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How To Repair No Power Symptom In Asus X555LD Laptop
My friend and Shop owner Chafik asked me to have a go on an Asus X555LD Laptop from a customer that was completely dead now. It didn’t do anything. No startup, no screen, no nothing. Those repairs I like the most because if I can’t fix it I didn’t make it worse either. See next photos below:
Apparently the Laptop broke down after a Beamer was connected to the external VGA connector.
I looked at the VGA components on the board but they all seemed fine. And when I came in the next day, Chafik already had tried if he could repair the Laptop by replacing the BIOS chip by a new version. After he had no luck he replaced the BIOS chip with the original Flash content. And I picked up the search on the compact mainboard that he already had completely removed from the Asus after opening the compact housing.
Next photo shows the mainboard side where the Power Adapter Input voltage goes into the Laptop by passing through the first 2 mosfets. The principle was explained in my previous Laptop repairs. And they protect the Laptop and the Li-ion Battery Pack from overcharging and over current (shorting currents). The first mosfet disconnects the Laptop from the input voltage coming from the Power Adapter. And Li-ions are normally never trickle charged after they are fully charged. Why the first mosfet is followed by two other mosfets. I can’t show any matching X555LD schematic because I found none, but the principle is the same with any Laptop.
Click on the photo to enlarge
Next photo zooms in on the first 2 mosfets, both MO 930 types. Replacing them won’t be easy because they are much smaller than the common mosfet types. And buying them can’t be easy either. I noticed that the input voltage didn’t pass the first mosfet and that was also the reason that the BIOS chip had no 3.3V working voltage. And the reason why nothing worked. That bad MO930 mosfet is surrounded by a blue circle. Because I had no other identical MO930 at hand I used a 0.1mm thin wire to bridge over the bad first mosfet. Which is also the only way to tell if the Asus will startup again. And afterwards when no parts get hot it means that the chance that it will have a positive change is very high now! Of course the wire is only temporarily because else the Battery could get overcharged or worse explode!
Now it was time to reconnect the Power Adapter to see if the Laptop would come back to life.
The BIOS chip got power again and indeed the Asus X555LD was running like new. Next photos prove that it again was only a small component at little cost that kept the Laptop from Booting up.
Buying MO930 mosfets won’t be easy but maybe we’ll find a equivalent type that fits too.
Below the bad bridged over MO 930 mosfet:
Although the customer told us that it must have been because of attaching his Laptop to his Beamer, it had nothing to do with it whatsoever afterall. And I know of repair companies that exploited this knowledge to price up the repair afterwards, as they said the complaint could come back within 3 months for the same reason. And that could mean a costly repair if true.
But since Chafik runs a reliable repair shop he does not do business that way!
I like to add a thing that revealed itself lateron. Following situation explains exactly what happened.
After the N-channel finally arrived 3 to 4 weeks later from China (A5 smd code) and my friend had replaced the first suspected bad mosfet something unexpected occured.
Because after the wire was removed and the ‘bad’ MO 0930 mosfet was replaced the Asus laptop didn’t boot at all. Which was not what we had anticipated at all!
So we started measuring on the gate of the first new mosfet to source. The resistance was only 30 ohms. So that explained why the first mosfet didn’t conduct. And why the Asus X555LD was dead again after the wire over the first mosfet was removed.
We continued our search and found that it was the second mosfet that already conducted and why the first mosfet failed to work all along!
We therefore replaced the second MO 0930 mosfet that turned out being ‘bad’ indeed.
After that procedure not only the Asus X555LD Laptop worked splendidly again by booting up into the Windows OS but also the Battery charging worked perfectly too! (which didn’t when the first suspected mosfet was bridged over by a tiny wire).
Because according to many forums on this model laptop, many people failed on repairing the Battery Charger. Probably because they only replaced the first mosfet instead of the second one. Like we succesfully did.
Following photo shows both N-channel mosfets PO 0930 that were okay now (they are sold with smd marking A5).
So this shows that although it was still a Laptop mainboard repair like the ones done before, this one was a little bit different after all! But keep in mind that these special mosfets are very tiny being about only 4 x 4 mm but they can handle more Ampèrage and Voltage than the normally much larger N-channel mosfets.
But nothing we can’t handle!
Until another fun repair!
PS: the importance of mosfets also shows in this repair that mentioned them 25 times! Must be my personal record!
Albert van Bemmelen, Weert, The Netherlands.
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Note: You can read his previous repair article in the below link: