PC loses video signal when touching headphone jack or usb on front panel of case

[usp8riot]

Edit: Found this post. I think I'll sleep on this and look at it more tomorrow. I wish you could delete posts here.

This seems to happen randomly on a motherboard I've had installed for about a year. I can't say it happened enough to say it's always done this but with the other motherboard it never happened. Anyone have an idea how to reproduce this to make it easier to narrow things down? Things I've did:

* Tried 3 different PSUs
* Reseated the motherboard
* Checked case wiring multiple times and rewired a few times already


It's an old Antec 900 case but I have no need to replace it. I can't say it doesn't happen with the back panel but I haven't tried it enough. I'm going to guess a motherboard issue. If it's a grounding issue somewhere, you'd think I could touch the ground on those components and get a measurable voltage but out of curiosity, I checked it with a DMM and only have 13.1mv between case and earth ground. I don't know if that's anything significant though. Is this something to worry about? I don't really want to replace the case or motherboard. Also it happens with the front panel usb/3.5mm audio jacks and the card reader/usb device on front connected to the motherboard. I've also had two HDD's fried at the same time months ago which I thought was suspicious of an electrical issue.

 

[ATMINSIDE]

Sounds like a case grounding issue to me.
New case solved it for a friend of mine, just for the record.

 

[Mpegger]

I would say it's also a grounding issue. Front panel USB ports may be wired incorrectly, though you may not have enough static being built up for it to happen all the time (which is a good thing). Since you have a DMM, the simplest way to test the front panel USB wiring is setting it to a continuity test. Most grounding issues on front panel USB ports come from the outer shield being tied to the Ground pin of the USB port(s), which should not be! It shouldn't be tied to any of the USB pins at all. It should instead make contact with the case. It is what is designed to take the static away first, before any of the USB pins make contact. If the outer shielding is not making continuity with any of the USB pins, double check by measuring the resistance between the outer shield and the 4 USB pins. It should be infinite resistance. If all is good, next check the continuity between the outer shield and the case, and the rear panel motherboard shielding. You should have continuity at pretty much any point of the case, and the rear mobo shielding. Next would be to check the resistance. You'll want to see as low a resistance as possible between the outer shielding and any point of the case/rear mobo shield. The lower, the better. High resistance would mean that the outer shield is not making good contact and you'll need to do something to provide a better contact, as you want any static to have a easy time going to ground. I'm trying to remember what the standards for home wiring was, but somewhere around 1-2 Ohm max resistance.

If grounding is not an issue, it could be just a specific issue with the ports/hardware that controls those ports.

 

[Itachiii]

Have to agree with the guys above, sounds like a grounding issue. If you got a spare case to put everything in that and test, it would be the best way to see if it solves things. Otherwise, just assemble everything on air and just plug the USB ports of the case on the mobo check if it affects the video signal again.

 

[usp8riot]

I would say it's also a grounding issue. Front panel USB ports may be wired incorrectly, though you may not have enough static being built up for it to happen all the time (which is a good thing). Since you have a DMM, the simplest way to test the front panel USB wiring is setting it to a continuity test. Most grounding issues on front panel USB ports come from the outer shield being tied to the Ground pin of the USB port(s), which should not be! It shouldn't be tied to any of the USB pins at all. It should instead make contact with the case. It is what is designed to take the static away first, before any of the USB pins make contact. If the outer shielding is not making continuity with any of the USB pins, double check by measuring the resistance between the outer shield and the 4 USB pins. It should be infinite resistance. If all is good, next check the continuity between the outer shield and the case, and the rear panel motherboard shielding. You should have continuity at pretty much any point of the case, and the rear mobo shielding. Next would be to check the resistance. You'll want to see as low a resistance as possible between the outer shielding and any point of the case/rear mobo shield. The lower, the better. High resistance would mean that the outer shield is not making good contact and you'll need to do something to provide a better contact, as you want any static to have a easy time going to ground. I'm trying to remember what the standards for home wiring was, but somewhere around 1-2 Ohm max resistance.

If grounding is not an issue, it could be just a specific issue with the ports/hardware that controls those ports.

Thanks for the info. I'm too tired to mess with it for now. I've had the thing apart multiple times double checking the wiring. The new motherboard did seem to have weird behavior, like in Windows, two USB ports would disappear and not appear until maybe a couple reboots. Also my secondary USB wireless keyboard's signal isn't as strong as my previous motherboard after a year of using different batteries.

But according to the link I mentioned, the 9th pin on the usb port shouldn't be connected to the mobo if it says NC, which it is. Should I just cut the ground wire on the USB front panel connector? I also had it wired the same on my previous mobo and didn't have the problem. I want to chalk it up to my ebay'd mobo I bought from some sketchy cheap seller. Anyway, I'll mess around with it tomorrow and update if I think it's fixed and how.