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nobusFlag for Belgium

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no start up at all

i have a system sitting right here with a P3 450 Celeron that displays the following strange and uncorrectable thing (until now) : if i leave the mobo under power, there is no problem, it runs fine.
When i cut it from the power with a hard switch, it will not start after i reapply the power. only after several tries, it will eventually power up. It will not have the cpu fan or the power supply fan running then. (when it does not pwr up)
I already replaced everything, memory, Pwr supply, disconnected everything, got it out the case, everything possible. So it is the mobo or the cpu.
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philby11

It could more than likely be the mobo.
normally with something like that I'd look at the PSU but you say that you have replaced that already.
Have you had any on screen errors?
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philby, if it does not start up at all, i do not get a light  or a screen or dollars !
The strange thing is that, if it powers up, i work fine without problems !
Are there any bulging caps on the mobo or any with a brownish stain on them?
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no , it is just a fine board, i list the type here :

Shuttle® SpacewalkerTM ME17/ME18

I want everybody answering this to realize the following :
if i try your solution, and it is the wrong one, it will not power up again, because to test it i have to cut the power.
it can take 2-3 days before it wants to power up, as easily as powering up the next hour; so i cannot try just this and that.

So if you've got the solution, please say so.
If you are not sure, please tell me.
check the power button connection where it goes into the MoBo make sure that connection is good.

H
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I even tried shorting the 2 pins on the mobo. The basic problem is, the mobo returns the power ok signal in a bad state, so it will not start the power supply; but sometimes it starts ok . I know this, because if i short this signal, the cpu fan starts turning, and when i open it again, it stops
If you havn't tried this, see what happens.  Hold down your power switch for 5-10 seconds.  If nothing, once you let go of the switch, try it again as normal.
It sounds like a MB problem. I had a simular problem with an ABIT BE7 MB. I had to keep toggling the switch on the back of the power supply and hiting the power button to get it to boot up. I did check the PS. I left it run 24 hrs because of this. It got to be a pain when the power went out.
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eccs19 i tried that already a dozen of times, without effect.
And i agree with crazyjoe that it is a mobo problem, i just want to know if somebody had other ideas about it.
I just delt with it till technology said "time to upgrade". I just realized I have been using the same case for 6 years.
SOLUTION
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OliWarner

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You mean Pray, like in prug and prey?
Will try it, but i must wait till it will startup again, i looked for some bios things, changed here and there something, and after powar off --> no start.....
yeah it sounds exactly like the problem i've seen a million times...
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I had a strange, similar problem with a compaq about 3 years ago.
One day, it just wouldn't power on. I hadn't made any changes to it. I simply could not figure it out. I figured the power supply might be bad, so I brought it in and hooked it up to another power supply. It worked like a champ. Upon further inspection, I found that one of the connectors was slightly bent and had shifted just enough to lose the connection, which in turn had left me without a frickin computer for 2 weeks. I bent the connector back into place and didn't have any problems after that.
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sorry guys, you'll have to wait a bit; after the first things i tried, i cut the power, and have not been able since to startup again. i am still trying......
does turning it on for a second then restarting it work?
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it does not start, i tried holding the pwr button for 20 sec, then starting; disconnecting the pwr cable and doing the same; - it does not start, and pwr ok stays false.
However, when the light falls good, and the wind is right, and the good lord allows it, i get pwr ok, and it starts

I will have to thrash this board, but was seduced to trying it again, and right now, it refuses all cooperation. Maybe speaking my mind out in front of it may help...
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hurray, hurray, the electron gods made my day !!
today sep 13 at 1400 it turned on!! I'm trying now your suggestions

tmj833
1- i swapped pwr sup, same story, i think you mix pwr - on and pwr ok signals; pwr-on is always there, but not pwr-ok
2- i swapped the CPU, same story, so the generation of pwr ok (or feedthrough) comes from the mobo
3- and 4- :  see 2-
So definitely it is the motherboard.

Oliwarner : the setting was set to PCI, and i use a PCI card. do you mean iset it to AGP?

If others have more ideas, please come,
also technical explanations like tmj833's are much appreciated
Power_Good(Power OK)5V signal from pin 8 must be continuously supplied from Power Supply to Mainboard. The Mainboard(microprocessor timer chip) stops sending the reset signal to the CPU and allows the system to start. It is the Mainboard at fault as it is failing to stop the reset signal as proved using the second cpu.
The Power_ON signal from Mainboard is used by Windows for Soft OFF or shutdown via software...this is how an ATX system is able to turn off completely by software or on by a keyboard stroke...not the same as Power_Good. Signal ON...system on, signal off...system off...it simply reports the status to the OS.
Hope this is more clear...
T
nobus, when you changed psu, did you try one with higher wattage?  I had to do this once, but I had added new periphals as well.
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monte0472, no, because i was runnin,g a minimum config.
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I'm closing this question. Neither comment has led to the solution, but i learned from some comments, so i'm splitting points.