raan@hp-pcd.UUCP (raan) (02/18/85)
I encountered the same phenomenon on my system. I believe that what is happening is that the tv cable shielding is picking up an induced voltage (relative to earth). The source of this induced voltage is the power lines that the cable shares the telephone poles with. (You get the same affect if you ground one of the incoming wires on your telephone. You'll be rewarded with a LOUD buzz in the telephone. The telephone works because it is a balanced line and the induced voltage is common to both lines. The cable is not balanced, since the shield is grounded.) The first thing to check is to make sure that the cable shield is grounded to a good earth ground (like a cold water pipe or a long metal stake in the ground). This should be at the point of entry to your house. I still had problems with attaching the cable to the rest of my system because of ground loops, so I constructed an isolation transformer by putting a 75 to 300 ohm adapter back-to-back with a 300 to 75 ohm adapter. (The 75 to 300 does not block the ground path, but the 300 to 75 does. This means that you need this combination even if the cable is being fed to a 300 ohm input.) The voltage on the cable should be very low current, in other words it should not have sufficent power to do anything useful (ie light a light bulb, I assume that your test light was a neon type, which uses low current.) If it does light up a regular bulb then you have a fault in the system (a potentially lethal and certainly damaging one) and should contact the power company and or cable company immediately! Raan Young (hp-pcd!raan)
slipson@bbnccv.UUCP (Sam Lipson) (02/25/85)
One of the folks here has cable at home, and has an
interesting problem. Whenever he hooks his stereo up to the
cable he gets an AC hum through the speakers [note he doesn't
need to turn the stereo on to hear the hum].
{The intent was to be able to tune the MTV FM stereo
portion.}
A test light shows that there is a significant AC potential
across his cable (perhaps about 110VAC).
Has anyone heard of this before? Is this a way for the
cable company to keep people from making their own unauthorized
connections?
The cable company's box does not derive its power from the
cable (i.e. is has an AC power cord like most).
Presumably to safely attach his stereo he would want to
isolate the FM signal from the AC. Any ideas on how to do this?
(Commercially available preferably)
I'm not so sure I'd want to connect this known live cable to
my television directly either, presumably the cable converter box
idoes a good job at isolation.
I guess one could always consider free power another advantage
of cable. (Then when the lights go out you could blame have two different
"utilities" to blame)
A
Afish@ihlpg.UUCP (Bob Fishell) (02/27/85)
> > One of the folks here has cable at home, and has an > interesting problem. Whenever he hooks his stereo up to the > cable he gets an AC hum through the speakers [note he doesn't > need to turn the stereo on to hear the hum]. > > {The intent was to be able to tune the MTV FM stereo > portion.} > > A test light shows that there is a significant AC potential > across his cable (perhaps about 110VAC). > > Has anyone heard of this before? *** REPLACE THIS MESS WITH YOUR LINEAGE *** My cable hookup gave me a similar problem. I had hooked up the cable to my VCR, which I then route to my stereo via the audio out jack. I noticed a pronounced AC hum which I traced to an AC potential between the cable's shielding and earth ground. Connecting a ground wire between the cable and the screw holding a wall socket cover in place solved the problem nicely. I don't believe the cable company is deliberately putting AC on the cable; rather, it's probably picking up some AC voltage inductively or through a power cross somewhere in the network. CATV lines are often hung on the same poles as power lines, so this kind of thing is bound to happen. In any case, grounding the outside of the cable will take care of the problem. /_\_ Bob Fishell ihnp4!ihlpg!fish
rfg@hound.UUCP (R.GRANTGES) (02/27/85)
[] If there is voltage between the cable tv and any ground, your friend had better call the cable company to fix their damn cable before it kills him or damages his equipment. If he doesn't want to do that because he is ripping off the cable company, then let it go ahead and kill him. -- "It's the thought, if any, that counts!" Dick Grantges hound!rfg
bam@sdchema.UUCP (Bret Marquis) (03/02/85)
Most cable tv companies send power down their lines. The line amplifiers in your neighborhood (or street) are powered that way. It should NEVER reach your house though. Either a faulty tap on the pole or you've tied yourself into a main distribution line improperly. If your connection is legit, you should call and complain. If not disconnect and have the service connected properly. The tap they use for the drop to your house has many uses, only one of which is protecting you from hi voltages. (Another major one is preventing your VCR from transmitting your home movies to your neighbors.) Good luck! -- Bret Marquis (sdcsvax,ihnp4)!bang!bam Bang World Communication Center - San Diego.