ECN Forum Forums General Topics of the Trade General Discussion Area IWire...or any other commercial guys.....(especially retail....)

I bet Yanci just pinned this. I had the same problems when running a closed loop system running remote processors. Fixed it after going back thru everything and making sure the shields were only tied in once at the same place.

LarryC has pretty much spelled out the solution. The monitors AND the equipment feeding signals to them need to be fed out of the same panel. And separate neutrals for each circuit can only help.

A quick and dirty solution would be isolation transformers in the audio and video lines. Something like this:
http://www.kramerelectronics.com/indexes/item.asp?desc=569

not sure about the sheilding on the cables. we didn't install them. How would we check this. All their cables are pre-made with RCA jacks.

they are all fed out of the same panel. separate neutrals are not an option at this point. (both due to conduit fill and derating)

this job was installed as spec'd.

We were tempted to run a new ground directly from building steel to the CPU. Didn't know the effect of it and did not want to be on the hook for $100K in electronics.

To check for any accidental grounds to the shield, remove the cable from its source and then see if there is any continuity to ground. Should be open to ground.

BTW I've had to run a separate EGC all the way to the Grounding Electrode on some old main frame computers. Similar to an isolated ground. Maybe the way to ground the CPU.

A while back I had a simular problem in my CCTV set up, and through the advice here at ECN and some other sources eventualy corrected it. Heres an article about it: http://www.epanorama.net/documents/groundloop/video_isolation.html
Hope it all works out, but I would not promote the lifting of ground with a two/three prong adaptor.

Oh, another question, is the video feed grounded at all? Still thinking about those static shocks from the begining of the thread....

I understand they do not have a cable company feed, it's most likely, that their in store distribution amp is faulty and i bet if you measured from the cable shield to ground you would be supprised at the reading you get, this is a common problem with distribution amps, poor design, or bad instalations, just remove the cable from one of the sets and feed it with a isolated signal, i bet it clears up, there is AC getting into the signal distribution.

[This message has been edited by LK (edited 10-29-2006).]

Grounds- should either be everywhere if you are shielding for RF or single point if you are shielding for low frequency like less than 50 KHz.

The shells of the Video lines are either capacitor coupled or direct coupled to the earth ground of the power cord.

Check if the Audio-Video patch panels are grounded to the building steel, thru the mounting cabinet.

Replace the PC video source with an isolated video source and see if the lines disappear. PC's and video have not been a good mix in my experience.

Larry C

Hello everyone. From reading through this thread I have a question about the diagnosis of a faulty distribution amp from the video feed. Mahlere mentioned that using a two/three prong adapter corrected the problem. By isolating the equipment from the egc, how is that correcting the problem?

Let me just say to that the faulty distribution amp diagnosis makes sense to me, the two/three prong adapter however, it's a curve ball for me.

This really sounds like a Ground Loop (Circulating) Current problem - mainly due to the fact that the noise disappears when the 3 wire to 2 wire "Cheater" adapter is used.

My guess is the signal lines (A/V patch cords) are grounded at both ends - the Monitor end and the Distribution Amplifier end.
(BTW, by "Grounded At Both Ends", I mean each device has a physical connection to an EGC, which is also physically connected to the signal line cables' shield).

If this is true, the only way to eliminate the problem(s) will involve a single point grounding approach for the signal leads.

The A/V cables will need to have the shields + drains ground bonded at one end only (if using a coaxial cable with RCA jacks, for instance, the outer "Ring" of the connector).
In the case of multiple monitors at various locations, the "Star Point" for grounding the shields might best be done at the Distribution Amplifier.

The really interesting thing now will be how to isolate the ground connection of the signal leads at each monitor!

Another player in the creation of the noise may be from an additional (or sole) ground loop between the computer and the Distribution Amplifier.
If the computer links to the Amp via an RS 232 Serial Cable, that would be an additional ground loop path.
If connected via an Ethernet NIC, there should be an opto - isolator already on the interfaces, but something may have caused the unused pins of the RJ45 modular jack to be pulled to ground (???).

When the cheater adapter is placed in the AC Power Circuit's cord ... (for the monitor(s), right???)... the noise disappears. This points to a ground loop issue, most likely looping through the signal lines.

If it's not possible to eliminate the shields from being grounded at the monitors, then you will have to eliminate the grounding at the Amplifier ends of the signal lines and leave the monitor ends as they are, having the signal lines grounded at the monitors (only).

Contact the client's preferred vendor (for the A/V equipment), and get them to figure out how to isolate the shields from the grounded enclosure of the Amplifier (let them suggest and/or supply an isolation means for the signal lines).

The 1.0 Volts read N-G is normal - and a lot less than I would have expected! As Bob (Iwire) mentioned, it's the result of normal Voltage Drop in the branch circuitry.

Got a question about the noise (the interference, or lines, displayed across the monitors):

Do the "Lines" roll Horizontally (either Up or Down, maybe both), Vertically (to the Left or Right, maybe both), or do they roll diagonal?
As they roll, are the lines "solid" or "dashed"?
Lastly, is there audible noise present with the rolling lines?

Good luck man! Really wish I had 100% definite answers for you - it just really points towards loop currents in the signal lines.

Scott35

Scott,

I'll read that in the morning. My head hurts and I'm dizzy trying to understand that all right now

E57 - thanks for the link to the article.

I also just realized that I am a complete moron sometimes. The reason that I was getting a voltage between neutral and ground? very obvious....the 3 phases were not exactly the same draw, the neutral was simply carrying the unbalanced load of the 3 phases. Doh!

Doesn't solve the problem, but I'm kicking myself for not realizing that right away.