Re: Moving Horizontal Lines - Help


"w_tom" <w_tom1@xxxxxxx> wrote in message
news:1145059597.435316.204220@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Leonard Caillouet wrote:
...
As usual, w_tom, you have taken an essentially correct view and injected
information that is simply incorrect in actual practice. Most television
sets and consumer electronics do not have isolation built in at the
signal
ground level. They do have isolation at the power supply side, but
nearly
all sets these days use a signal ground that is common to the "cold" side
ground throughout the set.

If cable connection did not have such isolation, then that TV set
must have a three prong AC power cord. Why does TV not have a three
prong power cord? Because all external parts must be isolated from
internal power as I stated earlier. That function provided by the RF
isolator must already be inside the TV - or that TV cannot obtain a UL
approval sticker.

This is simply incorrect. The isolation is at the power supply. The ground
at the tuner is the SAME ground as all of the signal processing and
everything on the secondary side of the power supply. This is why ground
loops are so common. You simply do not understand the implementation of
current television isolation. Most sets DO NOT have the antenna line ground
isolated from the rest of the set. That ground IS isolated from the a.c.
line, but at the power supply converter in the SMPS in nearly all
televisions sold in the US.

Any consumer electronics that does not have a three prong plug must
then have isolation built into every external electrical connection -
in direct contradiction of what Leonard Caillouet has posted.

This is a true statement but it does not contradict what I said at all.
Once again you are taking information that is correct and applying it in a
way that is inconsistent or out of context. Of course the electrical power
system is isolated, but it is not isolated at the tuner input, it is
isolated at the power supply. All of the signal grounds are common and
should be at the same potential as the earth electrode on the power system
and must be isolated from the hot and neutral. Look at the schematic or the
actual circuit in any set sold in the US today and you will find nearly all
have two-prong polarized a.c. inputs and grounds that are common on
virtually every circuit on the secondary side of the power converter in the
SMPS. I would be surprised if you found a single isolation transformer or
capacitor on the coaxial input.

This
easily understood if basic UL requirements - how electronic appliances
are designed - was known. "... television sets and consumer
electronics do not have isolation built in at the signal ground level"
are products that cannot have two prong AC power plugs. Obvious basic
human safety requirements.

Your condescending attitude would be more tolerable if your pedantry was
effective and you did not assume that others do not understand basic
electronics and safety sytems.

Any electrically conductive connection to a 'two prong AC' appliance
that is not isolated is then a violation of basic electrical safety.

Again, no argument. I only disagree when you assume that the RF input is
isolated from the rest of the signal processing and switching circuitry,
which it is generally not. If IS isolated from the a.c. line.

That cable TV signal input must already provide isolation making the RF
isolator recommendation redundant.

What creates the ground loop is a difference in potential between the a.c.
service ground and other ground paths in the system. Most can be eliminated
by correcting the grounding problems and this is what makes the RF isolator
unnecessary in most cases. In systems where long runs of cable connect
several types of equipment with multiple signal types, it is not uncommon to
have to creat isolation between components to eliminate small potentials
that can occur in even the best installation. It is rare, but sometimes
they are neccesary.

I am making an assumption here: TV
does meet basic human safety requirements that are also demanded for UL
approval. Poster should verify that TV is not defective - therefore a
threat to human life. If TV is defective, then the RF isolator is only
curing a symptom of a more serious problem.

It is your assumptions that generally make your posts confusing and
misleading to many readers. A more sound assumtion would be that a ground
is either not installed correctly or is made with a poor connection. In
reality, in practice, this is nearly always the source of ground loop
problems, not defective components. Your assumption that ground loop
isolators are just a patch and not needed in most cases is correct, but for
somewhat different reasons than you argue.

I install and service consumer a/v systems daily and we are called to deal
with ground loops quite often. I can't recall the last time that I needed
to use an isolation transformer to solve one. I can also not recall the
last time a ground loop was due to a defective product. Both have occurred,
but they are very rare in current products.

Leonard


.

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