Windows 7 - Laptop shuts down after few minutes

Hi,
first sorry for my english but i'll try to explain my problem anyway:
i got friends laptop (HP G61-410SA) which was completely dead.The battery for this laptop was dead before.Suddenly he unpluged power source from the socket while laptop was runing and after that he was unable to switch it on again.I checked first power adapter by multimeter,it shows correct voltage (14vdc-inner and 19vdc-middle), so i've decided to change motherboard (HP spare 577997-001).Got used one on ebay,changed it,checked vents/heatsink if it has any dust inside,cleaned,applied thermal grease as should be,assembled,switched laptop on,everything seemed goes well but after few minutes (3/5min) laptop shuts down.On every next try to switch on,time it keeps on,goes shorter and shorter.Seems like owerheat but vent is spinning normaly,without load,just normal,for start it spins faster after some short time goes to normal silent state but not stops at all,so it seems ok,tried to reseat heatsink and so on,just to ensure.Next what i did are i've removed componnents one by one,hard disk drive,cd-rom,one ram modules,tried to swap ram modules,got another one ddr2,reset cmos,set bios to safe defaults,nothing helped.Then got another power adaptor,same thing,even changed cpu,no way.Even if i will go to BIOS and leaving laptop in that state for a while it does the same thing,after 3/5 minutes goes off.What is very strange,i can't switch laptop on at all with battery removed,only on dc power cord,never had laptop before that i couldn't use it without battery.Is this normal?All those steps i have done with laptop assembled and with almost all componnets on the desk.Still confused,is there some known issue for this or just i got bad motherboard with some temperature sensor broken or something like that.Any help,ideas,is much appreciated.

You seem to have done everything bar what I would have done first!

Change the power supply! All problems started with it, so that's what I'd have changed first

Hi,first thanks for suggestion but i've already done it,i got original,new adapter from exact model,it didn't helped,that's one of the reasons why i'm still confused.Anyway thank you once again.Any more ideas?

In answer to your question:
What is very strange,i can't switch laptop on at all with battery removed,only on dc power cord,never had laptop before that i couldn't use it without battery.Is this normal?
This is not normal for most all laptops.

How do you know the eBay replacement didn't have the same problem yours does? This is why it was for sale on eBay.
Does this laptop have a separate power inverter circuit board?

Not being able to turn on without a battery installed leads me to believe there is a power inverter board which also monitors & controls battery charging.

Your issue sounds like a short circuit. The AC power adapter has a circuit breaker that cuts out if a short is detected. Since the AC adapter and the battery both feed the same power connecter leads on the motherboard I would focus my attention there.

If you have a multimeter you will be looking for connections reading approx. 3 ohms.

The thing that makes the above questionable is that you have replaced the motherboard. The odds that both motherboards have a short circuit in the power connector is pretty high. That would also tend to rule out a circuit board delamination.

When you remove all of the removable components did you try and turn the laptop on to see if it would stay on without tripping the breaker? One of those components could have an internal short.

And then there is the possibility that the short occurs in the connection between motherboard and display. You could disconnect the display to see if it will stay on.

EDIT: See here: http://www.uofr.net/~greg/hardware/l...oubleshooting/

How do you know the eBay replacement didn't have the same problem yours does? This is why it was for sale on eBay.
No,i don't know and of course it is possible the motherboard was sold to me faulty .I've buyed one more mb from this seler before and after week it died,he changed it without problems and second one works perfect till now,so that's the reason,i must be 100% sure there is issues with mb again before i'll contact him for replacement.

Not being able to turn on without a battery installed leads me to believe there is a power inverter board which also monitors & controls battery charging.
There is no separate power inverter or at least i can't visualy locate it,there is just power jack and some piece of cable goes to mb.Visualy there is no any damage or damaged components on motherboard.I'll try later another same model to switch on without battery and will see does it works.

Thanks a lot for any ideas !

Sounds daft, but try building it up without using the laptop chassis.

If there is metal on metal contact under the mobo with the chassis, that could cause a short.. But wouldn't explain the few minutes... Unless heat causes the chassis to flex.. Lol