Dead MoBo 12v issue

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
joerockheadAsked:
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Gary CaseConnect With a Mentor RetiredCommented:
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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joerockheadAuthor Commented:
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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nobusConnect With a Mentor Commented:
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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joerockheadAuthor Commented:
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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nobusCommented:
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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