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Homebuilt front panel is shorting out

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January 12, 2012 11:07:46 PM

In a separate (and previous thread) you all helped me get a machine that had slowed w ......a......y down to be back up to snuff.

After putting the computer back together and having it run beautifully, there is one very vexing problem

If I try to plug anything into the front panel, USB or audio, a tiny spark occurs and the system tries to reboot but hangs. (Also, perhaps part of the same problem, there is a 60 cycle hum in my headphones that I can stop by touching the outside of the case.)

Obviously it has something to do with a ground problem but where do I start to diagnose? If I plug something in and then turn on machine, everything (including the I/O for those front panel ports) is fine.

Where do I begin?

The system is:

CPU - AMD Phenom x4 9850 (quad core)
MOBO - MicroStar 785 GT”M-E45
BIOS – American Megatrends v 8.7
Memory – 4 gb DDR2
Graphics card – Nvida GeForce GTX 550 TI
HD - 500 gb SATA
Windows XP, SP3 with all updates
Power – Powerup LT 400W 270-210

Oh, and I have a really good multimeter if that is needed.

Thanks, you're all the best!

maestrogn
a b B Homebuilt system
January 14, 2012 11:03:32 AM

I would stop trying to use the internal usb port. Get a pci card with usb ports or use the back ports only, unless you can afford a new board. If your case connector is a standard 9 pin usb, then it should work unless you're trying to use the wrong pin cluster. If your usb pins are separated, then you may have them in the wrong order.
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January 14, 2012 11:27:36 AM

o1die said:
I would stop trying to use the internal usb port. Get a pci card with usb ports or use the back ports only, unless you can afford a new board. If your case connector is a standard 9 pin usb, then it should work unless you're trying to use the wrong pin cluster. If your usb pins are separated, then you may have them in the wrong order.



Thank you, o1die ... If this is a USB pin problem, why does it also happen with the plug in of an audio jack (specifically, the mike jack)? I will open up the case later this morning to check the pinning for both USB and Audio to the front panel to see if it is correct. I'll post the result.

I already have aux USB pci, but those ports are filled and in the back of the case that is on the floor under the desk. It is a convenience to have those front panel jacks.

maestrogn
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a c 122 B Homebuilt system
January 14, 2012 11:52:59 AM

Yes, you have a ground problem. From your PSU brand, I am guessing that you are in Europe. What kind of wall plug do you have on the end of your PSU power cord?

Is the computer plugged directly into the wall?

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January 14, 2012 12:02:12 PM

jsc said:
Yes, you have a ground problem. From your PSU brand, I am guessing that you are in Europe. What kind of wall plug do you have on the end of your PSU power cord?

Is the computer plugged directly into the wall?


Thanks for the reply. No, I am here in the States and the PSU was bought from TigerDirect last year. The power chord (3-prong) is plugged into our UPS and that is plugged directly into the wall.

I will try plugging the PSU directly to see if it is something in the line from the CPU through the UPS (this is starting to sound like a conversation in a cockpit).

maestrogn
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a c 122 B Homebuilt system
January 14, 2012 1:04:01 PM

It seems like that sometimes.

I figured you came back quickly enough that you were in Europe. I am at GMT+3 (Saudi Arabia). And you, surely you can find something better to do than mess with a tempermental computer on a Friday night. :sarcastic: 

What kind of case do you have?

OK, let's start eliminating things. We are going to go through this systematically, step-by-step.

Plug your computer directly into the wall. Eliminates the UPS. (Duh. :) )

Unplug your computer from the wall.

Check for a very low resistance reading from the center ground pin of the power plug to the metal case of the power supply. You may need to scrape a little paint away to get a good contact. Should be close to 0 ohms, basically a short circuit.

Check for a very low resistance reading from the center ground pin to the inside of the computer case. Looking for a short. The PSU is screwed to the case, so the case should be grounded through the PSU.

Check for a very low resistance reading from the center ground pin to any of the black wires of the main power plug going to the motherboard. Checks that the motherboard is grounded.

"Hip bone's connected to the thigh bone, thigh bone's connected ..." :) 

Honestly, I expect that all the above will check good. But I can't make house calls.

Here is where I think the problem is:
Check for a very low resistance reading from the center ground pin of the power plug to the metal shell of the front panel headset jack. You should be able to reach this from the outside of the case. DMM should read close to zero.

Check for a very low resistance reading from the center ground pin of the power plug to the metal shell to the metal shell of one of hte USB jacks. DMM should read close to zero.

If they do not read zero to the power plug, can you reach your front panel from the inside? You should see a common ground wire running through all of the connectors. The other end of that wire should be grounded to the case.

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January 14, 2012 1:23:48 PM

One could also ask what I'm doing on a Saturday morning. I appreciate that list of probes and will do them later this morning or afternoon and post back.

I may check the common ground on the front panel first because I don't get the same shut-down reaction on any of the back-connected USB or audio ports -- they connect just fine, only the 4 on the front panel. Failing that, I'll then use the DMM to check all of the other points.

Thanks so much, and greetings to Saudi Arabia!

maestrogn
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a c 122 B Homebuilt system
January 15, 2012 5:45:28 PM

I thought about jumping to the front panel, then working backwards if necessary. But I learned a long time ago, that it is better to underestimate someone's technical skills than to overestimate them. :) 
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January 21, 2012 12:13:47 AM

Best answer selected by maestrogn.
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January 21, 2012 12:14:09 AM

A huge thank you to everybody of that replied concerning my problem with the front panel shorting out. I especially appreciate the time the JSC took to give me his point-by-point multi-tester instructions (which I have saved for future reference).

On a hunch, before conducting the power tests with the multi-tester, I removed everything from the mobo, cleaned the surface with compressed air, then re-plugged everything including the connections to the front panel. Lo and behold, the front panel now works without shorting out when I plug something into it. Could it literally have been a bug?

Garyth
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!