PU401 = charger IC.

                      Where exactly are you measuring your battery voltage flow ?

                      Use only the names / labels from the schematic otherwise it is impossible to guess and follow.

                      Do you mean 20V_VINT_IN is VIN ??


                      (1) Vin with 12 V from my first post referrs to PWR_DCBATOUT_3D3V @ PU4501.


                      (2) Battery voltage flow I measured at BATT1 / F4301.

                      (3) 20V_VINT_IN at Q4202 measures 20.2 V too.

                      (4) PU4401 seems to be fine. No shorts around and the mosfets read in the MOhm range with exeption of Gate to Source (~170 KOhm). This applies for PU4403,PU4404 PU4405 and PU4406.

                      PQ4403 is fine. Everything is reading in the MOhm range.

                      (5) Will use now net labels entirely and try to avoid confusion!

                      So just to clear things out furter !

                      I have short on:
                      PL4501 (0.5 Ohm to GND; PU4501 was blown. After soldering a new IC PU4501, 4004 & PCH were heating up slightly)
                      PL4801 (6 Ohm to GND)
                      PL4701 (4 Ohm to GND)
                      PL4702 (4 Ohm to GND)
                      PL5201 (0.4 Ohm to GND; removed PU5201 no difference)
                      PL5301 (0.4 Ohm to GND)
                      PL4551 (20 Ohm to GND)


                      I guess(ed) some of the coils were related to the power rails, but there are the ones left with almost 0 Ohm.

                      I initially believed, that 3.3 V was not present at all.
                      However, I can confirm, that 3.3 V is present on 3D3V_LDO_PD_R @ Bios 2
                      @Bios1 3D3V_SPI is short with 0 Ohm.

                      PWR1 has no short on Pin 2 and is reading 0.2 V.

                      Can it be that this is EC related ?
                      Last edited by cool-oase; 08-12-2022, 12:27 PM.

                      Comment


                        #12
                        Re: Lenovo ThinkPad L13 18834-1M strange fault

                        Guys, what are you doing here? The PCH is already confirmed bad.
                        FairRepair on YouTube

                        Comment

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