HP desktop won't turn on after power failure

April 2, 2011 at 08:13:47
Specs: Vista64 Home Premium, AMD x4 9750, 8GB
Lon story short: we had a power failure, and my wife's system does not power up.

System: Store bought, HP Pavilion 9400f, AMD Phenom X4 9750, 8GB RAM, 750GB HD.

When power switch is pressed, absolutely nothing happens, no fans try to spin, no lights appear anywhere, nothing.

HP docs on this system are not good.

I pulled the covers and looked around. I see no obvious burns or signs of damage.

Motherboard appears to be a MCP61PM-HM, HP P/N 5189-4598. I think this is an Asus based mobo. It has a pretty standard and obvious layout.

PSU part number is HP 5188-2589. It is 350W, but there is not a lot of hardware to drive in this system. And has worked unchanged for about 2-3 years.

Further diagnostics: the PSU has an LED, which does show green when external power is applied. This seems suspicious to me. So I ....

pulled the PSU, and ran standard bench tests. They are clean: 3.3 where expected, 5 where expected ,......

I should mention: this mobo does not appear to have a single 'power present' LED. Either that or I am blind.

I am looking for suggestions as to what to try next, because I am baffled at this point. I have the feeling that throwing a new PSU at this is not going to fix anything.

Would appreciate any thoughts from folks more savvy about hardware than I am.

Thanks, Guy

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April 2, 2011 at 09:10:43
The +12v rail is the most important of the "big 3". But even if they all check to be within tolerance with a voltmeter, it doesn't mean they're capable of supplying the required amperage. Was the system plugged into a surge protector or just directly into a wall outlet?

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April 2, 2011 at 09:44:48
Mickliq is right, a voltage meter cannot test things like packaged goods and load tolerance. After a power failure, I tell people to unplug the power cord itself from the back from awhile, then plug back in and retry (after like 10 mins), the capacitors get stuck sometimes and the power needs to drain. After a power outage especially. Usually it's one of these things, 1) the above. 2) faulty power supply (most likely), 3) the mother board, or 4) if you are unlucky...nearly everything plugged into the board goes.

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April 2, 2011 at 11:22:08
Well, it has certainly been unplugged for a sufficient time. It took me a couple of days to get around to looking at it more closely.

I will try another PSU, even though I think that is unlikely to have an effect.

I also tried jumping across the Power on switch, thinking that maybe the front panel switch was somehow bad. No effect. Or rather same effect.

Is there any sure simple way to confirm/deny a mother board failure?

I'm now pondering how to extract the HD. This particular HP is not very friendly inside.

Thanks, Guy

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April 2, 2011 at 13:10:41
Forget capacitors changing if power is removed. Your computer has something called a power controller. Most do not even know it exists. One safety feature is a lockout. To reset the lockout in a controller, disconnect a power cord for a few seconds.

Never disconnect anything before making a measures (as another has noted). One minute with the meter could have answers a long list of unknowns. Start by measuring the purple wire (from power supply to motherboard) before powering the machine. It should measure about 5 VDC. But post the three digit number hear. Every digit contains useful information.

Do same with the green and gray wires both before and as the power switch is pressed. Those numbers will also result in facts that contain no speculation.

And finally measure any one red, orange, and yellow wire both before and as the power switch is pressed. Also report that behavior - in numbers.

What follows is a list of parts that are no longer only suspected good or bad. Listed will be 'definitive' answers - no more 'it could be ...'. How a computer really works (and it is simple) will also be learned.

Never disconnect or remove anything until hard facts (especially those above numbers) are first learned.

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April 2, 2011 at 15:08:57
I've got three psu's that all give a steady twelve volts at the twelve volt rail. They are all bad. Some of the fans will spin and with one the hard drive even spins still they are all bad. Get your hands on an AMP meter. Check with HP to find the amperage you should have at each the 3,5, and 12 volt lines. The twelve normally being the one that fails is also the one that is the most detrimental to your system. Better yet if you have the psu out of the system call a local shop and see what they charge to test it. If it is good my bet is the cpu is bad. The problem you describe I have seen several times so far it has not yet been the motherboard. Always the psu or the cpu at least in the cases I have seen.

You may want to check This page out and see if it helps.


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