A7A266 Wont Power - Get Green Standby Light

[Kathy]

Hi All...

I have an Asus A7A266 which was working fine earlier this week. Switched the
MB to a different case, and apparently it doesn't like the new home, as it
won't power up. I am thinking that the Mobo is fried somehow (as it was
working earlier), and am not sure why the green stand by light is on when
the PS is plugged in. I cant even get the fans on the PS to spin on start.
It is completely DEAD.

I checked the PS (550 Watt) which works fine with other sistems. I checked
the Power switch by using both the Power SW and the Reset SW leads to the
mobo connectors. They work fine on other system. I swapped the RAM, Removed
the CPU, and still nothing. All I have in there is a video card.

I also removed the MB from the case and tried running it on the cardboard
box just to kill the possibility of it being shorted on a stand-off post in
the case...

Any ideas on why this wouldn't power up? I am seriously frustrated. This was
working earlier this week...

Scott

 

[Sick Willie]

Hi All...

I have an Asus A7A266 which was working fine earlier this week. Switched the
MB to a different case, and apparently it doesn't like the new home, as it
won't power up. I am thinking that the Mobo is fried somehow (as it was
working earlier), and am not sure why the green stand by light is on when
the PS is plugged in. I cant even get the fans on the PS to spin on start.
It is completely DEAD.

I checked the PS (550 Watt) which works fine with other sistems. I checked
the Power switch by using both the Power SW and the Reset SW leads to the
mobo connectors. They work fine on other system. I swapped the RAM, Removed
the CPU, and still nothing. All I have in there is a video card.

I also removed the MB from the case and tried running it on the cardboard
box just to kill the possibility of it being shorted on a stand-off post in
the case...

Any ideas on why this wouldn't power up? I am seriously frustrated. This was
working earlier this week...

Scott

The most common cause of what you describe is that you have mis-identified
the pins that act as the switch for the power. Asus uses the 1st two pins
of the front panel header for reset, skips a pin, and then uses the next two
as the power switch. This is from the orientation of standing in front of
the board, looking at the header along the left hand side. The LED you
describe is not a standby light per se, its purpose it to let you know that
power, standby or otherwise, is applied to the board so that you do not
install/remove boards without either unplugging the power source, or (a
better alternative when available) flipping the power switch on the power
supply to the off position.

Sick Willie

 

[Kathy]

Yup. I had that correct. Does the LED being on indicate a good MB? Or, could
that mean that there is power getting to it, but something else crapped out?

-Scott=

 

[Tyler Porter]

The most common cause of what you describe is that you have mis-identified
the pins that act as the switch for the power. Asus uses the 1st two pins
of the front panel header for reset, skips a pin, and then uses the next two
as the power switch. This is from the orientation of standing in front of
the board, looking at the header along the left hand side. The LED you
describe is not a standby light per se, its purpose it to let you know that
power, standby or otherwise, is applied to the board so that you do not
install/remove boards without either unplugging the power source, or (a
better alternative when available) flipping the power switch on the power
supply to the off position.

Sick Willie

Im having the exact same problem with my A7N8X motherboard. Tried
everything you tried, plus resetting the bios. I have also checked
and rechecked the power switch plug about 100 times. Its something
else, and im pulling out my hair trying to figure it out.

 

[Paul]

Yup. I had that correct. Does the LED being on indicate a good MB? Or, could
that mean that there is power getting to it, but something else crapped out?

-Scott=

Are you referring to the green LED on the motherboard, next to the
three DIMM slots ? That LED is powered by +5VSB from the ATX power supply.

Power supplies have two "sections". The +5VSB supply operates whenever
the switch on the back of an ATX supply is in the ON position. The
other outputs of the supply on the other hand, are controllable via a
logic signal.

When a power supply is connected to a motherboard, some of the motherboard
circuits are powered off +5VSB. One of the circuits is the circuit that
listens to the power switch on the front of the case. Momentarily closing
the switch on the front of the case, results in a chip on the motherboard
latching the signal, and pulling down the PS_ON# signal which is on
the ATX 20 pin connector. This causes the rest of the PS outputs to be
switched on. The PS runs as long as PS_ON# is grounded (connected to
COM on the 20 pin connector). At shutdown, Windows tells the chip that
listens to the switch, to release the PS_ON# signal, and then the machine
is shut off.

