Bad CPU killed motherboard?

[WildW]

Okay, so this is one of those questions where I'm pretty sure I know the answer already, but I feel the need to ask it anyways.

I was going to turn an old Pentium 4 PC into a low-end HTPC for my sister - it was a machine she had sat in a cupboard. Anyhow, when I tried to start it up, it wouldn't POST. Fans ran at full speed but no video, no beeps, no nothing. So I pulled out any bits that might be useful - ram, CPU, etc, and binned the rest, assuming a dead motherboard.

I got hold of a Dell Dimension 4700 really really cheap - again, a Pentium 4 CPU, threw in a Radeon 5450 for HDMI type goodness, all was good. Then I remembered I had this faster CPU from the older dead machine, so thought it couldn't hurt to try it.

So I swapped in the CPU. Power on. . . . fans full speed, no video, no beeps, no POST. Ah well, must be that rarest of things, the actual dead CPU. No matter, I'll just swap back to the old working CPU. . . . fans full speed, no video, no beeps, no post. So, bad CPU appears to have killed my motherboard

About all I could think of to try was to do a CMOS clear, but that didn't make any difference. Any other suggestions before I throw the thing out?

 

[Blain]

Give us more details about the specific CPUs in the mix.

Not all socket 775 processors work with those Dells.
Was the faster CPU supported by the 4700 Dimension?

Exactly what steps did you take to clear the CMOS?

 

[WildW]

Well, it's socket 775. It had a 3.0GHz P4 530 (Prescot). I swapped in a 3.4 GHz, but don't honestly know exactly which - it's gone in the bin and is long gone now. Granted it would've had a higher TDP than the original CPU. From the looks of the 3.4GHz chips that existed, it can't have been too different from the original. I didn't do anything clever like checking a CPU compatibility list, so I guess I got what I deserved.

As far as clearing the bios, I removed the battery, did the BIOS jumper, replaced the battery. . . that usually does the trick.

I'm imagining that the "bad" CPU has probably fried a power phase on the motherboard or something, and there's nothing more to be done. . . I'm going to try the bios clear once more - at this point I have nothing to lose.

 

[mfenn]

Yeah, it sounds like you are pretty much boned on this one.

 

[Blain]

As far as clearing the bios, I removed the battery, did the BIOS jumper, replaced the battery. . . that usually does the trick.

I'm going to try the bios clear once more - at this point I have nothing to lose.

Make sure there isn't any power coming from the PS when you clear the CMOS.