Calling all techies - dead PC diagnosis!

PhiL_P_3

Got a PC which refuses to POST - powers up (lights on, fans and hard disk spinning) but no signal is sent to the monitor. Also there are no beeps from the motherboard.

Only sign of something amiss I've found is that the PSU fan is dead (and may have been a while), which is bad news as there was very little space in the case between the top of the CPU fan and the PSU so this would have caused extra heat build-up.

I've ascertained that the graphics card and memory work fine by testing them in another PC. I also tried the PSU on another PC and though it wouldn't fully boot due to the PSU's low wattage, it did at least POST, so there is some life in it still!

So the only things I haven't individually tested which could be the culprits are the motherboard and CPU. I'm inclined to think it's the motherboard, since if the CPU had blown the motherboard would at least voice it's displeasure with a series of angry beeps at power-up.

Anyone care to back up (or blow away) my theory before I spend on a replacement board? Either way I'll definitely be replacing the PSU anyway.

All help most appreciated!

Cymraeg_2

Sounds as if you're going the right way . Had similar problems to what you're describing and it was the processor in my case had part blown/ really upset the motherboard and they wouldn't play with each other .
At least it's sale time and you may get some kind of bargain.

c011410

try and reseating the cpu and memory sticks, also make sure all internal power leads are plugged into the system board, also disconnect one by one the hard drive then cd-rom and finally your floppy drive, each time you disconnect one power up the box..one of these devices may have a short circuit causing loss of activity on your PC, if this does not work try to reset the clear cmos on the system board, there should be instructions on the case on how to do this, its just moving a jumper over to another pin on the system board

PhiL_P_3

try and reseating the cpu and memory sticks, also make sure all internal power leads are plugged into the system board, also disconnect one by one the hard drive then cd-rom and finally your floppy drive, each time you disconnect one power up the box..one of these devices may have a short circuit causing loss of activity on your PC, if this does not work try to reset the clear cmos on the system board, there should be instructions on the case on how to do this, its just moving a jumper over to another pin on the system board

I've already disconnected all non-essential devices that aren't required for the PC to POST so that the only things connected are the PSU, motherboard, CPU, memory and graphics card.

I've also aready tried resetting the CMOS using the motherboard jumper and then removing and refitting the battery.

All components and connectors have been removed and refitted, as well as tested in another PC where possible, although as I mentioned if this was the problem I would expect the motherboard to issue warning beeps at power-up, and there are none at all.

Still thanks for your suggestions, all help greatly appreciated!

It seems all signs still point to a faulty motherboard..

PhiL_P_3

Sounds as if you're going the right way . Had similar problems to what you're describing and it was the processor in my case had part blown/ really upset the motherboard and they wouldn't play with each other .
At least it's sale time and you may get some kind of bargain.

Presuming from what you said your motherboard was ok and the fault was with the CPU, did the motherboard then issue the expected warning beeps at power-up?

Joe_Bloggs

Have you inspected the large capacitors on the motherboard. They hate being cooked and tend to be a main cause of motherboard failure in certain types of motherboard these days. The caps often distort and leak.
You did not say that you tested the suspect motherboard /CPU with another working power supply. This would have been the nail in the coffin of the CPU/ MB combination. It is a tricky problem to isolate the cause if you don't have a stock of easy to swap parts. The caps in the PSU with the faulty fan will be worse for wear also.
J_B.

PhiL_P_3

Both the motherboard and CPU (upon removal) look fine and normal, no sign of anything amiss to the eye, though I haven't studied the capacitors particularly closely.

Yes I did try another working PSU, to no avail, but why would that be the final nail in the coffin of the motherboard/CPU? ???

Joe_Bloggs

You said that you tried the PSU from the from the 'failed to post' system in another system. Where it failed at a slightly later stage. You did 'not' say that you tried a PSU from a healty system in your 'failed to post' system. Hopefully a test PSU can handle the load without excuses, and will fail gracefully when a faulty motherboard or processor are present.
If the PSU is good and MB and CPU are unresponsive then something is wrong but nothing is proven.
Have you measured the rail voltages of your PSU with a volmeter. ?
J_B.

PhiL_P_3

Sorry, I didn't expand on the PSU testing originally since I'd already taken it out of the equation since it's obviously on it's last legs and I'm going to replace it anyway. Yes, to confirm I did test the faulty system with a PSU of higher wattage rating from a healthy PC and the results were no different.

What remains is to establish whether it's the motherboard and/or CPU that has failed and needs replacing. Testing voltages and similar is going a bit beyond my diagnostic abilities I'm afraid!

Without being able to test the suspect motherboard & CPU individually this is going to have to be a best guess situation I think. The only thing I've got to work on is the lack of warning beeps from the motherboard at power-up..

StrikeEagle

U tried booting the pc outside of the case on an anti-static mat?

CityHunter_3

It sounds like you're reasoning is sound and you have isolated the problem down to the CPU or MoBo. My money's on the MoBo.

I would have asked whether you have overclocked any of the components in the bios as over-clocking the memory would stop the machine from posting, but seeing how you have reset the cmos this would have cleared all settings.

Before the machine stopped working did you notice the PC shutting down, or alarm, due to components exceeding temperature thresholds? Namely the CPU? Usually they are set within the Bios where if the CPU exceeds 70C, or something, it would issue warning beeps before shutting down.

You must have a mate who could test the CPU for you? What's the spec of it?