Adam said:
Okay, sounds like what the mobo manual calls the standby power (SB_PWR) LED,
which lights up no problem whenever mobo is connected to PS so far.
Make sure any USB loads are disconnected. No iPod or iPad charging.
Disconnect the memory card reader. The purpose of doing that,
is to try to get as much +5VSB power available as possible.
You would check the voltage level on the +5VSB wire of the PSU.
With a multimeter set to the 20V full scale range, probe pin 9 (purple)
and measure with respect to ground. I use an alligator clip on the
multimeter black lead, and clip to a ground screw on the back of the PC
to establish the black wire potential. Then, I have one hand free to work
on probing with the red multimeter wire.
When the 24 pin PSU connector is installed into the motherboard, there
is a little clearance where the PSU wire goes into the back of the connector.
It's possible to put the tip of the red lead, between the wire and the
plastic connector shell. And make electrical contact. That's how you
measure the voltage on the purple wire, while the system is in use.
The PS_ON# wire, when the computer isn't running, should be pulled
high (to the same voltage level as the purple wire you just checked).
So now you've measured green. If the voltage is low (and you haven't
attempted to turn on the PC yet), then the open collector motherboard
driver isn't working right.
But you've seen the green LED, which is a first order indicator
that it's working a bit on +5VSB. If the LED was dim, you'd suspect
PSU trouble.
Now comes the fun test. Connect PWR to GND on the motherboard
panel header (a momentary touch is enough). The fans should
all start to spin. If they don't, and you really think you were
successful in grounding the PWR pin, now you go back and check
the PS_ON# green wire. It should drop to 0.4V to 0.7V or so.
That is the open collector motherboard driver, pulling the
line low and asking the power supply to turn on the main rails.
If the PS_ON# signal is still high ( greater than +2.0V ), then
the motherboard needs to be replaced (under warranty). If the
PS_ON# wire is 0.4V like it is supposed to be, and the PSU
doesn't react, replace the PSU. You've already done the
"unconnected supply" test, so somehow I don't expect the
problem to be an un-reactive PSU. It's more likely at this
point, to be a motherboard open collector driver that
can't pull PS_ON# low. (Or as Vanguard mentioned, a defective
front panel switch.)
What's strange about this whole thing, is the failure frequency
of this interface. I just don't get it. The motherboard has
a couple thousand high tech electrical signals, and that damn
low tech signal is the one that fails. They must cover that
pin with the two minute functional test at the factory (or their
test bench wouldn't turn on), so they must be able to verify
PS_ON# drives low, by virtue that the woman doing the testing
is able to complete all her tests (in the two minutes allocated).
(There was a video available, of that testing process at Asus.
That's how I know it's a woman
)
Asus used to produce anywhere from 3 million to 5 million motherboards
per month, so that 2 minute functional test is one expensive test to
carry out. But it's essential. They can do boundary scan structural
testing until the cows come home, but a functional test covers things
like that PS_ON# signal. Just to prove the motherboard works. It's
really a final acceptance test, to make sure as many duff motherboards
don't get put into cardboard boxes, as possible.
So we're basically checking a couple voltages here. To fault isolate
and tell whether the PSU or the motherboard should be replaced.
The purple wire runs the Asus green motherboard LED. We want to
check that it reads 5.0V +/- 5% or so. The PS_ON# signal, when the
system is off, should also be 5V (same voltage as purple). When the
momentary switch on the front is pressed, the PS_ON# voltage
drops. And using your multimeter on the green wire, verifies it
dropped to around 0.4V or so. It won't drop to zero, because the
driver doesn't go all the way to exactly zero. It just has to pull
low enough, to be well below the 1.5V to 2.0V threshold on the
thing receiving the signal.
+5VSB (0.0V level +5VSB
| means "run please") |
Pullup \_ Pullup
Resistor \ Resistor
| PS_ON# |
PWR X----+---- Motherboard ---- Open -------------------+- ... control
/ logic Collector (to of main
| GND X----+ Driver ATX + PSU
| | supply) | section
(Front GND GND
Panel
Switch - normally open, momentarily close to operate)
On some supplies, if the PS_ON# signal rides around 1.5V to 2.0V
(in other words, right on the threshold), the power supply actually
comes on half-way. This leads to weak rails, and an inability to supply
full load current. Apparently, the on-off control isn't as "digital"
as it should be. So if the supply appears limp, sometimes it's not
actually the supply. And this is an important reason to be
measuring the green wire, and that it's driven to 0.4V or less.
Otherwise, the wrong component could get the blame. And this
whole procedure is for "fault isolation", identifying the defective
part.
Paul
