PC does virtually nothing when powered up - what's fried?

A friend's PC conked out recently, he tells me he'd been getting "incorrect frequency" messages. So he turned it off, and when he turned it on later he got another and soon after it died; no noise or lights when switched on.

Well, I instantly suspected the PSU, but some hours later (after trying a few PSUs) I came to the conclusion that the PSU was fine.

It looks to be either the CPU or Mobo or both, but I'd like to know how I can find out which it is.

Here are my results:
-Oldish P4, socket 478, D845WN mobo
-When everything connected as normal, nothing except a tiny LED next to the 20-pin connected labelled "power". This is on all the time that the PSU is connected to mains. Pushing the power button has no effect.
-I discovered that on removing the 4-pin connector from the mobo (the "P4" cable) and reconnected the PSU to mains, the fans would start, and the HDD light would go solid. This would only happen, however, if the CPU was in the socket, otherwise it would do nothing (but the LED would again be on). Front power button would not turn it off. The CPU and every other chip seemed cold, except for one labelled "American Megatrends" (which I presume is the BIOS) which was just slightly warm after a few minutes of running like this.
-Above behaviour was consistent no matter what else was connected. I took out all the RAM, PCI cards and peripherals and the behaviour remained. No beeps, ever, (and definitely no video output), although it does have a mobo speaker.
-Also tried 1 of the 2 RAM sticks at a time. Nothing new.
-Mobo battery seems fine.
-No obvious damaged components
-I took apart the original PSU and found that the fuse had been removed and bridged with a wire. Perhaps this caused a problem during a power surge or some such...

The fact that there was at one point a difference between processor in and processor out to me hints that the processor isn't a complete potato.

Any ideas on which component it might be, or maybe what else I could try? (I have a meter, but I don't have another socket 478 mobo or CPU - actually I have one Pentium 4, and I don't know what kind of socket it has because I can't remove the heatsink from the CPU (and I don't feel secure about pulling rather hard...))

Thanks,
Mikrondel

 

The incorrect frequencies message can only come from the Display or Monitor (from the Video Card)

First chech the Vid card (or basically put another one in)

Also clear the cmos (pins / jumpers on m/b)

The other cpu (stuck on) with the m/b out of the case, you can pry it off with a flat screwdriver (at your risk ! )

It could be a cpu fault by the way.

 

That's what I thought, but I really doubt it's the graphics card. This particular friend is notorious for getting things wrong so he may have had an entirely different message.

A fried video card shouldn't stop the fans and stuff coming on, particularly when the card is not plugged in (when I said I removed the "PCI cards" I did mean the graphics card as well).

Remembering the lesson I once learned about not making assumptions, I tried another graphics card anyway, but was not surprised to find that nothing happened.

One of the first things I did after realising it wasn't the PSU.

I'll keep that in mind but the broken PC isn't important enough. Once upon a time I yanked the heatsink out along with the processor and had to get a new mobo

Could it be a mobo fault? If not, then I think I'll have my answer...


Any more ideas?

Mikrondel

 

New development!

Seeing as the 4-pin power plug on the mobo is apparently the CPU power plug, I did some resistance testing and I found that the resistance between +12 and Ground in the broken PC is 1.7 ohms; in a working PC it jumps around a bit then stabilises at around 1800 ohms.

This makes me think processor... but perhaps there's something on the mobo - in between that connector and the processor - that's fried?

What would be your verdict?

Mikrondel

 

CPU

Probably cooked.

 

Yeah, I think that's just about definitive.

And for the record I also tried the video card from the dead PC in another one and it seemed to work.

Well, thanks!

 

I usually avoid swapping these cards from PC to PC, and just try to work on one at a time.
Otherwise you may have two dead computers.
But I'm glad you were able to confirm the video card anyway.

I have had similar faults in the past and here are the recommendations:

Remove the motherboard fully from the case
Inspect it (blown caps; a staple lying around !)
Bench test with No harddrive or CD or sound or mouse or keyboard or fan or video
(basically nothing except power, onboard video, ram and a flat screw driver to power up the power pins)

If that fails (ofcourse cmos fully cleared out)

Start replacing in sequence
Power supply
Ram
cpu
m/b !!