Haynes Retro Radio Instruction Manual

Build Your Own
FM RADIO
Instructions
Introduction
Electronics as a hobby can provide great pleasure and this
is especially true when building a radio. Building your own
radio and then using it on a regular basis is very rewarding
and provides a real sense of achievement.
Once the FM radio has been built, you’ll be able to listen to
your local FM channels with good quality audio. Firstly,
we’ll undertake a little experimentation, studying the
functions of the individual parts and gradually we’ll build
the complete circuit.
The FM radio is easy to build but nevertheless provides lots
of possibilities. There’s a wide range of variants and
options. Experiment with different length antennae to
receive close or distant stations. Ultimately, you’ll have
several possible circuits at your disposal. You must decide
what your own radio circuit will look like.
Happy building!
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Contents
1.
The Loudspeaker
5
2.
The Plug-in board
7
3.
A Switch Contact
8
4.
The Electrolytic Capacitor
10
5.
The Amplifier
11
6.
A Coupling Capacitor
13
7.
Tone Generator
15
8.
FM Receiver
16
9.
Improved Sound
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10. Tuning
20
11. Reset Button
21
12. Volume Control
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13. Potentiometer Tuning
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14. Restricted Tuning Range
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15. Fine Tuning
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16. Explanation of the Radio Circuit Board 31
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1. The Loudspeaker
Be careful with the loudspeaker - it’s a precision assembly and
one of the most important parts of a radio. Located on the front
of the loudspeaker is the diaphragm. If you gently tap it with your
finger it will make a noise. This is how the loudspeaker works any movement of the diaphragm will produce sound.
In the kit you’ll find some more components. Find the 1kΩ
resistor - it has coloured rings, brown (1), black (0) and red (00),
which indicate 1,000 Ohm. A fourth, golden ring signifies 5%
tolerance. Resistors are often used to limit the current. In this
case, the resistor should be connected from the loudspeaker to
the battery ensuring that the current that flows through the
loudspeaker is limited to approx. 9mA. Connect these
components to form a continuous circuit. On connecting the
battery, you will hear a soft crackling from the loudspeaker. A
sound also occurs when opening the electrical circuit. The current
that flows through the loudspeaker, creates small movements of
the diaphragm, which creates a sharp sound. On the underside
(of the loudspeaker) you will find a strong magnet, hidden inside
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is a wire coil, which is connected to the contacts by soldered
cables. This causes the diaphragm to move with an electric
current.
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2. The Plug-in Board
Building a complicated circuit is made much simpler with this
plug-in board. The patch panel with its 270 contacts in a 2.54mm
grid provides reliable connection of the components. For the
initial tests, the plug-in board can be used on the bench. Later, it
will be secured into the radio housing.
In the centre of the patch panel there are 230 contacts that are
each connected in vertical groups containing 5 contacts. In
addition, there are 40 contacts on the edge for the power supply,
consisting of two horizontal groups containing 20 contacts. This
allows the patch panel to support two independent power supply
lines.
Inserting the components requires quite a bit of force, therefore
the connecting wires can easily break. It is important that the
wires are inserted from above. It is best to use a pair of tweezers
or small pliers. It is important the wire should gripped just above
the plug-in board and pressed down vertically. Be careful with the
connection wires, make sure the tinned ends of the connection
wires, the battery clips and the loudspeaker are inserted without
bending them.
Build the simple circuit with the resistor and loudspeaker again
on the plug-in board. When attaching the battery clips you will
hear the crackling sound again coming from the loudspeaker.
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3. A Switch Contact
The construction kit contains two coils of up hook-up wire, each
one metre long. One of the wires is used whole as the antenna,
strip 5mm of insulation from each end of this wire. A good way
to strip the ends of the wire is to cut around the insulator with a
sharp knife. Take care not to score the wire conductor as it can
then easily break.
