ATX power supplies keep blowing

[le ténébreux]

Hi all,

I'm having a bit of a problem and hoping someone here
can at least give me a clue about what might be going on.
About 3 weeks ago my PC (ASUS P4SE mainboard,
Celeron 1.8GHz) suddenly started spitting out ATX power
supplies for no apparent reason. I've gone through four
power supplies since then, now onto my fifth.

The power supplies don't go completely dead, they still
supply some power to the mainboard and USB devices,
but nothing else. No fan, no CPU. It's like they suddenly
go into permanent "standby" mode and never work again.
Put a new one in, and everything is fine. For about a week.

Can anyone shed some light on what might be happening
here? Would I be right in thinking that it's not the fuse
blowing, otherwise it would be completely dead? And
is it likely to be a problem with the electricity supply to my
house, or could it be caused by something in the PC itself?

This is getting annoying and expensive, so any help greatly
appreciated.

 

[Vanguard]

Sounds more like you are getting underpowered cheap power supplies. If
you are drawing more power than the power supply is rated for (and some
are overrated in the first place as to how much power they will actually
supply) then the power supply heats up and something blows. Hopefully
it is a resettable fuse (after a cool down period the unit is ready
again). Sometimes it is a fuse so you have to open its case to replace
it (it might be in a snap-in holder or you have to do some snipping and
soldering). Other times something pops and you can't find what to fix
inside; a part might've exploded and you can see its remnants or it just
vaporized and you might find the leads that went to it. Some power
supplies recover after a prolonged overload, others can be repaired, and
some just self destruct.

Tally up the power consumed by everything that you have connected to the
power supply. See if you can find what are the real specs for the power
supply (see if you can find the maker's web site and if they provide
anything useful but unfortunately many only give the combined power for
the 5 and 12 volt taps). If it's a cheapy power supply, don't load it
more than three-fourths of its rated power because that might be all it
will really handle. If your system requires more than that, get a
bigger power supply. Or get one that really can handle loads up to
their rated power, like Antec, Fortron, Zalman, Enermax (I don't
remember all the good ones right now).

If you feel like doing some reading, Tom's Hardware has some interesting
articles:
http://www.tomshardware.com/column/20011012/index.html
http://www.tomshardware.com/howto/20021021/index.html
http://www.tomshardware.com/howto/20030609/index.html

Also, it could be surges on your power line taking out the power supply.
The best solution is a whole-home surge arrestor but sometimes that
isn't practical or even legal (it may not be your property). If you
have to use a surge protector, use only one. Don't use 2 or more surge
protectors because one doesn't have enough outlets. Get one with lots
of outlets or string outlet strips from the surge protector. You want
to keep ALL power connections upstream of the one surge protector.
Don't interconnect equipment that is connected to multiple surge
protectors. Due to impedance differences between the surge protectors,
you can end up with several hundred volts potential between them. And
expect to pay some hefty money for a good surge protector. A UPS lets
you keep your system up during a power outage but it may still provide
little or no surge protection. Again, check out what surge protection
features it has in addition to its power backup abilities.

 

[w_tom]

Some basic information. When one claims that surges on
power line might be causing problems, then poster is not
technically informed. Too many are easily made victims of
propaganda promoted by plug-in surge protector manufacturers.
Surges typically occur once every eight years - which they
don't want you to know. If suffering daily surges, then you
are trooping daily to the hardware store to replace electronic
appliances. Are you suffering daily electrical appliance
failures?

If not paying about $80 retail for that new power supply,
then immediately suspect that as where to begin seeking a
reason for failure. If power supply manufacturer is not
provided with a long list of specifications, then supply
probably is missing essential functions. However if doing
hardware repair / replacement - especially involving power
supplies - a 3.5 digit multimeter and associated data is
necessary. Data in chart at:
http://www.hardwaresite.net/faqpowersupply.html

Tom's Hardware, for example, demonstrated by experiment how
many supplies do not even output power as claimed. But it
goes farther - much farther. No power supply can be damaged
by too much load. Power supply outputs can even be shorted
together and power supply still would not be damaged. If
power supply fails under normal load, then most likely reasons
include manufacturing defects, undersized supply, or failures
associated with motherboard. Which one? That is what meter
data would first suggest.

You know one important number - how much was paid for that
supply. Other numbers essential to answer your question
include those provided by multimeter and the so many numbers
provided by manufacturer's specifications. If the latter is
not provided, then you all but know that supply is missing
essential internal functions.

