Refers to a Sounds - is a travelling

wave which is an oscillation of pressure

transmitted through a solid, liquid, or
gas, composed of frequencies within
the range of hearing and of a level
sufficiently strong to be heard, or the
sensation stimulated in organs of
hearing by such vibrations.
For humans, hearing is normally limited to frequencies between about
12 Hz and 20,000 Hz (20 kHz)[2], although these limits are not
definite. The upper limit generally decreases with age. Other species
have a different range of hearing. For example, dogs can perceive
vibrations higher than 20 kHz. As a signal perceived by one of the
major senses, sound is used by many species for detecting danger,
navigation, predation, and communication. Earth's atmosphere, water,
and virtually any physical phenomenon, such as fire, rain, wind, surf,
or earthquake, produces (and is characterized by) its unique sounds.
Many species, such as frogs, birds, marine and terrestrial mammals,
have also developed special organs to produce sound. In some
species, these have evolved to produce song and speech. Human ear
Furthermore, humans have developed culture and technology (such
as music, telephone and radio) that allows them to generate, record,
transmit, and broadcast sound.
The mechanical vibrations that can
be interpreted as sound are able to
travel through all forms of matter:
gases, liquids, solids, and plasmas.
The matter that supports the sound
is called the medium. Sound cannot
travel through vacuum.
Sound is transmitted through gases, plasma, and liquids as
longitudinal waves, also called compression waves. Through
solids, however, it can be transmitted as both longitudinal
waves and transverse waves. Longitudinal sound waves are
waves of alternating pressure deviations from the equilibrium
pressure, causing local regions of compression and rarefaction,
while transverse waves (in solids) are waves of alternating
shear stress at right angle to the direction of propagation.
Matter in the medium is periodically displaced by a sound
wave, and thus oscillates. The energy carried by the sound Sinusoidal waves of various
wave converts back and forth between the potential energy of frequencies; the bottom
the extra compression (in case of longitudinal waves) or lateral waves have higher
frequencies than those
displacement strain (in case of transverse waves) of the matter above. The horizontal axis
and the kinetic energy of the oscillations of the medium. represents time.
 Probably all of us have seen a
transistor radio. And perhaps some of
us may wonder and ponder how this
“queen looking-thing” bring to our
living rooms the mellow, haunting and
enchanting musical sounds and songs
played for us by our favorite disc
jockeys (DJ) from for broadcasting
radio station.
 Do you know that radio programs travel
in air and in space, from far-away radio
station to our homes, at the tremendous
speed of 186,000 miles in one second? This
186,000 miles per second is approximately
300,000,000 meters in one second. That is
why at the moment our favorite announcers
move their lips at the radio station, we hear
their voices simultaneously in our transistor
radio. Their voices travel with the radio
signals at the tremendous speed of light.
 Specifically, the transistor radio that we
have at home is called radio receiver, and the
one used in the radio broadcasting station is
called radio transmitter. The work or function
of the radio transmitter is to send out or
transmit throught the air or space the radio
programs from the radio broadcasting or
transmitting station, to the radio receivers.
Basically, the radio transmitter is made
up of an oscillator circuit, An oscillator is an
electronic circuit that capable of producing or
generating high-frequency electric current.
 Because the radio transmitter
is made u of an oscillator circuit the
said radio transmitter is therefore
capable of generating or producing
high-frequency electric current. This
high-frequency electric current is
used to produce radio signals.
 When the radio transmitter is fed to or
coupled in the transmitting antenna, its high-
frequency electric current cause alternating
current (AC current), also of high-frequency, to
flow back and forth along the entire length of
the said antenna.
As alternating current (AC current)
undergoes cycles, polarities are established on
the opposite ends of the antenna. On one
instant one end of the antenna is positive and
the other end is negative. On then other instant
another end is negative and the other end is
positive, and vise versa.
 The frequency of the radio signal
produced by the oscillator of each radio
station differs. Each radio station is assigned
a specific frequency to transmit its radio
signal, by the National Telecommunication
Commission, (NTC), and this specific
frequency becomes the carrier frequency or
carrier signal of the particular radio station.
 Radio Station Carrier Signal
DZXL -----------------------------558 kHz. (kiloHerz)
DZBB -----------------------------594 kHz.
DZMM ---------------------------- 630 kHz.
DZRH ----------------------------- 666 kHz.
DZAS ----------------------------- 702 kHz.
DZSR ----------------------------- 738 kHz.
The entire broadcasting band for AM
broadcasting range from 535 up to 1605 kHz. AM
Broadcasting the carrier frequencies shall not be
lower than 535 kHz. And not higher than 1605 kHz.
The frequency allocation for all radio communication
service is allocated or assigned by the (FCC) Federal
Communication Commission.
Technically, the process of adding
information to the carrier signal is
called modulation. Modulation can
therefore be defined as the
process whereby the information
or intelligence is added to the
carrier signal.
For clarity, the audio signal is a low-frequency
signal. Its frequency ranges from 20Hertz (Hz.) to
20,000 Hz. At this range of frequency the audio
signal is normally detected and heard by a human
ear. For transmission or broadcasting purpose,
however, the audio signal can not be transmitted
alone because its frequency is low. The audio
signal would be lost or it could travel a very short
distance only. This is the reason why we have to
combine the audio signal with the radio frequency
or carrier signal to transmit it.
Audio Signal Carrier Signal

