Samsung NP355 with QMLE4 motherboard troubleshooting

[Askic]

Hello guys,
the other night my laptop just turned off in the middle of a movie. It wasn't possible to start it back on. It is Samsung NP355 with QMLE4 motherboard.
I've tried usual things such as reset BIOS etc, but no luck.
Then I removed the motherboard and tried to check random components.
Here's what I have found and would like your opinion.

Please have a look at the attached picture. I have follow main 19V track from the charger. There are PL1 and PL2 (probably resistors or inductors since mark is PL and not R) and two MOSFETS. After that there is a current resistor with very small resistance (R010) and after that there are, I believe two bigger and one smaller capacitor, all in parallel.

My instrument is beeping if I measure any side against the ground and also, my multimeter shows 0.3 ohms resistance across any of the capacitors.
I think that is not normal and probably one is in short to ground.

So, I need to know, if those components are really capacitors and if it possible that this is actually OK.

Thank you

[Techtiger]

Re: Samsung Laptop Motherboard troubleshooting

test the resistance on PR211 if low you have a short in +19V lail check the first 2 Fets PQ203/205 S D G these 2 Fets protect the MB from EMI

[Askic]

Re: Samsung Laptop Motherboard troubleshooting

test the resistance on PR211 if low you have a short in +19V lail check the first 2 Fets PQ203/205 S D G these 2 Fets protect the MB from EMI

I did test that and it is very low, and that is expected. I measure about 0.3 ohms but that is resistance of multimeter cables. It is written on it R010 which is 0.01 ohms.

[Techtiger]

Re: Samsung Laptop Motherboard troubleshooting

test the resistance on PR211 if low you have a short in +19V lail check the first 2 Fets PQ203/205 S D G these 2 Fets protect the MB from EMI

I did test that and it is very low, and that is expected. I measure about 0.3 ohms but that is resistance of multimeter cables. It is written on it R010 which is 0.01 ohms.

did you check the 2Fets & PQ205/203 Source Drain Gate if you dont' know find out how to test , if you have the tools to remove R010 & update what side is short to GND

[Askic]

Re: Samsung NP355 with QMLE4 motherboard troubleshooting

Hello,

I don't have hot air station, but I'll take the board to repair shop for test.
First MOSFET is very strange, it is 6504, N channel and right now between source and drain there is about 5 ohm, which is not normal. However, this shouldn't be a problem, because if this MOSFET is shorted, than it should be acting as a wire passing current to second one.

Normally, I would do what you suggested, remove R010 and test to see if there is still short to ground on one of its legs. This way I could isolate area with the problem. But I don't have proper tools and this will go into shop. I'll try to find out what was the problem and post here.

[Askic]

Re: Samsung NP355 with QMLE4 motherboard troubleshooting

Just a small update, I have removed PR211 with a soldering iron and test. Left is in short to ground, right leg is OK. Then I removed all three caps, but the short is still present, which means my 19V power rail is in short. I guess this is a job for a repair shop.

Thank you for your time and advices.

[Techtiger]

Re: Samsung NP355 with QMLE4 motherboard troubleshooting

Just a small update, I have removed PR211 with a soldering iron and test. Left is in short to ground, right leg is OK. Then I removed all three caps, but the short is still present, which means my 19V power rail is in short. I guess this is a job for a repair shop.

Thank you for your time and advices.

Good job for PR211 your +B line or charging sector CHG_B+ is short to GND , test PQ207 ohms
S D G

[Askic]

Re: Samsung NP355 with QMLE4 motherboard troubleshooting

wow, you actually have schematics for this laptop??? It didn't cross my mind to ask. I assume No one has it, since it is not common lapot comparing with HP or Dell. Unfortunately, I laft motherboard together with laptop at a repair shop, so I can't test anything now. Too bad, it could have been great learning experience...

Is it possible to get this schematics for learning purpose, or at least its 19V power rail?

Thank you!

P.S: Like I said before, I removed two ceramic caps (according to schematics PC211 and PC212 + smaller third one, but that could be maybe resistor or tiny capacitor), and the short was still present.

