Know a lot about EMI? Read me. :)

[Nebben]

So there's this computer that works fine, except for one thing: Randomly the monitor (CRT) will go black on the left/right sides for just a split second. It almost looks like a 'wipe' in a film, but it's much more quick. It's quite annoying. My first guess was that the monitor was just going out, as it doesn't do the same thing when using another monitor on the same system. I haven't had a chance to try the monitor in question on another system yet.

But then on to the real issue: the monitor is getting interference of some kind. Depending on the used refreshrate and resolution, it varies in how much interference is noticeable. There is a constant flicker, easily noticed when reading text. It is somewhat like when you have two poorly shielded CRT monitors too close. But I've moved EVERYTHING far away and it changes nothing.

My guess is that there may be something inside the system that is causing EMI to occur with the monitor output. The board has onboard video. I'm thinking of trying a PCI video card, but I'm not sure if the board will let me disable the onboard video. It's a pretty old Intel board.

Is it possible that a bad standoff connection in the case, or a not-snug fitting I/O shield on the back, or something similar to these could be the problem?

I'll be swapping out everything that could possibly cause this, but I want to be sure it's actually possible for the above two things to be the problem before I rebuild the whole system in the case.

Thanks for any help, guys. This is something I don't know much about other than the basics and can't seem to find a lot of information on through web searches.

 

[RampantAndroid]

Well, EMI can occur from a few different things. For one, it can be internals. It could be your PSU letting some noise in. It could also be your onboard video.

Also, be your AC line. First, the AC wiring can cause intererence in your Dsub 19 video cable going from the vid card to the moniter. Try a different cable here, macking sure it has ferromagnets (those buldges on the cord) to minimize noise there. It could be noise in your AC line going to your moniter.

As for a bad standoff, so long as the mobo is grounded properly, this shouldn't be it. If the board were resting against the case, it'd probably be fried by now.

Try a different cable, a different surge protector/power cable, and even a different moniter.

 

[Nebben]

Hopefully it's not the PSU. It's a Sparkle, and brand new, and seems to be working very well.

I am inclined to think it's the motherboard in some way. I'll be trying a PCI video card first and going from there.

I'm pretty sure the motherboard is properly mounted and grounded, but I will check for anything out of the ordinary.

I've tried different outlets in the house, as I wondered about the AC power quality. I tried it on a different electrical circuit and it did the same thing... the only thing left to try is going completely out of the house, but I don't think that is very high on my to-try list. I also did try another monitor, with the same problem, except for the black-wiping thing.

 

[RampantAndroid]

I'm inclined to agree that its the mobo/onboard video then, since you eliminated all else. SHove a pCI card in and post the result.

 

[KGB]

"My first guess was that the monitor was just going out, as it doesn't do the same thing when using another monitor on the same system."

Just curious... why are you thinking the problem lies in the case?
Try the monitor on another system and see what happens.

 

[FlyingPenguin]

What you describe does not sound like any external interference. It wouldn't manifest itself as a "black wipe". That sounds like an internal component failing.

 

[Nebben]

Originally posted by: KGBMAN
"My first guess was that the monitor was just going out, as it doesn't do the same thing when using another monitor on the same system."

Just curious... why are you thinking the problem lies in the case?
Try the monitor on another system and see what happens.

Because the monitor has been used on another system in the past and this problem just recently manifested in this newly rebuilt system.

And I tried another monitor on the same system and it also had some interference-like behavior... but not the 'black wipe' thing.

I can't get over to the system right now, but as soon as I get there I'm trying a PCI card and hopefully will figure something out.

 

[imported_ssm0002]

I had a whole row of offices that had intermitted EMI problems showing up on CRT screens - solution...replaced them all with LCD screens, problem solved.