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Motherboard LED on, but computer won't start

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Asus Socket 478 MB
Pentium 4 1.3
AGP 4x video
SB Audigy sound
40GB Seagate HD
320w PSU

I built my computer about 2.5 years ago, using an Asus P4B motherboard. A few weeks ago, it started to make funny whining noise, but I thought it was just dust so I opened it up and used an air can to clean it. It was okay for a week, then started again. This time, after opening and blowing it out, it crashed in the middle of startup, and then shut itself off. I thought that maybe the CPU fan was bad, so I ordered a new one from TigerDirect.

I put the new fan on, but no joy. The onboard LED would light up, and the fan blade twitch when I plugged it in and hit the Power switch. I took it in for the tech guy to look at it, and the first thing he pointed out was that four of the capacitors around the CPU had blown up, with brown gunk leaking out the tops. I read about this, and the bad batch of capacitors, and the conclusion that boards with this problem were shot. So I ordered a new board from NewEgg. Only they didn't have any P4B boards, but a slightly newer Socket 478 board, that required DDR memory instead of the older SDRAM memory I had, so I also bought a new stick of RAM.

I got all the parts today, and replaced the motherboard, keeping the same CPU and peripherals. When I plugged it in and turned the PSU on, I got the green onboard LED for the motherboard, so it's getting some power. But when I try to turn the computer on, nothing happens. I have read through other threads on this board, and disconnected all PCI cards, graphics card, hard drives, everything. Only the CPU and CPU fan are left. The power supply is plugged into the mobo by the 20 pin and the 4 pin plug. The CPU fan is plugged into the correct spot. I have fiddled with the power switch and case LED plugs, they are not clearly marked as to which way to go, but it doesn't make any difference.

This is driving me crazy. I feel like I am just throwing new parts at it and hoping to get lucky, $220 and counting. Is it possible for a power supply to go bad, yet still light up the motherboard LED? Is it possible that when the old board started getting wacky and blowing capacitors, that the power was fluctuating, leading to the fan changing speeds and making that sound, burning out the CPU in the meantime? I was hoping to fix this myself, because I have never had anything but very, very bad experiences with computer repair shops, and I would hate to trust them once again.

Please help!
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I have sympathy for your plight having been there myself.

As far as potentially blowing your CPU, everything is possible given the micro-voltages chips work at (beware static from yourself also).

From my own experience, I can tell you those little cables that attach to the pins on your board to make those led light up, are important. You've got to get the right one's on the right pins. You can think you've got it right, but the chances of hitting on it experimentally is remote. E.g. power on led will light, but re-start button won't function.

If you've got a new board, this should come with a CD with a pdf manaul detailing the board and the options for these little critters.

Get them right, before considering giving up.

DaveB - LinuxWannabee
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Did you keep the old motherboard?
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I assume in all of this, you must have removed the heatsink from the cpu on at least one occasion?
Did you clean it up & reapply thermal compound each time?

For the soft power switch, consult your motherboard manual to check exactly which pins they are. It could be something as simple as that thats causing the system not to power up.

Also, be aware that if the last mobo suffered blown capacitors, its possible that may have blown the CPU as well.
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