Asus P5P800 SE - cold CPU, no voltage at the VRM

[SuperDuty]

Hi,

I have a P5P800 SE motherboard laying around, but it doesn't work. All caps are good quality and not bulging (mosty Panasonic FJ and six FL around the CPU), except five Chemicon KZG 16V 1000uF.
The board will turn on, but nothing happens. The CPU doesn't heat up at all, yet the chipset does get hot as well as the GPU of the video card I plug in.
And when I want to force it to turn off by shorting th two pins, it just won't so I have to unplug the PSU.
I've checked the voltages at the VRM near the CPU and there is none. They don't seem shorted.
I've soldered a buzzer where there was a room for it (but none originally installed) but it doesn't make any noise.

Any ideas ?

Thanks !

[spleenharvester]

Re: Asus P5P800 SE - cold CPU, no voltage at the VRM

You got a pic of the VRM section? My guess is something has gone open circuit. Are the VRMs supplied with any voltage at all?

[SuperDuty]

Re: Asus P5P800 SE - cold CPU, no voltage at the VRM

Yes I read close to 12V a the VRM supply. Do you want a photo of the VRM on my board ? Because nothing is burnt or obviously bad.

There's some close up pics of the VRM on a forum :
http://www.xtremesystems.org/forums/...r-p5p800se-plz

[spleenharvester]

Re: Asus P5P800 SE - cold CPU, no voltage at the VRM

Hmm, I'm not so used to repairing desktop boards - forgot there's never schematics!

If I were you I'd try and figure out how that whole VRM section is sequenced. Test each MOSFET to make sure they're fully functioning and the numbers are all in order.

Then check for shorts using the 200ohm setting between the legs of the MOSFETs and ground.

If you don't find anything, you're best off going from that 12V voltage you found, and working your way through to try and find where the voltage drops off. Is DDR2 voltage present?

Someone else probably has a better idea - I'm not too familiar with desktop VRMs.

[Th3_uN1Qu3]

Re: Asus P5P800 SE - cold CPU, no voltage at the VRM

First things first, try reflashing the BIOS to rule that out. Also look for any bent pins in the LGA775 socket.

[SuperDuty]

Re: Asus P5P800 SE - cold CPU, no voltage at the VRM

I've put a BIOS chip from another P5P800 SE, it didn't change anything.
The pins on the socket are fine.

[momaka]

Re: Asus P5P800 SE - cold CPU, no voltage at the VRM

Could be Southbridge issues, judging by the fact that you can't turn OFF the motherboard after you turn it ON.
Make sure all major circuits have proper power. You already found CPU VRM has no voltage on it. There's a few more to check:
RAM Vcc
Northbridge Vcc
CPU Vtt (if CPU has one)
Southbridge Vcc (unless motherboard NB and SB are merged together)
Any other linear regulators (1117, 1084, 1085, 1086, 7805, etc.)

[kc8adu]

Re: Asus P5P800 SE - cold CPU, no voltage at the VRM

is southbridge hot?
common failure on these.