Problem An expensive doorstop at present!

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PC will not boot up.
I have been using a spare system for some time now because I didn’t have the time to attend to this problem.

I replaced the old CPU fan on my regular system with a new and more efficient heat sink and fan. The new one was a bit harder to install than expected, and it took some time to get the lugs properly secured to the m’board.

When I first turned on the system nothing happened, the fans started up, but the HDD LED would flash on and off with a regular rhythm and the speed of the fan would fluctuate in time with that flashing.

If left for a couple of minutes, the fluctuating speed of the fan and rhythmic LED flashing would stop. The HDD LED would stay on, and the fan would settle down to a constant speed.

But that was it. It would sit there, the HDD LED staying on, and the CPU fan purring away. But it would not load the BIOS, or even try to.

I did the usual things, stripped down the PC to bare essentials, (as it now is still). No difference.

I took off the new heat sink and fan and carefully re-seated the CPU- redoing the thermal compound each time.
I reset and also cleared the BIOS. No difference.
I swapped HDs between the sick PC and the one I am now using instead. (not that this should make a difference if it’s not even loading the BIOS). It didn’t.

The HD from the sick PC started to work in the working PC.
The HD from the good PC made no difference in the sick PC.

I tried the procedure again this weekend. The only difference in result being that the rhythmic fluctuating in fan speed was absent, as was the corresponding flashing of the HDD LED. It just goes straight to the HDD LED staying on, and the CPU fan purring away. There is still no sign of the BIOS loading.

I have now used a POST card to check, which produces a code of FF.
The BIOS is a phoenix. The Mobo is a Foxconn P9657AA 8EKRS2H. the CPU is a Pentium D805.

If the BIOS is not loading, yet the motherboard clearly has power and is not dead, it seems to be pointing to either a motherboard or CPU problem.

I noticed that the two LEDs at either end of the POST card were flashing, (+5v, +3.3v) suggesting a possible PSU problem.

So I substituted the power supply from the PC that I presently use daily.

On hooking it up and turning on the power, for the first second or two, the CPU started very briefly and stopped, and the lights on the POST card flashed briefly, then went dead.
After a couple of seconds, the CPU fan started again, and the two end lights on the POST card came back on and flashed, as previously with the other PSU. The HDD LED stayed on as before.

So it appears that there is an issue with stable voltage at +5v and +3.3v (the two LEDs on POST card that are flashing), but as using a PSU from my daily PC does not affect that, it seems to me to be pointing to a motherboard problem.

Any thoughts anyone please? Many thanks in advance.

 

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I think at this point i would take the motherboard out and check that there isn't a screw or something else under it shorting something to chassis.

 

Did you take the mobo out to install the new fan?

 

Is the 24pin and 4 pin connection securely conneted to the motherboard?

what are the full specs of the system including both psu brands?

 

Hate to say it but i've seen this happen where a bit to much pressure's been applied when the replacing cpu/heatsink/fan and the motherboards got a crack in it

 

weird. I haven't seen this as it seems to me that the ZIF socket stops this from happening.

I highly suspect this issue is related to power either the PSU or mobo (most likely PSU).

 

Since the OP has tried a known good PSU and the POST card exhibited (almost) the same behaviour I'd say it was most likely the mobo.
It wasn't a straightforward installation either, which also points to potential mobo damage.

 

yep but is the OP using a PSU that can handle his system?

Recently I have seen many people with similar problems thinking that their psu isn't the issue because they think that they have some 700w psu that everything is ok but they don't seem to take in the amperage of the psu which is the main problem.

The original psu could have been defective and taken something with it when it died or started to fail.

The OP should look for capacitor damage on his mobo.

 

Since the OP has stripped the system to it's essentials I would assume yes.

See above.

True, but the fact that it is exhibiting the same behaviour as the known good PSU tends to point to both PSUs being good. The alternative being that they are both broken in exactly the same way.

 

or not good enough for the system.

A psu being pushed to or beyonds it's limits can exhibit good working order untill it fails then when you try a similar one it may work for a while then fail again or not work at all. I have seen this happen a lot over the last few months. (This is why we could use the full spec of the system including gpu and psu brand etc).

This is when the psu tends to take something with it like the ram or blows the capacitors on the mobo.

The reason why I think this could be a power issue was the OP was saying the HD led was on and the fan speed was fluctuating this could point to fluctuating voltages on the 12v which as you know is most important voltage and if so would cause issues with no POST etc being shown.

I would suggest the OP get a multimeter on the PSUs to see if they are working correctly, try a cmos reset to see if he can get into the BIOS so he can check voltages and temps there.

 

GBL.. this 'was' a working system, all the OP has changed is the CPU heat sink fan and he has tried another PSU that works. So, i very much doubt it's the PSU.

As i said earlier, something could be shorting the motherboard to the chassis, or as others have said, the motherboard may have received some physical damage when the OP was trying to get the heat sink lugs in place.

 

Yes that's why I said could.

That is why I also said a system can run fine for a long time whilst the psu is being pushed to it's limits and when it decides to fail it can take something with it or the system won't work properly even with a good known psu of same wattage and make.

It would be helpful if we actually knew the full specs of the system though.

 

I would suspect the motherboard also. The PCB may be cracked from the heatsink change. I don't like the heatsink mounting design on socket 775. I did a motherboard and CPU replacement for a customer recently and the board was actually bowed slightly after I mounted the heatsink. I much prefer AMDs designs with a lever action clip that hooks onto the CPU socket rather than mounting onto the motherboard itself. It's so much easier to install and doesn't stress the motherboard.

 

Is the new CPU definately plugged into the CPU fan header on the board and not one of the aux headers?

Had a system once that would not post unless a CPU fan was attached (changable in bios). Additionally as others have suggested remove the MB and reinstall, reinstall the heatsink too while the MB is out of the case, ensure all aux power ports on the MB are hooked up to the PSU, not just the 24 pin one.