Tale of two Precision M3800 laptops

Question

I recently came in to possession of two M3800's. They have some issues with charging and I'm not sure why. Read my story and give me some feedback if you can, please. One of them, as soon as I plug in the charger, the light on the charger turns off. I'm guessing that indicates the charger shuts itself off as a safety measure. I suspect there may be a short of some kind on the motherboard. Before the charger will turn back on I have to unplug it from the laptop and unplug it from the wall outlet. Then if I plug it back in to the wall the light on the charger will come back on. I have tried charging this one laptop with two different chargers (same model) and I get the same result of the light on the charger shutting off. This particular laptop won't turn on whether it is plugged in to a charger or not. I'd guess the battery in it is completely discharged. The other laptop will turn on while plugged in to a charger but will not charge the battery. This one seems to work mostly except for its charging issues. When I first plug in the charger an orange or amber light flashes on the side of the laptop. It is one of the lights in a row of lights next to a button you can press to check the charge of the battery. If I go in to the ePSA diagnostics it will tell me the battery is reaching the end of its life, error code 2000-0132 Validation 116781 but at the same time on the System Health tab it tells me Primary Battery Charge is at 1% and Primary Battery Health is at 98%. Why is it telling me the battery is reaching the end of its life and showing me the health is at 98%? This makes me think there may be a bad battery charging circuit on the motherboard but I'm not sure. The diagnostics say the charger is OK and I have tried a couple different chargers with the same result of the battery not charging. Just for fun I tried switching the batteries between the two laptops. The one that will run off a charger will still power on, doesn't blink the orange light, but still reports the same battery issue that the battery has reached the end of it's life while showing 1% charge and 98% battery health. The other one still causes the charger to shut off when it is plugged in to the laptop. So, other than the light not blinking, really no change.

Wiklen · Answer

With some help found on another thread here I have identified a bad capacitor on the M3800 motherboard in the laptop that causes the AC adapter to shut off when it is plugged in to the laptop. I used a multi-meter to check the capacitor and it is shorted.

mor2 · Answer

What happened next?  Were you able to get the m3800 capacitor replaced?  I have the same symptom.

Wiklen · Answer

Hi!

I did get my hands on a hot air station and replaced the cracked capacitor. Now the AC charger doesn't shut down when I plug it in to the laptop so it seems that one capacitor was the short the charger was detecting. However, the laptop still doesn't turn on when I press the power button. There is something more wrong with it but I'm not sure what. I'm a noob at board level repair and still learning a lot.

Wiklen · Answer

Just in case this needs to by typed: to any readers trying to repair their laptop, you may have similar symptoms but the causes may be different! This is not meant to be a literal step by step guide for board level repair. This is only an example of what you might find. Your malfunctioning components may be different than mine and you could still have the same result of a laptop not powering on.

Suspecting I might have a malfunctioning MOSFET, after more probing around the board, as I mentioned I'm not quite sure what I'm doing, I found that MOSFET PQ200 appears to be shorted. It is almost in the picture I posted earlier. It is in the upper right corner but it is covered up by the cable.

Wiklen · Answer

Actually, the MOSFET you can't see is identical to the one you can see just below the cable. It has the label MDS1521 on it.

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