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Re: TV feeding voltage into antenna/cable?

Is it possible that TV's are designed to be double insulated so that they don't have (and don't need ) a ground connection?

Unplug the TV and try measuring the resistance between the earth pin on the wall plug and the aerial socket outer. If that's open circuit then they probably are double insulated. In which case all you are seeing is a bit of mains pickup - your TV aren't "feeding voltage into the system" at all.

I am not an electrician or electrical engineer, but have questions for

those of you who are.

> I have been having problems with my telephone. My telephone and TV cable

are connected (in some

manner) from Comcast. Comcast sent a technician out and he discovered my

television sets were

feeding voltage into the system. He said the device is not designed to

take voltage from the

output side of the box. Their connecting box (I have no idea what it is

called) converts one

type of signal to another for both telephone and television reception. > > One TV (kitchen counter) had approximately 64 volts AC between the cable

antenna sheathing and

the wall circuit ground. (All of my wall outlets are properly grounded

and were recently

checked by an electrician.) After the technician left, I dug out my

multimeter and tested it

myself. I disconnected the cable from the kitchen TV and placed one test

lead on the threaded

portion of the TV connector and one test lead in the ground opening of the

wall outlet. The TV

was still plugged in to the wall outlet naturally. The voltage fluctuated

between 62 and 68

volts. Unplugging the TV loses the voltage as expected. > > The technician checked the other TV sets in my home and found they all had

a voltage across the

same ground connections but in the 8 to 20 volt range. He made some

wiring changes to the

outside connection box and said the cable feed now bypassed the phone

connection so I should no

longer have the problem on the my phone. > > My questions are: > Why is there a potential across the grounded antenna shielding and the

outlet ground?

Is this normal? > Why so high on one TV? > Should I consider replacing the TV with the high voltage? > Is this a dangerous situation? I am concerned. > > Thank you, > Jim Y > >
Reply to
CWatters
Loading thread data ...

those of you who are.

are connected (in some

television sets were

take voltage from the

called) converts one

antenna sheathing and

and were recently

multimeter and tested it

lead on the threaded

wall outlet. The TV

between 62 and 68

a voltage across the

wiring changes to the

connection so I should no

outlet ground?

Today's TV sets have one side of the AC input connected to the internal circuit board and its components. The TV should have a polarity plug (the neutral prong is wider) to the AC wall outlet. The cable input is an isolation device from the AC input. The TV technician has to check the Cable input with a leakage tester, and it must show less < .05ma! You may have one or both of the following problems: The cable input connector (isolating device) has become defective, allowing excessive current to flow. And you need to check your wall outlet, that it is wired correctly! Get an outlet tester (cheap

Reply to
scada

In article , snipped-for-privacy@NOSPAMatt.net mentioned...

of you who are.

connected (in some

television sets were

voltage from the

called) converts one

antenna sheathing and

were recently

multimeter and tested it

lead on the threaded

wall outlet. The TV

between 62 and 68

voltage across the

changes to the

connection so I should no

ground?

Somewhat. The usual method of wiring a TV or radio with a two prong power plug is to use a capacitor between each side of the AC line and the chassis. That makes the chassis at about half the AC line voltage.

The usual TV antenna jack has a coil of wire across the pins, and the signal is coupled to the TV thru this magnetic coupling so there's no electrical connection. But there has to be a way for static charge to bleed off the antenna and coax, so the TV has a 1 or more meg resistor between the antenna jack and the chassis. So if you measure, with a high impedance DMM, the voltage from jack to the AC ground, you should get something like 60VAC. In your case, the resistor may have changed to a lower value so that a lot more current is flowing thru to the chassis. This resistor should measure what the marked value is, or else it should be changed.

Another alternative is to use a pair of ceramic capacitors to isolate the antenna from the jack.

330 pF 1kV Antenna center lead-------||--------- TV jack center lead

Antenna shield------------||--------- TV jack shield 330 pF 1kV

This ceramic capacitor is often marked 331k 1kV or similar. A 470 pF might work okay. The high voltage is important for safety reasons. If you build this, keep all leads short. Put it in a box with a jack for the antenna at one end and a pigtail with the plug coming out the other. And anchor the cable so that it can't short out inside. Also it would be a good idea to put two 1 meg resistors from the antenna side to a ground wire going to the ground pin of the AC outlet.

I disclaim any and all responsibility for using this, and it may or may not help with the problem. So take this _free_ advice for what it's worth.

Also, another solution would be to use an isolation transformer on the TV, but that may be more expensive than a new TV.

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Reply to
Watson A.Name - "Watt Sun

I appreciate the comments, but some of it is going over my head. My electronic knowledge is limited. My multitester is an LCD Digital with a 20 megohm input impedance. I have an outlet tester - all duplex outlets in the house are properly grounded for three prong plugs. Several months ago, an electrician replaced my *old* fuse box replaced with circuit breakers. He checked all wiring at that time for proper grounding.

Unplugging the TV, I checked for resistance between the coax outer shield and the ground/earth pin in the wall outlet. It is open.

of you who are.

connected (in some

television sets were

voltage from the

called) converts one

antenna sheathing and

were recently

multimeter and tested it

lead on the threaded

wall outlet. The TV

between 62 and 68

voltage across the

changes to the

connection so I should no

ground?