Don't just wildly replace supplies. First collect facts so
that you know where reason for failure is located. Fact #1
comes from that multimeter. Even fans spinning or not
spinning just don't provide sufficient information. That
meter costs less than a new supply.

 

[Lane Lewis]

Hi all,

I'm having a bit of a problem and hoping someone here
can at least give me a clue about what might be going on.
About 3 weeks ago my PC (ASUS P4SE mainboard,
Celeron 1.8GHz) suddenly started spitting out ATX power
supplies for no apparent reason. I've gone through four
power supplies since then, now onto my fifth.

The power supplies don't go completely dead, they still
supply some power to the mainboard and USB devices,
but nothing else. No fan, no CPU. It's like they suddenly
go into permanent "standby" mode and never work again.
Put a new one in, and everything is fine. For about a week.

Can anyone shed some light on what might be happening
here? Would I be right in thinking that it's not the fuse
blowing, otherwise it would be completely dead? And
is it likely to be a problem with the electricity supply to my
house, or could it be caused by something in the PC itself?

This is getting annoying and expensive, so any help greatly
appreciated.

In addition to what was already said there are three other strong
possibilities.

1. Check the voltage at the receptacle should be between 105 and 125 volts,
if it isn't call an electrician. Next turn the computer on and have someone
play a game while you check the voltage. If it drops below 100v Call an
electrician. Also check to make sure there are not any large appliances on
the same circuit as your computer. Computers should really be on their own
circuit.

2. Your system may have a short that is overloading the PSU. They way to
check this is quite complex so generally if you have not had any electronics
training it's quit difficult to do. However if you have eliminated
everything else and don't want to send the unit to the shop let me know and
I'll see if I can walk you thru it.

3. the motherboard may indeed be putting the power supplies in standby mode.
Did you check to make sure the PSUs were bad.

Lane

 

[le ténébreux]

Sounds more like you are getting underpowered cheap power supplies.
If you are drawing more power than the power supply is rated for (and
some are overrated in the first place as to how much power they will
actually supply) then the power supply heats up and something blows.

Thanks for the reply. That was the first thing I suspected, because
I could actually hear the cooling fans slow down slightly when it was
doing something CPU-intensive (I'm pretty sure that's not normal).

When I explained this, the PC repair guy assured me I was delusional
and replaced it with the same thing, warning me not to bother coming
back if it blew again because the warranty wouldn't cover a fault
with my equipment (which was the only thing it could possibly be),
and would I be interested in purchasing a surge protector.

As I found out, it's not easy to get real answers from someone who's
standing behind a cash register.

Anyway, I've managed to got hold of a different brand this time which
was about twice the price of the others, and the PC actually sounds
a little healthier now. It would still be nice to know what sort of thing
could have caused it... no power supply problems in about six years,
and then four fail all at once.

 

[le ténébreux]

Some basic information. When one claims that surges on
power line might be causing problems, then poster is not
technically informed. Too many are easily made victims of
propaganda promoted by plug-in surge protector manufacturers.
Surges typically occur once every eight years - which they
don't want you to know.

:-(

I just bought one, thinking it might help.

If suffering daily surges, then you are trooping daily to the
hardware store to replace electronic appliances. Are you
suffering daily electrical appliance failures?

Nothing at all except ATX power supplies.

If not paying about $80 retail for that new power supply,
then immediately suspect that as where to begin seeking a
reason for failure. If power supply manufacturer is not
provided with a long list of specifications, then supply
probably is missing essential functions. ....
You know one important number - how much was paid
for that supply.

I have no idea which manufacturers are reputable and which
are not, but the first one that blew was a CODEGEN 350W
power supply that had been purchased new about 4 months
previously ($55 Australian) and had been running just fine up
until then. I replaced that with an older 300W supply that I
had lying around (no brand name, it came with a PC case).
That died within three days. I went back to the shop where
I got the first one, and bought another CODEGEN 350W
which blew up after a week. They replaced it free under
warranty but made it clear that they would not replace any
more (and I don't think I blame them). Again, it went after
only a week.

So although this latest one is my fifth power supply, it's only
the second replacement I've had to purchase. It's a "Herolchi"
300W. ($115 Australian).

Twice the price - here's hoping it lasts at least two weeks
instead of one.

