Installation guide

TROUBLESHOOTING
32
CB and/or Ham Radio Interference
Strong, local signals transmitted by Citizens Band
(CB) or Amateur Radio (Ham) operators can cause a
picture to roll, fade, or disappear completely.
Sometimes, horizontal lines appear on the screen
(Figure 11-7) and the voices of the radio operators are
heard in the TV audio.
Solution: If you know or can identify the radio oper-
ator, call the person and tell him/her about the inter-
ference. If the radio operator is cooperative (most
are), he/she usually can eliminate the interference by
equipment adjustment and/or by installing filters. If
the operator will not cooperate or is unknown, call
the FCC. If these efforts do not solve the problem,
purchase a high-pass filter that eliminates frequen-
cies below 54 MHz. Attach the filter to the input of the
receiver or prior to any amplifiers if used.
Electromagnetic Interference
Electromagnetic Interference is the most common
form of TV screen disturbance. It causes bright static
across the screen (Figure 11-8). Often, it also causes a
grinding or whining sound. This type of interference
can come from many different sources, including
household appliances, power lines, motor vehicles,
and hospital equipment. The motors of household
appliances such as furnaces, mixers, hairdryers and
humidifiers are principal causes of electromagnetic
interference. The interference from these devices is
carried to the receiver through the house wiring.
Solution: A highly directive antenna often solves this
problem if the sources from outside the house and
the interference is fed to the set by the antenna.
There are other measures you can take, however, if
the problem persists. If you suspect power line inter-
ference, contact the power company. In most cases
their linemen can quickly find the loose connector or
clamp that is causing the problem. Remember: The
use of coaxial cable reduces the probability of elec-
tromagnetic interference.
FM Interference
TV interference from the FM band is also a common
type of reception problem. FM interference most fre-
quently affects the TV high band (Channels 7-13), but
sometimes a weak Channel 6 will also be affected. FM
interference typically causes a “herringbone” pattern
on the screen (Figure 11-9).
Solution: Use coaxial cable instead of twinlead. Use
an FM trap (filter) for severe problems. (See the chap-
ter titled Antenna-Mounted Amplifiers for specific
instructions about tuning and installing FM traps.)
Preamp Overload
Preamp overload occurs when the transistorized cir-
cuitry of an antenna mounted amplifier is fed an
excessively strong signal. On the TV screen, preamp
overload looks exactly like FM interference.
Solution: When an installation that includes a pre-
amp seems to be experiencing FM interference,
always suspect the preamp. Use a preamp with a high
input capability (capacity). The input capabilities of
preamps are specified as microvolts (uV). Measure
the strength of the overload-causing signal. An FM
trap or other filter also may be necessary in extreme-
ly troublesome causes. Always mount the FM trap
prior to the preamp.
Fig. 11-9. A herringbone pattern caused by FM interference.
Fig. 11-7 Horizontal lines caused by Amateur Radio Interference.
Fig. 11-8. Bright static caused by electromagnetic interference.