Amplitude Modulation Signal

AM – Amplitude Modulation. In layman’s language
amplitude means height of the waveform of the
signal, current or voltage.
FM – Frequency Modulation. This is a kind of
modulation where the frequency of the carrier
signal is varied, but the amplitude remains the
same or constant.
There are different ways of modulating the radio
frequency carrier signal with the audio signal, to
produce the amplitude modulated radio signal. One
way of doing this is by the use of the transistor.
The radio frequency carrier signal is fed or coupled
to the base of the transistor through the primary
and secondary windings of transformer T1. This
radio frequency carrier signal comes from the
oscillator circuit of the radio transmitter
Another kind of modulation is by coupling the radio frequency
carrier signal at the base of the transistor. After the radio
frequency carrier signal is amplified it is modulated by the audio
signal (modulating signal at the collector. After the radio
frequency carrier signal is modulated, the produced modulated
radio signal is coupled to the transmitter antenna and is
radiated away to far distances.
In this kind of transistor modulator circuit, the radio frequency
(rf) carrier signal is coupled or fed to the base of the transistor,
while the audio signal (modulating signal) is coupled at the
emitter. Modulation takes place in the transistor, after which the
modulated radio signal is coupled to the transmitting antenna.
 The modulated radio signal is also called by such names as, radio
wave, electromagnetic wave and radio signal for short.
When the radio signal is transmitted, it goes out in all directions,
and passes in different paths. Of the different paths, the most important
to consider are the radio signals that travel along the ground and toward
the sky.
THE GROUND WAVE- the radio signal that goes out from the
transmitter and travels along the ground.
In AM (Amplitude Modulation) transmission, the radio signal such
as the ground wave is effective up to 200 miles. In other words, from the
transmitter stations up to a distance of 200 miles away the ground wave
of the AM radio transmitter can be received loud and clear, normally.
This of course depends upon the power rating of the radio
transmitter. In other words the radio transmitter with higher power rating
gives loud and clear transmitted radio signals even in far distances while
the transmitter with lower power rating gives weaker radio signals.
 In AM transmission the transmitted radio signal such as ground
wave are effective only up to the 200 mile-distance. Beyond 200 miles, the
ground wave is too weak for reception. This is because some of the radio
signals, such as the ground wave, are absorbed as they travel along the
ground. Another reason that may weaken the ground waves are the
obstructions along the path on which the said radio signals travel.
Example of the these obstructions are the tall buildings, trees, hills and
mountains.
 The radio signal that goes out from the radio transmitter antenna
and travels towards the sky. This is effective for along distance
transmissions.
when the radio signal, such as the sky wave, leaves the antenna it
goes out into space. A part of this sky wave continues to flow and is lost in
space. However, a bigger part of this sky wave is reflected back to earth
by the ionosphere. This reflected sky wave is what our radio receivers
receive even many hundred miles away from the transmitting station.
 Is made up of several layers of negative and positive ions. It is
located approximately 50 miles above the earth, and extends 300 miles
upward.
Negative ions- are atoms that have gained one or more extra
electrons. Positive ions are atoms that have lost one or mor of its
electrons. These negative and positive ions are formed by the heat
radiation from the sun. so the sun has an important role in shifting the
layers of the ionosphere.
 Theoretically, during daytime the negative ions are more
concentrated at the upper layer of the ionosphere. In other words the
concentration of the negative ions is at the upper layer of the ionosphere
at daytime. This concentration of negative ions becomes thinner towards
the lower layers. At the second layer of the ionosphere, the layer of the
negative ions is very thin. The first layer of the ionosphere is concentrated
with positive ions, during daytime. We must make it clear at this point
that the formation of the positive ions on the first layer, and the gradual
concentrations of the negative ions on the second, third and fourth layers
of the ionosphere is due entirely to the influence of the heat energy of the
sun.
 Theoretically, during daytime the negative ions are more concentrated at
the upper layer of the ionosphere. In other words the concentration of the negative
ions is at the upper layer of the ionosphere at daytime. This concentration of
negative ions becomes thinner towards the lower layers. At the second layer of the
ionosphere, the layer of the negative ions is very thin. The first layer of the
ionosphere is concentrated with positive ions, during daytime. We must make it clear
