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 TM 9-8000
b. Recognition. Looseness in the hood, brackets,
by a frying or snapping sound. It can be detected only
when the vehicle is in motion; or, when moving the loose
and bolts can cause considerable noise in a receiver.
parts, it can be recognized by a scratching sound in a
This type of disturbance is intermittent, varies in value,
receiver.
and can be detected
Section II. SUPPRESSION
and the capacitor wire is connected to the terminal. A
18-5. Description of Suppression Methods. Various
capacitor allows the interfering voltage to pass freely to
methods are used to suppress radio interference caused
ground (frame and body of vehicle), and at the same time
by a vehicle.
These methods include resistor-
prevents any loss of the useful direct current. Thus, the
suppressors, capacitors, filters, bonding, and shielding.
surges are conducted away from the wiring and cannot
cause interference.
Application of one of these methods usually is sufficient
to suppress the interference from any one source
c. Filters. An assembly made of a closely wound
adequately. However, in some instances, it may be
coil of heavy wire and one or more capacitors mounted in
necessary to use a combination of these methods to
a metal container is called a filter (fig. 18-2). The
obtain the desired amount of suppression.
capacitors act in the same manner described in
paragraph b above, and the coil of wire acts to block the
a.
Resistor-Suppressors. A resistor-suppressor
interfering voltage from getting farther in the circuit.
consists of a short carbon rod of high resistance,
Filters are used In some generator, regulator, and low-
protected by a plastic cover. Resistor-suppresssors are
tension ignition circuits.
connected in the high-tension wires at the spark plugs
and distributor to reduce the intensity of surges and thus
reduce interference from these sources. The resistance
method of electrically connecting individual metal
of the suppressors is high enough to control the surges
sections to each other and to the frame or hull of the
but not high enough to affect the operation of the engine
vehicle. Such bonding is necessary to provide an easy
in any way. Some special-purpose spark plugs have a
path for grounding static charges.
Bonding is
built-in resistor (para 15-3).
accomplished by internal-external toothed lockwashers,
and by bond straps. The better the connection between
b. Capacitors. These are units of metal foils
metal parts, the greater is the effect in preventing
separated by paper insulation and protected by a metal
interfering waves from being thrown off to affect radio
case. The case is filled with an impregnating compound
reception.
to keep the moisture out. A wire connected to one side
of the capacitor is provided for connection to a circuit.
e. Shielding. This term is applied to the method of
The other side of the capacitor is connected internally to
covering all wiring carrying interfering voltages or surges
the case. Surges created in the wiring by sparks at the
with a metal shield (fig. 18-2). Woven metal conduit is
generator brushes, regulator, and gage contacts are not
used where flexibility is required, while solid conduit is
as strong as those produced by the high-tension ignition
used elsewhere. Units causing interference, such as the
circuit because the voltage is low, but they are strong
spark plugs, ignition coil, distributor, and regulator, ate
enough to cause interference in a radio set. Resistor-
enclosed in metal boxes. This shielding does not reduce
suppressors cannot be used in these circuits because
the intensity of the interfering surges, but prevents their
their resistance would reduce the low-voltage current too
radiation. While such shielding is effective in preventing
much. However, capacitors, because of their inherent
the radiation of interfering waves, filters and capacitors
capacitance, may be used to dissipate these surges.
are necessary to eliminate any interfering surges that
They are attached to the circuit as near as possible to the
would otherwise travel on the wires and affect the radio
point at which the spark occurs. The case of the
set through the power supply.
Such filters and
capacitor is mounted on the metal frame of the unit
condensers are enclosed in metal
causing interference,
18-3
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