Radio Interference Diagnosis - Jeep 1979 CJ-5 Technical & Service Manual

3V-6 RADIO SOUND SYSTEMS
(1) Always fine-tune radio manually for clearest
sound and minimum noise.
(2) Weak FM stereo signals are inherently noisier
than monaural ones when received on an FM stereo
radio. To prevent this type of noise from being heard,
the FM Stereo radio automatically switches from stereo
to the monaural mode. The Stereo-Indicator light goes
out, both speakers still operate, but without the stereo
effect. When the signal strength increases to a noise-free
level, the receiver switches back to the stereo mode. This
action is automatic and requires no adjustment.
(3) Occasionally, conditions will be such that noise-
free reception simply cannot be attained. If this occurs,
set the tone control to the bass (counterclockwise) posi-
tion to reduce the noise level. When out of the noisy
area, set the tone control back to its normal position.
CB Radio Reception
The range of CB broadcasts and reception depends on
weather conditions, amount of air traffic, strength and
condition of transmitter and receiver, and the adjust-
ment of antenna and splitter box. Range also depends on
the movement between the transmitter and receiver.
Reception will be clearer in open spaces with little air
traffic than in congested metropolitan areas. Hilly ter-
rain may also affect operation. Electrical interference
affects reception similar to AM broadcasts.
Reception should not be fuzzy or screechy unless the
network is congested.
R A D I O INTERFERENCE DIAONOSIS
The object of this diagnosis is to present a systematic
approach to troubleshooting interference (noise) prob-
lems. First, determine if the noise is normal by referring
to Radio Reception Characteristics. If the noise is abnor-
mal, the following procedures outline methods of deter-
mining interference point of entry and elimination.
There are two major ways interference enters the
radio—the antenna and the A-line or power feed wire to
the radio.
12 INCHES
CLIPS
A J 4 2 2 4 3
Fig. 3V-11 Improvised Noise Suppression Tool
Lay the screen across the top of the dash and attach
the clips to body grounds. If the noise is diminished or
disappears, the noise is being radiated up through the
dash.
To determine exactly where the noise source is, a
useful noise probe can be improvised from an antenna
lead-in cable.
To make the probe, cut or remove the lead-in from the
antenna at the antenna, remove approximately 2 inches
of the outer plastic covering and the woven wire shield
(fig. 3V-12).
COAXIAL
SHIELD '
CENTER
CONDUCTOR WIRE
WHITE
PLASTIC
INSULATOR
L
ANTENNA LEAD IN
• WITH ANTENNA BASE
CONNECTOR REMOVED
CUT 2-INCHES
OF COAXIAL "*
SHIELD
A J 4 2 2 4 4
FI§.3V-12 Noise Protii Filrloitloi
Interference Entry—Antenna
Disconnect the antenna. If this causes the noise to
stop, the problem is reduced to three possibilities:
• A defective antenna; refer to Radio Antenna Ohm-
meter Tests
• Noise radiated upward from the dash
• Noise radiated from the engine compartment
Noise Radiated Upward from Dash
This noise can be determined by fabricating a tool out
of a piece of aluminum or copper screen approximately
36 inches by 12 inches (fig. 3V-11).
(1) Disconnect original antenna lead-in and plug in
noise probe.
(2) Turn radio on and use probe to discover hotspot
or source of noise. Do not touch the end of the probe with
your hand as this would give an incorrect indication. As
the probe comes closer to the noise source, the loudness
of the noise will increase.
(3) If the source is found to be a switch, connect a
0.5-mfd capacitor from the power feed side of the switch
to a good chassis ground.
(4) Gauges and sender units generally can be si-
lenced by installing 0.5-mfd capacitors at their termi-
nals. Install a 0.5-mfd capacitor at the battery terminal