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Trying to bring IBM L40SX back to life

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    Trying to bring IBM L40SX back to life

    I have this L40SX that appears dead. The display and the small display indicators are the only signs of life, the display backlight turns on and I can control the brightness, but that's all.
    The rest of the machine looks dead, no chips gets warm, no sound, nothing.

    I'm using a modern power supply rated at 15V.
    I've removed all the batteries inside, I've noticed some corrosion on the battery terminals in the pcb, but doesn't look bad.
    Also I see that some capacitors have leaked, but I see no traces damaged.

    I've noticed this component marked "SOC 5A" that looks like a fuse (check pic attached). Is it a fuse? because it doesn't have continuity on my multimeter.
    Will this machine power on without any of the batteries installed?

    Any help will be appreciated.

    IMG_1891.jpg IMG_1881.jpg IMG_1877.jpg IMG_1879.jpg IMG_1889.jpg IMG_1885.jpg

    Looks to me like 50C 5A. Thermistor? Thermal Fuse? Kinda weird it's off on it's own plug-in board.


      SOC makes fuses, so a 5A fuse would make sense.

      A blown fuse would explain the lack of life.


        How many amps does that power supply put out? Looks like the original was 2.7, which is pretty hefty.
        Looks like it should be able to run without the battery
        Lots of power info at the following link


          From the relevant wiki page:

          "The Wall Street Journal reported that IBM had received 15 complaints of a short circuit occurring between the circuitry and a conductive coating inside the case which, in some instances, has melted a small hole in the case. The short occurs when the laptop is run on batteries, and IBM reported it will install a fuse to stop overheating. They had to issue a recall for 150,000 machines"

          I think this might be it.


            Originally posted by eswan View Post
            How many amps does that power supply put out? Looks like the original was 2.7, which is pretty hefty.
            Looks like it should be able to run without the battery
            Lots of power info at the following link
            4.5 Amps.
            I have a 4 amps pico fuse, it's smaller than the one in the machine, but for testing maybe it works.
            Anybody with this machine should replace the caps, as they are leaking.


              Well, after fuse replacement machine now looks shorted.
              Upon power it on thr display lights up for a fraction of a second.

              I think some component is shorted.
              Iíll replace all the caps as they are leaking.


                Caps replaced, machine still not booting. any ideas where to look at/test?
                Last edited by gleegum; February 8, 2021, 04:20 PM.


                  Originally posted by gleegum View Post
                  Caps replaced, machine still not booting. any ideas where to look at/test?
                  Have you checked the rails for shorts at all?


                    Originally posted by TheDrip View Post
                    Have you checked the rails for shorts at all?
                    I did some test in individual components like 74xx chips and they weren't shorted. So I didn't found any shorts in this rail (5v?)
                    I'm assuming there's a 12v rail also, not sure where to test.
                    There's no much info about fixing this machine, service manual says just replace the entire board. Machine it's beyond my repair knowledge.
                    One tantalum cap (pic attached) was shorted upon measurement, unsoldered one leg and it was ok, Maybe this is the 12v rail and it's shorted somewhere.

                    I found no schematics about this machine, also the multilayer board is a PITA to solder/unsolder components.

                    Last edited by gleegum; February 9, 2021, 11:02 AM.