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power up problem after unplugged


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#1
jgalt

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This has got me stumped. I can start up my computer just fine as long as I don't unplug it. If the computer has been unplugged for more than a couple of minutes it won't start up. I get the fans to all start turning, all the LED lights come on but I don't get any beep codes and nothing comes up on the screen. I have tried updating the BIOS, changing the CMOS battery, reseating all the components, starting up without anything connected. Any suggestions would be helpful (other than the obvious one of don't unplug the computer).
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#2
Samm

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I don't know but I can have a guess.
When the computer is turned off but still plugged in, there will be a standby voltage that is constantly fed to the PSU. This, I think, will ensure that the PSU's capacitors are not discharged completely & will also keep the components from getting cold.

When the computer is turned on, there is often a very short period between the PSU powering up & the system powering up. This is because the mobo won't power on until it receives the POWER GOOD signal from the PSU. The power good signal is only sent after the psu has stablised. Its possible therefore, that it is taking longer to stablise after the power has been disconnected for any length of time due to the reasons stated above.

You could probably test this theory by running the PC, turning it off, disconnect the power lead for a couple of seconds then reconnect & power the system up. If it works OK after doing that, then I could be right.

I am guessing completely though & anyone else please feel free to tell me that I am taking a load of b****ks here.
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#3
jgalt

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Alright, I tried what you suggested and the computer did start up just fine. Would you recommend an new PSU?
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#4
Doby

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Well Samm in therory that is a very good guess and must be the case but I wonder why the capacitors would discharge over a short period, I could see this happenning if left unpluged for months. I am thinking that one or more of them are going to fail or are leaking, what do you think?

jgalt how long do you have to leave it pluged in for till the computer will finally start?

Also DO NOT under any circumstance try and open the psu for a look doing so can kill you because of the voltages they store even when unpluged

Rick
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#5
jgalt

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The time required for it to be restarted varies but doesn't seem to be a function of how long it was unplugged. Thank you for your advice on not opening the psu but I do feel comfortable enough around high voltage capacitors that I feel I could do it safely but I think the best bet would to just buy a new psu and toss the old one out.

Thank you both for your help.
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#6
Samm

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Well Samm in therory that is a very good guess and must be the case but I wonder why the capacitors would discharge over a short period, I could see this happenning if left unpluged for months. I am thinking that one or more of them are going to fail or are leaking, what do you think?

Rick

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Doby - as usual, I think you're right. I have already been informed that you are the PSU expert! :tazz:
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#7
Doby

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Well I don't know about a expert but I due tend to make sure the psu is adequate for the system weather its troubleshooting or just a recommend for new builds.

I see far to many of these cheap ( I hate that term lets say inexpensive) psu's not having enough amps to support modern cpu's mobo's and graphics cards.

jgalt in this case I think I would replace it just to be on the safe side cause when a psu fails it can be troublesome to pinpoint not to mention the fact that it can also damage other components.

Rick
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