 

[le ténébreux]

1. Check the voltage at the receptacle should be between 105 and 125
volts, if it isn't call an electrician. Next turn the computer on and
have someone play a game while you check the voltage. If it drops
below 100v Call an electrician. Also check to make sure there are not
any large appliances on the same circuit as your computer. Computers
should really be on their own circuit.

I'm in Australia, where it's ~240 volts. Point taken though, I'll get an
electrician to check that it's working as it should. The computer
room only has a single power socket, and the only things plugged
into that are the PC, monitor, printer and ADSL modem. Nothing
else ever malfunctions, blows fuses, or does anything weird. Ever.

2. Your system may have a short that is overloading the PSU.
They way to check this is quite complex so generally if you have
not had any electronics training it's quit difficult to do. However
if you have eliminated everything else and don't want to send
the unit to the shop let me know and I'll see if I can walk you thru it.

I do have some basic knowledge of electronics, but no testing
equipment. I would have thought that if something was shorting
out, there would be some other symptoms? Something ought to
be malfunctioning at the very least, if functioning at all.

3. the motherboard may indeed be putting the power supplies in
standby mode. Did you check to make sure the PSUs were bad.

This can happen? Really? I didn't do any testing beyond noting
that the power supply failed to start the PC. When they "fail", it's
very much like going into standby mode. Everything shuts down
instantly, and the only signs of life are the LED on the mainboard
and the light in the optical mouse. But no amount of unplugging,
button pushing, or expletives can convince it to power up again.

I've still got two of them here. Is there any way to check if this
has happened, and maybe reset them?

 

[kony]

I have no idea which manufacturers are reputable and which
are not, but the first one that blew was a CODEGEN 350W
power supply that had been purchased new about 4 months
previously ($55 Australian) and had been running just fine up
until then. I replaced that with an older 300W supply that I
had lying around (no brand name, it came with a PC case).
That died within three days. I went back to the shop where
I got the first one, and bought another CODEGEN 350W
which blew up after a week. They replaced it free under
warranty but made it clear that they would not replace any
more (and I don't think I blame them). Again, it went after
only a week.

This is where your problem is, you keep using junk power supplies.
After you'd (told them?) why you needed a power supply, they should've
recommended something suitable, but instead your components were
risked so they could make an extra $. Luckily the system is working
still.

You should blame the shop, because they knew the relative quality of
the power supply, else are grossly incompetent and sold a part with no
idea whether it'd work. Your system is not particularly power hungry
or unique, it's just that a Codengen or other generic is not worth the
capacity printed on the label, and cut corners wherever possible.

So although this latest one is my fifth power supply, it's only
the second replacement I've had to purchase. It's a "Herolchi"
300W. ($115 Australian).

Twice the price - here's hoping it lasts at least two weeks
instead of one.

The Herolchi 300W versions I've seen are quite a bit better than the
Codegen, it's a mid-grade power supply, was relabeled by Antec to be
their model PP303XP.


Dave

 

[larrymoencurly]

I'm having a bit of a problem and hoping someone here
can at least give me a clue about what might be going on.
About 3 weeks ago my PC (ASUS P4SE mainboard,
Celeron 1.8GHz) suddenly started spitting out ATX power
supplies for no apparent reason. I've gone through four
power supplies since then, now onto my fifth.

Does your Codegen look anything like this one?

www.mikhailtech.com/articles/psu/codegen350/codegen350-07.jpg

In the lower left corner, near the donut choke wrapped with red and
white wires, is an empty space on the circuit board where I think a
capacitor would normally go to help reduce EMI. As far as I can tell,
this PSU doesn't have another EMI filter, such as at the AC
receptacle, and in another PSU made by the same company, Deer, all of
the EMI filter components were left out, including the choke. Also in
front of the missing capacitor, next to the fuse, is a place labelled
TR1 with a jumper soldered there, but that should be a thermistor to
help reduce the surge at turn-on.

If you ever decide to buy an inexpensive PSU, it's possible that the
Q-tec 300-350W models are good, even though the higher powered ones
are supposedly junk.
http://forums.overclockers.com.au/showthread.php?s=&threadid=115831&highlight=killed+psu
shows a VIP brand 350W version of the same PSU in this photo
http://members.optusnet.com.au/funked/psu/7.jpg , and the thread's
originator thinks it's of decent quality. The other PSU may be a
Deer, with some components upgraded or added to improve the quality.

