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Dead MoBo 12v issue

Posted on 2011-09-10
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Last Modified: 2012-05-12
Stared out w/ a PC that won't boot. NOTHING happens, no POST, no nothing.
Tried a different (known good) power supply, same thing.

Started unplugging all extraneous devices (like the CD drive & hard drive) just to see if I could even get the CPU fan to spin. No help.

CLUE #1: With nothing from the power supply connected to anything except the main 24-pin MoBo connector the fan does now spin BUT if I also connect the 4-pin 12v aux power connector to the MoBo the fan again will not spin.  (This happens with either power supply.)

QUESTION: Is my most likely failed component a MoBo or a CPU? (And w/o out a replacement CPU how can I test this?)

Thx all
JR
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Question by:joerockhead
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garycase earned 500 total points
ID: 36517667
Could be either -- and it's very difficult to say which.    If you hadn't tried two different power supplies, I'd think it was a dead 2nd 12v rail ... but since you've effectively eliminated the PSU from the mix, it sounds like you're getting an excessive draw on the 12v line from the auxiliary CPU connector.

This could be either a shorted CPU, or a short in the motherboard.   It's virtually impossible to say which without very expensive test equipment, or spare parts to substitute (i.e. a spare CPU).

One thing you can do:   remove the CPU, but leave the heatsink/fan in the case plugged into the CPU fan connector.    Then see if the fan spins okay with the auxiliary connector connected.     Clearly the system won't run ... but if it then spins the fans, it's a good indication that there's not a direct short in the motherboard.     Doesn't guarantee anything ... but if the fan spins okay, you may want to buy a replacement CPU to continue testing.

Note:   Depending on the age of the motherboard & CPU, you may want to simply replace both with more up-to-date components and give your system a nice performance boost in the process.
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Author Comment

by:joerockhead
ID: 36517773
Good idea. I will try that.
Both chip & board were bought recently only 4 or 5 months ago in a kit. I don't know offhand if the chip may still be covered but I know the board is.
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Assisted Solution

by:nobus
nobus earned 500 total points
ID: 36518300
from my experience, mostly motherboards die - it's about 95% versus 5% for CPU
but did you remove the ram also?  a bad stick can block also the system (had one last week like that)
so test with other ram - or test this ram on another system
you can also reset the bios by removing AC and bios battery for a minute

check also the capacitors on the mobo (near the cpu) for leaking or bulging ones :
caps.jpg
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Author Closing Comment

by:joerockhead
ID: 36538211
The MoBo w/o the CPU still does not spin the fan. That helpful (but inconclusive?) test taken into account along with the statement above that experience says MoBo more likely that CPU to burn out suggests strongly to me that most likely a bad MoBo.  Am RMAing it today. Will let you know!
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Expert Comment

by:nobus
ID: 36540997
in this case, a PCI post card could help you decide better : http://www.amazon.com/StarTech-com-System-Diagnostics-Segment-PCIPOST/dp/B00006B8EZ
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