Question about Biostar : NF4ST-A9 Socket939/nForce4/DDR400/SATA/ATX Motherboard (MOTBI15)

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No power up when pressing switch

PSU and case switch proven good with back-up mobo. Pressing case switch functioned only intermitantly until yesterday. VSB is 4.56VDC, PS_on reads 0.748VDC and does not go up when switch is disconnected. Shorting PS_on to ground turns fans on but does not post the CPU. Reset has no effect. I am wondering what could be wrong since I am leaning towards Motherboard due to a different issue where the onboard nic would drop connection all the time. Motherboard is less than a year old with no overclocking.

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  • polisan Sep 12, 2007

    I disconnected the power supply main connector and tested the PS_on for voltage and found 4.5VDC. While metering this, I plugged it into the motherboard and it immediatly dropped to 0.748VDC leading me to believe I have a bad Motherboard. power supply functions with a different motherboard



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Power suplly problem

Posted on Sep 12, 2007


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Mobo boots up when powering PSU without pressing power button on case.

What do you mean apparently?
"Apparently, no swollen or leaking caps."

Helping someone remotely?

The top round flat seal on the Radial Aluminum Electrolytic Capacitor is called the Vent Cover.

The bottom round flat synthetic rubber seal is called the Bung.
Sometimes just one edge of the Bung pushes down, and out of the capacitor's case; and this allows Electrolytic Paste to be pushed out.

The capacitor may only be on a slight lean, and a lot of capacitors wind up being that way when installed.

You/they need to L@@K CLOSE.

This really means motherboard in hand.
Not peering at it installed in a computer case, with hardware components, or cables in the way.

The capacitor case could just be slightly swelled up.
Hard to discern sometimes. I use a bright light, and have been known to use a magnifying glass, lol!

I realize it is just an assumption on your part, and now mine; as to if there are bad 'caps'.
But I do go along with your hypothesis.

Obviously (IMHO) the motherboard is at fault.

To post back post in a Comment.


Feb 04, 2013 | Intel D865GVHZ Motherboard

1 Answer

Hp slimline s 3000 wont start green light for ps goes off when start button pushed

You seem to have eliminated the PSU as a cause of the problem.

It might be that there is short circuit somewhere in your case, possibly one between the m/b and the case. This could cause your PSU to shut down (if it has s/c protection). Holding the power button for more than a few seconds would cause a power down (mine is power on when the button is pressed, but pressing again/continuing to press for 5s causes power down: to power down, the power button must be pressed for the 5s - to prevent accidentally pressing the power button from switching your machine off by touching the power button).
If this is the cause, probably the only way to solve it is to remove the m/b from the case and clean out the case and the m/b, the latter with canned air perhaps. (you should inspect the surface of the m/b for objects which might be causing such a s/c, but most likely any object has become lodged behind the m/b. Some computers allow removal of both side panels) Be careful during re-assembly because some m/b-cases require (either insulating or conducting) spacers/stand-offs between the m/b and mounting points in the case.
Of course, there may be another cause...


Jul 08, 2012 | HP Pavilion Slimline S3123W Desktop PC -...

2 Answers

I seem to have a problem with the power to my computer. After 2 days of searching all kinds of forums I still haven't found a solution. The computer won't power up, no fans are spinning, not a...

First remove the power plug from the smps and press the power switch few time and wait for few minutes the re plug the power connector to the smps and switch on the computer if you still dont have power then you will have to check whether the smps work to test that its not enough to test the voltage in the 21/24 pin you will have to short the green wire and the black wire and see whether the smps fan is working if the fan is not working then the problem is with smps

Jul 30, 2011 | ASUS Computers & Internet

1 Answer

Compaq Presario Media Center SR1750NX - Will not come up. Fan starts on high and stays there. No Keyboard lights flash. No monitor. No beeps. Replaced memory, disconnected harddrive, tested and then put...

the PSU has fan, does it spin and blow.
Seems to me. the PSU is bad. 5/12 is ok but there 20/24 pin connector is way more important that that legacy 4 pin .
my guess is the Cd driver ejects ok.

re-seated memory , i guess. (tried 1 at a time is the way !)
tried in bank 0 (lowest number) one at a time.

the voltage are here, do the bench test on it , all power pins.
see that 3.3v?
unplug it, jumper wire the correct 2 pins
pin 16 to 17 on modern PCs (24pin)
your PSU stays on, but some have really poor power checks.
so the unstated SCREEN , with power on last.
shows, no SIGnAL, no sync, warnings.? (or out of range)
no text ever. on said screen.
what video card is there, or is this onboard video.,
on mobo direct vga jack? yes it does but is there a addin video card there.? count VGA jacks, 2 means it has adding card.
onboard is here.
ATI Radeon Xpress 200 shared video memory (UMA)

I think the PSU is bad, one other way it can go bad is to
incorrectly time the Power _OK pin, timing.(output)
see this many times, this pin tells the mobo power is ok.
if it sticks at ok, the Mobo can not function, if this pin lies.
the PSU tells the mobo power is ok
then if the mobo power regs fail, then the mobo sends
Power_on to false and the PSU shuts down.
in your case the Power_ok seems stuck ok. falsely.
or can be.
the thing to know is the power regulators are in the PSU and the MOBO, and both must be good.
or it will be dead.
we bench test all PSUs
and even swap them , to be sure it is ok,
if yes, ok then the mobo is bad (if all loads removed first)
all hdd,/dvd/cd drive. out. (cables pulled)
all addin cards removed ! all of them. if present (look for them)
all memory out and put back 1 by 1.

