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PostPosted: Sat Jul 07, 2018 10:03 am 
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Pilgrim
Pilgrim

Joined: Thu Jul 05, 2018 8:17 pm
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HI all :)

I'd like to direct this question to those of you, who has some experience in repairing VRMs of motherboards...

OK, I have a Rampage II Extreme I burned about 2 years ago. It was solely my fault. I tried to mount a cooler in a DIY manner, and the result was: scraped protective lacker, a trace shorted to the ground, a little smoke and smell of burned plastic, and finally a dead PC.

However now I have a perfect use case for that PC. Also I'm itching to see if I could pull it off. I know it's not worth the effort. The CPU might be dead as well so it could all be in vain. Although I do have reason to believe it survived.

The PC did not die immediately and it wasn't a flashy death. Nothing popped, there were no sparks. I turned it on (after I had done the nuisance) and it posted. I entered the BIOS and saw that two of the RAM modules are not recognized. Then the PC shut down on it's own as it was in the BIOS. I turned it on again and then I felt the smell. It shut down again and that was it... I saw the smoke but is was too little for me to see exactly where it was coming from.

That was 2 years ago...

Now. There is no visible damage, no disintegrated MOSFETS, no blackened spots, nothing. The diagnostics I've done so far: Using the buzzer of my multimeter I was able to check all MOSFETs in in the VRMs of the CPU and RAM.
As I touch the probes to Drain-Source of any MOSFET I hear a short beep (less than a second) and then it stops. I can also see the reading of the multimeter increases. The "resistance" is increasing. Obviously a capacitor is charging. I consider this to be normal. All MOSFETs responded this way, but three of them. Those three show a constant whine between drain and source. I think those are the culprits. It's the same between drain and ground, that shouldn't be right. They are part of the memory powering VRM. I know that because there is an inscription on the board. And it makes complete sense, because the two modules that weren't found right before the PC died ... Supposedly because they weren't receiving any power. So I think if something should've burned out with the board that should've been a RAM module or two not the CPU?! Of course I can't be sure.

... Oh and all the RAM modules were fine, they all work in other PCs ever since.

I was able to follow the shorted trace (not shorted any more, it was shorted by a washer with a sharp edge that scraped the lacker) to a PWM chip. I couldn't find the datasheet of that one. My theory is that the short circuit obstructed the normal operation of that chip that supposedly drives those MOSFETs, therefore those three remained constantly open and thus they burned. Should be something of that sort...

So my question is should I just replace those MOSFETs or should I continue to investigate for other damaged components?
Or am I no the right track at all?!

Thanks a lot in advance :)


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PostPosted: Sun Jul 08, 2018 8:03 am 
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Mobo-fu Master
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Joined: Wed Nov 12, 2003 11:57 am
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Hope the board still has life in it, and no other components were damaged. Parts have to be replaced with exact originals, concerning capacitors and regulators. VRM components regulate voltages to the CPU precisely, and if higher voltage than specified is delivered, possibly instant death. I would first test removable components in another system that is compatible, to see if they are indeed good. That includes memory, the CPU, and any primary device cards.
Unsoldering and soldering components on a motherboard is a special task, so as not to damage other components. "Get in and get out with the heat" is the golden rule.
Depending on if the MOSFET or transistor is an NPN or PNP design, possibly there will be a resistance reading between two contact leads unless a bias voltage is applied. The specs sheet for each will disclose what should be or not be.
One type of transistor requires a bias voltage to "turn off", reducing voltage or current. Another requires a bias voltage to "turn on" or increase voltage or current.
https://www.electronics-tutorials.ws/tr ... ran_2.html
https://www.electronics-tutorials.ws/tr ... ran_3.html
https://electronics.stackexchange.com/q ... datasheets
https://en.wikipedia.org/wiki/Power_MOSFET
https://en.wikipedia.org/wiki/Bipolar_j ... transistor

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PostPosted: Fri Jul 20, 2018 6:28 am 
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Mobo-fu Master
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Joined: Sun May 06, 2001 12:01 am
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what is the part number of the PWM chip?

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