[mythtv-users] Electrical power in my cable line? Is that bad?

Brian Wood beww at beww.org
Thu Sep 6 13:38:47 UTC 2007

Dan Wilga wrote:
> In addition to what the other posters said, you should also not rule 
> out the possibility that the fault is in your house's *electrical* 
> wiring. If the computer is plugged-into an outlet (or powerstrip) 
> that is not properly grounded, there could very easily be a 
> difference in potential between it and a cable TV wire--even a 
> properly-grounded cable TV wire.
> Just having a 3-prong outlet at the wall does not always mean that 
> third prong is connected to anything. If in doubt, you can pick up a 
> tester cheaply at any Radio Shack or home improvement store.

Absolutely correct. It is fairly common to have significant ground
potential differences between the AC power ground and the CATV ground.
This can be caused by problems in the AC system, poor grounding of the
cable system or, most often, a combination of both.

You can of course measure this difference with a multi-meter. What you
want to know is whether the voltage is "ground-to-ground" or actually
between the center conductor and ground of the cable coax.

Such problems can show up as AC hum in the video, usually as a double
bar slowly rising through the picture (assuming NTSC).

Basically both systems (power and cable) should be checked by a
competent technician. Unfortunately, the word "competent" sometimes does
not apply to the cable companies "expert installation technicians",
especially if they are contract installers. You might want to call and
insist on a "service tech" coming out and checking the installation.

For what it's worth, Cable systems use 60-volt AC power running through
the trunk and feeder lines to power the amplifiers. The directional
couplers used to tap off the signal for a subscriber feed are supposed
to block this power, and usually do, but I have seen defective taps that
allow all or part of this system power to pass through to the subscriber
drop. It's unlikely, but possible, that this is the OP's problem.

Some STBs ("converters"), especially older ones, can leak power onto the
cable line as well, but the OP didn't mention an STB.

I've also seen problems caused by "hot chassis" type TV sets (the ones
that usually have polarized AC plugs) where the TV cable is connected
directly to an "F" connector (not to 300-ohm terminals via a balun
("transformer"), and the user has either defeated the polarized plug or
the AC outlet had reversed polarity. Again, I don't think this is the
problem being discussed here, as it results in 115VAC on the CATV shield.


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