 

[le ténébreux]

This is where your problem is, you keep using junk power supplies.
After you'd (told them?) why you needed a power supply, they should've
recommended something suitable, but instead your components were
risked so they could make an extra $. Luckily the system is working
still.

It's good to know I at least got something a bit better this time.

Seriously, up until the past few days I had no idea these things
could vary that much in quality, or that they could break so easily.
When you ask PC hardware suppliers here about these things,
they tend to dodge the question by looking at you like you're
mentally retarded and asking you what essential safety equipment
you have installed.

I'll be a bit more careful in future about letting these guys tell me
what I want.

 

[le ténébreux]

Does your Codegen look anything like this one?

www.mikhailtech.com/articles/psu/codegen350/codegen350-07.jpg

That's it. Almost. Except this one DOES have a capacitor in
the space next to the donut. A big yellow rectangular one -
" .33µF K MPX-X2 GPF 250V~ 275V~ " There's another
one like it (but smaller) in that rectangular space just behind the
heatsink.

The thermistor is also there - MF71 SD-11 and also a resistor
between that and the capacitor. In fact, I can see a few other
resistors and capacitors in that area that are missing from that
picture. Very very naughty.

But interesting.

 

[Trent©]

Hi all,

I'm having a bit of a problem and hoping someone here
can at least give me a clue about what might be going on.
About 3 weeks ago my PC (ASUS P4SE mainboard,
Celeron 1.8GHz) suddenly started spitting out ATX power
supplies for no apparent reason. I've gone through four
power supplies since then, now onto my fifth.

What did you do differently 3 weeks ago? New system?...new
mainboard?...changed some configuration?

The power supplies don't go completely dead, they still
supply some power to the mainboard and USB devices,
but nothing else.

Can you elaborate? What kind of power to the mainboard? Are you
booting into the hard drive?...into an operating system?

No fan, no CPU. It's like they suddenly
go into permanent "standby" mode and never work again.
Put a new one in, and everything is fine. For about a week.

Does this happen immediately when the machine boots?...or when its
been running for awhile. Maybe it IS going into standby? Did you try
resetting/shorting the BIOS?

Can anyone shed some light on what might be happening
here? Would I be right in thinking that it's not the fuse
blowing, otherwise it would be completely dead?

Correct. Did you check your BIOS settings?...to make sure you don't
have the machine going into standby? It VERY unlikely that you'd get
4 power supplies that would exhibit the same symptoms...no matter HOW
inexpensive the supplies are.

And
is it likely to be a problem with the electricity supply to my
house, or could it be caused by something in the PC itself?

Very unlikely.

You never mentioned your operating system. It almost sounds like its
kicking into power-saver mode...especially if it happens after the
machine has been on for awhile.

Good luck...let us know.


Have a nice week...

Trent

Fighting for peace is like screwing for virginity!

 

[Trent©]

It's good to know I at least got something a bit better this time.

Seriously, up until the past few days I had no idea these things
could vary that much in quality,

They do vary in quality. But even the inexpensive ones are very
good...and usually come with many seals of approval...marked right on
the ps.

or that they could break so easily.

They DON'T break that easily...and definitely not 4 in a row. You
have a particular problem...that is not the problem of the power
supply.

When you ask PC hardware suppliers here about these things,
they tend to dodge the question by looking at you like you're
mentally retarded and asking you what essential safety equipment
you have installed.

That's because your problem is unique...and unusual.

I'll be a bit more careful in future about letting these guys tell me
what I want.

Millions of inexpensive power supplies are sold each year. Few have
problems.


Have a nice week...

Trent

Fighting for peace is like screwing for virginity!

 

[le ténébreux]

What did you do differently 3 weeks ago? New system?...new
mainboard?...changed some configuration?

Nothing different at that time. I put it together about
3 or 4 months ago -

NEW -
* case
* mainboard (ASUS P4SE)
* power supply (CODEGEN 350W)
* CPU (Celeron 1.8GHz)
* RAM (3 × 256MB)

OLD (but all working fine) -
* keyboard, mouse, monitor, various USB devices, etc
* hard drive 20GB IBM Deskstar
* CD-RW drive
* floppy disk drive
* PCI - Soundblaster Live!
* PCI - Banshee video card
* PCI - TV tuner card

That's it. It's not a complicated setup, there is nothing weird
connected to it, nothing is overclocked or tampered with.
I've kept the original hardware settings in the BIOS (aside
from things like bootup sequence and HD configuration)
and haven't touched any jumpers on the mainboard that
I shouldn't. None of the devices misbehave in any way
before or after these "failures" (if that's what they are).