Dec 20, 2017 | HP Compaq Presario Media Center SR1750NX...

1 Answer

I have a Compaq Presario SR1830NX desktop, I leave it on all the time, when I got up this morning, it was off, so I pressed the power button, but it doesn't power on, nothing happens.. I checked the power...

step 2 the TECH uses a voltmeter and checks all wires on PSU for correct voltage.

PSU in Desktop dead, my guess fans dead too right>
you never said, what it does inside PC ,except nothing
does nothing mean dead silence?
the fan must run for 1second, then goes off.
a 1 second run tells me the PC power switch told the MOBO to turn on and in turn it tells the PSU to turn on,
the PSU then turns on for at least 1second,and if the voltages are ok
then the PSU stays on, else off goes.

15 and 16?
the 20 pin older supplies those are both ground
but 24 pin ATX pins are correct there.
the voltages are here.

There are 2 causes, of dead power button.
1: the buttons 2 wire cable ends fell off. at button or PC mobo.
2: The PSU is bad.
3: the MOBO is bad.

The mobo can also shut off any PSU, if the PSU fails
or it sees the regulators in the MOBO fail.
the Desktop runs on many power rails.
some in the PSU, others on the MOBO.
THe mobo creates special low voltage for the CPU.

the power good pin, does that #8,
One way to see if the MOBO is overload
is to remove things from it, I start at RAM
lacking a voltmeter, this is very hard to fix any thing electric.
only guessing or pulling objects PC can find shorts.
pull all power cables there, hdd, power, cdrom.
all things that use red/yellow wires.
all SATA cables, all USB , then ram. even a fan can short
a good tech keeps an open mind, and finds why the PSU shut down.
in most cases, the PSU is bad. but not always.

This is a feature, of ATX it keeps you from blowing up say a
$300 microprocessor.... or fancy gaming Video card (have one)?
yes, pull the video card too.

Jun 25, 2017 | HP Compaq Presario SR1630NX PC Desktop

1 Answer

Emachine D2880 no power clarification

your getting power to your PSU, via the green light on your mobo.
verify the PSU has a green light(most emachines do)
it sort of sounds stupid but, check the connection of the cable coming from the front of the case(the wires that connect to the mobo to the power button on the outside of the case.

if all else fails, take the pc out of the case.. put all components on a static free bag and start to rebuild your PC(without a case)
...this will rule out a short on the PC case, and faulty wiring from the case...

pretty much sounds like you've been through almost every step except replacing the PSU(you just said you tested/but your getting power to the PSU)

hope some of this helped

Sep 04, 2009 | E-Machines eMachines Desktop PC

1 Answer

Nothing happens when I press the power button (good PSU)

You've tested the PSU for volts, although you didn't mention the +12v, the critical thing with all PSU's though is Amps, if there is insufficient amps the CPU will not initiate properly, modern CPU's mobo north and south bridge chips Graphics chips etc are power hungry, volts are one thing amps are everything. Try and borrow a friends PSU to test, then buy a good one if it works with at least 14 amps on the +12v rail, the higher the better.

Jul 05, 2009 | Intel D915GAG Socket 775 Motherboard -...

1 Answer

No boot up.

Power supply is easier to troubleshoot my local pc shop has used ones for $5 or if you have one laying around make sure you have one with the proper mobo connector 24pin will sometimes fit 20 pin as long as there are no transistors to close to plug just turn pc off unplug all drives and mobo plug in good supply and turn on if it works thats the problem if not probably mobo

Apr 10, 2009 | Shuttle SK43G (tsk43g) Barebone

1 Answer

Wont post.

Try a few things for me OK? From what you typed, turned on the psu "the toggle switch on the psu", pc seemed to be starting up. I need you to keep the toggle switch on the power supply in the on position. Press and hold the start button on the PC case to shut it down. Give it a few seconds & power up by pressing the power switch on the case. Notice a differance? Next, unplug everything connected to the pc expect the monitor, mouse & keyboard. Try to boot. Still no? Remove the side of the case exposing the board. Turn the psu off by flipping the toggle switch on the psu, wait till the green light on the board goes out. Grab the Mb ( I know it's tight in there) and see if you can move the board in any direction. Not loose? Re-check all data & power connections for a solid connect. Re-check your case to Mb connections. Also check the PSU, there is a red slide switch it should be set to 115v. Post your findings..

Jan 28, 2008 | ASUS P5ND2-SLI Deluxe Motherboard

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