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How do I check for ground voltage coming in to my cable modem?

How do I check for ground voltage coming in to my cable modem?

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  #1  
Old 06-23-11, 12:53 PM
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How do I check for ground voltage coming in to my cable modem?

I've been having cable internet issues for a while now. Getting random disconnects during games and streaming content periodically.

I posted the whole story here: WOW connectivity issues even with new equipment - W.O.W. | DSLReports Forums

The answer I got makes sense for most part, but I'm not much of an electrician. He said to check the ground voltage from the shielding (I'm assuming of the coax cable?) to the ground. What ground? Where do I put the other multimeter prong to test ground on a coax cable plugged into a cable modem? Also, what setting do I put my multimeter on? I've scoured the internet for a simple guide but haven't found what I'm looking for.
 
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  #2  
Old 06-23-11, 01:28 PM
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Disconnect the cable from the TV/modem/BR player whatever and measure between the shield of the cable and the ground of an electrical outlet.

I believe it's been said to disconnect ALL cable connections before testing..but not sure of that.

Might want to try it with everything connected as well as a baseline kinda thing..I may be mis-remembering.

You would normally be in the AC voltage setting, but you might want to check in DC voltage as well.

Post back all your specific readings...more info is always better than less.

You also want to try to keep power cords away from cable or data connections.
 
  #3  
Old 06-23-11, 01:31 PM
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Set your multimeter to the AC voltage scale. Touch one probe to the exposed screw part of the coax cable and stick the other probe into the round hole of a nearby power receptacle. The measurement should be pretty close to zero.
 
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Old 06-23-11, 01:36 PM
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Has the coax cable from your modem to the cable company service drop been replaced? In my experience your type of problem is caused by low quality cables and/or splitters somewhere in the house degrading the cable modem signal. The best thing to do is run a brand new RG6 coax directly from the modem to the cable company service drop, use the right connectors and make sure they're installed well. Put in no splices or splitters. The cable company can then install a splitter right at the service entrance between the TV lines throughout the house and the modem line. You can also do a test here by hooking up only the cable modem line to the cable service to rule out a problematic television somewhere in your house.
 
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Old 06-23-11, 02:38 PM
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Thanks for the replies, I'll be trying these suggestions out tonight. Also, the cable line coming to the house was a brand new drop when I setup with WOW. Oddly enough, they didn't use the comcast line that was working just fine.

As far as the line from the cable ground block into the house, I don't know what the status of that is but I hope to find out tonight. Will post results later.
 
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Old 06-23-11, 02:50 PM
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Originally Posted by Rock_Lobster View Post
As far as the line from the cable ground block into the house, I don't know what the status of that is but I hope to find out tonight. Will post results later.
I'm going to bet this is your problem. If a builder wired the house, chances are it's very cheap cable poorly installed barely suitable for analog cable tv. Digital cable and cable modems really need good quality RG59 or RG6 with few or no splitters inline.
 
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Old 06-23-11, 03:39 PM
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Comcast owns that cable that comes from the pole (or underground) to your house. Any other provider has to run their own line to the interface with your in house wiring.
 
  #8  
Old 06-23-11, 04:04 PM
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RG59 is not used anymore for anything other than very short jumpers between devices.

If you have recently purchased an LCD, plasma, or LED TV it could be causing the ground loop.

The shield, or outer metal edge of the "F" (cable) connector, is where you want to check for AC voltage. Disconnect the cable from the TV and put the meter leads between that metal connector and the screw threads on the TV. Do the same with every other device that's connected to the cable. Post back with the readings.
 
  #9  
Old 06-24-11, 11:41 AM
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(A little off topic) Rick, I thought RG59 was still good for most digital cable systems and that RG6 was really only required for higher frequency applications like S/Ka/Ku-band satellite dishes?
 
  #10  
Old 06-24-11, 12:19 PM
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You or one of your neighbors may also have an open neutral which is seeking through the cable shielding.
 
  #11  
Old 06-24-11, 10:39 PM
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Originally Posted by ibpooks View Post
(A little off topic) Rick, I thought RG59 was still good for most digital cable systems and that RG6 was really only required for higher frequency applications like S/Ka/Ku-band satellite dishes?
RG-6 has been the standard for anything digital (cable and satellite) for years (it is even recommended as a fix to signal problems with analog cable and OTA). Basically it has a heavier center conductor and denser shielding so it can tolerate longer runs and more splits with less loss than 59. The higher frequency range is bonus.
 
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Old 06-24-11, 10:45 PM
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Originally Posted by Justin Smith View Post
You or one of your neighbors may also have an open neutral which is seeking through the cable shielding.
Very unlikely. For it to be coming from a neighbor there would have to be three bad ground points - yours, your neighbor's, and the tap at the pole - all of these would drain the current before it got into your house. And I doubt the OP has an open neutral with no symptoms other than AC on his cable line.
 
  #13  
Old 06-25-11, 04:53 AM
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Ben, have a look at this Cable Attenuation Calculator.

The difference doesn't seem like much, but 3dB is half the power and the losses are additive. Most cable installations have a bandwidth of 50MHz to 750MHZ with passives rated up to 1GHz. Satellite dish feeds are up in the 2GHz range, plus they use a pure copper center conductor instead of CATV's commonly-used copper-clad steel center conductor.

RG7 is typically used for serial digital (SDI) in broadcast applications. RG11 is used in schools, hospitals, and other commercial installations as trunk lines between taps.

To bring this back on topic: Voltage on a cable line is usually there because of a connection to chassis ground on one or more of the connected devices. Because chassis ground is ultimately bonded to neutral in the service panel, ground loops can develop that result in small currents in the cable ground. As I said, some newer TVs are notorious for causing this. The fix is to (1) make sure there is no voltage on the incoming cable line, (2) make sure the cable is properly grounded only at the entrance, (3) pull connected devices one at a time to find the culprit, and (4) install a filter, hum eliminator, voltage blocker, or transformer between the cable and the device.
 
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