So we know from your description, it's a desktop
computer, and it has an Asus motherboard in it.
Asus is pretty consistent about placing a green
monitoring LED on the +5VSB supply.
Your Antec is making +5VSB, but the step that is
failing, is PS_ON# from the motherboard, which
travels from the motherboard to the PSU, is not
making good logic levels. Either the driving end
(motherboard) is defective, or the Antec component
receiving the signal is defective.
A typical standalone test, is to remove the Antec
and drive the PS_ON# with a ground strap. Which
would cause the Antec to start and the cooling
fan on the Antec to spin. It's normally advised
to have a test electrical load connected to the
Antec, and with a test load connected, you can
use a multimeter to verify the voltages.
Doing such a test, verifies the Antec recognizes
a zero volt (logic "0") on the PS_ON# signal wire.
The supply remains powered, for as long as there
is a logic 0 on the PS_ON# wire. The "#" symbol in
the signal name, means "active low" and the signal
is active, when a logic "0" is present.
On the motherboard side, the drive is open collector
logic. A resistor (like maybe a 2.2K or a 10K) pulls
PS_ON# to approximately the same voltage as +5VSB
(5.0V). The open collector driver on the motherboard,
pulls the signal wire to around 0.4V or so when it
wants the PSU to come on. And normally, that would be
sufficient to pull down the PS_ON# on the power supply.
At one time, monster chips were used to drive PS_ON#.
Like maybe a 7407 or a 74F07. In modern times, they
don't like to waste money on additional components,
so perhaps one of the larger chips is driving it.
And in such a case, the drive capability might be
a bit less.
This interface doesn't have a good track record, and
users are called upon to debug this particular wire,
way more than they should have to. As that user,
you don't know if it is the motherboard driver that
is defective, or the Antec end which is defective.
Doing the ground strap on PS_ON# test, is one way
to verify the power supply is holding up its end.
Knowing it's an Antec, probably a Truepower series,
my guess is the Antec has given up. The Antec supplies
built by ChannelWell weren't the best, as I had two
fail here. The main transformer inside says "CWT" on
some of the printed text, which is one identifier.
But mine didn't fail on PS_ON#, and instead they
had leaking caps (sizzling noise at startup). There
were some later Antecs built by Delta, and each OEM
supplier will have their own failure characteristics.
(Ways they like to fail.) Antec doesn't build their
own supplies, but buys them from an OEM. This is
quite common in the power supply industry - although
several of the majors do actually make their own.
Since the wiring on PS_ON# is wired OR logic, you
can also ground the PS_ON# signal while the system
is completely assembled. But that's physically a
bit difficult to do. The system will start if you
do that, but it also overrides THERMTRIP and you
would have no protection against a CPU overheat
if run that way.
For more info, there are three power supply specs on
formfactors, and the one I'm including here is for
24 pin (modern) supplies. The second link gives an overview
on the connectors used on ATX, in a general way. The second
link shows you that the 20 and 24 pin cables share a
subset of pins, so work the same. It's just the pin
numbering which is a bit screwy. So I might use wire
color to identify my PS_ON# signal. The spec has
a wire color table in it.
http://www.formfactors.org/developer/specs/ATX12V_PSDG_2_2_public_br2.pdf
http://www.playtool.com/pages/psuconnectors/connectors.html
Paul