I've got a few strange symptoms on a machine I built:

  • The machine won't boot (sometimes) with certain USB thumb drives plugged into USB ports; disconnecting the front panel USB/firewire/audio seems to have helped, but I'm not sure...
  • On-board audio has noticeable static/hum; it seems to only happen when ethernet is plugged in. I bought a Sound Blaster X-Fi XtremeGamer (wanted a nicer sound card anyway) and that seems to have resolved this issue, but...
  • ...the new sound card occasionally loses track of the speaker jack and gives me only SPDIF out (and occasionally an "internal aux" output shows up in the available outputs list). From what I understand, modern cards often automatically switch based on measured resistence

I think I've got a ground loop, maybe at my cable modem.

I found an article on solving ground loops, but I'd love to confirm that these symptoms are all related to that issue, and make sure I'm not dealing with multiple loops.

I've got an ohmmeter around somewhere, but not a lot of experience using it.

(I assume I should be stripping the machine down, disconnecting and unplugging it, and adding components one at a time to test...)

Any general advice on hunting down the issue(s)? Anything I should be paying special attention to? If it is only the cable modem, what's the best way to solve it -- an isolator?

  • Make sure your outlet is actually grounded first. Jul 17, 2009 at 1:56
  • @Brad: fair enough. Most of the place is on old bx cable; I've specifically gone to a separately-breakered romex circuit for these machines -- but I've never tested grounding on this circuit. (Need to google for how.)
    – leander
    Jul 17, 2009 at 3:58
  • 2
    Test grounding by buying a cheap electrical outlet tester at your local Home Depot/Lowes/etc.
    – derobert
    Jul 17, 2009 at 5:04
  • 1
    I have a Wacom tablet that causes every machine it's plugged into to not reboot. I don't think the USB issue is related.
    – Moose
    Oct 10, 2009 at 14:22

2 Answers 2


If you already suspect the modem, I'd suggest that rather than stripping the machine down and adding one component at a time, it may be easier to first remove the modem and see if that fixes it.

No concrete suggestions on what to measure tho.

  • That means going without internet for a long enough time to perceive if it makes a difference -- I'll try to suppress my tears. Part of the issue is that I'm not sure all of the three bullets above are actually related. Some of them are not terribly reproducible. So I'd love a way to actually positively diagnose issues rather than just try removing things and waiting... But yeah, the suggestion is legit, lacking other options, it's probably what I'll do next. (I prefer breaking out proper debugging tools to history bisection when coding, too -- reduces overlapping variables...)
    – leander
    Jul 17, 2009 at 4:07
  • If you suspect the modem itself you can always see if you can borrow someone else's modem, or someone's old modem so that you are not cut off from us here all that time. ;)
    – jerryjvl
    Jul 17, 2009 at 4:19

I had a computer a couple years ago which wouldn't reboot if a flash drive (or anything USB with storage) was plugged in. That part turned out to be a BIOS setting, it was trying all 'removable drives' before the hard drive, and the hard drive turned out to be the boot device of last resort after a 10-minute time out (it was a freaky BIOS, let me tell you).

The sound card issue could be grounding, but a simple check would be the volts (not ohms) between the bare metal of the case and a known ground, like an outlet ground pin if you know that's good or a water pipe. Then you could check voltage between your PC and the cable connector as well. Running your ethernet cable through a UPS 'filter' should match that ground to the PC's as well.

The audio connection part: Are you running Vista/Windows 7? Windows post-Vista contains 'protection' routines that disable your analog audio outs when 'protected' (hi-def) content is played with a media player. Only digital (in this case your digital out and an internal Aux) outputs are supported so that you can't redirect audio data to a tape recorder (the 'analog hole'). More here.

So, at least I've given you the possibility that all three of your issues are completely unrelated. I certainly can't prove that they aren't related, but these are just the first three things that popped into my head.

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