Dead PC, help sought

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18 Oct 2009
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I am pretty sure my CPU has passed away but I am a learner when it comes to PC's, following instructions, building, upgrading e.t.c. seem fine but I have not been that good at diagnosis! And just want to check a few things before I commit myself to a replacement CPU.

CPU is a year old i5 750k LGA1155. Mobo is an Asus P7P55D Pro. Wen't to turn it on an nothing happens, opened her up and despite the mobo power supply light being on nothing at all. At 1st i thought I had a broken switch so stripped this down tried shorting etc but nothing. I then realised if I detach PC from mains and start it the fans start momentarily a big red LED flashes on once then off. Reading my manual my PC has seperate LEDS to show if their is an issue with RAM, VGA card, CPU or Boot device. It was the CPU light so I am pretty sure this leaves no mystery.

But it seems weird to me that nothing happens when I try to turn it on again not even the red led. I have to unplug from mains then try turn it on to recreate the LED. Is this odd?

Could it be something repairable i.e. reseating the CPU (I hear they don't really like this) or renewing the Thermal Paste?

I have run the Pc for a year quite a lot (mainly gaming) and while I have not overclocked anything I am just using the stock cooler. Did I have this coming for me?

Could it be anything else I have overlooked and anything I can check before I shell out for a new CPU?

Finally as I expect my only option will be to buy a new CPU :( why do all the LGA1155 now say Sandybridge? Any i5/i7 LGA1155 Sandybridge will work right?

Sorry for so many questions i think I am just in denial, I don't really have 150-250 quid lying around :(
 
Could be a power issue. The CPU light could be indicating poor power to the EPS12v connector or something.

Try a different PSU.

By the way what is the PSU brand and wattage?
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It is a cosaire HX750. The lights on all peripherals keyboard, mouse e.t.c. light up so their is some juice going through the mobo to them at least. It does seem as though it is going through the boot process deciding the CPU won't work flash the relevant LED then stop trying. Maybe the mobo caches the fact that the CPU is dead and does not try to power on again to protect itself. That would explain why i need to disconnect from the mains (clearing the cache) and reconnect before it will try the boot sequence again?
 
On the lates today and tomorrow so do not want to open the PC. Will have a good play on Wednesday, I will try the PSU from this machine just to rule the PSU out.
 
sounds like a psu problem - my psu did this about 4 times before it eventually just died

would power all the fans etc for a second then die

do you not have a spare psu laying around to power board ?
 
Peripherals such as mouse and keyboard use the 5v rail which doesn't mean that the 12v is working properly at all. Motherboard standby and things work off Lower voltages as well.

The reason I say PSU is because everything appears to be right until you hit that start switch, when something goes wrong and shuts down the whole PSU. The CPU can't do this and neither can the motherboard. If the CPU was bad you still get some fans running and mobo lights would remain powered. I've also had a dead PSU behave something like you describe.

Anyway jut slave your second PSU on to the 24 way and 8 way connectors as there's no need to power anything else to prove it. Place your two rigs next to each other to save having to remove the PSU from the second rig.
 
Excellent thanks for the feedback, I had no idea there were seperate power rails but your explanation sounds good especially as the fans don't turn! Makes a lot of sense.

Have to sleep now, work running me ragged. Will test with this PSU weds, and if you are right (which I think and hope you are i will have saved myself 150 quid from buying a new Processer that I don't need! (might buy a new PSU as my spare won't be a permanent solution but that is cheaper than a processor).

Just shows it is always worth asking, thanks for the feedback both and especially for the details Tealc.
 
Ok, trying to fix this now and think i have hit a wall.

Some power runs through the PSU to make mobo light turn on. When i hit power the PSU fan spins for a few revs then stops. So it is dead right? (note currently still wired up to system)

While powerful enough the PSU in my old PC (Pentium 4 system) is non modular and has only one PCIE lead which used to power an x1800xt. My 5850 seems to need 2 of these cables. Can i install the old psu into the new machine without powering the 5850 (but leaving it plugged in) and rely on onboard graphics? I just want to test that the CPU works.
 
I would give it ago on the on board graphics, but it does sound to me that the PSU is in fact dead. it should still power the cpu and board use a PS2 mouse and keyboard as these general use lower power ratings then there USB counterparts.
 
Thanks for that tip ethermaster.

Typical after stripping the PSUs from 2 PCs I realise the one in the old machine only has a 4 pin ATX12V connector, new system uses 8 pin so I can't use it anyway :/

However I did confirm my Corsair HX750W is definitely dead by doing the paperclip test with a case fan as load, duplicated the test on the old PSU which does it fine.

Knackered now so tomorrow I get to work out how to act on the 84 month warrantee as I have never returned anything in my life. Bet it is not a quick process lol
 
Just worked out I can use a molex - 6 pin PCIe converter and can just stick the 4 pin EPS/ATX 12v plug into one half of the mobos 8 pin EPS connector!

So i should be able to switch the old PSU in afterall to keep me going until the RMA is sorted.
 
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