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(This is jeopardy-style; I found the answer eventually, but it is not what I expected. Posting here for search etc).

I have a Dell laptop and external monitor (on DisplayPort). It was fine for ages, but recently it has started cutting out randomly, displaying "no signal". Turning the monitor off and on, or disabling and re-enabling via Windows both make it work again, but it is annoying and disruptive.

To investigate, I've disabled all the power settings / screen-saver / etc, so it shouldn't be the OS disabling it electively.

What might cause this? And how can I fix it?

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Hi Marc, lost on Super User are we? Or did they force you to pay us visits nowadays? ;-) – Ivo Flipse Jun 19 '10 at 21:10
@Ivo - well, it turns out that I can't blame this one on the compiler... – Marc Gravell Jun 19 '10 at 21:58
up vote 3 down vote accepted

This could simply be a cabling problem. Check that your video cable doesn't run too close to anything like a power pack, or something else that could cause interference to the signal.

OK; since I'm the OP I'll elaborate: the DisplayPort cable had slipped, and was directly next to my (powered) USB hub. Simply relocating the cable solved the problem completely. I guess with a digital signal, some kinds of interference can have a significant effect, requiring a signal reset.

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Interesting, is the DisplayPort cable fully shielded, ferrite cores on the end and everything? I've taken my share of electricity and magnetism classes, and honestly I'm surprised that more cables don't have issues, but using fully shielded cables I haven't had issues with 3 power cables and 3 DVI cables run together as a single bundle. – Darth Android Jun 19 '10 at 21:06
@Darth - it wasn't the cables that were the problem - it was the powered hub itself that messed with it. – Marc Gravell Jun 19 '10 at 21:57
@Darth - another embarrassing point. I'm not using the cable that came with my monitor, but one that I had lying around. Maybe Dell ship better-shielded cables than whatever I picked up off amazon. Maybe not; who knows. I might swap them at some point - but relocating fixed it for now. – Marc Gravell Jun 19 '10 at 22:02
Yeah, I always make sure I get maximum shielding; I bought an offbrand VGA cable from local store a while back, and it looked like someone had taken a Photoshop blur filter to my monitor. While certainly not the first thing I'd think of, I'm not entirely surprised the hub was causing interference. – Darth Android Jun 20 '10 at 0:33

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