Can you elaborate? What kind of power to the mainboard?

The mainboard has an LED on it that indicates when the
power is connected (even when turned off). This always
lights up, even with a "faulty" unit. So I know there is
current getting through. And I also have an optical mouse
which remains lit when this happens.

Are you booting into the hard drive?...into an operating system?

It boots into a partition manager which activates either
Windows XP or Windows 98.

Does this happen immediately when the machine boots?...
or when its been running for awhile.

It has done this when the machine had been running for
several days, it has done it five seconds after being switched
on. It has done it in both versions of Windows, and it
has done it at the boot manager before any operating system
has been loaded at all. It has happened at all times of night
and day, in hot, cool, humid and dry weather. It has
happened twice while I was doing something, and twice
while I wasn't anywhere near it.

There is NO common factor here, that's the freaky thing.

Maybe it IS going into
standby? Did you try resetting/shorting the BIOS?

I tried that too, and still nothing. Even after I get it working,
if I put one of the old power supplies back in, it stops
working again until I put the new one back.

Correct. Did you check your BIOS settings?...to make
sure you don't have the machine going into standby?

Yes, I've done that. I don't usually use standby mode
(I prefer to switch it right off) but I've used it a few times
just to test it, and the difference is that the machine
switches back on again without too much difficulty.

It VERY unlikely that you'd get
4 power supplies that would exhibit the same symptoms...
no matter HOW inexpensive the supplies are.

Yes, I agree with that. I only went through about 4
in the past 8 years. All were cheap ones, and only
one hardware failure (the rest were system upgrades).

Now as many in 3 weeks? Ludicrous. Something is up.

 

[kony]

Nothing different at that time. I put it together about
3 or 4 months ago -

NEW -
* case
* mainboard (ASUS P4SE)
* power supply (CODEGEN 350W)
* CPU (Celeron 1.8GHz)
* RAM (3 × 256MB)

Try the system with fewer memory modules.

OLD (but all working fine) -
* keyboard, mouse, monitor, various USB devices, etc
* hard drive 20GB IBM Deskstar
* CD-RW drive
* floppy disk drive
* PCI - Soundblaster Live!
* PCI - Banshee video card
* PCI - TV tuner card

That's it. It's not a complicated setup, there is nothing weird
connected to it, nothing is overclocked or tampered with.
I've kept the original hardware settings in the BIOS (aside
from things like bootup sequence and HD configuration)
and haven't touched any jumpers on the mainboard that
I shouldn't. None of the devices misbehave in any way
before or after these "failures" (if that's what they are).


The mainboard has an LED on it that indicates when the
power is connected (even when turned off). This always
lights up, even with a "faulty" unit. So I know there is
current getting through. And I also have an optical mouse
which remains lit when this happens.

Try it without any USB devices attached... unplug them all while the
AC cord is pulled, THEN plug the AC cord in, wait a few seconds or
longer just for the heck of it, and then try to power-on.

There is NO common factor here, that's the freaky thing.

Well, there is the one, that none of those power supplies were
actually capable of 300W, and might easily not be capable of their
rated (2A?) 5VSB either.

Yes, I agree with that. I only went through about 4
in the past 8 years. All were cheap ones, and only
one hardware failure (the rest were system upgrades).

Now as many in 3 weeks? Ludicrous. Something is up.

These past systems, probably didn't draw nearly as much 12V power, did
they? Your current system is a departure in power distribution, and
you did mention the fan not spinning. If you had a voltmeter I'd
adivse you to check the 12V rail while it's in the on-but-dead state,
and check the 5VSB rail when the system is soft-off (AC cord plugged
in but off by the front switch).


I have a power supply here that's quite similar to that Codegen, it's
brand new, never even been plugged in because I've seen too many of
these hunks of junk fail... I have a few > 4 years old in the basement
for some odd reason, and oddly enough, they're virtually identical
inside but the old ones were only rated for 200-250W.... It seems the
only thing modernized about them is the sticker.


