Conrad UKW User manual

Imprint
© 2011 Franzis Verlag GmbH, 85586 Poing
www.franzis.de
Author: Burkhard Kainka
Art & Design, Composition: www.ideehoch2.de
ISBN 978-3-645-10057-1
Produced on the order of Conrad Electronic SE, Klaus-Conrad-Straße 1, D92240 Hirschau
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do not assume any liability for incorrect information and their consequences.
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This product complies with the relevant CE directives if used according to the included instructions. The descrip-
tion belongs with the product and must be passed on with it.
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UKW Retro Radio
The UKW Retro Radio 4
Components 5
Installation of Control Elements 6
Soldering 8
First Test and Settings 12
Reception Practice 13
Explanations on the Circuit Diagram 13
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4
The UKW Retro Radio
This state-of-the-art VHF radio with traditional design receives FM stations in the
range of 87.5 MHz to 108 MHz with good reception performance. You will mainly
hear the strong local stations with good sound. However, the receiver is sensitive
enough to also receive remote stations at times.
VHF radio was not widely introduced before 1945. Initially, there were still many
radios that were only able to receive the AM ranges long wave, medium wave
and short wave. Many devices could be retrofit to receive VHF. The tube superhet
with VHF range became generally popular in the 1950s.
This radio looks reminiscent of a typical portable radio from the 1960s. With in-
vention of the transistor, radios could be built that could be operated at lower
energy than tube receivers. They could therefore also be battery-operated. Apart
from this, they remained technically similar to older tube radios.

D1 varactor diode 1SV101
R1 4.7 kO(yellow, violet, red)
R2 220 kO(red, red, yellow)
R3 1 kO(brown, black, red)
R5 330 kO(orange, orange, yellow)
R6 33 O(orange, orange, black)
C15 Elko 100 µF
C17 100 nF ceramic (104)
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With the highly integrated receiver IC TDA7088, construction of a dedicated VHF
radio has become so simple that anyone is able to successfully solder together
this radio. The single-ended low frequency amplifier works similar to the historic
tube radio template. Your nostalgic radio uses a two-level transistor amplifier
with medium volume at low battery voltage. Now, two 1.5 V alkaline batteries are
sufficient for up to 100 hours of radio reception. Your self-made radio will make
listening to the radio even more fun. Enjoy the diversity of the VHF stations.
Components
Pre-equipped PCB with TDA7088
rod aerial
speakers 8 O, 0.5 W
volume control 22 kOwith switch
tuning control 22 kO
insulated wire
battery compartment with connection wires
T1 PNP transistor BC557B
T2 NPN transistor BC547B

