MSI K8N NEO2 3.3V short

[pdavid]

A friend got this dead nforce3 based motherboard from the junk and he asked me to try and fix it. It looks clean and undamaged but it has a 3.3V rail short all over and I don't know where it is exactly. Some fets where blown next to tha SATA controller. So far I got those off but the short is still present. The board has chemicon kzg before the vrm, matsushita and gluxon caps everywhere else. Should we just ditch this board? If it's some important ic's defect then we definitely will.

[c_hegge]

Re: MSI K8N NEO2 3.3V short

Another suspect would be the RAM VRM. I'm pretty sure it uses the 3.3V rail.

[tmiha71]

Re: MSI K8N NEO2 3.3V short

As dead IC's (supplied directly from +3.3V ), sound chip, or LAN chip are good candidates for removing :-))...

[momaka]

Re: MSI K8N NEO2 3.3V short

Check Northbridge and Southbridge VRM. NB likely uses a buck regulator (i.e. 2 MOSFETs and a coil). SB probably a linear (1084/85/86 or similar). If not, check what the RAM VRM uses for power, as c_hegge suggested. There's also the AGP VRM - that usually uses 3.3V as well (for AGP 3.0, the 3.3V is brought down to either 1.5 or 0.8V). Those are the 4 major things that use 3.3V rail. Audio and LAN I usually find are powered by 5V through a 7805 or 3.3V through a 1084/85/86 linear regulator.

I guess start removing until the short is gone. IMO motherboard should be salvageable. Now whether or not it's worth your time - that is up to you. If you want to take it as a challenge or learning experience, this would be a good opportunity.

[shortchange]

Re: MSI K8N NEO2 3.3V short

To find a short or multiple shorts.
Say the 3.3v supply has 4 ohms.
attach your ohm meter to the 3.3 volt line.
And using chilling spray or a heat gun.
The component that causes a change in the ohm is or are most likely at fault.

[pdavid]

Re: MSI K8N NEO2 3.3V short

Thanks everyone! So far it looks that the ram vrm took some damage too. I removed another shorted fet from there and the resistance is higher but still a constant 3 ohm. I'm definitely going to try the freeze spay method!
I'm not in a hurry with this thing so I poke around some more. If some chip is fried then it's recycling time for the board.