waco

In the olden days, we used to call "drive lines" the horizontal line that would slowly move down the TV screen and it was usually due to a noisy neutral. Does anybody know of a test device which is good for measuring noise on the neutral? I used to use an oscilloscope, but that was a long time ago.

Anyway, the HO told me the cable folks are grounded to the meter socket which tells me they aren't grounded at all, but the cable guys furnished alternative power from their van and the drive line went away, thus, the drive line is due to the house power and not their cable system.

I'm going to look at it right now.

forqnc

I had this on and LED TV I bought a while back. Had the satelite company out twice and they couldn't solve it. I fixed it by plugging it into an outlet which has a ground. The first outlet is an older part of the house which has no grounds.

Magnettica

Check to make sure there is a solid connection to the grounding electrode system and the CATV block on the outside of the house. If not, run a solid #12 wire from the CATV block to the ground buss inside the panel. I would also verify that the grounding electrode system is still intact and wired correctly.

It's because of crap like this (little lines on the tv set) that I would prefer to not install any cable television lines. I was a much stronger supporter of this way of thinking before I went into business for myself. Now I'll do just about anything even if I don't what the hell I'm doing!

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BuzzKill

Good suggestions.
And doing about anything is a point I am nearing; I'm thinking on getting my low voltage license and doing all things data also. Security work especially; I've always thought of it but the other day I was t my accountants office and he asked if i did security camera work because he needs some...a big old light bulb went off in my head.

waco

Findings.

The CATV has a ground running to the weather connection at the top of the meter box and, of course, the local PUCO doesn't allow grounding of meter boxes. There is a grounding electrode right below the connection and the primary and secondary GECs are good and intact. I used a multimeter and measured the voltage between the CATV grounding wire and the GEC and had enough to move the meter. A temporary hookup between the CATV and the GEC improved the drive line, but didn't eliminate it.

I figure the ball is back in the CATV court. If I was their guy, I'd hook up to the GEC. I don't know why they didn't.

The service is one of those Square Ds which have more breaker places than ground/neutral bar connections. Wonder why they do that.

Magnettica

I have a ton trade magazines sent to my house and this is one of the magazines I get for cameras and that sort of thing. I don't do any of that work either but I know where to get stuff if I need to.

http://www.scdlink.com

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BuzzKill

thanks for the link...I have some research to do; I did some low voltage for an EC I used to work for and it isn't that big a deal really; works easier and you still can make similar money, if not more, than electric work.

BuzzKill

I suppose you were metering at the meter base, where the N and G combine? This is all very interesting...what if you ran a seperate bond to the panel, or water pipe? And why don't poco's allow grounding to the meter box for4 low voltage? Seems ridiculous.

egads

Here's something else to try: I had a client with one of these lines on a large (over 60") LCD flat screen. At the suggestion of an AV pro, I removed the ground from the display. (by plugging the power cord into a grounding adapter, like one you would use when plugging grounded plugs into ungrounded outlets) Crazy? unsafe? maybe, but it worked.

I_get_shocked

i have seen this same problem vanish when the tv was plugged into a surge protector (strip type). I would give that a try. If that failed I would remove the coax line in and lv ground and meter the outer shielding to gec looking for substantial voltage.

srm

The generally accepted term for what you are seeing is "hum bar". If you use the search term "hum bar" on google you will see a lot of pages that have suggested solutions for this sort of problem.


I assume you have CATV entering the residence and connected to a cable box. The cable box is then connected to the TV through some kind of analog connection like composite RCA cables or component RCA cables.

I think what is happing is that there is a voltage difference between the shield of the CATV coax cable and the signal ground of cable box and/or TV.

You can test this hypothesis by first disconnecting the CATV coax from the cable box. Then take your meter and measure the voltage between the metal shield of the CATV coax cable connector and the outer metal part of the coax jack on the cable box. You can also take measurements between the outer part of the RCA connectors on the TV and the shield of CATV coax cable connector.

If everything is working correctly you should get a reading of zero volts - however, I suspect that in your case you will see something higher than zero volts. A reading of higher than zero volts means that there is a potential difference between the signal grounds on the equipment and the signal ground on the CATV system. A small amount of current is flowing over shield of the signal cables which is causing the picture distortion that you describe.

Couple of possible solutions:

1. As you suggested, make sure the CATV system is bonded as described in 820.93 and 820.100 -- In theory that should put the CATV shield at the same potential as the signal grounds for all the AV equipment in the residence and should therefore eliminate current from being carried on the signal cables. However, it is possible that this solution won't work entirely if the CATV cable run inside the residence is fairly long or if there is some other issue inducing voltage on the CATV cable. You could find that even with the CATV system bonded there will still be some voltage difference between the coax and the equipment.

2. Isolate the CATV system from the AV equipment with an isolation transformer. Use something like the VRD-1FF from Jensen ( http://www.jensentransformers.com/datashts/vrd1ff.pdf ) to isolate the CATV shield from the cable box. As you will see from that PDF, that sort of transformer is specifically designed "hum bar" issues. Just connect it with short piece of coax cable directly behind the cable box as shown in the diagram on page two of that PDF.

hope this helps - these things can be tricky to track down
SRM

BuzzKill

more good suggestions, and welcome to the forum.

waco

I agree. The owners want to turn it back over to the CATV company before I do anything else. As I wrote, I don't know why the CATV company chose to tie their ground to the top of the meter socket when the secondary GEC is right below the meter socket, but they did.