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Dell 1558 1.5v high side mosfet very hot?

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    #1

    Dell 1558 1.5v high side mosfet very hot?

    Hope someone can help with diagnosing/fixing a dell 1558 that I've been working on, with the aid of Quanta FM9C schematic and boardview, (downloaded from this site...Thanks

    Original symptom was shorted Mainboard, cutting out the psu.

    PQ41 (high side Mosfet on 1.5v Rail) got warm when 19v rail was injected with 5V-2a.

    The Mosfet was changed out from a donor board, and the short disappeared, but the laptop wouldn't start and would only charge the battery.

    On inspection the voltage regulator chip Tps51116 showed signs of a previous repair .... a blown trace next to pin 23 (VBST).

    I've re-bridged the trace today and the laptop now starts, as soon as 19v is plugged in. Fan spins at full chat immediately, but no display. PQ41 gets hot, very quickly. I get a low ohm reading from 1.5v coil to ground 3ohm..?

    I took off a couple of caps, but then i notice quite a few on Dram that I,m reluctant to lift, as the problem may lay elswhere. (When coil is lifted, the low ohm short apears on 1.5v sus... Any advice? (other than the bin)
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    #2
    Re: Dell 1558 1.5v high side mosfet very hot?

    I think you changed the PQ40 and PU5 together with PQ41. When the coil is removed, if the low ohm short circuit appears at 1.5v sus, not only the DDR3 ram but also the VGA_RAM chpip may be broken.
    I'm not happy to be so strong. because it's not humanly

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      #3
      Re: Dell 1558 1.5v high side mosfet very hot?

      Thanks for quick reply Sametbey.

      (I acknowledge changing PQ40 and PU5 too).

      With regards to VGA_RAM, I,m not familiar with the term chpip?

      Comment

        #4
        Re: Dell 1558 1.5v high side mosfet very hot?

        Hi Sametbey , Im fairly new to repairing motherboards, I assume chpip = chip...I get it now..short is present without DDR3 installed.What would you recommend for checking a VGA RAM short?

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          #5
          Re: Dell 1558 1.5v high side mosfet very hot?

          It goes to many places like RAM, VGA, PCH. You can find the heated part by removing the PL7 coil by giving it 1.2V. Or by removing the chips. and you can check after PQ11.
          I'm not happy to be so strong. because it's not humanly

          Comment

            #6
            Re: Dell 1558 1.5v high side mosfet very hot?

            Thanks again for your advice.

            Comment

              #7
              Re: Dell 1558 1.5v high side mosfet very hot?

              Update:

              Voltage injection on 1.5v reveals a bad cap (C565)
              now removed and PL7 reconnected.

              On start up, (no ram istalled) fan spins for about 5 seconds, then board shut down?
              PQ40 lowside fet was getting hot prior to laptop shutting itself down.?

              Checking PU5 and fet condition,
              Last edited by coltron; 02-01-2021, 09:57 AM. Reason: missprint

              Comment

                #8
                Re: Dell 1558 1.5v high side mosfet very hot?

                I believe the 1.5v is now sorted, though other issues are being investigated on this link

                https://www.badcaps.net/forum/showth...ight=1558+dell

                Thanks again to Sametbey for his input

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