ASUS A7M266 - negative voltages dropping.

[LEM]

Hello,

I'm seeking an advise on a problem I'm having with my so far trouble-less
ASUS K7M266 board... A short list of HW on board: AMD Athlon XP 1500 (NOT
overclocked, running at 1.3Ghz, as it's supposed to), 2 hd's, cd-rw, dvd-
rw, floppy, Nvidia GEForce2 MX/400 in AGP slot, PCI SLOTS: SB Live,
10/100 ethernet adapter (installed a few weeks ago in place of 10MB, but
no problems then), 56K modem, ATI TV Wonder capture card. Then a range of
USB devices, and that's about all there is...

As of a couple of days ago I noticed weired behavior of the system. Sound
disappearing, serial ports not working properly... When tried to reboot
system it stopped on hardware monitor warning, and indicated that what
was supposed to be -12v is showing as -0.86v. So I started checking MBM
(mother board monitor) and got the following behaviors:

- On cold power up after some time off the voltages are good. After about
10 minutes of working -12v dropped to -0.86.
- On reboot -12v was showing at -0.86 to start with.

So, my first thought was the power supply went boo.. I went to a local
retailer and bought a new one. Old = 300 watts, new=430 watts. Installed,
powered up.. and... same thing. Except now -12 drops to around -2 volts.
But now I also notice some drop in -5v down to about -1.15v... Strange.

So, I thought... hm. maybe the old PS is not that bad. Plugged it in
(without computer), shorted green+black wires to get it going and started
taking measurments. It's been about an hour since, and -12 didn't drop
even 1/10th of a volt. It's been hanging around -11.33 give or take
0.03v... Of course this test is without any load on the unit, but still
tells me something about power supply not being a bad guy here.

I searched google far and wide, but found nothing yet. So here are my
questions: Has anyone seen a behavior like this before? Should I start
shopping for a new mother board (anyone would recommend a replacement
that has specs similar to my current A7M266)? Anything I can do to
pinpoint the problem more exactly? Any other suggestions?

Thanks in advance,

LEM
(NG reply appreciated, but to e-mail remove all digits from address)

 

[Roger Hamlett]

Hello,

I'm seeking an advise on a problem I'm having with my so far
trouble-less
ASUS K7M266 board... A short list of HW on board: AMD Athlon XP 1500
(NOT
overclocked, running at 1.3Ghz, as it's supposed to), 2 hd's, cd-rw,
dvd-
rw, floppy, Nvidia GEForce2 MX/400 in AGP slot, PCI SLOTS: SB Live,
10/100 ethernet adapter (installed a few weeks ago in place of 10MB, but
no problems then), 56K modem, ATI TV Wonder capture card. Then a range
of
USB devices, and that's about all there is...

As of a couple of days ago I noticed weired behavior of the system.
Sound
disappearing, serial ports not working properly... When tried to reboot
system it stopped on hardware monitor warning, and indicated that what
was supposed to be -12v is showing as -0.86v. So I started checking MBM
(mother board monitor) and got the following behaviors:

- On cold power up after some time off the voltages are good. After
about
10 minutes of working -12v dropped to -0.86.
- On reboot -12v was showing at -0.86 to start with.

So, my first thought was the power supply went boo.. I went to a local
retailer and bought a new one. Old = 300 watts, new=430 watts.
Installed,
powered up.. and... same thing. Except now -12 drops to around -2 volts.
But now I also notice some drop in -5v down to about -1.15v... Strange.

So, I thought... hm. maybe the old PS is not that bad. Plugged it in
(without computer), shorted green+black wires to get it going and
started
taking measurments. It's been about an hour since, and -12 didn't drop
even 1/10th of a volt. It's been hanging around -11.33 give or take
0.03v... Of course this test is without any load on the unit, but still
tells me something about power supply not being a bad guy here.

First comment, you should never run a PC PSU without load. You are lucky
that if unit has not been damaged by doing this. Most have a minimum load
specification in the order of 5% of their quoted full power, and without
this cannot maintain regulation. Better units will see the +5v rail go
overvoltage, and trigger a crowbar shutdown, possibly blowing the fuse.
Units without this protection, can overvoltage the output capacitors on
the main power rails....

I searched google far and wide, but found nothing yet. So here are my
questions: Has anyone seen a behavior like this before? Should I start
shopping for a new mother board (anyone would recommend a replacement
that has specs similar to my current A7M266)? Anything I can do to
pinpoint the problem more exactly? Any other suggestions?

