Question Asus P9X79 pro

[kaehligj]

I bought this mb cheap second hand as a non starter.
After lengthy fault tracing I found bad connections on +5V on the mb pwr conn.
The solder joints had cracked due to thermal cycling.
I also found an exploded fan driver mosfet, the previous owner seemed to have played around in despair, creating more problems for himself.

A word of warning: if you try repair this yourself you MUST be certified or at least very experienced with soldering tools and methods.
Its all too easy to completely destroy the mb applying too much soldering heat using wrong methods.
Having a background as an electronics engineer from the air force in the 70's, I had no fear attacking the problem.

However I still have a recurring problem: the USB 5V power on all connectors are unstable, even with only keyboard and mouse connected from a separately powered USB hub.
I later noticed, that my other PC mb in fact does the same: the USB +5V seems to alternate between some 4.6V and 5.1V
Only occasionally I get some weird problems like a notice bong in my (bluetooth) headphones.

Anyone knows whats going on ?
Thank you / Anders

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[helpstar]

Between 4.45 V to 5.5 V should be the normal behavior of USB port´s tolerance.

Which power supply are you using?

 

[kaehligj]

OK thanks
Its an Energon EPS650W CM PSU.
At present the mb setup is running bare on my desk with only a SSD, my test GT610 graphics card and a mediocre WiFi dongle.
In the cabinet, it will work with my good old medium performance GTX660 card.

I've got an issue with one (of 8) ram blocks not working, but I'll start a new thread on this problem.

The wide tolerance on the 5V supply would explain why I cant feed my raspberry off this supply and some other minor issues.
The RPI will not accept less than 4,9V on the micro usb pwr feed to my experience.

 

[kaehligj]

I believe I did not phrase my question right.
I actually meant that the USB voltage fluctuates, which is a fast mode of voltage variation
And that is not right in my book.

The PSU voltages at the pwr connector (PSU side) does not fluctuate.

I think I have to trace down the USB power circuit, to find the point where the voltage is stable.
AFAIK that would be poly fuses, any mosfet circuit switches or regulators off the 12V line.