Reply to
Jim Y

Unlikely to effect ALL his TV's I would have thought?

Reply to
CWatters

Both sides, actually. How else would the TV work?

Joking aside, most TVs made in the last twenty years have a switch mode power supply with a bridge rectifier across the AC supply, and the rest of the circuitry isolated.

The old practice of using a half wave circuit with the chassis connected to one side of the suppply died when supply companies started getting tetchy about DC components getting onto their nice clean (?) AC mains.

A few dozen half-wave thyristor power supplies could really screw things up.

This may be urban legend, but there was a story about a TV factory in Europe putting their first model using a half-wave thyristor PSU into production. The first day they started up the test line with a few hundred sets on, the substation tripped on the peak current.

Then there's duct tape ... (Garrison Keillor) snipped-for-privacy@sbhevre.pbzchyvax.pb.hx

Reply to
Fred Abse

First, if what kind of area are you living? Is the mains voltage there 110(/120) or 220(/230)V? If it is 220V, then a voltage of approx. 60++V at the antenna connector of a TV is not a rarity, but rather too small. It may indeed cause problems, but some that can be dealt with. If the mains voltage is at most 110V, then the

64V appear a little too much as there should be losses making it a little less then half of the mains voltage. But because 64 * 2 is only little more than half of the assumed mains voltage (which is never exact), and 2 would-be identical capacitors are sometimes not really identical, I do not think that it will mean real insulation problems. In any case, as the antenna shielding and most likely the TV's non-HV part is obviosly not grounded (not a failure, this is meant to be so), a little improvised patchwork will easily solve this problem. Consider using a decoupling antenna transformer (Ver 1) or a set of 2 capacitors if an appropriate transformer is unavailable (Ver 2).

The following schematic shows the connection of an average TV's power supply to explain how the antenna voltage comes into existance.

-> Fixed Width Font Needed

Reply to
Dimirtij Klingbeil

Using my multitester, the voltage from the wall outlet read 122.5 volts AC. With the TV plugged into an outlet, coaxial antenna cable connected to the TV and one test lead on the ground of the cable (the outer sheath) and one test lead to the *hot* (small opening) opening of the outlet, I read 122.5 volts. Moving the second lead from the small opening of the outlet to the large one (white or ground), there is NO potential as it should be. This tells me that the outer sheathing of the coaxial cable is connected to ground when the TV is plugged in and the coaxial cable connected to the TV.

It is only when the coaxial cable is disconnected from the TV and I place one lead to the

*grounded* cable connection (threaded portion of TV cable connection) and the other lead to the outlet ground opening that I read 64 volts. As long as the coaxial antenna cable is connected to the TV that voltage is going into the ground.

Jim Y

electronic knowledge is

I have an outlet

plugs. Several

breakers. He

the ground/earth

of you who are.

are connected (in

television sets were

voltage from the

called) converts one

antenna sheathing and

were recently

multimeter and tested it

lead on the

wall outlet. The

between 62 and 68

voltage across

changes to the

connection so I should

outlet ground?

Reply to
Jim Y

Most of the service manuals I have show something like this section at the front...

----

"After correcting the original service problem, perform the following safety checks before releasing the set to the customer:

"Check the metal trim, "metallized" knobs, screws, and all other exposed metal parts for AC leakage. Check leakage as described below.

"LEAKAGE TEST

"The AC leakage from any exposed metal part to earth ground and from all exposed metal parts to any ex- powered metal part having a return to chassis, must not exceed 0.05 mA (50 microamperes). Leakage current can be measured by any one of three methods.

"1. A commercial leakage tester, such as the Simpson 229 or RCA WT-540A. Follow the manufacturers' instructions to use these instru- ments

"2. A battery-operated AC milliammeter. The Data Precision 245 digital multimeter is suitable for this job.

"3. Measuring the voltage drop across a resistor by means of a VOM or battery-operated AC volt- meter. The "limit" indication is 50mV, so analog meters must have an accurate low-voltage scale. The Simpson 250 and Sanwa SH-63Trd are examples of a passive VOM that is suitable."

----

Then they show a diagram of a "probe circuit" used to measure leakage with a 50mV voltmeter.

One side of a 1.0K resistor is connected to earth ground. The other side is the "probe" that is used to contact various exposed metal parts of the equipment under test.

In parallel with this 1K resistor is a series combination of a 10.2 ohm resistor and a 0.15 microfarad capacitor. The 50mV meter is also in parallel with the 1K resistor.

This seems to apply to the US models of international equipment. Other countries likely have similar limits and measurement standards.

-----

The OP could at least try using a 1K resistor as a "probe" and see how much leakage current is coming out of his TV receiver. If it is more than 100 mV (1/10 of one volt), I'd be concerned enough to discontinue using the equipment until/unless repaired.

----

!!! WARNING AND DISCLAIMER !!!

NOTE THAT THIS IS A POTENTIALLY LIFE-THREATENING SITUATION.

USE ALL DUE CAUTION AND DILIGENCE.

NOBODY SHOULD ATTEMPT TO MAKE THIS SORT OF MEASUREMENT OR TO DECLARE ANY EQUIPMENT "SAFE" OR "FAULTY" WITHOUT ADEQUATE TRAINING AND/OR EXPERIENCE.

Reply to
Richard Crowley

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