at this point that the formation of the positive ions on the first layer, and the gradual
concentrations of the negative ions on the second, third and fourth layers of the
ionosphere is due entirely to the influence of the heat energy of the sun.
At daytime when AM radio signal is transmitted, the sky wave upon
reaching the first layer of the ionosphere is absorbed mostly by this layer. A portion
of this sky wave passes through the said first layer but is reflected back to earth by
the second, third and fourth layers. This reflected sky wave is received by the radio
receivers many hundred miles away from radio transmitting antenna station.
However, because this reflected sky wave is only a very small part of the sky wave
absorbed by the first layer, the said reflected sky wave received by the radio receiver
is very weak and noisy. This is during the day.
 At night the composition of the layers of the ionosphere changes
because of the absence of the sun or heat energy. The first layer
disappears, the second, third and fourth layers merge into one and the
concentration of negative ions is very high.
With this condition of the ionosphere, the whole of the sky wave is
reflected back to earth at night. This is the reason that AM radio signals
are received loud and clear at night even in far places.
We must add that FM radio signals can not be deflected back to
earth by the ionosphere. This is because the frequency of the FM radio
signal is very high, and high frequency signal penetrate the whole layers
of the ionosphere easily and continue flowing into space.
The FM radio broadcasting band is 88 megahertz (MHz.) to 108
MHz. while the AM broadcasting band is 535 kHz. To 1605 kHz. Mega
means million, while kilo means thousand.
 We learned that the audio frequency signal (AF signal)
and the radio frequency carrier signal (RF signal) are combine by
the AM radio transmitter in a process called amplitude modulated
radio signal or AM radio signal.
The Radio Receiver. When we listen to our transistor radio
receiver at home, we listen to the audio frequency signal, not to
the radio frequency carrier signal.
 In the early day of radio, variety of radio receiver circuit
were develop. First of these is the crystal or cat-whisker radio
receiver. Then came the one tube radio receiver and the tuned-
radio-frequency radio receiver (TRF radio receiver). These kind of
radio receivers are no longer used today.
Now we are using the transistor radio receiver. It is
composed of the following:
1. A tuning Circuit
2. A detector
3. An audio amplifier
 The tuning circuit is made up of the ferrite antenna coil
and the tuning capacitor. Its function is to receive the radio signal
that comes from the radio transmitting station.
The detector is the crystal diode. Its work is to detect and
receive the original audio signal that come from the radio
broadcasting or transmitting station. In simple word, the function
of the detector diode is to separate the audio signal from the
radio carrier signal.
The transistor work as the audio amplifier. It amplifies or
strengthens the audio signal that is separated from the radio
carrier signal by the detector.
 Forward bias voltage is applied at the emitter of the
transistor by the positive post of the batteries. Another forward
bias voltage is applied to the base of the negative post of the
batteries via the base bias resistor. At the collector reversed bias
voltage is applied by the negative post of the batteries through
the 2 kilo ohms earphone.
 Current-flow in the transistor is different from the bias voltage
applied. Bias voltage is applied to the transistor to make the said
transistor work or function, while current is the amount of electrons
(negative current carriers) and holes (positive current carriers) that flow
in the transistor when required bias voltage are applied. Bias voltage
does not flow, while current flows.
In transistor, as well as other semiconductor devices, the flow of
hole current is confined inside the transistor only, but the electron
current flows inside and outside the transistor as well as in its circuit
connections. Because it is the flow of electron current that we consider
when repairing transistor radios.
 Electron current flows from the negative post of the batteries. A
bigger part (95%) of this electron current, or current for short, flow in
the collector. A small portion of this current (5%) flows in the base.
These two current add or combine inside the transistor and flow out at
the emitter and return to the positive post of the batteries.
We must emphasize at this point that circuit connecting the
batteries and the transistor is very important to consider.
For example, if a loose connection or disconnection develops, the
necessary bias voltages cannot reach the transistor. As a consequence
no current will flow in the transistor and surely no radio signal be heard.
 Electron current flows in PNP (Positive Negative Positive)
transistor is opposite the direction of the flow in the NPN (Negative
Positive Negative) transistor.
 There are three methods of coupling used to add another
transistor to provide additional stage of amplification. These method are:

