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Discussion Starter · #1 ·
Has anyone solved a problem with hum bars -- horizontal lines that move up through a cable picture?

This website does a good job of explaing the problem (see the picture at the top of the page for an idea of the problem). Fortunately, my problem isn't that bad. Just one or two lines and it happens only about half the time and not on all channels.


My system is digital cable (Cablevision iO). The problem is only on analog channels (2-38) and it's intermittent. I had a Cablevision service call and he found some bad cable connectors and that reduced the problem. He confirmed that the signal into the house is fine so it's my problem to fix. It also was present before I got digital, when we had only the analog box. (it's also present on TVs in the house that have no box, but take the cable directly).


I have gone through every line and checked all connectors, and I'm left with the fact that somehow, there is a ground loop in the system. I am using an amplifier but if I remove it, the hum bars are still there (but with a weaker signal). The system is grounded. One suggestion I had was that if it was ground together with the electrical system (which it was), that could cause it. So I ground it separately with no change.


According to the article at that link above, there are hum reduction devices, but I can't see to find them. If you've dealt with this, please offer suggestions.
 

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Find where the CATV enters the house. It should be grounded to your electrical service ground there.


You got the typical cable tech service call, snip a few ends, put some connectors on, then call it your problem.


Ground loop isn't going away until something is done about it.


You can use a Jensen Transformer VRD-1FF to isolate the cable where you're seeing the problem or perhaps at the entrance to your in-house system.

http://www.jensen-transformers.com/
 

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Discussion Starter · #3 ·
Those are some serious prices at the Jensen site, but they also have a really good FAQ for free.


I'll see if some of those tips can help me debug it first. This is the most detailed discussion of the issue that I've seen, so it is a big help. Thanks.
 

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Discussion Starter · #5 ·
Well I moved the ground last night and it improved the situation but did not completely eliminate the bars. They're lighter now and appear maybe 1/3 of the time.


The ground had been connected to the amplifer's grounding screw. The amplifer was connected just after a 2-way splitter (this spliiter has 1 line to the cable modem, and the second line into the amplifier; I know this is not optimal, but then the cable guy had seen it and didn't think it was a problem).


So last ight I moved the ground so that it is connected to the grounding screw on the first splitter.


I am guessing that the ground should be connected directly to the incoming cable. But I'm not sure how I would attach the ground wire to the cable connector. Does anyone know of a diagram or can tell me the right way to make this connection?


If this change doesn't work, I'll go to the Jensen products, but I am a little confused. (I'm just an ordinary computer guy, and don't follow a lot of the electrical lingo.)

Quote:
You can use a Jensen Transformer VRD-1FF to isolate the cable where you're seeing the problem or perhaps at the entrance to your in-house system.
Would this transformer plug into the cable line and essentially condition the line so it's clean? Or is this a diagnostic technique to help me find the problem. I am beginning to suspect that dimmers in the house are contributing to the problem. We have quite a few overhead lights that use dimmers in the house. I would hate to have to rip them all out. So my real question is whether this transformer would fix the interference that may be coming from dimmers?
 

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The Jensen transformers are top quality, price goes with the territory.


They won't help with induced nosie from dimmers and so on. However new dimmers aren't bad from a noise perspective. I've got 5 dimmers in and near my TV room, but they are recently made ones. No fuzz there.


Ground loops are a PITA to sort out.


In some cases, just one transformer is all you need, at the point where the catv feeds into your wiring. That's all I needed. The cable feed's ground just disagreed with my building's ground reference.


Other times, if you have a lot of stuff hooked up, some grounded some not to the same common coax system, you might need more than one ground isolator.


There are cheaper ways to do the transformers, but they might not work well with a cable modem or digital cable in all cases. You can just hook up two 75-300 ohm transformers back to back and they will usually isolate a ground loop.
 

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I am guessing that the ground should be connected directly to the incoming cable. But I'm not sure how I would attach the ground wire to the cable connector. Does anyone know of a diagram or can tell me the right way to make this connection?


Radio Shack,for one, sells connector blocks with mounting lugs that are designed to connect two coax connectors and include a grounding wire lug ,with screw, to connect to a ground rod which should be in the ground not too far away. These rods are about 5 feet long and should be in earth that gets rain to keep it as wet as possible. The cable TV coax will have to be cut and connectors attached to connect to the grounding block. R Shack also the connectors,strippers and crimpers to do this. Practice on some spare coax first !!
 

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Discussion Starter · #8 ·
Re-grounding didn't help (used an RCA cable grounding block from Home Depot).


And then I tried that trick of combing two 75/300 ohm transformers as described on this page but it didn't help.


So I'm ordering the Jensen VRD-1FF and keeping my fingers crossed.


