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Recent build was working; now no POST, no beep

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November 21, 2009 9:52:07 PM

I've built quite a few, and this one was working fine for several months - and no recent changes. Suddenly, it won't boot at all. No POST, no beeps, no signal to the monitor. The Power light comes on, the fans run, the HDD starts up, the CDROM light comes on - but that's it.

I disconnected various things, first externally, then internally, to see if I could get any beeps at least. No difference, even if the only thing still connected to the motherboard was the processor - and power, of course, plus the case speaker, which used to give me the customary short beep when booting.

I put in a POST diagnostics PCI card at some stages, and it always showed a steady "00" - nothing else.

So, it must be either the PSU or the motherboard, right?

With power applied but the system off, I get these voltages at the 24-pin connector:
green 5.05V
purple 5.06V
gray 0V

A few seconds after hitting the Power button, I get:
green 21.7mV
purple 5.06V
gray 4.98V

Various other colors give me a fraction of a millivolt with the system off, then with the system powered up:
blue -11.56 V
yellow 12.16 V (also on the 4-pin connector)
red 4.98 V
orange 3.397V

These readings are all with motherboard, CPU, RAM, HDD, FDD, CDROM, fans connected, plus power button and light, speaker, HDD light.

I don't run wires under the motherboard, and always check for the possibility of shorts when originally installing it, so have not breadboarded it. This is a very straightforward PC for my wife's use, not overclocked, no exotic hardware!

One thing I have not done is to clear CMOS. In its current state, the keyboard if connected can still be used to wake up the system. I raised a query on Gigabyte's website a week ago and they have still not even acknowledged it, let alone read it. So - I'm your hands, guys, please.

You'll want to know a bit about the system, I guess, so here are the unexciting essentials:

MB: Gigabyte GA-P31-ES3G Rev 1.0 (45nm, FSB 1333, DDR2 1066)
BIOS: ah, I can't see that, because... (you know the rest)
Processor: Intel E1200 (dual-core Celeron) LGA775 1.6GHz
RAM: 1GB Crucial (2x500MB, each CT6464AA667) PC2-5300 unbuffered, non-ECC, 240-pin, 8 chips
Graphics: PowerColor AX3450 256MD2-S (ATI Radeon 256MB 64bit DDR2, passive cooling)
PSU: Silver Power 500W, model SP-SS500
HDD: Seagate 7200.12 SATA 250GB ST3250318AS

All relevant bits (MB, RAM, PSU, processor) were brand new, mostly this year, a couple a bit older; case is an older Supermicro.

Have I done enough checks on the PSU to eliminate it? Can I incriminate the motherboard and RMA it? Anything else I should have tried (and tell me why)? Any questions?
a c 156 V Motherboard
November 21, 2009 10:43:46 PM

Welcome to the forums.

Nice summary of what you have done.

Now here's what is going on:

With power applied but the system off, I get these voltages at the 24-pin connector:
green 5.05V
purple 5.06V
gray 0V

The green wire is the control line to the PSU. Means it's off but ready to be turned on. The purple wire is the output of the standby power supply. It's always on. It also provides the power to make the green wire work. The gray wire is the PowerOK line. If the PSU is off, it's at a Logic LOW (0 volts). So far so good.

A few seconds after hitting the Power button, I get:
green 21.7mV
purple 5.06V
gray 4.98V

The power button grounds the green wire, turning on the power section of the PSU. Somewhere within 500 msecs, the power monitoring circuits detect good 3.3, 5, & 12 volt levels and the gray wire goes HIGH. When the gray wire goes HIGH, it removes a hardware reset signal from the CPU and the system boots.

Various other colors give me a fraction of a millivolt with the system off, then with the system powered up:
blue -11.56 V
yellow 12.16 V (also on the 4-pin connector)
red 4.98 V
orange 3.397V

All normal.

The one thing that I do not know (because it's something I forgot to check) is what happens to the green wire after the power switch is pushed and released. I think it should go back HIGH when it is released.

Unfortunately, I am on vacation away from my desktop systems, so I cannot check.

The reason that I think the green wire is a problem is that one way of powering down a system is to press and hold the case power switch. If the green wire is already at 0.0 volts, pressing the case power switch wouldn't do anything.

On the other hand, you have all of your voltages and the PowerOK signal. I think your system should boot.

The only two things that I can suggest are:
Try to check how the green wire acts in another system.
Swap the power switch with the reset switch and try to boot with reset switch while monitoring the voltage on the green wire.

Our standard checklist and troubleshooting thread:
http://www.tomshardware.com/forum/261145-31-read-postin...

There's some more troubleshooting ideas here, but it does look like either your motherboard (most likely) or CPU is bad and your power supply is OK.
November 22, 2009 10:57:09 AM

jsc said:

The one thing that I do not know (because it's something I forgot to check) is what happens to the green wire after the power switch is pushed and released. I think it should go back HIGH when it is released.

The reason that I think the green wire is a problem is that one way of powering down a system is to press and hold the case power switch. If the green wire is already at 0.0 volts, pressing the case power switch wouldn't do anything.

The only two things that I can suggest are:
Try to check how the green wire acts in another system.
Swap the power switch with the reset switch and try to boot with reset switch while monitoring the voltage on the green wire.


Thanks, jsc, for your welcome and help.

I understand your logic regarding the green wire after power on. However, holding in the case power switch does indeed power off the system. To eliminate any possible switch problem, I disconnected it completely and used a screwdriver to short the MB terminals, and the system started up as before – but no further. So I guess that eliminates the switch, and we are lacking some info about green wire logic. I checked the On voltage of the green wire on my own workstation, and got 66mV, so all looks well in that respect.

Enjoy the rest of your vacation!
November 25, 2009 4:11:38 PM

DJKR said:
...I've built quite a few, and this one was working fine for several months - and no recent changes...
I don't run wires under the motherboard, and always check for the possibility of shorts when originally installing it, so have not breadboarded it...
One thing I have not done is to clear CMOS...

GUILTY! :(  Me, that is.

I breadboarded the system with just bare essentials - no change, no beep. Then I cleared CMOS. Success - it worked!!

Why? Well, the clue was in the fact that the case was old, and had housed a different motherboard, with of course slightly different stand-off positions. I had loosened all the ones to be relocated, to correspond with the mounting holes in the new motherboard. One new position couldn't be used, and so one stand-off was left slightly loose and not relocated - in the wrong position. :non:  It must have touched the underside of the MB briefly, with the results that you know.

So, all is now well again, with nothing replaced, and I have another spare stand-off.

To be fair to Gigabyte, they did eventually reply, and suggested the action above, which I followed.

Thanks folks!
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