Old PC needs to warm up for 3 mins (If left unplugged) - then it starts

A green light comes on the mother board after about 3 mins then it starts.
Before that nada if its left unplugged.

Bad PSU or Bad mobo

It works .... just this quirk

Part on the way out ?

Old PC with ddr400 memory :-)  memory test fine by the way
feck1Asked:
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Dr. KlahnConnect With a Mentor Principal Software EngineerCommented:
You didn't state how old the system is, but if it dates from the 1999 through 2007 timeframe, it probably has bad capacitors in both the power supply and on the motherboard due to the so-called "capacitor plague".  Nearly every electronic product built in the early years of that era had "inexpensive" Taiwanese capacitors with bad electrolytes.

The behavior you've described is consistent with a bad cap trying to "re-form" (i.e., rebuild the insulating layer) on the damaged aluminum substrate and succeeding very slowly.  As the cap re-forms, the voltage across it will rise and eventually it will get to the point where the system can start.  If this is the case, then at some point the cap will no longer be able to re-form, there will be a loud POP, and the failing cap will burst.

If you intend to keep the system operating, inspect the motherboard carefully and replace any suspicious capacitors with low-ESR electrolytics or (if you have money to burn) tantalums.
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JohnBusiness Consultant (Owner)Commented:
You could have a old BIOS battery. You could try replacing it.

The other, and I think more likely, cause would be defective capacitors on the motherboard or in the power supply that may not be working correctly.

There could be cold solder joint problems as well.
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_Commented:
+1 ^

If you have to set the time/date in the bios , try the cmos battery first.
Otherwise beg, borrow, steal, another power supply, and see if it makes a difference.    ; )
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nobusCommented:
if it does not start, probably the power good signal is wrong
check it  with a multimeter, or repalce the power supply
here the pinouts : http://pinouts.ru/Power/atxpower_pinout.shtml
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