Mobo diode burnt out

July 25, 2006 at 14:20:12
Specs: Asus A7S-VM, Athlon 1Ghz 512Mb RAM

I recently built a new pc and it booted no probs and posted Ok to the BIOS as a new CPU had been fitted.
Asus A7S-VM Motherboard,
2x Sticks of 256MB SDRAM
Athlon Thunderbird 1.0Ghz 100Mhz FSB CPU.
300 watt PSU.

The next day I tried to boot it and there seemed to be no response after pressing the power switch. Noticng the PSU made a "squeak" sound upon unplugging the cord I tried another (400w) Power Supply, this time there was still no response, although when I turned the main switch off on the PSU the fans would spin briefly. I then decided to repeatedly press the power switch on the front of the pc in frustration when the pc suddenly started to post, i heard the normal beep then the monitor came on.
At this point I heard a crackle from inside the case, followed by a billow of acrid smoke so I immediately unplugged the entire setup.

Upon inspection, the diode labelled D12 (seems to be a IN4001?) was all burnt and the component cracked. There is also a slight scorch mark around the solder joints of the diode on the underside of the mobo.
I have uploaded 2 pictures of the component in question (front and rear view) which are not that clear but gives an idea.
They are here:
and here:

I think a metal mounting post may have been left in the case which i later discovered could have been the cause of the problem. Unless anyone has any other ideas why this diode burnt out so quickly.
And also, what are the odds on replacing this diode and recovering what is now a dead board? Memory, CPU etc where all connected at the time of the incident, what are the chances of these items being destroyed by the falilure of the diode?
Sorry for so many questions! I appreciate any advice.

See More: Mobo diode burnt out

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July 25, 2006 at 14:42:16

If a diode actually burns there is usually some other cause, such as a capacitor or chip short. Maybe your mounting post was the answer.

If that is the only problem then I suppose you have a fighting chance of replacing it if you are OK at soldering.

Obviously you need to try and find the full number - Google might narrow your guesses. As I'm sure you know it has to be the right way round. Maybe a meter will give some clue if you dare switch the mobo on.

You'd have to know a lot of detail about the board to say whether any other component has suffered. Maybe, maybe not, but burn outs mean a large current has flowed somewhere.

Mostly component changes on boards don't work out and I'm not hopeful. I suppose you've not much to lose as the mobo is defunct anyway. I should think any other damage, if any, has already been done.


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July 25, 2006 at 14:59:26

"I think a metal mounting post may have been left in the case which i later discovered could have been the cause of the problem."

I would be willing to be that you are right Hector. That's a very common mistake that builders make.

I would say the motherboard is shot, but I doubt if the CPU, memory, etc were damaged.

Do yourself a favor BACKUP!

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July 25, 2006 at 15:30:24

OK thanks guys, i'll have a got at replacing the diode. Im pretty competent at soldering so its worth a shot as a last resort to save the mobo.
Im no expert but from looking at the positioning of the diode and through some experience in circuits, this diode is near the input power source amongst the caps and voltage regulators, meaning its there to prevent voltage from flowing the wrong way into the rest of the board or for the caps to leak voltage elsewhere, i.e in one direction only. Taking into account this has burnt out meant voltage was shorted somewhere and tried to leak back the wrong way causing the diode to burn out... in turn acting as a fuse or cut out as the voltage was reversed (correct me if im wrong) so with my theory the damage may have been limited to the diode provided it did not let much voltage pass through it (the wrong way) and ceased functioning in time.
I will let you all know the outcome once I get hold of a spare diode of this type - the markings appear to be still visible as a likely IN40??. If anyone has any further comments, please feel free to add them.

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Related Solutions

July 25, 2006 at 15:59:11

A relatively high voltage shorted to the wrong side could possibly cause a burn out as you described. Mostly it is due to short circuit on the diode's output causing a heavy current in the normal forward direction.

Probably new mobo time.


