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- No Power
September 7, 2008, 12:26 am
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My wife's computer stopped working the other day. She would push the power
button and nothing would happen. I guessed ot was either the power button
or the power supply. I bought a new case but it didn't start up. Next
thought it was the motherboard, so I bought a new mother board. Still no
power. Any ideas?
Re: No Power
You bought a new case, but to clarify this means the new
case came with a new PSU? If so, was the new psu a know
good brand, accurately rated and sufficient for the system?
I'd take the motherboard out and lay it on a non-conductive
surface like a desk. Put both PSU next to it, take all
cards out of the mobo, disconnect all cables except the
couple for the ATX 20(+4) and 4+ CPU power connector.
Momentarily short the power pins on the motherboard together
with a metal object like a screwdriver tip to see if PSU
comes on and stays on. Recheck the motherboard manual to
be sure you're shorting the correct two pins.
If still no activity, plug only a hard drive into new PSU
(unplug motherboard) and short the PSU's PS-On pin to a
ground pin with a paperclip to see if it turns on. The
PS-ON pin is normally a green wire, pin 14 on a 20 pin
connector or pin 16 on a 24 pin connector. Ground is any of
the adjacent black wires.
- Andrew Smallshaw
September 7, 2008, 8:49 pm
Re: No Power
You've already had some good suggestions, but there is very little
to go on here - that isn't a criticism of you, it is just the nature
of the problem, so a strutured approach is best rather than blindly
swapping components. A basic DMM comes in very handy here.
1. Make sure the wall socket works, the lead is plugged in and if
appropriate, the PSU rocker switch is on and the voltage selector
is on the right range. I know you've already done this but do it
again. Humour me here.
2. Disconnect everything from the PSU. Take the main power connector
to the motherboard and test for +5VSB - use your multimeter on DC
volts (0-20V on most cheap meters) between the purple and any black
lead. If you get 4.5-5.5V (ish - the spec is actually slightly
tighter) that's good to go. If you don't, check for mains voltage
on the IEC plug end of the power lead - if there is voltage there
it must be the PSU.
3. Strip the motherboard down the less than minimum - all expansion
cards, memory, and any data cables to drives of any description.
Plug the power supply back in. Leave the processor, internal
speaker (if fitted), CPU fan and front panel switch and LEDS
connected. Disconnect the keyboard, mouse and any USB devices.
Try turning the machine on.
Obviously in this state the machine won't boot but you should see
signs of life - the power LED should come on and you should get
error codes from the speaker (don't wrry about those - they'll just
be telling you about missing RAM and graphics). If you do go to
step 4, if not try dropping in a stick of RAM. It would be a dodgy
mobo that wouldn't work without any memory but there is plenty of
dodgy hardware out there. Try reseating the processor but most
motherboards will show _some_ kind of life even with no chip in
If still no signs of life then it is looking like the mobo. Check
the front panel switch is going to the right place and actually
works (you may need to fashion some fine tip extensions to your
multimeter probes out of plain steel paper clips). If that is good
then you've isolated it to the mobo with near certainty.
4. Load a single stick of memory and the graphics card, plug in
the monitor and retest. Don't forget the graphics card's additional
power connector if it has one. If if fails here try swapping the
memory with another stick to isolate whether it is the memory of
graphics card causing the problem.
5. Reconnect the keyboard, primary hard drive, mouse, remaining
memory, and everything else (in that order) one at a time, testing
after each addition until you hit a problem. If you hit a problem
at this stage try a different slot, power connector or whatever
other options you have top see if you can persuade it to work.
Bear in mind though that if you find a problem adding somethign
then while it is the most likely culprit, the problem may actually
be somewhere else - adding a disk may show up a problem with the
disk controller for instance. However, this is less likely in this
situation where the machine flatly refuses to do anything.
6. There is no step 6. If you've got this far you've completely
reassembled your system and it is working normally. Who knows what
the problem was? Who cares? You can spend an awful lot of time
trying to diagnose non-problems, effort that is almost invariably
wasted. Get on with your life and forget about it.
- Andrew Smallshaw
September 7, 2008, 9:16 pm
Re: No Power
[Somehow managed to miss testing the PSU properly which is a prime
This doesn't check the PSU fully however, and doing that is not
straightforward. The plug in PSU testers do the job simply and
don't cost that much (under a tenner here in the UK) but I don't
always trust them since they tend not to say what load they put on
the PSU. By all means use one if handy but another way is with
you multimeter and a couple of resistors to bring the PSU to its
minimum load - you can't expect your PSU to work when there is
nothing connected to it.
The ATX specification does not specify a minimum current draw but
hints at 1A per rail. For this you need a 5R resistor for 5V and
12R for 12V - if you can't those exact sizes, go lower, not higher,
and make sure they are high wattage types. Connect the 5R resistor
between a red and a black wire on your PSU's mobo connector and
the 12R between a yellow and a black wire. Then start up the PSU
in the manner Kony described. With your meter, and with one lead
connected to ground (black) check for a nominal 3.3V on the orange
wires, 5V on the red wires and 12V on the yellow wires. Carry on
checking these voltages periodically as you reassemble the system
to ensure the power supply is coping with the load.
Re: No Power
On Sun, 7 Sep 2008 23:16:05 +0200 (CEST), Andrew Smallshaw
If you don't load the 3.3V rail you can't necessarily expect
it to be within regulation... or at least not all that close
to the proper value. Often the PSU manufacturer will
provide the minimum current per rail on the PSU spec sheet.
It tends to range inbetween 0.5A and 2A for at least the 5V
rail and often 12V rail, although many PSU will run with
only a jumper between PS-On and ground with no external
load. I tend to always see if the PSU will run like this
first to check whether any rail is wildly out of spec
because they usually will run like this but on occasion I
have found some that won't run if the 3.3V rail isn't loaded
- especially server PSU.
Since the old PSU is presumably out of warranty, another
thing the OP might do is leave it unplugged for a few
minutes then open and inspect it for burnt components or
vented capacitors especially around the attachment point of
the wiring harness (may need to move those wires out of the
way to see all the capacitors buried under them).
Re: No Power
finger to keyboard and composed:
Monitor the voltage on the PSU's PS_ON pin in the ATX power connector
Verify that this pin switches from +5V to 0V when you press the power
switch. If it doesn't, even for a brief time, then suspect the
motherboard. If everything is working properly, then pin 14 should
remain at 0V while the PSU is on.
- Franc Zabkar
Please remove one 'i' from my address when replying by email.