So, you have +5VSB present, but for some reason, either your momentary
switch from the front panel is not working properly, the motherboard
chip driving PS_ON# is malfunctioning, or the power supply is not
listening to PS_ON# for some reason. That makes at least three possible
pieces of broken hardware.

Some people test by actuating PS_ON# manually, while the motherboard
is connected, but I don't recommend that unless you know whether the
chip driving the PS_ON# signal is safe to short to ground or not.

A safer way to test, is to test the power supply by itself, by using
the 20 pin connector and connecting PS_ON# to COM. Even if you don't
own a voltmeter to check the voltages, you can at least see the PS
fan spinning when the PS has successfully been switched on. Inside
the supply, +5VSB is applied through a pullup resistor, to the PS_ON#
signal, and only a small current should have to be shunted to ground
by the chip on the motherboard - sometimes the PS puts too much
current on this lead, and then the chip on the motherboard cannot
fully shunt the current to ground to start the supply.

Otherwise, if you've isolated the problem to the motherboard via
swapping out other pieces of hardware, it could actually be a motherboard
fault. I have a motherboard here, where the PS_ON# signal went nuts
when one of the IDE cables was half seated, so make sure all of the
cabling is correctly installed before blaming the motherboard.

In terms of failure rates, the PS is tops on the list of dud
hardware, no matter what brand is stamped on it. If you have another
computer handy, use the supply from it to power your A7A266 and see
if they get along together.

HTH,
Paul

 

[Tyler Porter]

I tried checking the power supply by connecting it to another working board,
and it started up fine. I then tried to plug an older power supply I know
works to the asus board, and again nothing started. Seems like it must be
the motherboard. Is it repairable?

 

[Sick Willie]

...clip..

A safer way to test, is to test the power supply by itself, by using
the 20 pin connector and connecting PS_ON# to COM. Even if you don't
own a voltmeter to check the voltages, you can at least see the PS
fan spinning when the PS has successfully been switched on. Inside
the supply, +5VSB is applied through a pullup resistor, to the PS_ON#
signal, and only a small current should have to be shunted to ground
by the chip on the motherboard - sometimes the PS puts too much
current on this lead, and then the chip on the motherboard cannot
fully shunt the current to ground to start the supply.

This is not really a very safe way to test a supply. True enough, it won't
fry a board, but a power supply expects to find a load. Powering one up
without some load is *not* good for the supply any more than powering an
amplifier without a load is good for an amplifier.

Sick Willie

 

[Sick Willie]

Yup. I had that correct. Does the LED being on indicate a good MB? Or, could
that mean that there is power getting to it, but something else crapped out?

All the LED signifies is that some voltage is being applied to the board -
enough to power an LED. The LED is only there to remind you that power is
present on the board. It does not indicate that the board is good.

Sick Willie

 

[Sick Willie]

Yup. I had that correct. Does the LED being on indicate a good MB? Or, could
that mean that there is power getting to it, but something else crapped out?

-Scott=

Have you tried just shorting the two pins rather than using a switch?

Sick Willie

 

[Paul]

Yup. I had that correct. Does the LED being on indicate a good MB? Or, could
that mean that there is power getting to it, but something else crapped out?

-Scott=

Another thing. I wonder if the CMOS battery is dead. The CR2032 is
available at Radio Shack. When new, it reads at least 3V, and the
Southbridge circuit it powers can work down to about 2V. Maybe that
would be worth checking as well.

Paul

 

[Kathy]

That's what I ended up doing. I used the power supply to power up a separate
system fine. I then used a different power supply with the "dead" system,
and nothing.

I have no drives plugged in, no cables in, all I have in is the memory, CPU,
and Video Card (in the AGP slot). I should be able to get power one would
think, unless the mb was dead... but then again, I am sitting here with a
dead system...

=Scott=

 

[Paul]

This is not really a very safe way to test a supply. True enough, it won't
fry a board, but a power supply expects to find a load. Powering one up
without some load is *not* good for the supply any more than powering an
amplifier without a load is good for an amplifier.

Sick Willie

I have to admit Willie, operating a supply without a load is not a
"best practice". I own a set of load resistors for the specific purpose
of testing supplies, but they weren't exactly easy to find. The problem
is, I cannot suggest to people that they go out and acquire a set of
resistors and a connector to make their own dummy load, so the quick
power up test is the best I can come up with. If I recommend they just
go out and buy another power supply, that isn't much of an answer either.