With the other piece of wire you can make a simple switch. It
consists of two pieces of bare wire positioned so they can easily
be made to touch. Cut the pieces of wire 2 cm long and remove
all of the insulation. With your finger you can push together both
bare wires, creating a closed circuit. Every time the switch is
operated you will hear a soft crackling from the loudspeaker.
Another short piece of wire can be used as an integrated cable
clamp in order to keep the wires together. The battery clip should
always be connected (and not used as a switch), so that the
connections do not become excessively worn.
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9
4. The Electrolytic Capacitor
A loud noise can be generated using the 100μF (microfarad)
electrolytic capacitor (Elko). Note the polarity when building the
unit. The negative terminal is identified by a white stripe and has
the shorter pin. A capacitor contains two metal sheets that are
insulated from each other these can be charged up and therefore
the capacitor can store electrical energy.
In this experiment the electrolytic capacitor charges up to a
voltage of about 9 V. It stores so much energy that when you
close the switch there is a loud crack. For an instant, a large
amount of power goes through the loudspeaker; approximately
100 times more than through the resistor.
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5. The Amplifier
The LM386, eight leaded integrated circuit (IC), is a complete
loudspeaker amplifier for battery operation. Inside it there are
lots of transistors and resistors. In order to fit the IC into the plugin board, the eight leads of the integrated circuit must be pulled
apart and aligned parallel. The correct orientation of the IC is
important, a mark on the left side identifies Pin 1 and Pin 8. If you
want to remove the IC, you should cautiously lift it out with a
screwdriver, ensuring the connection pins do not snap.
Pin 4 of the IC is connected to the negative terminal of the
battery, the positive terminal is connected to Pin 6. Pin 5 is the
output. The loudspeaker is connected here via an electrolytic
capacitor. The voltage on Pin 5 of the LM386 is the middle output
voltage swing approx. 4 V. Therefore the positive terminal of the
electrolytic capacitor must be connected to the IC, whilst the
negative terminal with a white stripe is connected to the
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loudspeaker. The input is situated on Pin 2 of the IC. Connect a
piece of wire to pin 2. When you touch the free end of the wire
you will hear faint noises such as hum or buzzing from the
loudspeaker. Emissions from electrical leads and devices in the
room and are picked up by your body, as a sort of antenna, which
are amplified and made audible. This simple buzzing test is
helpful for checking the amplifier and can be used later once the
radio is built, e.g. to test for any faults.
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6. A Coupling Capacitor
Capacitors are often used for passing audio frequency signals.
Here, a ceramic disc capacitor, with a capacitance of 100nF is
used. The text 104 stands for 100,000pF (picofarad). This
capacitance amounts to just one thousandth of the 100μF
electrolytic capacitor. A 100nF capacitor is ideally suited as an
input coupling capacitor for the amplifier.
With the finger test, you hear the same sounds as in the previous
test. The audio signals therefore remain unchanged. This
capacitor is used to separate the DC voltage from the audio AC
voltage later in the radio circuit. In fact, the LM386 has two
inputs on Pin 2 (inverting input) and Pin 3 (non-inverting input).
For this experiment, both inputs have the same effect.
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7. Tone Generator
This circuit uses a 10kΩ resistor (brown, black, orange) in order to
make an oscillator out of the amplifier. In order for oscillations to
occur, the non-inverting input on Pin 3 of LM386 should be
connected to the output via a capacitor and a resistor. This
feedback leads to oscillations in the amplifier, which can be heard
via the loudspeaker.
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8. FM Receiver
The receiver board, comprising the FM integrated circuit
TDA7088 is the heart of the FM radio. As well as the IC, there are
a lot of small capacitors, tuning diode, and two printed coils on
the circuit board. For the first experiment, only three connections
are required. The stabilised operating voltage of 3V is supplied via
GND (-) and BAT (+). The audio signal (from the radio) is on the
NF Output. Two resistors provide the correct input voltage for the
power amplifier. The newly added 1kΩ resistor is marked brown,
black and red.