Thanks in advance,

The -12v line on many machines is not used at all by the motherboard (it
is not one of the main regulated lines from the switcher), the only thing
left on a modern machine that may use it is the serial port, and many do
not. The -5v rail is also generally not used (it was originally a bias
voltage for the memory, that was required on older systems). These two
rails are possibly alone, in normally using a seperate linear regulator
chip, and not relying on the main switching inverter. The -5v is often
derived from the -12v, which explains the behaviour on the new supply.
Some PCI cards do use this rail. Try running without any PCI cards. Unplug
the SBLive card first. This is one of the few cards that used a reasonable
current on the -12v rail, and my first suspicion would be that the audio
amp chip on this has blown, and is overloading the regulator on the -ve
rail. I don't know about the 'TV Wonder' card (some UHF modulators use
these rails), so if removing the SB, does not let the rail run as it
should, try this next.

Best Wishes

 

[LEM]

Thanks Roger, for your quick reply!
I spent the last couple of hours crawling under my desk, and here are a
few observations:

First comment, you should never run a PC PSU without load.

Thanks! I didn't know that. Hopefully the old PS was not damaged. At
least all voltages were correct last time I checked them...

The -12v line on many machines is not used at all by the motherboard
(it is not one of the main regulated lines from the switcher), the
only thing left on a modern machine that may use it is the serial
port, and many do not.

I'm pretty sure it's used for serial on mine, because one of the symptoms
I had with old PS - my PC could not communicate with GPS, which is
connected through a serial port.. read on on this...

The -5v rail is also generally not used (it was
originally a bias voltage for the memory, that was required on older
systems). These two rails are possibly alone, in normally using a
seperate linear regulator chip, and not relying on the main switching
inverter. The -5v is often derived from the -12v, which explains the
behaviour on the new supply.

Ok, from this I can derive that whatever is going on is affecting the
PS's ability to generate appropriate voltage, rather than it being lost
somewhere between PS and the sensor. Correct me if I'm wrong.

Some PCI cards do use this rail. Try
running without any PCI cards. Unplug the SBLive card first. This is
one of the few cards that used a reasonable current on the -12v rail,
and my first suspicion would be that the audio amp chip on this has
blown, and is overloading the regulator on the -ve rail. I don't know
about the 'TV Wonder' card (some UHF modulators use these rails), so
if removing the SB, does not let the rail run as it should, try this
next.

I unplugged all PCI cards one by one starting with SB then TV Wonder.
Nothing changed. Still getting -2V... Now a few other things that did
change between the old and new power supplies, which I observed why
messing around:

1. I am now able to use the serial port even with -2v. Something that
didn't work on the old PS, while it was providing less than -1v. In fact,
while I was downloading a track from GPS, the voltage dropped to -1.91,
and reverted back to -2 afterwards. This tells me that indeed that -12
rail is used for serial port... And that -2v is enough for it to operate
but not -0.86...

2. SoundBlaster is also usable now (I had no sound when -12v dropped down
on old PS). However, it seems to output less power than before. I can't
tell for sure, but I used to have to set the main volume control to about
second mark from the bottom to avoid getting red bars on the stereo
system through witch it outputs sound. Now I had to slide it all the way
up. But it's not main concern, since I don't have a way to really measure
that, or maybe I'm even wrong about what was before. But sometimes
(usually right after reboot) it does this: System comes up, the welcome
sound is played (normally). Then, as soon as things finish loading I
start playing an mp3 file using winamp. The first 1/2 a second is played
at normal volume, and then it drops dramatically, so that I can only
barely hear. Closing winamp and re-opening it gets normal sound again...
Sooooo... if it is the Sound Blaster giving me trouble it may have
already burned something and now just fooling around. Other thing I did -
pulled out the SB and enabled onboard sound chip. No problems with sound
at all, although I still have to pull the volume slider all the way up.

3. When rebooting I noticed that hardware monitor did not complain. So, I
broke into the BIOS set-up and looked there. It showed a perfect -12.33v
on that one (with old PS it still showed -0.86 when rebooting and
triggered error). As soon as the system booted up and MBM loaded - it
shows -2.03v +/- 0.1 (mostly minus).

4. Not related to new PS, but while digging inside the case I noticed
that the North Bridge fan is dead. Since it's not supposed to be involved
with -12v, I suppose it won't affect this problem, if I understand your
explanation about nothing on MB using that -12v -- but I'm including this
observation just in case.

So in the end, I have to ask myself this question: did I ever have that
dreaded -12v at full power on my PC? If everything seems to work (apart
from strange SB behavior with that sudden output drops that sometimes
occur), then maybe it's been that way for a while now? But something made
it drop to unacceptable levels, and I am pretty certain it happened if
not suddenly, then at pretty fast rate, because I never had problems
communicating with GPS or using sound until that one day last week when
both problems showed up...