1. Direct coupling

2. RC (Resistance-Capacitance) coupling

3. Transformer Coupling
 In this kind of circuit connection, the elements of the two
transistors are directly connected. Signal from one transistor is directly
coupled to the next transistor.
 The collector of the first transistor is connected to the base of
the second transistor trough the capacitor.
Signal is coupled to the second transistor from the first
transistor by means of the coupling capacitor C2.
We will assume that the coupling capacitor C2 is open or
disconnected. In this case the signal from the first transistor cannot be
coupled to the second transistor. The defect or trouble symptom of our
two transistor radio receiver then will be no sound output at the
earphone.
 On the other hand, if the coupling capacitor C2 is shorted the
deflect will also be no sound output. The reason for this is that with
shorted coupling capacitor C2 the DC voltage at the collector of the first
transistor will enter the base of the second transistor. This DC voltage
will apply reversed bias to the base of the said second transistor and
cause it not to conduct current. Without current flowing at the second
transistor no sound output is produce at the earphone.
 In this kind of circuit, signal from the first transistor is coupled
to the next transistor by means of the transformer.
The transformer does not function as a coupled device only but
also as path for the bias voltage of the transistors. We can see that the
DC bias voltage for the collector of the first transistor passes trough the
primary winding of the coupling transformer. On the other hand the
forward DC bias voltage for the base of the second transistor is applied
through the second winding of the said coupling transformer.
 The Super heterodyne radio receiver is composed of six (6)
section. These section are;
1. The tuner section
2. The IF (Intermediate Frequency) section
3. The detector section
4. The audio Amplifier Section
5. The reproduction or speaker section
6. The power supply section
Each of these section has its specific function and each
contributes its share in the collective effort to make the whole radio
receiver function efficiently.
If one section becomes defective and stops functioning the
whole radio receiver will be badly or adversely affected. It could be that
its sound output will be very weak, noisy, distorted or it can also be that
the radio receiver has no sound output at all.
CONVERTER STAGE
IF AMPLIFIER STAGE
DETECTOR STAGE
VOLTAGE AMPLIFIER STAGE
DRIVER TRANSISTOR STAGE
POWER AUDIO OUTPUT
AMPLIFIER STAGE
 Servicing FM tuner is no different from servicing AM
tuner except for few considerations, Understanding
fundamental block diagram of combined AM/FM tuner can
help locate a defective stage easily. It is important to
identity the different stages and their functions so that
check could be done right at the suspected stage.
 There are two sets of intermediate frequency (IF)
transformer used in the combination tuner, one for AM
frequency and the other for FM. The frequencies in which
they tuned are far apart: 450 kHz for AM and 110.8 MHz
for FM. In the IF amplifier stage, a single transistor
amplifies both AM and FM signals. The IF transformers are
connected in series to the transistor. The numbers of IF
transformers depend on how many amplifier stages are in
the tuner. AM and FM signals not frequency of FM
transformer is very high while AM signal passes the IF
transformer as low pass filter. FM signals pass the AM IF
transformers as high pass filters.
 Similar, there are two set of local oscillators. Each
oscillator works independently from the other, each
having a transistor oscillator of its own. Likewise, two
tuning capacitors ganged together handle AM and FM
signals. A ferrite bar with an antenna coil is used to pick
up AM signals whereas an aerial or whip antenna pick up
FM signals.
There are two different types of detector in the
combination tuner. A single diode detects AM signal and
the two diode facing opposite directions detect FM signals.
Separate AM and FM output signals are pick up after
passing the detectors.
 Trouble Symptoms
A defective stage in AM/FM tuner can be located
through its symptom. The most common trouble
symptoms are as follows;
1. No reception both in AM and FM
2. Good FM reception but no AM or vice-versa
3. Poor sensitivity or weak reception
4. Twit-twits and whistle
5. Distorted sound with hum.
 1. No Reception both AM and FM

