If they are wiped out on all channels (most likely), it's
overload - buy them a RadioShack filter. These are cheap -
get a bunch. Don't try to convince them their TVs and VCRs
are junk (even though it's their fault), but tell them it
will reduce interference from all kinds of things like
illegal CB'ers and AM broadcast antennas. Put one on your
TV and tell them it's the greatest thing since sliced
bread.>>
The largest problem I've run into is virtually all high-pass
75 ohm filters and nearly all 300 ohm filters have no common
mode rejection. 99% of overload is common mode related.
I've found the most effective cure is to ground the cable
shield to the safety ground of the equipment. With masthead
preamps, it is to ground the shield to the antenna mounting
mast and do a common mode ground from the antenna amplifier
input to the same point.
I inherited a CATV system in an apartment complex that had
5kW and 50kW BC transmitters next door. The complex was
built on top of the radial system. The CATV system
installed triple shield cables and everything else, but the
real cure was grounding the CATV wallplates to the
electrical safety grounds at every TV set. After going
through the complex and bonding the CATV lines to the power
mains grounds, and doing the outlets in the apartments, only
one or two TV's out of hundreds had any issues at all with
TVI.
They kept shielding and filtering, and the entire issue was
common mode currents flowing through the TV sets that were
connected between the CATV lines and power line. I wound up
with thousands of feet of triple shield cable and boxes of
surplus high-pass filters.
In my own house, I use these little CATV and power line
surge suppression outlet strips. They ground the CATV line
to the power mains ground. They not only reduce the odds of
lightning, they allow my wife's 15 soap operas being
recorded on multiple VCR's to come through without any loss
of artistic quality.
73 Tom
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