Asus Laptop No power Not Charging Repair- How Mosfets work and short circuit diagnosis

we're going to be working on the same laptop that we did in the previous video this is the Asus q3 o2l where we changed the charging port connector on it after we changed the charging port I tested the voltage at this point and it's given 19 volts when the customer brought this in we were not getting 19 volts at the positive side of the connector so the connector had to be changed now after we changed the charging port connector laptop is still not powering on using the charging cable it is powering on using the battery but not the charging cable and the cable is not able to charge this laptop so there is obviously something else going on right now if we remove the battery and try to power this laptop on using the charging cable the laptop will not power on it will only power on on a charged battery let's go near the charging connector that we did right over here and we should have two MOSFETs nearby right there these are most likely and type MOSFETs the number one two or three are usually the source pin number four is the gate and these four pins here are the drain one moment the purpose of those two MOSFETs is to protect the circuit in case maybe you have another charger plugged in where the polarity is opposite you do not fry this board so we need to have voltage on the gate in order for voltage to pass from drain to source right now if we test drain we should have 19 volts if we test drain we should have 19 volts last time what we did when we changed the charging port connector is we tested this pin here and this one is given us 19 volts when the customer brought this computer and this pin was not giving us 19 volts because the charging conductor was bad so right now we are getting 19 volts and as you can see so I'm testing the positive pin of the connector and we get 19 volts now if we go to the MOSFETs on the drain we should be getting 19 volts any one of those pins is the drain we are getting 19 volts of course 19 19 and 19 this is all connected as one pin let's measure the gate the gate is given us zero because there is no voltage coming on to the gate drain is Nayan up as the voltage onto the source if we test the source right now zero everything is zero let's test the drain here this is the drain we should be getting 19 volts now look at that we are getting 10 point 5 volts so there's a leak a leak could be caused by a partial short do we have any voltage on the gate here we do not zero volt when we have voltage on the gate above the threshold of the chip resistance of the chip will go down which allows current to go from drain to source when we have zero volts on the gate then resistance goes up which acts as an insulator between the drain and source when the voltage will flow between drain to source so right now we are getting 19 volts here but no voltage on gate since we do not have voltage on gate this voltage is not on a pass on to the source so if we test the MOSFET here the drain we should be getting 19 volts and this 19 volts is coming directly from the charging connector from here since we do not have voltage on the gate then this voltage is Diana Pass itself from drain to source which means the voltage is now in a pass to the rest of the board meter and diode mode let me test the drain that is the source test source and drain look at that drain is given as a short drain on this I see is given a short no short no short on the drain here this explains why we have 10 volts on the drain here and not 19 there is a short going on here meter and dired modred probe on ground and probe on drain we have a short which laptop is it so let's go out and we move this chip thank that's it let's put that ship on the side and quickly clean up this area right now we tested a short on this side of the mosfet which is the drain let's go ahead and test again to see if we still have a short and we do so I want to see if I have the charging cable plugged in do we still get down volts on the drain let's test and yes we still have 10 volts here what I'm going to do is remove this chip as well so what can cause a short on the board we have capacitors that can cause a short or maybe some other ICS on the board that could cause a short what we're gonna do is start by testing the capacitor is nearby randomly and we do not have a short here let's test this one here now we do not have a short let's test this one not short anything else here resisters do not cause a short so we do not need to test any of the resistors here oh look at that we have a short here so this whole line is shorted we have a short here we do not have a short here but we have a short here so this whole 19 volt pad here is connected to the drain side of the MOSFET let's continue working on the board I had to leave this on Saturday because time was running out today's Monday brand-new day and what we did so far is we remove the MOSFETs but we still have a short on the board let's go back to the area we do not have a circuit diagram for this board but we figured out that the drain is connected it shorted because of these capacitors here we do not know which one is causing the short like let's measure this one short the ground so what I'm going to do is apply voltage to the drain here and then we're gonna monitor the board under a thermal can to see what gets hot and that way we can pinpoint the component that is causing the short so in order to do this we're going to solder a wire onto the drain and then we gonna feed voltage onto it so the calves that tested for a short all right over here let me bump the voltage up to 19 volts let's see we're not gonna be able to use our floor cam we have the wire connected to the drain of the MOSFET so we can feed 19 volts onto it as soon as we turn the 19 volts on the power supply shows 3 amp current row which means that the motherboard is drawing a lot of amps there's a short on the board but then it goes down to zero and draw so the circuit is shutting itself down so it doesn't cause any damage on to other components on the board we will not be able to use the floor cam or the thermal cam so we have to do this manually try to see if we can figure this out so we tested all those caps as being short starting from this one here this is shorted short short short short which one is the bad one I have no idea can we guess maybe one of them looks discolored which is the last one here we can start with this one we will remove it and test to see if we still have a short if not then we put it back and remove the other ones so let's move it on to the side and let's touch that we still have a short and look at that the short is gone we do not have a short anymore let's measure the grain to see if they have a short still and no more shorty awesome we're gonna put the MOSFETs back I believe the MOSFETs are good and we're gonna plug the charger in and hopefully the laptop will turn on using the charger and it will charge the laptop the capacitor is a bypass capacitor so we do not have to put it back there are other bypass capacitors that will do the job we do not even know the value of the capacitor but it doesn't matter the laptop was still power on bypass capacitors are used to filter noise our battery is disconnected as you can see right here no connection and let's try to power it on using the charger awesome so I was calling to repair made me a my so as you can see the laptop is charging you see the amber light here and everything is good I hope you enjoyed the video don't forget to Like and subscribe and we'll do something else in the next video

Channel: NorthridgeFix

Views: 711,954

Rating: 4.928823 out of 5

Keywords: laptop no power, laptop not charging, no charge, mosfets, laptop mosfets, how mosfets work in laptops, short circuit, repair, northridgefix, mosfet

Id: tLPeG2YVM3Y

Length: 13min 10sec (790 seconds)

Published: Sun May 26 2019