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    #1

    NM-B891 (T490s) no picture

    Hi,

    Recently I received a NM-B891 board from a ThinkPad T490s with signs a abuse: around the TBT and BIOS chip many components were missing (probably from too much hot air) and the board was flooded with flux in many places. I replaced all missing parts according to the schematic and boardview, I replaced the TBT and the BIOS chip with new and flashed chips and I replaced a blown fuse of the power LED. Now the situation is as follows: when plugging in the charger the charge LED lights up and the power LED blinks 3 times. When I press the power button the power LED lights up, the fan starts spinning for a few seconds, the current briefly jumps to 600 mA, then drops to 500 mA and stays there, but there is no picture. All rails are ok, the CPU gets warm. A thermocam doesn't show any hotspots. The CLK port of the BIOS chip shows activity on an osci for about 1 second, but no more. All 3 crystals have a clear, undistorted signal. I measured the following signals:
    CPU_PWRGD 1,0 V on TP1301
    VCCST_PWRGD 1,0 V on R1322
    CPUCORE_PWRGD 3,3 V on PR0807
    DRAMRST 1,2 V on R0504
    RSMRST 3,3 V on ThinkEngine
    RTCRST 3 V on J1701
    PLTRST# 3,3 V on U1301 Pin 4

    What else can I do?

    schematics and boardview can be found here: https://www.badcaps.net/forum/showthread.php?p=1152211
    Last edited by rumpumpel1; 10-08-2023, 01:23 PM.
    Tags: None
    #2
    Re: NM-B891 (T490s) no picture

    Ram is onboard or its an external slot? To me it seems ram is not read by system properly. Does it goes straight to 600ma and then drops to 500mA or is there sufficient variation?

    Comment

      #3
      Re: NM-B891 (T490s) no picture

      Ram is onboard. There is not enough current variation.
      Recently I fixed some defective RAM modules. When using these RAM modules the ThinkPad showed the same behaviour: no beep code, no picture and the current quickly drops to 400 - 500 mA.
      But here DRAMRST = 1,2V. How else could I check the RAM?

      Comment

        #4
        Re: NM-B891 (T490s) no picture

        Presence of DRAMRST# is a good sign and proves cpu is alive as its reset by CPU. However, the most important is communication of RAM SDA/SCL with PCH. usually there is N channel 6 leg mosfet between ram and PCH. Look for SDA/SCL on schematic for ram section. Check whether any of these 2 smbus lines are pulled low. Best to test this communication with a DSO.

        Comment

          #5
          Re: NM-B891 (T490s) no picture

          In the schematics I found a lot of SMB/I2C connections using SDA /SCL, but there is none between PCH and RAM. The Block diagram on page 2 shows Channel A+B as the only connection between RAM and CPU/PCH.
          By looking into the schematics of some older ThinkPads it looks like, this SDA/SCL between RAM and PCH ist present for DDR3 but not anymore for DDR4.
          Last edited by rumpumpel1; 10-10-2023, 03:41 AM.

          Comment

            #6
            Re: NM-B891 (T490s) no picture

            SPD is generally inside BIOS for soldered RAM, so there is no external ROM to read.
            OpenBoardView — https://github.com/OpenBoardView/OpenBoardView

            Comment

              #7
              Re: NM-B891 (T490s) no picture

              Thank you for clarification.
              But then: how can I check if RAM is working except for looking on DRAMRST?

              Comment

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