ATX power problems??

[j.p]

Hi folks,

I recently had to replace my power supply and motherboard.
Everything has been fine for about 3 weeks but when i went to turn the
machine on yesterday it was 'dead', no power to anything.
Took the cover off thinking the PSU had gone again but found the power
light on the motherboard was lit and that the PSU was working fine.
Powered off the PSU, unplugged the main power cable, checked it, plugged
it back in again and when i powered up the PSU and pressed the on/off
switch on the case it powered up. I figured it was just a glitch and
left the PC on all day yesterday with no problems. Powered down the PC
using the shutdown command on windows but when i came to the machine
this morning it is dead again, ie. nothing happens when i press the
switch on the case. Unplugged the power cable and reconnected it again
and the system powers up fine.
ANY IDEAS???
Could it just be a dodgy on/off switch that is not triggering whatever
it has to trigger on the motherboard or is the motherboard faulty? What
does the handbook mean when it says that the switch puts the pc into a
'soft off' mode? i have only ever worked with PC's that had a proper
on/off power switch so i don't know exactly how this system works.
I'd be grateful of any advice before i go out and buy another mother
board if that is not the problem!!
Cheers,
John.

 

[w_tom]

Could be a long list of things causing your 'dead on
powerup' problem. Without necessary information, then all one
can expect is answers based upon speculation. Your question
should be about what information is important to solve this
problem.

The power supply system is more than just a Power Supply.
Which is defective? That is why you must provide numbers.
Required is that always important tool: a 3.5 digit
multimeter. In your case, important numbers are 5VSB when
power is not on. Also numbers in the first second and then
many seconds after the power switch is pressed for 3.3, 5, 12,
Power Good and Power On signals. Chart for where to take
those measurements (ie wire colors) is provided in the second
discussion: "Computer doesnt start at all" in
alt.comp.hardware on 10 Jan 2004 at
http://tinyurl.com/2t69q and
"I think my power supply is dead" in alt.comp.hardware on 5
Feb 2004 at
http://tinyurl.com/yvbw9

To get useful responses to what is not understood, post the
numbers. An illuminated light does not even mean the power
supply is working. Too many assume otherwise. The light can
glow and fans spin - when a power supply is 100% defective.
No way to say that power supply is or is not working without
numbers which is why the meter is so important.

 

[M. L. Kaspar]

ATX power supply testers are relatively cheap. I bought a new one for $15.00
on EBay.

Anytime I build a new machine..... I always test the power supply before I
install the MoBo.

If a "Dead" machine comes into my shop....the very first thing I do is use
my ATX power supply tester. It not only tests the voltage, but it tests the
voltage under a dummy load....this info is far more conclusive than a
voltage test with a high-impedance multimeter.

Good luck,
Kascomp

 

[w_tom]

The power supply tester applies a minimal and trivial load
on the power supply. Therefore a power supply that tests good
with the $15 tester can still be defective inside the
computer. However for $20, one tests a power supply under
FULL load, one observes if other parts of the power supply
system operate, one determines which other part of that system
are not working, one can immediately learn if the power supply
that passes a GO-NOGO test can also support the real world
(full) load, one identifies a power supply that is marginal
but still passes on that power supply tester (failing but not
yet fully failed), AND one has a meter (an essential tool)
that also identifies and solves other problems. $15 for the
incomplete and inconclusive power supply tester or $20 for a
tool to fully diagnosis the system? That power supply tester
is not an effective solution for too many reasons - including
this need to test new power supplies.

The advantages of a meter are massive - one sided. So one
sided that it is embarrassing. A computer assembler would not
even know what little that power supply tester accomplishes.
But then so many computer assemblers never first learn
electrical basics. The clone computer industry is chock full
of power supplies that are missing essential functions. Then
the assembler uses a tester and feels everything is OK? Sure
its OK to the tester. But defective by design. Damage
created by the power supply (missing essential functions) was
created by the mythical surge? Many computer assemblers never
learn basic electrical concepts such as advantages of a meter
or those missing 'essential functions' in power supplies that
cost less.

Some fancy power power supply tester MUST be better because
it only tests supplies? Nonsense. Get the meter. Identify
which of three parts in a power supply 'system' is defective
without disconnecting anything. And don't even look back at
that silly power supply tester recommendation. Two minutes to
identify which of three power supply 'system' components is
defective without swapping parts and changing cables. Meter
provides useful numbers and can provide facts necessary make
educated conclusions.