Radio And Tape Diagnosis; Servicing - GMC 1976 ZEO 6083 Maintenance Manual

The FM section of the AM-FM and AM-FM
Stereo radios is equipped with automatic frequency
control, which aids tuning to a station. The FM re
ceiver should be tuned directly on station frequency
for minimum noise interference, however, the auto-
matic frequency control will tune directly to and lock
on station frequency when slight mistuning is en-
countered.
Another feature of FM tuning is signal separa-
tion. When two FM stations are close in frequency,
the FM tuner selects the stronger signal, rejecting the
weaker one. This is in contrast to AM performance,
where it is not always possible to separate two sta-
tions.
The AM-FM radio incorporates an AM receiv-
ing circuit and an FM receiving circuit. The audio
system is common to both receiving circuits .
On stereo radios, an indicator light on the right
side of the dial, will indicate when the FM station
tuned is capable of transmitting stereo programs .
With the AM/FM stereo radio and eight-track
tape player, the operator may insert a tape cartridge
into the opening behind the face of the receiver dial .
When the dial face swings up, this automatically
removes power from the radio and switches control
of the speakers to the tape player . To change tape
programs, momentarily depress the volume control
knob. To eject the tape cartridge from the engaged
position, depress the "EJECT" pushbutton . A car-
tridge should not be left inserted in the tape player
due to possible tape damage.
SPEAKER CONTROL
The right radio knob is used to tune stations
manually. The ring around the knob is the speaker
control. When the ring is turned all the way counter
clockwise, 90% of the volume will be shifted to the
front speakers. Turning the ring progressively clock-
wise will shift volume to the rear. When the radio is
tuned to an FM-Stereo program, stereo separation
will be from side to side.
PUSH BUTTON TUNING
Each one of the five push buttons may be preset
to a favorite AM station when on the AM band.
Each one of these same five push buttons may be set
to a favorite FM or stereo station, giving a total of
10 preset favorite stations . To preset push buttons,
proceed as follows:
Select the desired band (AM or FM) and manu-
ally tune a favorite station until maximum signal is
received . Pull push button out and then push all the
way in to relatch. Ten favorite stations (5-AM and
5-FM) can be obtained in this manner.
CHASSIS ELECTRICAL
12- 3 1
CAUTION :
Do not move the AM--FM slide
bar band selector while any push button is
pulled out. Damage to the tuner mechanism
may result.
RADIO NOISE SUPPRESSORS
Various types of ignition suppressors are used to
prevent spark noise from interfering with radio
reception.
1 . Ignition noise is suppressed by use of resistance
core ignition cables . The resistance of these cables is
2000 to 6000 ohms-per foot.
2. Make sure resistance spark plugs are being
used to minimize ignition noise.
3 . It is particularly important that the terminals
in the ignition secondary cables make good mechani-
cal contact with the spark plug terminals and dis
tributor cap terminals. A loose connection at these
points will result in excessive ignition noise, seriously
reducing FM performance .
4. A capacitor mounted on the outside of the
ignition coil may be checked by running the engine
at medium speed and then quickly turning the igni
tion switch to the Accessory position . If the noise is
eliminated, while the engine is coasting to a stop,
replace faulty coil capacitor.
5. A capacitor is mounted on the blower motor
assembly for suppressing radio noise at high blower
speeds . If a whine is eliminated when the blower is
switched from HI to LO, this capacitor should be
replaced.
RADIO AND TAPE DIAGNOSIS
Many conditions that affect radio operation may
be corrected without removing set from vehicle. If
the checks in Figures 46 and 47 are performed and
problem cannot be found, the radio should be
removed and repaired at an authorized radio service
station.
CAUTION :
Do not turn on radio with any
speaker disconnected, as the radio transistor
may be permanently damaged.
SERVICING
ANTENNA TRIMMER ADJUSTMENT
NOTE :
If antenna is not trimmed, the set will
have weak and fading AM reception. Antenna
trimming should always be performed after any
radio or antenna repair work.