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Computer Will Not Turn On


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#1 sctig

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Posted 28 June 2006 - 09:43 PM

Only indication of power is that power on button on front of tower is lit.

When the power button is depressed, absolutely nothing happens.

I thought it was the power supply, but the green glow of light from the power button leads me to think otherwise.

Any comments/suggestions/advice/opinions?

Thank you in advance.

(2.5GHz Celeron, 256MBDDR. Computer has been in operation for a couple of years, and just recently died. Nothing that I'm aware of has changed).

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#2 sctig

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Posted 29 June 2006 - 08:19 AM

I changed out the power supply from another computer, however the "new" power supply does not have one of the connections the "old" power supply had, the P8 square connection to the motherboard beside the capacitors (2yellow, 2 black).

I connected without that, and it's still giving me the same problem. i.e. nothing happens. The only thing I'm getting in either case is a green led on the power on button.

When the power button is depressed, absolutely nothing happens.

So I'm not sure if it the power supply unless that one connection is critical to things happening.

Any thoughts?

#3 DaveM59

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Posted 29 June 2006 - 11:11 AM

That one connection _is_ critical. It supplies the power to the processor.

#4 sctig

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Posted 30 June 2006 - 06:59 AM

Thank you. Before I buy another power supply, do you think it is likely that it is a power supply problem, given that the only indication of power is the green glow from the power on button, and that absolutely nothing happens when the button is pressed?

#5 DaveM59

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Posted 30 June 2006 - 08:42 AM

PSU is always the first thing to try because they fail pretty frequently and they are the easiest and cheapest thing to replace. Until you do a proper test i.e. with power to the 4-pin socket as well as the ATX 20-pin, you have not ruled it out.

You might just want to look here and take a look at the pictures of some bad motherboard capacitors. Then look carefully at your own motherboard capacitors -- use a flashlight if necessary. If you see bulging caps, you know where the problem lies. If the motherboard caps look okay then the PSU is the prime suspect.

Rather than buy a new PSU you could try to find an adapter for the old one you have in there now. There are adapters available that will allow you to use an older ATX PSU with a newer ("Pentium 4 style") motherboard. Basically they have a hard drive power socket on one end and the square 4-pin P4 plug on the other. There's a picture here. You might be able to track one of these down locally, but I couldn't find one listed on CompUSA's website. You could call around to some "mom & pop" type shops, they might have one.

If you do get a new PSU, make sure the amp ratings for +12V, +5V, and +3.3V at least equal the original power supply that came with your computer.

Good luck.

#6 Dngrsone

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Posted 30 June 2006 - 01:51 PM

You might want to ensure that the switch itself is still good.
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#7 sctig

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Posted 01 July 2006 - 09:20 AM

Thanks for your help guys. Before I received the last two responses, I went out and bought a proper power supply.

Still the same problem.

Which narrows it down to the actual power on button, or the capacitors.

I'll check those.

If you have any other ideas, I'm listening.

Thank you.

#8 DaveM59

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Posted 01 July 2006 - 11:44 AM

I'm glad Dngrsone mentioned the power switch, I had forgotten about it. You might just check to make sure the wires from the switch to the mobo are attached.

Sorry you bought a new PSU for no reason. -- maybe. Sometimes when power supplies fail they take the motherboard with them. Look not only for the bulging capacitors but for any smaller components that look burnt or are surrounded by an area of discoloration. However those are not infallible signs, sometimes a motherboard dies leaving no visible clues.

It's also possible your CPU (processor) got damaged in the same event but less likely. My only personal run-in with bad caps was a socket 478 P3 that a co-worker brought in. His CPU worked fine in the new mobo. But this was not a power supply failure.

#9 sctig

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Posted 03 July 2006 - 10:44 PM

What should I be seeing on the leads from the power supply?
I'm assuming 12V, 5V and 3.3 V. But where?
(e.g where should I measure: between yellow, red, black???)
The reason I ask is I've measured both the old power supply and the new one (in both cases not connected to the computer) between a variety of leads and I'm getting voltages all down in the mV range, no matter where I measure. I'm wondering if possibly I got a new faulty power supply, or more likely the power supply was damaged when connected to the computer, or perhaps I'm measuring wrong (although I have measured across a number of leads).

Thanks in advance for your assistance.

#10 DaveM59

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Posted 04 July 2006 - 07:43 AM

It's unlikely that a bad motherboard would take down a good power supply, what I've seen is the opposite -- power supply blows and fries the mobo. However I'm open to correction on this.

The color codes are: Yellow, 12V; Red, 5V; Orange, 3.3V; Black, ground. The other colors are not as important, but you can go here to see a picture (scroll down the page) with all the color codes called out.

#11 sctig

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Posted 12 July 2006 - 01:32 PM

I've opened another working computer and what I've discovered is that power doesn't show up on the power supply leads until the power button is depressed. Given that I was testing the power supplies with a faulty computer, the power supply may not be receiving the trigger it needs to start outputing power on the leads, and therefore I would not be reading anything on the multimeter, therefore I'll assume that the power supplies are fine. I suppose I could test them on the working computer, but I'm a little leary of doing that.

Can anyone suggest another way of testing the power supplies? Or have any other suggestions? The computer is new enough that I don't want to abandon it altogether. I have also looked at the caps and they do not appear to exhibit the bulging cap issue mentioned earlier as a possibility.

Please help.

#12 acklan

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Posted 12 July 2006 - 02:46 PM

I have and use a tester. It is not very sensitive, but it will tell you if you have power to the plug. Tester
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#13 sctig

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Posted 12 July 2006 - 06:18 PM

Even with the computer not on? If that's the case, I should be able to detect the same thing with my multimeter. What am I missing here?

#14 Dngrsone

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Posted 12 July 2006 - 07:25 PM

With the PSU switch on (and plugged in), there should be 5vdc from the purple wire to ground.

The purple wire is the standby voltage that powers the switch sense for the motherboard.

Shorting the green wire on the PSU to a black wire (ground) should turn the PSU on, though some ATX power supplies require a certain amount of load on them to turn on.
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#15 sctig

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Posted 16 July 2006 - 10:59 PM

Thank you... most helpful.

The two power supplies I was using check out OK (i.e 5V between purple and black).

So I would imagine it's gotta be the motherboard then.




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