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Dell Latitude E7440-P40G Laptop Repair

By on September 16, 2020
dell laptop repair

 

 

 

 

 

 

 

My ICT shop owner friend Chafik asked me to help him checking on a Dell P40G laptop that no longer worked with the original power supply. As soon the adapter was connected to the laptop it went into protective mode because of the short circuiting mainboard that also produced smoke and some overheating components. And I helped him to disassemble the laptop and find the culprit.

dell latitute laptop repair

These Dell power adapters are different in the way that they also have an extra wire connection to check on the battery pack. And other universal adapters lack this communication wire to the battery pack and normally therefore won’t charge the pack because of it. This P40G Dell adapter also has a white led build into the cable plug that only works when the short circuit is gone. And it needs to be reset by re-plugging in the power cord to escape the protective mode the adapter was in.

dell laptop repair

Above photos show the original Dell adapter that came with this laptop and shows the white led that only is active after the problem is solved! Disassembling the laptop was doable but took some time to open the thinner made construction since no information was found online on how to exactly do that.

After all screws on the bottom side were removed we just needed to push the keyboard up, but first also remove the top rectangular frame with the text LATITUDE E7440 on it that covered the keyboard screws that were under it.

latitude motherboard repair

And next photo shows the mainboard bottom side.

dell laptop motherboard repair e7440

Because the original adapter kept going in protective mode we used another universal adapter to find the spot on the mainboard where the heat was generated and the smoke was coming from. This way we found the components that overheated. Two n-channel mosfets type AON7401 and 7403. With marking PQ4 and PQ700 on the mainboard.

aon7401 ic

We removed both mosfets that clearly where defect and needed to be replaced. The AON7401/PQ4 is a 35A 30V mosfet and the AON7403/PQ700 a 29A 30V mosfet. We also used a special thermal camera to check the spot that generated heat. Next photo shows a close-up on both mosfets taken with a digital microscope camera. It clearly shows a damaged and overheated 7401 mosfet.

aon7403 ic

Below shows some info on the datasheets of both mosfets.

aon7401 datasheet

aon7403 datasheet

But replacing both mosfets sadly didn’t help as both mosfets were destroyed right after we connected our universal adapter again. So they were not the cause of the problem and we started measuring where the real reason was. It turned out that the capacitor next to PQ700 was completely shorting. And this also caused shorting all other capacitors on both sides of the mainboard! Look at the brownish coloured capacitor on next, with the special microscope camera taken, photo. The culprit is marked by an orange arrow.

Only after this capacitor was removed the complete shorting of the mainboard was gone. What made it more difficult to find this capacitor and its position in the mainboard schematic was that even although we also found the correct laptop circuits, none of the capacitors on the mainboard were numbered! And almost all capacitors on both sides of the mainboard measured shorting the mainboard because of this single shorting capacitor on the mainboard bottom side. See also that photo with the mainboard label marking on it.

bad smd capacitor

dell latitude laptop motherboard repair

Above photo of the labelled mainboard top side where mosfets PQ704 and PQ706 show on their right the also shorting (green arrowed) capacitors because of the mentioned culprit capacitor next to position PQ700 on the bottom side.

dell latitude laptop motherboard repair

Above photo shows the mainboard top side label marking.




Conclusion: after the shorting capacitor was replaced followed by AON7401 and AON7403 mosfets , the laptop was fixed and ready and the original Dell adapter worked and also charged the battery pack. And after some additional tests my shop owner friend’s work was done!

albert from netherlands

Albert van Bemmelen, Weert, The Netherlands.

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Note: You can read his previous repair article in the below link:

https://jestineyong.com/vibrograf-b200-mechanical-watch-calibrator-repair/

 

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12 Comments

  1. Parasuraman S

    September 16, 2020 at 4:46 pm

    Great work! Very professional and thorough! Many thanks for sharing! These capacitors getting shorted is regular and it causes all sorts of trouble to the power supply!

    Likes(2)Dislikes(0)
    • Albert van Bemmelen

      September 16, 2020 at 10:27 pm

      You are so right Parasuraman! It are not only the polarized e-caps but also the smaller bipolar capacitors that deteriorate and cause problems. Even if they do not leak they still do short circuits.

