wont power up, sometimes

Aries / 2600
October 7, 2009 at 15:12:35
Specs: Windows XP, SP3
In some situations, my PC wont power up. I press the power on button, but it is totally dead - no lights, no fans spinning. But most of the time, it all powers up and runs perfectly ok.

In the past few months, I've found that the machine always fails to power up if it is connected to a router that is already switched on (Before that, I never used to leave my router switched on). If I switch off (or unplug from) the router before I push the PC power button, then the PC starts OK. If I push the power button while the router is on, then nothing happens. The moment I unplug the network cable, the PC springs into life by itself. Usually.

But it gets worse. I normally switch my PC on every day. If I go away for a few days, then when I come back, it often wont power up, even if I've switched off the router first. If I switch off the PC power switch on the back and wait 10 secs, I see a brief flash of the power button LED, and brief flick of the fans. The when I switch back on, I can sometimes power up ok.

But not always. Sometimes even then it remains dead. The other day, after many off-on cycles with no luck, I left it. 15 mins later, all by itself, it sprang into life.

The problem seems to be gradually getting worse.

Does this sound like a power supply problem?

I've tried re-seating all the power connectors.

The Machine is an Aries 2600, about 4 years old, with WinFast 760GXK8MC motherboard. I did add some extra memory and an extra hard drive a few months ago.

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October 7, 2009 at 17:41:35
I'm assuming that it used to work correctly at one time. Otherwise might be a different issue.

I'd see the web pages on how to diag a power supply. Usually you connect a hard drive and use a paperclip to try to start the power supply. Have your voltmeter along to see voltages. If you can power on your power supply that way even after days then you need to consider your board is funny. To simulate days you might try to unplug and press power button like 10 times. Either the power on switch at the case or how the signal gets through the motherboard or how the motherboard sets the lock to keep ps on. That leads back to did it ever work.

Playing to the angels
Les Paul (1915-2009)

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October 8, 2009 at 14:01:07
Hi, yes it worked fine for years (although in the past I never left the router switched on - but I never had any trouble with it failing to start after being off for a few days).

I have tracked down some pages which explain how to diagnose a power supply. The ones I found helpful were: http://www.fonerbooks.com/power.htm, http://www.pavouk.org/hw/en_atxps.h... http://www.ehow.com/how_2270681_dia... So I'll get my multimeter out and I will have a go at trying to start the PS with a paperclip shorting out pins 16 (green) and 17 (black), and with a hard drive connected.

However, before I try that, I will leave all the power connectors attached, and just measure the voltage between pins 16 & 17. I should see 5V (with PC "off"), and this should go to zero when I push the power on button. If it doesnt go to zero, then that will suggest a motherboard fault. If it does go to zero, but the PC stays dead, then it will suggest a PS fault.

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October 8, 2009 at 15:25:05
OK, I've done some poking around with my multimeter. Very interesting. First of all, with the power connected but the PC off and the router off, there's about 4.5V on the purple wire and the same on the green wire. When I turn my router on, a green and a yellow LED light up on the back of the PC by the network socket. The voltage on purple and green falls to 3.7V, no doubt due to the load on the power supply.

Then, when I push the power button, the voltage on green falls to 26mV, but the whole thing stays "dead". When I unplug the network cable, the voltage on purple rises to 5.1V, and everything springs to life.

So, it seems clear to me that the motherboard is doing everything right, but the PS is failing to turn on when its asked to. Time to replace the PS!

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