[Askic]

Update

Ok, here‘s the latest development. The board is declared not worth to be repaired, so I found and ordered another used one. In the meantime, this can be a valuable learning experience. I have found that drain (4 legs) of the PQ207 are in short (about 1 ohm to the ground which is a short). I investigated further and found shorted leg on PU200. This leg is number 1 since has a white triangle below it. Also I have number 6 on the first leg on the right side of the IC. I manged to find the schematics and this means that B+ is shorted.
According to the schematics ACN is shorted and since PR211 is shorted on the line to the inductor PL 201, this means that most probably PC235 cap is shorted to the ground. However I have a hard time finding this cap on the board, since there are no marks for caps. I know it must be very close to the IC PU200.

One of two tiny caps below PU200 is in short cca 1ohm to the ground but I winder if this is the cap I'm looking for. It seems that this question can be answered by Boardview or somerhing similar.

[Techtiger]

Re: Update

Ok, here‘s the latest development. The board is declared not worth to be repaired, so I found and ordered another used one. In the meantime, this can be a valuable learning experience. I have found that drain (4 legs) of the PQ207 are in short (about 1 ohm to the ground which is a short). I investigated further and found shorted leg on PU200. This leg is number 1 since has a white triangle below it. Also I have number 6 on the first leg on the right side of the IC. I manged to find the schematics and this means that B+ is shorted.
According to the schematics ACN is shorted and since PR211 is shorted on the line to the inductor PL 201, this means that most probably PC235 cap is shorted to the ground. However I have a hard time finding this cap on the board, since there are no marks for caps. I know it must be very close to the IC PU200.

One of two tiny caps below PU200 is in short cca 1ohm to the ground but I winder if this is the cap I'm looking for. It seems that this question can be answered by Boardview or somerhing similar.

wow, you actually have schematics for this laptop??? It didn't cross my mind to ask. I assume No one has it, since it is not common lapot comparing with HP or Dell. Unfortunately, I laft motherboard together with laptop at a repair shop, so I can't test anything now. Too bad, it could have been great learning experience...

Is it possible to get this schematics for learning purpose, or at least its 19V power rail?

Thank you!

P.S: Like I said before, I removed two ceramic caps (according to schematics PC211 and PC212 + smaller third one, but that could be maybe resistor or tiny capacitor), and the short was still present.

is that the actuel motherboard pic if so your charging IC is missing repair shop have removed it ,when B+ is short drain (4 legs) of the PQ207 will show low resistance , pin 1 is ACIN you are correct this line will show low resistance

[Askic]

Re: Samsung NP355 with QMLE4 motherboard troubleshooting

Yes it is the actual motherboard. Pq207 4 pins (drain) had short to ground. After removing IC chip the short was still there and also pin 1 was short to ground. I use this motherboard to gain some more knowledge about troubleshooting. I even found the schematics, but I cannot find my prime suspect and that’s this cap pc235. It must have been somewhere close to the charger IC. I starter removing all caps one by one that measure low resistance to ground.

[Askic]

Solved

Hello guys,

just want to share that the problem is finally solved. I have managed to find a shorted capacitor on the +B line, very near to PQ501.
I have found a short buy using 5 V from PSU with long wires (in order to prevent short circuit protection to activate and turn off PSU). I used compressed air turned upside down and I sprayed the surface of the motherboard. Immediately I noticed an area ner to PQ501 and PQ502. There were two bigger caps there and few smaller ones. After removing a cap, the short was not there any more.

This was a good learning experience. Maybe someone will find it useful too.

[Techtiger]

Re: Solved

Hello guys,

just want to share that the problem is finally solved. I have managed to find a shorted capacitor on the +B line, very near to PQ501.
I have found a short buy using 5 V from PSU with long wires (in order to prevent short circuit protection to activate and turn off PSU). I used compressed air turned upside down and I sprayed the surface of the motherboard. Immediately I noticed an area ner to PQ501 and PQ502. There were two bigger caps there and few smaller ones. After removing a cap, the short was not there any more.

This was a good learning experience. Maybe someone will find it useful too.

Good job well done 👍