I am still open to all and any suggestions, as to what would be the next
step to try apart from starting to look for a new MB (and if the later
will solve the problem, provided that some of the peripherals may be
responsible here).

Thanks again for any help!

LEM
(NG reply preferred. For correct e-mail address remove all digits)

 

[Paul]

Hello,

I'm seeking an advise on a problem I'm having with my so far trouble-less
ASUS K7M266 board... A short list of HW on board: AMD Athlon XP 1500 (NOT
overclocked, running at 1.3Ghz, as it's supposed to), 2 hd's, cd-rw, dvd-
rw, floppy, Nvidia GEForce2 MX/400 in AGP slot, PCI SLOTS: SB Live,
10/100 ethernet adapter (installed a few weeks ago in place of 10MB, but
no problems then), 56K modem, ATI TV Wonder capture card. Then a range of
USB devices, and that's about all there is...

As of a couple of days ago I noticed weired behavior of the system. Sound
disappearing, serial ports not working properly... When tried to reboot
system it stopped on hardware monitor warning, and indicated that what
was supposed to be -12v is showing as -0.86v. So I started checking MBM
(mother board monitor) and got the following behaviors:

- On cold power up after some time off the voltages are good. After about
10 minutes of working -12v dropped to -0.86.
- On reboot -12v was showing at -0.86 to start with.

So, my first thought was the power supply went boo.. I went to a local
retailer and bought a new one. Old = 300 watts, new=430 watts. Installed,
powered up.. and... same thing. Except now -12 drops to around -2 volts.
But now I also notice some drop in -5v down to about -1.15v... Strange.

So, I thought... hm. maybe the old PS is not that bad. Plugged it in
(without computer), shorted green+black wires to get it going and started
taking measurments. It's been about an hour since, and -12 didn't drop
even 1/10th of a volt. It's been hanging around -11.33 give or take
0.03v... Of course this test is without any load on the unit, but still
tells me something about power supply not being a bad guy here.

I searched google far and wide, but found nothing yet. So here are my
questions: Has anyone seen a behavior like this before? Should I start
shopping for a new mother board (anyone would recommend a replacement
that has specs similar to my current A7M266)? Anything I can do to
pinpoint the problem more exactly? Any other suggestions?

Thanks in advance,

LEM
(NG reply appreciated, but to e-mail remove all digits from address)

Have you tried the cardboard test ? Maybe the screws or the brass
standoffs underneath the board, are touching something they aren't
supposed to. Try removing the motherboard from the case, and assemble
it on your work table. Place a thick telephone book underneath it,
to support it. You can put whatever cards you want in it, as long
as every time before you turn on the power, you remember to
reseat the cards in their sockets. For example, pulling on the
monitor cable could pull the AGP card out of its socket, so you
do have to be careful with the cables.

If you are curious about what a power supply looks like, a guy
reverse engineered an older power supply and drew a schematic.
This is the only schematic I know of.

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

You could try measuring the current flow from -12V or -5V. If
you have a clamp-on ammeter that measures DC, it would be a simple
matter to measure the current, without cutting any wires. If
all you can dig up, is a normal multimeter, then cutting the -12V
wire (on a ATX 20 pin extender cable, if you want) and splicing
the ammeter in line with that wire, will allow you to see how much
current is flowing. Make sure the meter is secured and connected
well, before turning on the PSU switch. Set the meter to its
highest current rating, in case the number of amps flowing is
significant. (That is one area that the clamp-on meter has
the other kind beat - I have a clamp-on meter, and it can measure
up to 400 amps. Most ordinary multimeters cannot manage more
than about 10 amps, and have a fuse to protect them against
overcurrent.)

Ex. PSU ------- extender_cable -----x x------ to motherboard
| |
+--A--+

Splicing an ammeter in series with a wire in the cable

HTH,
Paul

 

[ASI Industries]

Hi Lem, with regards to your negative voltages please ignore as it even
states that in the manual if you have read it, however concentrate on the
positive voltages, the +5v and +12v line rails seem to be common at
failing.

If the +5v drops below +4.75v or +12v drops below +11.70v usually cause the
odd BSODs,that when i consider replacing the psu, further drops the worse
the system stability becomes.

Since you have recently replaced the psu and the systems stability has not
improved and your mainboard is not exactly new, i would consider doing a
visual inspection of all capacitors on the mainboard and even the psu if
possible.
http://www.badcaps.com/