A friend of mine had this same problem...it was showing 57 volts from the coaxial cable (center wire) to ground, and causing a really bad picture on all the tv's in the house. The cable company said it was something for an electrician, that it was not their problem. So, we unpluged each device one by one...turns out it was one of the cable company's tv-top digital cable boxes causing the problem. Once that was shown to them, they reluctantly brought out a new one...no more problem.
I currently am a tech for one of the cable providers and have been running into this problem EVERYWHERE this summer, im currently in the northeast and we've had an unusually bad summer for thunderstorms, im not posting is cause i know it all, i assure you i dont, but this is what ive found. Ill go into a house having intermittent internet, bad phone reception, or on demand errors and immediately test the cable for current with my fluke meter, i disconnect the cable from the tv and get no voltage, ok, then ill test the jack on the tv, red to the jack and black to the cable housing after making sure our cable is grounded, i get anywhere from 9-24 volts depending on the house, i then test our box, mind you that the only thing connected to these at this time is power, no cable. I get the same reading, test vcr, same reading, dvd same reading....change outlets same room, same reading change outlets different room, power GONE, all clear, Ive never been to a house where there is electricity running into it on our system from the pole, maybe its happened but i havent seen it, what i have seen are bad house grounds and bad neautrals feeding back power which then travels through our cable to our ground on the side of the house. Any ideas or help or knowledge or questions, id love to hear em and ill check this post regularly. Oh and jenweather, this is a known problem with one of our set top models and its voltage issue if the tech installing did not use the proper AC adapter with the box it will backfeed onto the cable line, there are several models of adapters and if the tech doesnt double check it when he gets it from the warehouse it can be an issue that will interfere with your system.
Last edited by autobotron; 08-13-2008 at 06:36 PM.
Ya know .... with all the talk about electrical grounds in other threads got me thinking about this issue.
Here's a theory ..... ground loop.
Low current wiring is particularly susceptible to ground loops. If two pieces of equipment are plugged into different power outlets, there will often be a difference in their respective ground potentials. If a signal is passed from one to the other via a cable with the ground wire intact, this potential difference causes a spurious current through the cable.
With audio equipment an audible buzz called " mains hum " at AC frequency ( 60 Hz) and the harmonics (120 Hz, 240 Hz, and so on) ...... with video ground loops " hum bars " can be seen scrolling vertically up the screen.
These are frequently seen with Video equipment where the display device has its case grounded via the wall plug and the other components have a floating ground connected to the CATV coax. In this case the video cable is grounded at the display end to the home electrical system, and at the other end to the cable TV's ground, inducing a current through the cable which distorts the picture.
Ground loop issues with television coaxial cable can also affect any connected audio and video equipment in a home theater system ..... even if all are plugged into the same power outlet which all share the same ground ...... the coaxial cable entering the TV is actually grounded at the cable company.
The potential of this ground is likely slightly different from the potential of the house's ground, so a ground loop occurs.
I'm thinking this problem can be solved by placing an isolation transformer on the cable-tv coax ...... or ..... use a surge protector that includes coax protection. If the cable is routed through the same surge protector as the equipment it connects to .... both will be regrounded to the surge protector.
A cheap "poor man's" isolation transformer ..... buy a 75-Ohm Coax Combiner-Splitter and a "Matching Transformer". The 75-Ohm Coax Combiner/Splitter converts the impedance from 75 ohms to 300 ohms and the "Matching Transformer" converts the impedance from 300 ohms to 75 ohms.
Just a thought.
"" an ounce of perception -- a pound of obscure "
I am a cable tech as well, one of the things I see more often than I would like(one of those wake you up tingles to your hand) is that the a faulty cable modem is feeding voltage back onto the cable system.
Try a little test for me. Take a standard television set that does not have a 3 prong, grounded plug. Disconnect your coaxial cable and any other AV connections. Take a voltmeter. Measure the voltage from any metal (coax connector shield/hot, RCA shield/hot) and you will find 60+ volts on it. This is the way ungrounded TV's are designed. Any plasma/LCD TV will have a grounded plug, you will not see the same voltage on a grounded TV.
So, if the CATV system is not bonded/grounded in accordance with NFPA 70 250.94, then the CATV system itself will see this voltage on the lines feeding back.
Simple little experiment, it's nothing more than the voltage from the TV power supply capacitors bleeding off. Not enough to really shock you, but enough to startle you for sure.
I am currently investigating this situation in the house which I bought. When the cable company came out to hook my cable up they saw a small arc when checking the existing main connection outside. They were hesitant to go through with the hook up, but I persuaded them. They said that they did not know what to do. I have found about 8 volts coming from the coax line going to the modem and 45 volts from the coax going to the tv. I tried e-engineer's suggestion and found 54 volts on the back of one tv. This now makes sense that the juice would use the coax to seek ground once it is connected.
From reading the various posts it seems that this is more prevalent than some people are aware. The cable company suggested having an electrician come check my neutral connections.
Does everyone agree that as long as the splitters and cable are grounded that there is nothing to worry about?
There should be absolutley nothing , nada , zero volts let alone 60 volts out putting from the TV's F connector at the tuner block or any other point.
There is no way UL would allow this to be a safe normal condition.
50 or 60 volts is a fair charge and if there is 5 ma or more this potentionally can kill.
The caps from the power supply don't normally feed back through the circuitry on a 2 conductor power cord.
If you had 50 or 60 volts feeding back through the tuner block I doubt the TV would be working or not very well at least.
Back in the day when electronic equipment used tubes instead of solid state components .... then maybe you might find feed back voltages . The metal chassis were notorious for being hot for grid voltages..... this is not the way it is with solid state componets used for over 35 years.
If there is voltage on the CATV cable coming into the home then it's the cable company's issue ...... either problems with the distribution amps or from their plant.
If the cable shows clean coming to the house but shows a voltage on the cable in the home then there is a ground issue .... perhaps the cable company ground block or grounding issues within the home.
If there is a cable box involved and the cable coming into the box is clean but has voltage coming out .... it's the cable box.
If there is only voltage coming out of the TV .... the TV is faulty.
"" an ounce of perception -- a pound of obscure "
I dont even know what to say to that. except maybe the cable company should check references a little better.
I know its an old thread but...
You live in a house without a ground loop, your gonna have all kinds of grounding issues, first correct the house wiring then address the cable if theres still an issue.
add an egc to all devices including switches and fixtures, electrodes, water main, bonding jumpers, and make sure all the splitters are bonded as well.
bet the problem goes away there, and probably a few other problems you didnt even know you had.
You can NOT diagnose ANY audio/video/data quality issues in a house with an unstable ground.
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I have this problem and I am a technician. The cable guys are at a loss except to blame it on my house. We have traced the source of the backfeed to the cable modem itself. That means the problem has to be from my house current. We tested this from an outlet with a solid ground (according to those little testers) and found 50 some volts. The cable techs tested the same modem outside from a generator and there was no voltage. I am racking my brain to try to solve this issue. The techs were surprised to find the voltage comes from the modem itself. Apparently it stores in the splitters (which are grounded) until it reaches a threshold that takes out my service. Then I can discharge it and get it back up. Any ideas?