ATX Power Supply Failure?

[Peter Hemmings]

Hi,

My friend asked me to look at his PC which intermittently goes dead and
sometimes starts up again.
Had a look today and checked the following:
power switch OK
Mains cable and fuse OK
Checked inside supply for poor connections/joints OK

Put it back and it started pressed power switch and it stopped but would
not start again.
After several minutes I got it to go again but it stopped after about 10
seconds without touching button.
Removed Power Supply and have it now working fine in my PC while sending
this e-mail.
The mother board in question is a Soyo, before deciding to replace M/B,
can someone tell me how to check pin 14 on M/B power connector without
it being powered up if possible?
Thanks

 

[Pen]

If you have a 3.5 digit meter, or any vom for that matter, you
just measure the volts on 14 and 15(Ground). Should be less than 0.7
V.
It is held low by the mobo after a momentary application of the
power on switch. It can't be checked unless power is on, since
the mobo has to be active to pull the line down.

 

[ric]

What is the supply being pulled down
is it 5.5volts?

To test an ATX PSU:

First, disconnect everything from the power supply (including MB.) Plug
AC cord into the rear of the PS. IF the PS has its own rocker on/off
switch, turn it ON. There should be +5v on pin 9 of the 20 pin connector
(+5vsb, usually a violet wire.) There should be >0.8v on pin 14 (PS-ON,
usually a green wire.) This is from an internal pull-up to the +5vsb.

Connect PS power leads to 1 or 2 IDE HDs (for a load). Jumper pin 14
(PS-ON) to pin 13 (ground, black wire.) At this point, fan in PS should
start spinning, drives should spin, and + 5/12v, -5/12v, +3.3v, and
+5vsb should be present at 20 pin connector.

Disconnecting pin 14-13 jumper should turn supply back off.

Further:

When your system fails to start, what is the voltage on pin 9? If it is
less than 4.75v, either a) the logic on your motherboard is shorting
b) your +5vsb is under powered, or c) your PS is bad. It is
most likely a combination of b) and c).

If your +5vsb is OK, measure pin 14 of the PS/MB connector. If it is <0.8v
(and the +5vsb is OK), the power supply should be on. If pin 14 is <0.8v,
the +5vsb is OK, and the PS is OFF, the PS is bad. If pin 14 is >2.0v, the
power supply is being told to stay OFF by the MB. If depressing the front
panel ON/OFF switch does NOT lower pin 14 to <0.8v (and the +5vsb is OK),
then the problem is NOT the power supply, but rather MB related.

 

[Peter Hemmings]

Yep just checked on my PC with suspect PSU that there is 4.78volts on
pin14 "pull-up"
Yes I assume this is OK as I switched it on and off at least 15 times
this afternoon and it was OK.
Will, check but will be Wednesday/Thursday and post results/conclusions
for information.

Just for information:
Fault was due to power supply 5vsb being 4.19volts off load/on load.
Took me longer than expected as it worked fine in my box!
(my M/B obviously works with wider limits, just!)
bought new PS and fitted.
tested OK
Noticed the new PS have what I assume to be a new HD connector (it also
has a small square one as well as the normal ones).

Unlikely as he has not got much in his machine.

This is (unfortunately) what my money is on!
BTW do you know if this is a common fault or if APM of "Wake on LAN"
settings could hold it off (just out of interest)?
Thanks for your very explicit information.

Thanks

 

[Pen]

That 4 pin extra connector is for P4 boards. It's an
additional 12V lead.

 

[Larc]

| Just for information:
| Fault was due to power supply 5vsb being 4.19volts off load/on load.
| Took me longer than expected as it worked fine in my box!
| (my M/B obviously works with wider limits, just!)
| bought new PS and fitted.
| tested OK
| Noticed the new PS have what I assume to be a new HD connector (it also
| has a small square one as well as the normal ones).

The square plug is a 12v connector for a Pentium 4 board.

Larc



§§§ - Please raise temperature of mail to reply by e-mail - §§§

 

[Peter Hemmings]

| Just for information:
| Fault was due to power supply 5vsb being 4.19volts off load/on load.
| Took me longer than expected as it worked fine in my box!
| (my M/B obviously works with wider limits, just!)
| bought new PS and fitted.
| tested OK
| Noticed the new PS have what I assume to be a new HD connector (it also
| has a small square one as well as the normal ones).

The square plug is a 12v connector for a Pentium 4 board.

Thanks for the information