Repairing short circuit in laptop  - Page 1
Forum section: electronics / beginners

author: mdijkens
author_rank: Regular Contributor
timestamp: November 18, 2018, 09:27:34 pm
content: Hi,I am trying to repair the ASUS ROG G751JY of my son which has a short in the power circuit.Without having much knowledge, experience or information I am finding my way throught the power circuit part.Near the 2 mosfets there's a small SMD component which I assume is a tantalum (??) or maybe a diode (?) but can't find any info on.See attachment.I desoldered it and it measures a dead short both ways and no capacitance.Does anyone know what component this is?For problem finding I am thinking of putting my lab power on it with current limiting on 1A (for a brief moment) and spray alcohol over the components to see which ones get hot. Is this a good approach?

author: wraper
author_rank: Unknown
timestamp: November 18, 2018, 09:44:01 pm
content: Looks like Panasonic poscap (tantalum polymer). Seem to be 15uF, not sure about series and voltage rating though. EDIT: but it seem to be rather big, so I'd guess it could be 35V. Is it connected in parallel to charger input?

author: mdijkens
author_rank: Regular Contributor
timestamp: November 18, 2018, 10:00:04 pm
content: Yep that looks like it. Probably 25V or higher (for a 19.5V power supply)If I measure it (off board), it's a dead short and no capacitance.Since I don't have a spare, Just trying to power the board without it, but that doesn't work: No short but also no power consumption 0.000ACould that be a critical piece? I would at least expect the laptop to consume something if this cap was the only problem ...

author: ArthurDent
author_rank: Super Contributor
timestamp: November 18, 2018, 10:16:06 pm
content: The capacitor shorting could have taken out other parts as well. The 2 parts marked 4983NZ are MOSFETs which could have been used as switches. Here's the data sheet.https://www.onsemi.com/pub/Collateral/NTMFS4983NF-D.PDF

author: wraper
author_rank: Unknown
timestamp: November 18, 2018, 10:16:39 pm
content: Probably blown fuse as well. Also measure resistance of R005 part on top right corner which seem to be 5 mOhm shunt resistor.

author: mdijkens
author_rank: Regular Contributor
timestamp: November 18, 2018, 10:37:22 pm
content: ThanksHow can I test the mosfets?Fuse? How to look for it?I will measure the R005

author: wraper
author_rank: Unknown
timestamp: November 18, 2018, 10:56:08 pm
content: Look for something like this in between circuit on the photo and charging connector.

author: rstofer
author_rank: Super Contributor
timestamp: November 19, 2018, 01:27:56 am
content: Is it my imagination or is the soldering butched around the MOSFETs?Did something melt the solder?

author: mdijkens
author_rank: Regular Contributor
timestamp: November 19, 2018, 09:08:35 am
content: The picture isn't mineI had to put the laptop back together as not to forget where all the stupid screws goWhen I have a new tantalum I'm opening it up again and do all the testing/measuring. I just want to be more prepared then because it's a terrible laptop to take apart; 100 screws and everything needs to get off

author: 6PTsocket
author_rank: Regular Contributor
timestamp: November 21, 2018, 07:32:15 pm
content: I replaced a motherboard aftrer a video fail. I learned the hard way. Make lots of drawings or take pictures. Note screw lengths and where they go and where cables plug in. That was my first tome inside a laptop. I did get it working ,learned a bit but it was a PIA. You can't do too much documentation. You think you remember where something came from until it is time to put it back.Sent from my SM-G900V using Tapatalk

author: mdijkens
author_rank: Regular Contributor
timestamp: March 11, 2019, 05:06:55 pm
content: I have put in the new tantalum, but still no power.I tried measuring as much as I can / understand, but I am not going to solve thisThe problem is in the initial path near the power-adapter. There's no power going to the charge-circuit and not to the laptop itselfI there anyone here who lives (or knows someone) in The Netherlands and wants to give it a try.I am more then happy to pay 100 euro's if someone can fix this...

author: WildMOSFET
author_rank: Contributor
timestamp: March 15, 2019, 09:20:36 pm
content: Quote from: mdijkens on November 18, 2018, 10:37:22 pmhttps://www.utm.edu/staff/leeb/mostest.htmThis is the fastest but not very rigorous method.

author: KL27x
author_rank: Super Contributor
timestamp: March 16, 2019, 01:52:13 am
content: QuoteThis why I justwhen someone watching too much Louis Rossman comes around and complains about the cost of a laptop repair. Laptops are essentially not made to be repaired, at all. This is like a car where every service item is completely inaccessible until you strip the thing down to parts. And after "repairing it" there's often no way to even verify if you succeeded until most of those pieces are put back together. Most broken laptops should just go in the trash.

author: WildMOSFET
author_rank: Contributor
timestamp: March 17, 2019, 04:08:01 pm
content: QuoteYou're lucky, because many famous producers use glue instead of screws.And in that case the expletives are unlimited.