Note: The receiver board must not be connected directly to 9V
but requires an operating voltage of 3V. A three pin voltage
regulator, type 7530, is used to provide the required 3V. The
positive battery voltage is connected to the input (middle pin),
the negative battery voltage is connected to the ground pin
(left).There will be a stable 3V on the output (right pin). Ensure
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the orientation of the voltage regulator is correct, the flat printed
face towards the middle of the board.
Even though the radio is far from finished, with luck you should
be able to hear a radio station. The wires on the circuit board
function as small antennas. As the connections for the tuner are
not yet in place, the receiving frequency is random. By briefly
touching the contacts, “+”, “S”, “R” and “–” on the receiver board
you will be able to switch through to other stations.
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9. Improved Sound
Another 100μF electrolytic capacitor is connected in parallel with
the battery, again it is important to connect the capacitor the
correct way round. The negative terminal on the electrolytic
capacitor is marked by the white bar. In particular, a run-down
battery can lead to distortion, this can be avoided with the
additional capacitor. In addition, a wire antenna can be
connected to port A. Use a 20 cm long piece of wire. A short
antenna of this length delivers improved audible reception. Poor
reception can be improved by holding your hand near the
antenna.
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10. Tuning
The radio integrated circuit has a scan input (S) for starting the
scan for stations. The push button switch is connected between
the BAT connection (on the receiver board) and the S input. Build
a push button switch from wire that connects the S input to the
BAT connection. You are now a step further towards a usable
tuner. A quick press on the push button causes the tuner to
search for the next highest frequency station. There is not
currently a way to go back to the last station. You can, however
remove the battery for a few seconds to receive a station on a
lower frequency and start a new search.
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11. Reset Button
Another switch connected to the receiver reset input R. Pressing
on the reset button will reset the receiver frequency to the
lowest in the FM range. With the scan button you can then start a
new search again.
There is a varactor diode on the receiver circuit board, the
capacitance (of the diode) changes dependant on the applied DC
voltage. The smaller the capacitance the higher the frequency.
The receiver board, has on the R input a connection to the
varactor diode. Operation of the reset button causes a
connection to the BAT input resulting in zero voltage on the
diode therefore the frequency becomes very low. It sets the
lowest frequency of just below 87.5MHz. Actually, you will find an
additional 100nFcapacitor receiver board that holds the current
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tuning voltage. This capacitor is discharged by the reset button. A
new search is started each time the scan button is pressed. A
larger DC voltage between the BAT and the R input increases the
frequency. The tuning voltage changes until a new station is
found. The Automatic Frequency Control (AFC) ensures that the
frequency is maintained in the event of any deviations.
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12. Volume Control
In the construction kit you will find two potentiometers. One is
for the volume control and is fitted with a switch contact for
turning the radio off. The second potentiometer is for the tuner.
Insert both potentiometers and the loudspeaker into the radio
housing. Stick the plug-in board between the potentiometers and
the loud speaker. To facilitate this, the plug-in board is covered
with an adhesive film. Identify the best position for the plug-in
board. Remove the protective film and stick the plug-in board in
place. Note: The position must be correct from the start, as it is
very difficult to correct it. Whilst the major components for the
previous experiments have their set position, all other
components can be arranged to provide the shortest connection
to the controls.
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Connect the volume control in place of the previous voltage
divider from two resistors. Connect the potentiometer switch in
the negative lead of the battery allowing a very short connection.
After rebuilding the wiring, the radio should function as before. In
addition, you can now also adjust the volume and turn the radio
on and off. The maximum volume depends on the battery
condition. A run down battery only allows a medium volume
level. If you turn the volume control too far, you will hear
distortion and background noise.
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13. Potentiometer Tuning
Once the Potentiometer is connected you can close up the Radio
and operate it from the outside. The advantage of the
potentiometer tuning compared to the tuner with two buttons is
that you can search for a station in both directions. The
previously tuned station is still tuned when the radio is turned on
again. The tuning voltage is set by the potentiometer and is
connected to the reset contact via a 1MΩ resistor (Brown, black,
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green) and therefore appears on the varactor diode. When the
Potentiometer wiper is at +3V, the frequency is a minimum as the
reverse voltage across the varactor diode is 0V and the
capacitance a maximum. With this simple circuit, the tuning
range is still too large. An improvement follows in the next
experiment.