      Likes(4)Dislikes(0)
  2. Mark

    September 16, 2020 at 7:17 pm

    Well done Albert!
    How did you decide what capacitor to use? I can't see in your article that you had a schematic which gave you the correct one.
    Great diagnosis 🙂

    Likes(2)Dislikes(0)
    • Albert van Bemmelen

      September 16, 2020 at 9:35 pm

      We just assumed to take of the strange colored capacitor, and after we measured it with our DMM in diode/beep test we were completely confident it was the culprit. And because the short circuits on both sides of the board were gone afterwards it only confirmed our diagnostics.
      And replacing such a small capacitor is so much easier compared to replacing those tiny 3x3 mm mosfets. In this case it is important to know that the culprit capacitor didn't generate any heat because its resistance was zero! But it killed the mosfets that got over-heated because of it.

      Likes(2)Dislikes(0)
      • Albert van Bemmelen

        September 17, 2020 at 1:10 am

        About your question about the schematic and what mosfet and circuit to see Mark, look at page 47 of this E7440 complete service manual here

        And search for Mosfets PQ700,PQ4 and PQ704, PQ706.

        Likes(3)Dislikes(0)
  3. babu ms

    September 16, 2020 at 9:59 pm

    Fantastic work.Many many thanks for sharing.Such a articles added knowledge to me.Once more thanks very much.

    Likes(1)Dislikes(0)
    • A van Bemmelen

      September 16, 2020 at 11:27 pm

      No problem Babu. Glad you liked it! Just know that this laptop design principle often applies to many if not all laptops that exist today! One mosfet for the about 19V adapter DC input voltage followed by a second mosfet for battery pack charging control, and current control, and 5V,3.3V and other voltage IC controllers that are fed by this input voltage too. And the processor runs on a special 1V or lower voltage controller. If only one of them fails the laptop won't run.

      Likes(1)Dislikes(0)
  4. Yogesh Panchal

    September 16, 2020 at 11:38 pm

    Albert,
    Good! Repair
    Now a days 50% faults in Smart Gadgets is because of this tiny capacitors and for this type of inspections HDMI Microscope & Thermal Video Camera Is must. A technician must have to invest in if want to survive in this Repair Business. Because parts and circuits are now getting microscopic compress in the size.

    Likes(1)Dislikes(0)
    • Albert van Bemmelen

      September 17, 2020 at 1:07 pm

      You're completely right Yogesh! My shop ownwer friend therefore has several quite expensive cameras to be able to quick diagnostic any repair. At home I have an extensive set of hardware tools and testers too, and even spare hot air solder stations and at least 6 oscilloscopes (3 of which were repaired, one build as kit, and the other 2 were bought new), amongst many build projects and added testers. Probably even more than my shopowner friend does but he also has an ultrasonic cleaning tank, and a simple hotfix machine for the more difficult repairs that involve issues as moisture damaged boards. And luckily I do not have to fix anything professional to survive running an ICT shop but I do love my electronics hobby more than anything else!

      Likes(2)Dislikes(0)
  5. dicksy

    September 17, 2020 at 6:14 pm

    nice article and very help full.i need to ask a question,from all of you,are there any solutions to increase our eyesight

    Likes(0)Dislikes(0)
    • Albert van Bemmelen

      September 17, 2020 at 9:09 pm

      I wish we could say that we nowadays added the most modern equipment that exists to our equipment, like say an electronic microscope or a scanning (electronic) microscope, but those sadly are still not available moneywise. Although the first electronic microscope was already invented and designed around 1933. We use (stereo) microscopes and other common microscopes, and the expensive thermic camera. Why is it that you ask Dicksy?

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  6. Chris

    September 18, 2020 at 1:56 pm

    I am using a cheap USB microscope (camera) hooked up to PC and modified to fit on my magnifier arm works very well they are available very cheaply on many e sites.
    Great for Id of smd components and bad tracks normally comes with software for viewing but some really simple and better options on the web

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