Dave

 

[Lane Lewis]

I'm in Australia, where it's ~240 volts. Point taken though, I'll get an
electrician to check that it's working as it should. The computer
room only has a single power socket, and the only things plugged
into that are the PC, monitor, printer and ADSL modem. Nothing
else ever malfunctions, blows fuses, or does anything weird. Ever.

Best to check the voltage yourself, a small voltage tester is all you need.
should be between 220 and 240

I do have some basic knowledge of electronics, but no testing
equipment. I would have thought that if something was shorting
out, there would be some other symptoms? Something ought to
be malfunctioning at the very least, if functioning at all.

Not neccesarily.

This can happen? Really? I didn't do any testing beyond noting
that the power supply failed to start the PC. When they "fail", it's
very much like going into standby mode. Everything shuts down
instantly, and the only signs of life are the LED on the mainboard
and the light in the optical mouse. But no amount of unplugging,
button pushing, or expletives can convince it to power up again.

I've still got two of them here. Is there any way to check if this
has happened, and maybe reset them?

The best way to check is to put them in another unit if possible.

 

[le ténébreux]

Try the system with fewer memory modules.

That's one thing I didn't try, so I'll give that a go.
Does RAM draw a lot of power from the system?

Try it without any USB devices attached... unplug them all
while the AC cord is pulled, THEN plug the AC cord in,
wait a few seconds or longer just for the heck of it, and
then try to power-on.

I'm pretty sure I tried that, and still nothing. At one point I
even took out all the PCI cards and the CD-RW, and unplugged
all device cables from the back to lighten the load. Still the
same thing.

These past systems, probably didn't draw nearly as much 12V
power, did they? Your current system is a departure in power
distribution

That's true. This mainboard is the first one I've had that uses
the extra 12V.

 

[~misfit~]

That's one thing I didn't try, so I'll give that a go.
Does RAM draw a lot of power from the system?


I'm pretty sure I tried that, and still nothing. At one point I
even took out all the PCI cards and the CD-RW, and unplugged
all device cables from the back to lighten the load. Still the
same thing.


That's true. This mainboard is the first one I've had that uses
the extra 12V.

If the USB mouse and the mobo LED stay lit with these other PSUs it suggests
to me that either the 3.3v or 5v rails are still working in the blown PSUs.
(I"m not sure which rail stay's 'live' when powered off so you can 'wake on
LAN' or modem or keypress, I think it's the 5v?) It seems like something is
killing the 12v rail. Maybe one of your old drives has an intermittent (the
hardest to diagnose) fault or short on the 12v rail?

 

[kony]

That's one thing I didn't try, so I'll give that a go.
Does RAM draw a lot of power from the system?

No, not relative to other components, but it may be powered by the
5VSB rail in soft-off mode. Too much load on the 5VSB can be a
problem.

I'm pretty sure I tried that, and still nothing. At one point I
even took out all the PCI cards and the CD-RW, and unplugged
all device cables from the back to lighten the load. Still the
same thing.

Being a Celeron system, it isn't going to have a (relatively) large
load on the 5V rail unless that's also being used for a video card, it
may not make as much difference on your system as on a P3 or most
Athlon platforms.

That's true. This mainboard is the first one I've had that uses
the extra 12V.

Many people who can successfully use the low-end power suppiles, are
just barely able to... it's not a good situation to be in. If the
low-end PSUs worked fine on a consistent basis, nobody would shell out
2-3X as much for a better PSU.


Dave

 

[V W Wall]

If the USB mouse and the mobo LED stay lit with these other PSUs it suggests
to me that either the 3.3v or 5v rails are still working in the blown PSUs.
(I"m not sure which rail stay's 'live' when powered off so you can 'wake on
LAN' or modem or keypress, I think it's the 5v?) It seems like something is
killing the 12v rail. Maybe one of your old drives has an intermittent (the
hardest to diagnose) fault or short on the 12v rail?

ATX power supplies have a separate supply to produce +5V SB. It's completely
independant of the other supply rails. It's primary purpose is to provide
power to start the main switching supply. In addition it's used for "wake
on LAN" as you mention. Older supplies had very linited current available
from this "always on" supply, but newer units can supply more current. The
main supply can fail, or fail to be started, and the +5V SB will keep the
mobo light on. A short on any main supply rail will keep it from starting,
or kill it once it has started.

The first ATX supplied computer I built, I got the floppy drive power plug
on incorrectly. The supply would not start. I kept looking for a problem
in the start switch, even took the front case panel off. ;-(

Virg Wall