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July 25, 2006 at 16:07:30

A 1N4001 is a very common diode. I used to work at a parts store and we sold tons of these. You can even pick one up at Radio Shack. If memory serves me correctly ta 1N4001 is rated at 50 volts 3 amps.

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July 25, 2006 at 16:13:29

If it's a brand new board, contact ASUS & inquire about an RMA. What have you got to lose?

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July 25, 2006 at 20:01:49

Sounds like too much voltage for one almost like a possible psu or wall plug problem

check the voltage you are getting from the wall before you plug anything else into it

how is your psu running?

I often do whole store installs during construction and luckily plugged in a porer strip before the work station was plugged in
it imediatly fried the strip
when I tested the outlet it was pulling 250 volts YIKES
we found a bad circuit in the main transformer
which brought it back to normal 119

I have also seen this happen to the first resistors on video cards

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July 26, 2006 at 09:19:56

The motherboard was bought second hand from ebay and it's not even listed on their (UK) site for details so I doubt I can RMA it.

The PSU was running fine, the voltage was set correctly (240vac as I am in the UK)
One thing I noticed the Squeaking sound it made when I turned the PSU off at the wall, but didnt and doesnt make that sound when connected to a working PC or if disconnected leading me to think that there was some sort of short for it not to turn on in the first place.

The diode is question is a IN5817 which I can pick up for a few pence. So i will try that over the weekend.
Im also disputing the fact that the mounting post may not have been the cause in the first place as after a count they all appear there except for the one that came off whilst I was removing the board from the chassis as all the screws where secure prior to this.
One question about this: There are red washers on all the motherboard mounting screws (i assume to prevent the mobo connecting to the chassis.)Does the PSU need to be physically atached to the case to provide a ground? As it was not during my initial power up.
Also, i'd be grateful if somebody knows what the two jumpers labelled: "USB_UP" and "KB_UP" as these are connected by the pcb track to the diode that went pop. Both jumpers are set in the UP position of the 3 pins. Thank you.

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July 26, 2006 at 13:37:35

I'm not a gambling man but I am a qualified electronics/electrical engineer, with TV repair experience etc.

Here's my bet. When you've replaced the diode it will burn out again as soon as you power up because there is something shorting on the mobo (unless the short has blown itself away too, in which case the board still won't work).

I never mind being proved wrong, so I'm all ready to eat that humble LOL.


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July 27, 2006 at 01:44:31

CHeers Derek, I guess you are probably right. But for 12pence its worth a shot.

Would the fact that I left the molex connectors of the other power supply connected to the drives and floppy when I tried the other psu have caused this? The other psu was not plugged in though but the IDE leads where still conected.

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July 27, 2006 at 11:11:35

It's not a good board anyway so you have little to lose...the SiS730S was a poor performer, as are most SiS chipsets.

"Would the fact that I left the molex connectors of the other power supply connected to the drives and floppy when I tried the other psu have caused this?"

I don't know if what you did was the cause of your problem, but it certainly wasn't a wise move

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July 27, 2006 at 12:15:23

Phew, for a minute in #12 I thought you were laying 12p down for the bet (although I think my bank balance would stand it).

Yes, sure, nothing lost by trying to replace diode.

Hard to be certain about connectors. If I've read you correctly, my initial reaction was this isn't much different to running with a few connectors removed - something with which boards can usually cope.

I'm definitely not risking my pennies this time tho - LOL. After all, the diode did pop straight after you did it.


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July 31, 2006 at 15:45:45

Hi folks!
I replaced the diode and.... everything works fine now! It posts, beeps once and i've finally finished installing WinXP on it. That was the best 12pence ever spent :)
I know its not a very good board but i only want it for wordprocessing etc. Thanks again folks. Your help was much appreciated - keep up the good work!

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July 31, 2006 at 16:53:54

This is one of those times when I'm well pleased for being wrong about the likely outcome (such lack of confidence on my part LOL). Good job I didn't put money on my bet! Just proves that it's always worth trying.

Great news, thx for popping back to let us know.


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September 23, 2006 at 20:22:34

Please ignore this post, I'm just checking out something on active links.


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