Bad mosfet on this motherboard?? - Page 1
Forum section: electronics / beginners

author: azigta
author_rank: Regular Contributor
timestamp: August 22, 2014, 01:29:42 am
content: Hi, Some weeks ago I recieved a dead mini itx board. It had the audio connector (the one in wich you plug the audio jack, not the internal audio connector) Cutted! The audio input jack was not there because some guy just cutted it, he told me that after cutting it, It won´t power up again, so I replaced the audio connector with a new one thinking that, that would solve the problem. I did continuity tests and everything is ok on the audio connector. But still doesn´t power up at allAll I get is a green light and when I try to power it up, the case fans attempt to spin for less than half a second. If I try again, they don´t do a thing. If I remove the power cord for some minutes and try again, they attempt to spin again the same wayCould it be a bad mosfet? Capacitors are not blown up, I guess that tells me they´re alright.The thing is that I have no idea on how to test this mosfets.. I have some experience testing mosfets, but I have only tested PSU mosfets. You know, those big mosfets, you can desolder them easily and test, but In a motherboard they are all soldered smd and they are so hard to desolder so I don´t really know If I have to take them out to make some testing.I mean... there are a lot of mosfets, I don´t really want to guess wich one it´s the bad one. Is there any way to know wich one is the bad one before desoldering? how can I test them?What else could be the problem?Also I want to say that I have checked the board for scratches or blown smd´s and I didn´t found any sign.Thanks

author: fubar.gr
author_rank: Unknown
timestamp: August 22, 2014, 07:19:05 am
content: This is a motherboard with a bad mosfet:The first sign was the little blob of overheated solder mask. I then measured the drain-source resistance and it was a few ohms. THe resistance of all other mosfets was in the Mohm region as it should be.Unfortunately the board was severely damaged already. The mosfet, solder mask and copper pad had become a single mass and I couldn't desolder it without doing more damage.

author: Kjelt
author_rank: Super Contributor
timestamp: August 22, 2014, 08:06:55 am
content: Those huge pcb (ground) planes that also serve as heatsink are a crime.The only remedy is a really good soldering iron that can deliver fast amounts of power in short amount of time.But you can not solder at three places at the same time, so next time cut the legs first and desolder them first which should not be a big problem, then go for the body with heatsink. If youre iron is not upto the job then you could use a dremel with lot of suction to grind the whole package away till you hit the heatsink, then you can solder the new part onto the old heatsink, but again you still then need a good solderiron with lots of quick power.

author: fubar.gr
author_rank: Unknown
timestamp: August 22, 2014, 08:13:02 am
content: The PCB material itself was burned up. There was a small crater below these mosfets. There was nothing I could do to save it.

author: Kjelt
author_rank: Super Contributor
timestamp: August 22, 2014, 08:23:01 am
content: Ah ok in that case it is indeed game over

author: azigta
author_rank: Regular Contributor
timestamp: August 22, 2014, 11:28:09 am
content: This motherboard doesn't have any burnt components. For what I know a mosfet doesn't have to be burnt to be bad. Some time ago I repaired a common problem on a dell laptop, it had a bad smd mosfet. It was a small one, with 4 legs in each side, not like the ones shown in the photos. The laptop was dead and after replacing the component it came back to life. I knew that it was that component because it seems to be a common problem on that dell laptop.So I think now I have to find wich mosfet is bad. Does anyone know how to test a mosfet? Do I have to desolder it In order to test it?

author: ConKbot
author_rank: Super Contributor
timestamp: August 22, 2014, 04:41:06 pm
content: That motherboard is a mess! once you get the black goo leaking out of PCBs, they are done for.

author: azigta
author_rank: Regular Contributor
timestamp: August 24, 2014, 02:19:27 am
content: anyone?

author: wraper
author_rank: Unknown
timestamp: August 24, 2014, 02:28:27 am
content: Suspect that this is not mosfet. Personally would verify voltages on the converters first during that short power up. Also it will be helpful to verify that all usb ports have no short between +5V and GND.

author: mzzj
author_rank: Super Contributor
timestamp: August 24, 2014, 12:13:04 pm
content: Quote from: azigta on August 22, 2014, 01:29:42 amPlenty of capacitors dry out without any external signs so you cant tell anything based on that.Unfortunately there is tons of other problems in laptops besides mosfets and capacitors.Bad solder joints on bga chips are nro 1 cause of failure in most of the laptops in my experience.