Vanagon Syncro AWD F4-2109cc 2.1L (MV) (1989)
Technical Service Bulletin # 919402
Date: 940801
Radio - Diagnosis Instructions
GROUP: 91
NUMBER: 94-02
DATE: Aug 1, 1994
SUBJECT:
Radio Diagnosis Instructions
MODEL(s):
All m.y. All
Car Radios
NOTE
Additional information can be obtained in the Operating instructions for car radios.
CAR RADIO RECEPTION BASICS
The reception conditions for a car radio are constantly changing. At one point, reception is outstanding but even at a small distance from this point, it can
be very poor. Often, even opening or closing a car door can have a considerable effect on reception and thus the quality of reproduction.
Radio Wave - Basics
RADIO WAVES
Radio waves move at the speed of light 186,000 mi./sec. (300,000 km/s). Depending upon the wave-length, these waves are affected to differing degrees
by the weather, mountains, buildings, tunnels, trees and other obstacles.
AM RADIO WAVES
AM = amplitude modulation (changing the amplitude of the transmitter frequency as a function of the transmitted speech and music).
This method is used in the long wave, medium and short wave bands
AM radio waves have relatively low frequencies (approx. 150 kHz to approx. 6 MHz). If certain conditions are met, the path of these waves follows the
curvature of the earth and the waves are reflected by various layers of the atmosphere.
At these frequencies, the range of a transmitter is considerably greater than is the case for FM radio waves (particularly during the evening and night).
FM RADIO WAVES
FM = frequency modulation (changing the frequency of the transmitter signal as a function of the transmitted speech and music). This method is used in
the very high frequency (VHF) band.
FM radio waves have much higher frequencies (approx. 87.5 MHz to approx. 104 MHz). They follow along straight lines, do not follow the curvature of
the earth and are not reflected by various layers of the atmosphere. In addition to the dependence on the wavelength of the signal, the signal strength
received is affected by the height of the transmitting antenna, the transmitted power and the conditions between the transmitter and receiver.
Reception is best when the car antenna can see: the transmitting antenna. However, this is almost never the case. Many times, the signal is reflected by
stationary objects, subjected to interference and only then received by the vehicles antenna. In this manner, the signal is delayed and weakened.
SUSCEPTIBILITY TO INTERFERENCE OF AM AND FM RADIO WAVES
Since the music and speech information are contained as changes in amplitude of the transmission frequency in the entire transmitter signal and since
interference pulses mainly affect the amplitude of a transmitter signal, the susceptibility to interference of AM radio waves is considerably greater than
that of FM radio waves.
FADING
Fading is a typical AM phenomenon characterized by irregular fluctuations in volume in conjunction with distortion. This phenomenon occurs if the
distance between the transmitter and receiver is very large and if the vehicles antenna has not been completely extended. Fading is compensated to a
large degree by the automatic fader control provided in all radios.