I did come across some other cable ground isolators, like this one for $40 but considering the endorsements here of Jensen, I'll spend the extra $18 because I'm tired of playing with this.


I'll report back, hopefully with a success story.
 

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Discussion Starter · #10 ·
The Jensen VRD-1FF arrived today and I tried it out immediately.


No improvement.


The horizontal hum bars are no different -- they're still on the analog channels and not on the digital channels (above 96). Still showing up about half the time on these channels.


I tried it at three different places --

1. at the first split, before the CATV hits the amplifier, and just after the grounding bloock.

2. I tried it at the output directly from the amp before the 80-foot run to 3 TVs. (there are 2 other runs out of that amp. one to a single TV, the other to the cable modem)

3. I tried it at the wall socket before it reaches one of the 2 digital cable boxes (SA 4200) in the house. (At this TV, CATV goes into the cable box, then to a Tivo; from the Tivo, its a composite cable to the TV.)


Same problem with all 3 tries. Any other ideas?
 

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I had the same problem-isolators did not help nor various grounding schemes. I finally got the cable company to believe that it was NOT my set up causing the problem. They sent out a repair individual who really knew his stuff,technically,and using a spectrum analyzer he found it was my neighbor downstream pumping out garbage with about a 5% component of 60 hertz hum. Probably has an ac/dc tv in his hook-up ! Anyway,he put in a good isolator and a trap in the downstream feed and my picture looked like new. He had to disconnect other subscribers temporarily to do this but did not get any complaints since apparently nobody else was watching TV in the early morning.

It pays to have a $1 a month service deal with the cable co !!:p
 

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Discussion Starter · #13 ·
I don't believe I have an RF amplifier in the system but I do have these ..


* a Terk IR repeater at one TV -- it echoes the Tivo remote signal so that the Tivo can be inside a cabinet.


* a Linksys WiFi router connected to the cable modem.


* a RadioShack CATV amplifier (bi-directional).


There is no "time of day" pattern though on some days it is much better than others. The house, by the way, is in a suburban setting. Nearest house is 50 feet away. No businesses within a mile.
 

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Time for more advanced troubleshooting.


1. Ditch the Radio Shack Amplifier. You should NOT need one at all.


2. Find the point where the CATV comes to your house and figure out how to connect just 1 TV and nothing else (no branches to other rooms or anything else) there.


3. When problem surfaces again, connect a TV at the CATV entrance while it is isolated from the rest of your house.


If you still get bars, then the problem is in the CATV system and you'll have to complain until they fix it.


If the problem is not present, then begin connecting things one by one until the problem re-appears.


It may not be your CATV, though commonly, it is. Ground loops are a PITA to resolve sometimes and it may be some other bit of equipment in your home that is causing the problem.


The first thing to ascertain is whether the CATV signal entering your home is "clean" or not.
 

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Discussion Starter · #16 ·
OK, I'll try it, though the amplifier made a huge difference in image to a TV that was at the end of about a 180 ft run.
 

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Discussion Starter · #17 ·
I'm picking this up because it took a little time to have the cable company come in. But I did establish it is there in the line with no amplifier and when a TV is hooked up directly to the incoming wire.


The cable guy used some meter that loops around the lines and found there was current in the line only after it was inside the house.


He suggested there was probably an improperly grounded electric socket.


By turning off different electric circuits I could make the problem go away, suggesting something is wrong with wiring.


I've only owned the house 5 years and there are a couple of outlets that look suspicious. So I'm going to have an electrician look into it.
 

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Quote:
Originally posted by tech_rich
The cable guy used some meter that loops around the lines and found there was current in the line only after it was inside the house.


I've only owned the house 5 years and there are a couple of outlets that look suspicious. So I'm going to have an electrician look into it.


Good call on the electrician, hopefully he can sort it out.


In the mean time, I've found a **very** handy thing to have is an outlet tester. It is just a plug with three lights on it. You plug it in the outlet and the lights indicate whether the polarity is correct and whether the ground is correctly connected.


Well worth the $7 they cost at Home Depot and most hardware stores.


I went around the house when we were buying it and checked each room. Luckily, everything was correctly wired.


You should have seen the rent house I was living in! Holy guacamole, nearly every outlet in the place was either wired backwards, not actually connected to a ground or worse!


You'd think whoever installed the outlets would have noticed the words "black" and "white" on the back of the outlets and got the corresponding colors connected.


Keep after them. Ground loops are a pain in the rear.
 

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Discussion Starter · #19 ·
Thanks for the tip. I've got to get one.


Obviously, it will be cheaper for me if I can check polarity myself instead of having the electrician check everything.
 
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