Dell Latitude E7440-P40G 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.
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.
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.
And next photo shows the mainboard bottom side.
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.
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.
Below shows some info on the datasheets of both mosfets.
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.
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.
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 van Bemmelen, Weert, The Netherlands.
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Note: You can read his previous repair article in the below link: