wrote:

On Sun, 03 Apr 2005 22:02:53 -0500, Bob Urz
wrote:



wrote:

On Sun, 03 Apr 2005 16:50:03 -0600, Luke wrote:



Our TV reception, from a roof antenna, was intermittently flaky.
Trying to find the problem, I got a shock when I touched both the TV
F-connector and coax cable antenna lead. So I put a tester on and to
my surprise got a 75 volt reading.


DVD player around to plug it into all circuits. I also get ~30 volts
off the housing of an old stand mixer in the kitchen.

This isn't normal, is it?


NO. It's dangerous. It's gonna kill someone.

Ignore the illiterate hacks Matt and Turtle aka Weasel, and
get a licensed electrician in there NOW to find out what's going on,
before it kills you. You have neither the tools nor the training to
continue.

Well, licensed electricians don't work on TV's. TV repair men and
electronic techs do. And in most locations there NOT licensed
like electricians or HVAC men are.


I do agree. If its truly leakage, it can be deadly. Most modern TV's
are hot chassis. That means they don't have a direct from the power line
power isolation transformer like in the old days. They turn the AC line
into a high DC voltage and then switch it with a secondary transformer
into the low DC rails required for the TV. This is the "cold" side of
the TV.

This cold side of the TV is where the signal processing and the antenna
input are. If there is a leakage fault in the TV, it could cause a
voltage on the antenna terminal. But you have to differentiate between a
phantom voltage and a fault. A good TV shop will have a tool
like a Sencore that can easily measure if any surface of the TV has
a abnormal amount of AC leakage.


Yes - but when you have 30 V to ground on the case of a
kitchen mixer, you don't need to go there. You need an electrician,
not a bench tech.



Well, if its only the mixer you need an appliance tech then.
A smart electrician should be able to understand the problem.

One big issue on leakage is phantom voltage. And understanding
what that really means. Most modern DVM's have such a high input
impedance that they can read a voltage that really not there.
A older analog meter like a simpson 260 would not have this problem
since it has a much lower input impedance. Some people have run into
this issue measuring a voltage on a un energized circuit in house
wiring. They put the DVM on the dead circuit and read 30 to 60 volts or
so and think they have a problem. When in reality its the capacitive
coupling of wires close together generating this "phantom" voltage.
Put any kind of load on the wires and it goes away. Capacitive coupling
can cause this effect in other devices too. A small load and it goes
away. In any case, a leakage test with proper test equipment is the
best way to put an end to the issue.

I have had and fixed TV's with excessive voltage on the F terminal. And
the TV's DID have an internal fault.

Bob


Leakage can go both ways. EC&M magazine has had some doozy examples of
how this can happen. Like a guy who had the foil insulation of his house
at AC line level when a nail went through the siding and nicked the hot
side of a piece of romax.


Yep. Who you gonna call ? :-)


Another issue is reverse wired AC outlets. A $10 home depot AC outlet
and polarity checker will tell you if the hot and ground and neutral are
intact in any household outlet.

You either have a lifted neutral, or ground, and either one
can kill you, or burn your house down.

Since most modern TV's are two wire, if the TV works its would be
pretty hard to have a lifted neutral at that outlet.


Not really. Lifted open. High resistance connection is a
strong possiblity.


Reversed hot/neutral maybe. For those that don't know, ground and
neutral are suppose to be bonded together in the service panel.
Even though there at the same potential, the ground is for safety
only and NOT suppose to be a current carrying conductor in a non fault
situation. Many cable TV drops develop a voltage problem when the ground
strap to the households ground rod or water pipe is missing or is
corroded or loose.


IE 'lifted' :-)

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