Is my asus p3b-f dead?

  • Thread starter Thread starter Peter Santoro
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Peter Santoro

I have an asus p3b-f rev. 1.04 that's been running fine for a number of
years - until now. It originally had a 600Mhz P3 and I switched it to a
1.4Ghz tualatin celeron over a year ago. It was in an Inwin tower case
with a 300W psu.

I recently bought a new Antec case and True Power 430W psu. I believe I
was very careful in transferring the motherboard to the new case;
however, when I went to power it up, nothing happened. (Yes, it was
plugged in.) There was no power to the fans or motherboard. The psu
fan didn't even turn on and there was no sound of any kind. I had a
CompUSA tech test the brand new Antec psu. He found it to be defective
(no ground). The tech mentioned that this condition may have damaged my
motherboard.

I decided to put the motherboard back in the old case with the old psu,
to see if it still ran ok as before. I replaced all cables/connectors,
as before. The motherboard no longer appears to work with the old
case/psu either. When I press the power button, all is silent and the
green light on the MB is off. There is no visible damage and nothing
smells burnt.

Can a faulty psu (no ground) damage a motherboard? Is there anything
else I can try?

I probably should have left my trusty p3b-f alone.

Thanks for your assistance,

Peter
 
Peter Santoro said:
I have an asus p3b-f rev. 1.04 that's been running fine for a number of
years - until now. It originally had a 600Mhz P3 and I switched it to a
1.4Ghz tualatin celeron over a year ago. It was in an Inwin tower case
with a 300W psu.

I recently bought a new Antec case and True Power 430W psu. I believe I
was very careful in transferring the motherboard to the new case;
however, when I went to power it up, nothing happened. (Yes, it was
plugged in.) There was no power to the fans or motherboard. The psu
fan didn't even turn on and there was no sound of any kind. I had a
CompUSA tech test the brand new Antec psu. He found it to be defective
(no ground). The tech mentioned that this condition may have damaged my
motherboard.

I decided to put the motherboard back in the old case with the old psu,
to see if it still ran ok as before. I replaced all cables/connectors,
as before. The motherboard no longer appears to work with the old
case/psu either. When I press the power button, all is silent and the
green light on the MB is off. There is no visible damage and nothing
smells burnt.

Can a faulty psu (no ground) damage a motherboard? Is there anything
else I can try?

I probably should have left my trusty p3b-f alone.

Thanks for your assistance,

Peter
Click to expand...

If the green LED on the motherboard is off, that means that +5VSB
is not coming from the supply.

When you flip the switch on the back of the computer to "ON", the
ATX PSU responds by applying +5VSB to the motherboard. That
voltage powers logic on the motherboard, that interfaces to the
power switch on the front of the case. This is for the "soft on/off"
function. If the +5VSB is "flattened" for any reason, then the
motherboard won't have the energy to power the driver that
grounds the PS_ON# signal on the ATX power connector. That is
what turns on the supply.

So, you need to figure out why and how the motherboard is
managing to short out the +5VSB (plus five stand by). That rail
is pretty weak and most power supplies cannot put out more than
a couple of amps on that rail. Quite likely, there is thermal
overload protection inside the PSU, for when that supply gets
shorted.

As for tracing on the motherboard, where exactly that signal
goes, I would say it goes everywhere. Something could be
damaged enough in there to affect circuit operation, but with
only 2 amps or less of current to work with, not be burned
bad enough to be a visual fault.

Try the motherboard outside the case. Just the motherboard and
the old PSU. Touch an (ESD discharged) screwdriver tip to the
two power switch terminals on the PANEL header will turn it
on, assuming the green LED is lit. All you are looking for in
this case, is for the PSU fan to start spinning, so you don't
need to have a whole lot of components installed on the
motherboard for this basic test.