The 1MΩ resistor in the circuit, allows the automatic frequency
control (AFC) to have a large influence on the tuning. Therefore a
poorly tuned radio is automatically accurately tuned. This is
noticeable when slowly tuning across the FM range, you will
notice a large capture range which contains one station. Hence
station selection is simplified.
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14. Restricted Tuning Range
So far, the tuning range has been too large, resulting in the useful
FM range occupying only a small range of the scale. With two
resistors, the range can be narrowed to around 87.5 MHz to
108MHz. The broadcasting sector occupies almost the entire
scale. The 1kΩ resistor determines the lower frequency and the
10kΩ the upper. In fact, you will find some variability with tuning
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range, so that you can experiment with the band limits by
selecting slightly different resistors. Use for example the 15kΩ
resistor (brown, green, orange) to reduce the upper frequency or
replace the 1kΩ resistor with a wire link to reach lower
frequencies.
Another way of improving the radio is to use a longer aerial. For
this purpose, use a loop antenna made from 1 metre long wire.
The end of the wire loop should be guided through the preprepared holes of the housing. The loop can be installed either
upright or lying flat and will have a small amount of directivity, so
that by twisting the antenna when on a weak signal station, a
better reception may be achieved.
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15. Fine Tuning
The last change of the circuit uses a 220kΩ resistor (red, red,
yellow) instead of the previously used 1MΩ resistor and so the
capture range of the tuner is reduced. This does have an
advantage when stations are very close to each other. Especially
as, different signal strengths can sometimes result in weak
stations being skipped. You can now adjust the frequency more
accurately. This feature is most useful for reception of distant
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stations. It is up to you whether you prefer a radio with a 1MΩ
resistor and a larger capture range. Test the radio again outdoors,
preferably on elevated ground with an unrestricted view. You
should then receive distant stations. For even better reception,
use a dipole antenna with two 75cm long wires or a loop dipole
with a wire length of 3 metres in total. The connection of a roof
antenna would also be useful. Try connecting a large
loudspeaker, which will allow the radio will deliver a much better
sound.
The finished radio has a stand-by power consumption of approx.
10mA and for high volume requires up to 50mA. A 9V alkaline
battery with a typical 500mAh capacity should supply up to 50
hours of power on low volume. Alternatively the radio can also
operate from 6 V and last four times longer with four AAA
batteries.
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16. Explanation of the Radio Circuit Board
Standard FM superhetrodyne receivers utilise an intermediate
frequency of around 10.7 MHz. The receiving frequency is initially
converted to the intermediate frequency and then filtered,
amplified and demodulated. Also, this FM radio is a superhet that
can convert its received signal to an intermediate frequency.
However the intermediate frequency is considerably lower
approx. 70 kHz. As a result, the intermediate frequency filter is
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implemented without inductors. The FM demodulator is simpler
and is less susceptible to distortion. All essential elements fit into
a single 16 pin SMD-IC, the TDA7088. Rather than a variable
capacitor, like in older receivers, the radio uses a varactor diode,
D1. The larger the voltage on the diode, the lower its capacitance
and as a result, the receiving frequency is higher. Most of the
capacitors on the radio module belong to the IF amplifier and
band pass filter. C1 and C3 are part of the input circuit and
antenna matching. The varactor diode D1 adjusts the oscillator
frequency. The tuning voltage is applied across C4 and reaches
the diode through R1. Pin 16 of TDA7088 is the AFC output and
regulates the tuning voltage to C4. Via the connection R, the
external tuning is controlled by the reset button or an external
tuner. The scan input on Pin 15 allows, the AFC to be temporarily
shut down, so that a new station can be found. The demodulated
AF signal appears on Pin 2 of the TDA7088 C14 removes any IF
signals.
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