projector waves

[ceb58]

I have a strange one. In a church i wired I have 3 sets of can lights 16 lights per set. Each set is on a dedicated 20 amp circ. all in mc-ap. In the sound room I have 3 2000 watt 3 way dimmers for these lights the other 3 way switches are on the out side wall of the sound room in a hall way. When they show slides on the screen the picture has "waves" going through it. If you turn off all three sets of lights the waves go away. My first thought was the church people that ran the av cable laid the cable too close to the mc cable and it was picking up mag. field from the cable. However it still dose this when the far left side lights are on by them self and the av cable is not close to this run. The computer that runs the projector is sitting on a counter about 2 ft. from the dimmers. The projector is on its own circ. also. Any ideas?

 

[480sparky]

Harmonics or line noise caused by the dimmers?

 

[ultramegabob]

try moving the computer, feed computer with an extension cord from a differnt circuit. run extension cord and feed projector from differnt circuit and see what happens.... try installing single pole switches and see if it still has waves with lights on....

 

[dbuckley]

Is the projector hanging off the roof running off a local socket? If so, try running the projector on an extension lead plugged in in the control room, twofered off where the computer or whatever is plugged in. If the interference goes away, then you know why, and how to fix it.

 

[e57]

Ground loop - a minute amount of AC on the shield or grounding of the cable feeding the projector from the computer - look up 'video ground loop' and see if the symtoms match and how to correct it, e.g. isolators, or baluns....

 

[RayS]

Sounds like "60 hz hum bars", kind of a standard interference issue.

I'd try 2 tests- put up color bars if the projector can do it by itself, otherwise feed it with a local source (short cable) away from the dimmers. See if dimmers still interfere. If that clears it up, try relocating the computer away from the dimmers. If that works, consider a remote keyboard/video/mouse setup to keep the computer away from the dimmers.

Remote dimming would be great, but the $$$....
Our church has remote dimmer racks backstage. Works great.

 

[ceb58]

Ground loop - a minute amount of AC on the shield or grounding of the cable feeding the projector from the computer - look up 'video ground loop' and see if the symtoms match and how to correct it, e.g. isolators, or baluns....

Sounds like "60 hz hum bars", kind of a standard interference issue.

Thanks what I found on the search is exactly what is happening "hum bars". I told them I would ask others with more experience with this and see if I could find a answer. I did ask and I found the answer. I had nothing to do with installing any of this video equipment and I will not have any thing to do with correcting there problem except to tell them what is going on and they can solve this them selves.

 

[dbuckley]

Not necessarily so, grasshopper, assuming you are the electrician on the job (though not the AV installer)

If you re-read my post above, and it applies, and you follow what it says, and the problem goes away then it is your problem to solve.

 

[e57]

Is the projector hanging off the roof running off a local socket? If so, try running the projector on an extension lead plugged in in the control room, twofered off where the computer or whatever is plugged in. If the interference goes away, then you know why, and how to fix it.

~If you re-read my post above, and it applies, and you follow what it says, and the problem goes away then it is your problem to solve.

IMO - even if it does come off the same outlet and it doesn't have the hum - still not a lot he can do about it. (Except leave the cord there...) Sure, if on the same phase it would be less, but the distance (LV cable run from the comp to projector) is still an issue. Even with some pretty hefty grounding it will not solve it... Single point grounding of the AV system might - but still not his problem - unless he was responsable for that, but as it sounds as if he did not spec., or cable it....

Short of the dreaded words - isolated ground - back to the main panel with say a #4 from each/every AV location to a single point on the main bonding jumper - which still might not solve it.... (Due to distance and other equipment that may be on it, or even in contact with the chassis grounds. Isolators, or baluns are an easy fix....

There are many poor examples of this everywhere on the net; but here are a few...
http://www.siber-sonic.com/electronics/GLoopFix.html
This one is not bad....
http://www.contractorsav.com/article/2007/01/11/troubleshooting

Using this pic as an example; The TV reciever picks up a slight electrical noise at it's ground, then attenuates it to the sub woofer. In his case one ground point at the projector gets a slightly different voltage potential than the computer. And yes it works the same way with audio, and video.

 

[ceb58]

Not necessarily so, grasshopper, assuming you are the electrician on the job (though not the AV installer)

If you re-read my post above, and it applies, and you follow what it says, and the problem goes away then it is your problem to solve.

Yes necessarily so, weedhopper. They wanted these dimmers in the sound room against my recommendations. They pulled the 75 ft of av cable laying the cable with my bx. These are the same people that would come behind my back and try and add things to the elect. portion of the job and think I would just hook it up, trim it out and not say anything or charge for it. It finally got so bad I had to call the inspector in and in front of the gc and the people that were doing this behind my back and told them that if I found one more thing that they added I was voiding all warranty and placing a lien for money owed. The inspector told them if I complained about this one more time they would be cited for preforming elect. work without a license. The bottom line is I told one of the guys that I had a good working relationship with during this job I would try and find a answer for him. And I have found the answer. The lights come on they get bright when you slide the dimmer up, they dim when you slide it down. My job done

 

[dbuckley]

Please - try the experiment, and report back.

The grounding requirements for AV equipment including the power sockets that feed same are more exacting than that that satisfies the NEC. I suspect whoever installed the outlet that runs the projector did it the same way that a GDO socket would be installed (they're both on the roof - right?) and for AV purposes, that may be inadequate.