No reception both on AM and FM is probably caused by the failure of an IF

amplifier. Both AM and FM signals pass the IF amplifiers. Thus, a defective IF stage
breaks the signal path. A defective IF, either AM and FM, cause both signals to stop
because the signals pass the IF transformers in series. Similarly, malfunctioning IF
amplifier transistor cuts off both signals resulting in this symptom. A low supply
voltage or its complete loss will bring about the same symptom of no reception in both
frequencies. The AGC does not constitute a signal path. It controls the signal strength
at the IF amplifiers. If defective, it can partly or totally block out both receptions. A
shorted signal or zener diode in the circuit possibly stops the tuner from working. The
zener diode is used to regulate the voltage in some points of the supply line in the
circuit. Summarizing the possible cause of no reception both on AM and FM are;
a. Defective IF transformer
b. Defective IF amplifier transistor
c. Low or zero supply voltage
d. Defective AGC
e. Shorted signal diode
f. Shorted zener diode.
 2. Good FM reception but no AM or
vice-versa
The trouble symptom of good FM reception but no
AM or vice-versa may be traced to their corresponding
defectors as well as to their respective converters or
mixers/oscillators. Defective RF amplifiers preceding the
oscillators are likewise good suspects. Often, the selector
can be the cause of failure of one if not both receptions.
Knowledge about AM tuners is helpful in pinpointing the
defective components.
 3. Poor Sensitivity or Weak
Reception
Poor sensitivity of both AM and FM signals is caused
by the trouble in the AGC. Other causes are misaligned IF
transformers and low supply voltage. Touch – up IF
alignment and check AGC filter capacitors. Check by-pass
capacitors. A defective aerial can cause poor sensitivity of
tuner. Touch-up the sensitivity coil adjustment.
 4. Twit – twits and Whistle
Twit – twits and whistle are caused by misaligned
IF transformers and defective AGC. Touch –up alignment
and check AGC filter capacitors. Check also AFC circuit. A
defective or detuned IF trans former is indicated by
whistle and howling when it is tuned to a radio station.
Replace defective or detuned IF transformers.
 5. Distorted Sound with Hum
Poor filtering of the supply to the tuner can cause
distorted sound with hum, check electrolytic capacitors in
the supply line of the tuner.