Any suggestions on how I should answer the question ?

Power supplies come in a couple of architectures, and the old ones
have a common primary feeding a set of secondary transformer outputs.
I cannot honestly say how close the outputs stay to their nominal
voltages when they are unloaded. There is some (light) feedback from
each output back to the primary side, so they shouldn't really shoot
to infinity.

As a test, I just took a "Touch" brand 250W power supply manufactured
in 1998, and shorted PS_ON# to COM while using no other loads. When
I measure with my voltmeter, the +3.3V and +5V outputs are sane, while
the other outputs like +12V, -12V, and -5V are all on the low side.
This implies that the feedback in the supply is working so well as to
shut down all but the lowest voltage outputs.

While a sample size of one is insignificant, at least I don't see
anything on this old architecture power supply to suggest the supply
is producing voltages high enough to cause breakdown. AFAIK the supply
simply won't meet its regulation spec, when it is unloaded.

Here is a sample schematic of an old architecture switching power
supply. See what you think. (The thing I like best about this
design, is the two 470uF caps charged to line voltage. Talk about
potential energy...)

http://www.pavouk.comp.cz/hw/en_atxps.html

Paul

 

[Sick Willie]

I have to admit Willie, operating a supply without a load is not a
"best practice". I own a set of load resistors for the specific purpose
of testing supplies, but they weren't exactly easy to find. The problem
is, I cannot suggest to people that they go out and acquire a set of
resistors and a connector to make their own dummy load, so the quick
power up test is the best I can come up with. If I recommend they just
go out and buy another power supply, that isn't much of an answer either.

Any suggestions on how I should answer the question ?

Have them either plug it into another board. If this is not available, most
smaller computer repair stores, such as mine, will, if the customer's
attitude is friendly, plug in a supply, while they wait. And, having a
spare supply is actually an ideal situation.

If your entire goal is to see if the fans spins, you can probably run it
safely long enough to do so. However, a fan spinning, or a standby LED, are
poor indicators of available power.

Power supplies come in a couple of architectures, and the old ones
have a common primary feeding a set of secondary transformer outputs.
I cannot honestly say how close the outputs stay to their nominal
voltages when they are unloaded. There is some (light) feedback from
each output back to the primary side, so they shouldn't really shoot
to infinity.

The primary purpose of have a "power good" signal as feedback from the
board, in both the old AT style and the newer ATX, et al, power supplies is
to assure that there is a load present. Again, just as in the case of an
amplifier, you can probably run it unloaded for very short periods of time,
but all you can really ascertain is that the unit powers up. Even after
having personally practiced something for a period of years, (in fact, I
practice alot of things that I can't recommend <g>) I stil find myself
loathe to suggest that others indulge in these same practices. <g>

Sick Willie

 

[Sick Willie]

Another thing. I wonder if the CMOS battery is dead. The CR2032 is
available at Radio Shack. When new, it reads at least 3V, and the
Southbridge circuit it powers can work down to about 2V. Maybe that
would be worth checking as well.

Paul

The CMOS battery being dead should not keep the board from powering up.

Sick Willie

 

[Sick Willie]

That's what I ended up doing. I used the power supply to power up a separate
system fine. I then used a different power supply with the "dead" system,
and nothing.

I have no drives plugged in, no cables in, all I have in is the memory, CPU,
and Video Card (in the AGP slot). I should be able to get power one would
think, unless the mb was dead... but then again, I am sitting here with a
dead system...

=Scott=

I would say pull it out of the case and try, but you said you already did
this. The #1 scenario is a standoff shorting the board.

You might try removing the heatsink and processor. Reseat the processor in
the socket and re-attach the heatsink, preferrably after re-applying some
thermal compound. I know it sounds like voodoo - after all the processor is
already in the socket - but a ZIF socket, just like any other socket,
provides a metal on metal pressure contact connection and can, over a period
of time, especially with a multitude of heat up/cool down cycles, oxidize to
the point where a good connection is not being made. With the Athlon, there
is the potential for somewhere around 462 bad connections.

Sick Willie

 

[Kathy]

Already Beat you to that idea. Attempted a boot with nothing attached with
the exception of the power supply in order to try and force a boot error. No
power. Fans wont even spin up on PSU.

I would definitely think this was the PSU if I didn't try it on a couple of
separate systems, and try good PSUs on this board which won't boot.

-Scott=