Asus Laptop Repaired
Recently my friend Chafik asked my help in fixing an Asus Laptop of a customer that had a very big Short Circuit right after the DC Power input jack. The 19.5 V DC immediately short circuited in a degree that the Mainboard multilayer pcb completely had disappeared over at least 2 cm2 right behind the Input Jack. The current had ‘eaten’ the copper spores of the perhaps 6 inner layers and maybe also some of the outer layers on top and bottom. The epoxy had kind of melted over the complete area involved.
And the burning smell of copper and epoxy was really filling the service room with dislike because of the fumes after every new burning short current.
However the short circuit didn’t dissolve and the pcb kept on burning further. Something that is sadly nothing new in Repair, because bad short circuiting pcb’s keep on popping up so now and then.
The Asus laptop type N71VG we are talking about in this case, just wasn’t going to work and I noticed that the green led on the Power Supply (4.74A / 90Watt) went off after every new Short Current. But restored itself completely after about 15 minutes again! These Power Supplies are very new because normally they would break down internally after big Short Circuit Currents. And a repair would usually be necessary. But not with these very good totally new designed Power Supplies!
Next photo shows the gap that was burned because of the short circuit in the mainboard pcb.
Above photo also shows what needed to be modified to the pcb in order to dissolve the short circuit problem in the N71VG laptop multilayer board.
The 2 black parallel mounted input coils had to be removed because the short circuit was on their inputs. And therefore the 19.5V DC input voltage had to be connected at the endsides of both Coils (without leaving the coils in place of course). Also the Mosfet a typ 4538 (30V 9.6A) on the right of both coils was defect, being 490 ohm in one direction and about 370 ohm in the other direction. So that was another error that a replacement Mosfet type (also P-channel 30V 10A) 4407 couldn’t fix. Because that one also got defect after connecting with the measured Power supply 19.5V DC input. The replacement 4407 also was afterwards defect in 2 directions just like the original replaced Mosfet did. And measured out of the circuit equally defect like the 4538.
So because we also knew that the N71VG Asus laptop only worked okay when the input resistance at the Power Jack measured about 490 Ohm in one direction and very high in the other direction, we also simply had to remove the first Bad Mosfet 4538/or 4407 from the board.
The reverse polarity Diode that also was mounted at the board input circuit was still okay. And previous measurements on the Input Jack were taken with the Polarity Diode removed. This Diode is normally protecting the complete Mainboard behind it, from idiot users that try wrong Power supplies with reversed polarized Voltages. Simply by shorting the input voltage because it only conducts at wrong polarity.
And by connecting the 19.5V DC at the output side of the now removed first input Mosfet position. The Asus N71VG laptop should be working again!
But to show what had changed I will show you an example circuit from a HP Laptop/notebook that is compatible with the way in which most, if not all , Laptops nowadays are designed and working.
Above circuit clearly shows the incoming DC Voltage from the Power Supply going to the first 4538 P-Channel Mosfet (30V 9.6A).
The first Mosfet switches through the 19.5V DC to every other circuit on Board if the AC Power Voltage is active. So if our Mainboard pcb in this case is so bad at the input side that it keeps the entire Laptop from working we simply disable these components. And we put our 19.5V DC there ourself without having the necessity of this first switching Mosfet component that also acted as kind of protective circuit. But it is either this or a completely defect – throw it in the Bin – Laptop.
And I’m sure you’ll agree that there is no better way to solve the problem then the one presented here!
Hope that you all enjoyed this repair.
Albert van Bemmelen, Weert , The Netherlands.
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P.P.S- Note: You can also read his previous repair article below:
P.P.P.S- If you wish to learn how to repair laptop, you can check out the info HERE.