PSU & MB - Help - have I blown up my PC?? :(

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I'm fairly computer competent but don't profess to being an expert. Recently
one of my computers stopped working correctly. What happened is you'd turn
it on and often it wouldn't even light the screen up (no beeps either) and
occassionally Windows would start to boot or even get as far as the desktop
and then either completly lock up or black the screen (any ideas??). The PC
is a P4 (2.6Ghz). Thru a process of elimination I was down to testing the
power supply when I scruffed one from an older PC (600mhz Celeron). The P4
PSU had an extra 4-wire molex connector but the ATX plugs matched so I
thought what the hell put the new PSU ATX plug on the motherboard and
drives, removed the four pin molex connector near the CPU and powered her

This is where things got interesting. Straight away I could tell something
was wrong - the cooling fans were spinning too slowly and before I could
turn it off the PSU went up in a very thick black, smelly lot of smoke. I
put the original PSU back on the board and reattached all plugs and turned
the computer back on. Nothing now happens.

Now - can anyone help me here - PLEASE! Any idea why the original PSU now
won't even turn the computer on, and also why the new one (from the Celeron)
went up in smoke? Both are ATX factor. Also, any idea with the problem that
was initially happening (turning on but usually not actually getting
anywhere, and would then freeze at the desktop or before or during Windows
startup, even in safe mode).

For what it's worth, the original PSU hasn't blown up (well at least it
doesn't smell and hasn't put out a plume of smoke) but I'm not game enough
to leave it plugged in to find out.

Thanks all,

Re: PSU & MB - Help - have I blown up my PC?? :(

Jane Miller wrote:
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I think you blew it up, at least a dead motherboard, yes. That 4 prong
Molex is important, but most motherboards that need it, won't start at
all without it. You got one of the dumb ones. If you're super crazy
lucky, a replacement PSU MIGHT cure it. I'm thinking new motherboard
AND new powersupply is your only cure, however. Or a whole new machine.

 BTW was the 2.6GhZ celeron a Black and Silver Emachine?

Re: PSU & MB - Help - have I blown up my PC?? :(

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Did you pull the second power supply from a Dell ? A non-standard
pinout was used for a few years by Dell, and maybe you had the
misfortune to use something that is not standard ATX.

If you look at the connector pins on the P4 2.6Ghz motherboard,
is there any damage to the connector ? Where did the thick black
smoke come from ? My guess would be, it came out of the second
power supply, but if something on the motherboard smoked, then
it could be the motherboard that got cooked as well.

Some more detail about the two machines in terms of hardware, might
help. Especially the "donor" machine, at least for the purposes
of identifying what kind of supply it might have had in it.

A P4 based machine needs a power supply with at least 12V @ 15A
printed on the label on the side. An older supply, especially one
without the 2x2 ATX12V connector, might not have such a high rating.
If the ATX12V square connector is not connected to a P4 motherboard,
the CPU does not get any power. In rare cases, it does get power,
through the single 12V pin on the main connector. (The majority of
motherboards separate the 12V on the square connector, from the 12V
on the 20 pin connector, but there are some boards where a mistake
was made, and they are joined together.) The single 12V wire on the
main connector, is only rated to carry 6 amps, and it is not healthy
for the wire, if a high power processor is drawing current through
that wire. That is why normally the 12V on the square connector is
separated from the 12V on the main 20 pin connector. The main 20 pin
connector normally is carrying 12V for use by the fan headers on the
motherboard, and perhaps for use by any serial I/O chips on the

The square connector powers the Vcore conversion circuitry around
the P4 CPU socket. If the square connector is not connected, the
Vcore is not powered, and the CPU cannot run and execute any code.
Normally, there would not be any damage from doing so. It sounds
like the second PSU either had a non-standard pinout, _or_ the
second PSU was severely overloaded for the amount of power the
second PSU had to offer.

To guess what has happened to the motherboard, I would visually
inspect the motherboard, looking for clues. And perhaps using a
multimeter, check to see if any connections have burned out.
Simplifying the hardware setup, by removing components, might
uncover something that got damaged (say, disconnect hard drives
and CDROM cables, unplug the hard drive and CDROM etc). Maybe
it will show signs of life with less stuff plugged in. This
would all be part of disassembling the machine anyway, if it
turns out that the motherboard will need to be replaced.
Finding a nice S478 board for your processor, will be a

I don't see any reason to suspect the CPU or memory were
killed. But you are the observer, and can see better what
happened than we can.


Re: PSU & MB - Help - have I blown up my PC?? :(


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i am by no means an expert either, but i will share what i have
experienced, hopefully it'll help.

when this has happened to me in the past, the culprit has usually been
one of the following:
1. Overclocked CPU causes the computer to hang / become unstable.
2. Hardware conflict: mixing different frequencies of RAM, installing a
hard drive(s) without properly setting the jumpers, dying hard drives,
3. a type of virus.
4. fried mobo/psu

obviously, if you haven't overclocked your CPU or messed with your BIOS
settings you can most probably rule #1 out. if you haven't installed
any new hardware (which is what it sounds like), you can also rule out
most of #2 as well.

was your hard drive acting up (making loud, unusual noises) when you
were experiencing the problems with windows? if yes, your hard drive
could very well be failing and could be the source of your windows
woes. if your computer were still functional, i would recommend
disconnecting any new hardware you may have installed recently and try
to boot with the bare minimum (mobo, cpu, psu, hd, 1 stick of RAM,
video card) and try to POST consistently; an incompatible piece of
hardware will cause your system to hang. also, if you already hadn't, i
would check the connections of all your hardware, making sure they are
tight and snug in their respective places. lastly, the windows troubles
could be caused by a virus, but i am doubtful of that.

i believe you are on the right track; the hardware being the source of
the problem.

if indeed your mobo is fried, don't throw away your whole pc! salvage
the good parts, most importantly, the cpu. motherboards are not very
expensive and can be replaced fairly easily. i have had a number of
fried motherboards whose cpu's were perfectly fine. you can find good
prices for motherboards and other computer hardware at or


Re: PSU & MB - Help - have I blown up my PC?? :(

Hi guys,

I thought I'd just reply here to answer some of the questions and tell you
all where I'm at.

Essentially the original power supply is working fine - the hard drive is
dead and the PC will turn on when the hard disk is removed. So good to know
that everything else is _probably okay_.

There is a weird issue thats still nagging that didn't register before (as I
hadn't used the PC regularly for a long time). This was actually happening
earlier (before the PSU incident). When I turn the PC on it often won't
light the screen up. However, this happens 100% of the time - if I detach
the computer from the power point for a few minutes and turn it back on it
boots and POSTs fine. What's going here? Tested with two power supplies.

Re: PSU & MB - Help - have I blown up my PC?? :(

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Some motherboard designs are known for cold boot problems.
The only oddity I remember, is some computers would start
properly, if the monitor was unplugged. Or if the room
temperature was a bit higher (in other words, computer
starts properly in summer, but not in winter). At the
time this was happening, there didn't seem to be a cure.


Re: PSU & MB - Help - have I blown up my PC?? :(

P4 based computers require a special upgraded PSU, which your substitute PSU
was not.  You may have fried the motherboard.  I would obtain a current
production PSU that you know works and is of adquate power output and see if
that fixes things.


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