As for the fault with the PSU, a three prong plug has three
terminals (duh). The safety ground carries a small current,
and this current is the result of having a common mode filter
on the input side of the PSU. The filter, by design, has to leak
a little AC current into the safety ground, as part of the
filtering function. If the safety ground is left floating (say
you are in an old house, without safety ground), this generally
shouldn't be an issue, except for the fact you'll get a mild
shock from your conputer case :-) If, on the other hand, neutral
was missing for some reason, I still don't see that being an
issue. So, for now, I don't understand how the claimed failure
mode of the power supply, lines up with the symptoms. Power
supplies have isolation between primary and secondary (that is
what the "hipot tested" sticker indicates on the PSU), and
can have 1000V difference applied between primary and secondary
without breaking down, so having floating AC on one side of the
supply, shouldn't be visible on the other side of the supply.
Only if the hot wire was connected in place of the safety ground,
would there be an electric light show.

It looks like something is wrong with +5VSB, but we don't know
if the rest of the rails are fried as well or not. This will
be tough to diagnose. Motherboards don't tend to use conventional
fuses (they use polyfuses instead), so it isn't likely to be
a fuse problem.

HTH,
Paul
 
Paul said:
If the green LED on the motherboard is off, that means that +5VSB
is not coming from the supply.

When you flip the switch on the back of the computer to "ON", the
ATX PSU responds by applying +5VSB to the motherboard. That
voltage powers logic on the motherboard, that interfaces to the
power switch on the front of the case. This is for the "soft on/off"
function. If the +5VSB is "flattened" for any reason, then the
motherboard won't have the energy to power the driver that
grounds the PS_ON# signal on the ATX power connector. That is
what turns on the supply.

So, you need to figure out why and how the motherboard is
managing to short out the +5VSB (plus five stand by). That rail
is pretty weak and most power supplies cannot put out more than
a couple of amps on that rail. Quite likely, there is thermal
overload protection inside the PSU, for when that supply gets
shorted.

As for tracing on the motherboard, where exactly that signal
goes, I would say it goes everywhere. Something could be
damaged enough in there to affect circuit operation, but with
only 2 amps or less of current to work with, not be burned
bad enough to be a visual fault.

Try the motherboard outside the case. Just the motherboard and
the old PSU. Touch an (ESD discharged) screwdriver tip to the
two power switch terminals on the PANEL header will turn it
on, assuming the green LED is lit. All you are looking for in
this case, is for the PSU fan to start spinning, so you don't
need to have a whole lot of components installed on the
motherboard for this basic test.

As for the fault with the PSU, a three prong plug has three
terminals (duh). The safety ground carries a small current,
and this current is the result of having a common mode filter
on the input side of the PSU. The filter, by design, has to leak
a little AC current into the safety ground, as part of the
filtering function. If the safety ground is left floating (say
you are in an old house, without safety ground), this generally
shouldn't be an issue, except for the fact you'll get a mild
shock from your conputer case :-) If, on the other hand, neutral
was missing for some reason, I still don't see that being an
issue. So, for now, I don't understand how the claimed failure
mode of the power supply, lines up with the symptoms. Power
supplies have isolation between primary and secondary (that is
what the "hipot tested" sticker indicates on the PSU), and
can have 1000V difference applied between primary and secondary
without breaking down, so having floating AC on one side of the
supply, shouldn't be visible on the other side of the supply.
Only if the hot wire was connected in place of the safety ground,
would there be an electric light show.

It looks like something is wrong with +5VSB, but we don't know
if the rest of the rails are fried as well or not. This will
be tough to diagnose. Motherboards don't tend to use conventional
fuses (they use polyfuses instead), so it isn't likely to be
a fuse problem.

HTH,
Paul
Click to expand...

Thanks, Paul. I figured out my problem. What the CompUSA tech told me
did not make sense and I just couldn't believe that two power supplies
(one new and one that was just working) would die at the same time. I
rechecked my power switch connector on the motherboard. And sure
enough, it was off by one pin (connected to two ground pins). I know
better (it's clearly marked in the manual), but my eyesight isn't what
it used to be.

Everything appears to run fine now. The new case and power supply are
much quieter!


Peter
 
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