Assembly of the Control Elements
The radio has two rotary controls. One for frequency and one for volume. The
volume controller with three connections additionally also carries the on/off
switch with two connections. If you turn the axis all the way to the left, the
switch opens. Place the volume controller in the left assembly hole. A small tab
prevents twisted insertion. Attach the control with the ring nut and do not forget
the washer.
The volume control (poti) with switch
The second potentiometer (poti) with 22 kΩ is used for frequency tuning. It is in-
stalled on the right. The connections for both potis should point inwards so that
the PCB can be installed between them later. Then screw the two rotary controls
onto the axes so that the end stops correspond to the printed scales.
Insert the speaker by pushing it into the matching slot. The connections should
point upwards to keep the connections to the PCB short later. The speaker will
be anchored sufficiently firmly in its intended slot. However, you can also apply
an additional drop of adhesive or hot glue.
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Speaker
Screw the solder eye to the base of the telescopic aerial. Then push the aerial
through the casing opening from the inside and push the end into the flat holder,
which you have to attach properly in the casing with the included adhesive tape.
The aerial is then installed sufficiently firmly, but can later be additionally at-
tached with some adhesive tape.
The aerial
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Soldering
The PCB already contains many soldered-on SMD components (surface-mounted
device): the receiver IC TDA7088, 15 capacitors and one resistor. Only a few of
these components still must be soldered in with connection wires. This includes
all components of the NF amplifier, the coils and components around the radio’s
diode tuning.
The SMD components
Now the PCBs are soldered together. The circuit diagram of the complete receiver
on the last page of this manual serves orientation purposes. For a small solder-
ing course, see the online magazine ELO (www.elo-web.de).
Components on the PCB
Equip the PCB with the electronic components according to the equipment plan.
First install the two coils SP1 and SP2.
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Both coils must be made of the included equipment wire. Each of them has
three turns with an inner diameter of 5 mm and a length of approx. 7 mm. Use
the 100-µF-Elko as a coiling mandrel. First wind three turns close to each other.
Then draw the turns apart so that the coil has a total length of approx. 7 mm. Ac-
curacy does not need to be perfect, because the coil can still be changed a little
after installation. Strip the wire ends of insulation. Remove the coil from its coil-
ing mandrel only after this. Solder on the two wires at the bottom. Then cut off
protruding wires with sharp pliers about 2 mm above the PCV.
Coil installation
Insert the resistors R1 (4.7 kO, yellow, violet, red), R2 (220 kO, red, red, yellow)
close to the connections for Poti PT2. The resistors R3 (1 kO, brown, black, red),
R5 (330 kO, orange, orange, yellow) and R6 (33 O, orange, orange, black) belong
to the LF amplifier on the other side of the PCB. The resistor R4 (5.6 kO) is al-
ready soldered on as an SMD component. Bend the connection wires appropri-
ately for standing installation.
The resistors
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The varactor diode D1
Now solder in the electrolyte capacitator C15 (100 µF). Observe installation direc-
tion. The plus pole is marked on the PCB. The minus pole is marked with a white
dash. It points towards the IC. Then install the ceramics of the capacitator C17 with
100 nF (print: 104). The installation direction is not important.
The capacitators
Last, you need to install the transistors. Do not swap the two types. T1 is a PNP
transistor BC557B, T2 is an NPN transistor BC547B.
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The PCB completely equipped
Next, solder on the matching wire pieces for the two potis. For their lengths, see
the wiring plan. Either push the wire ends through the holes and solder them on
like the remaining components, or solder them to the solder areas flat. The PCB
should be installed suspended between the two potis. It is held by six wires.
Additionally, one wire runs to the volume poti switch and two to the amplifier.
The black battery compartment wire must be connected to the GND connection.
The red connection leads to the volume controller switch. The remaining wire
should be soldered to the aerial connection. If this connection wire is longer
than required, this will improve reception by increasing the effective aerial
length.
The wiring plan
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The complete wiring
The radio is now assembled completely and can be tested. Draw the set-up with
your name and date in the circuit diagram on the last page of these instructions.
Then copy or tear out the page and glue it into the radio casing. This will make it
possible to understand everything even after years and to perform repairs if they
become necessary. That is what old radios are like as well: They can be repaired
even after many years, because the circuit diagram is always inside the radio.
First Test and Settings
Insert two 1.5 V mignon alkaline batteries. Switch on the radio and turn up the
volume control all the way. Using the frequency button, you will quickly find a
station that sounds from the speaker clearly.
Now the frequency range must be set. Use a present radio for comparison. The
bottom-most station in the VHF range should appear at the left stop. The recep-
tion range can be relocated by adjusting the SP1 coil. Push the turns together
more closely to receive lower frequencies. If a large empty area is present to the
left of the bottom-most station, pull the coil apart a little.
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For fine tuning, push a screwdriver between two turns to bend them apart a lit-
tle. When the lower band threshold is perfectly adjusted, you can also adjust the
upper threshold of 108 MHz. The frequency depends on battery voltage, how-
ever. If you find in current operation that the upper-most station can no longer
be set, replace the batteries.
Reception Practice
During station tuning, the receiver’s AFC (Automatic Frequency Control) will lock
on the precise frequency. The station is then audible in a certain range of the fre-
quency controller. Set this control as precisely to the centre of this range as pos-
sible. If the battery voltage drops considerably in operation, the frequency may
change. Then you have to re-set the transmitter. Depending on station modula-
tion, sound distortions may occur from over-modulation of the radio end ampli-
fier. In this case, turn the volume control back a little. The FM retro radio with its
moderate volume is ideally suitable for relaxed listening to the radio in the evening.
With the telescopic aerial extended all the way, the radio is sufficiently sensitive
for all strong local stations. If you connect a longer aerial wire, you will also be
able to listen to weaker stations (e.g. local stations of the adjacent towns). For
even higher reception performance, additionally connect a second aerial wire to
the GND connection.
You are then using a dipole aerial. The perfect length is 75 cm per wire. Careful
alignment of both wires can improve reception of a weak station.
Explanations on the Circuit Diagram
Most VHF superhet receivers use an intermediate frequency of 10.7 MHz. The re-
ception frequency is first transposed to the intermediate frequency, then filtered,
amplified and demodulated. The VHF
retro radio also is a superhet and transposes the signal it receives to an interme-
diate frequency. However, the intermediate frequency is much lower at 70 kHz.
This makes it possible for the intermediate frequency filters to do without bal-
anced coils. The FM demodulator becomes simpler and much more distortion-
proof. All essential stages fit in a single SMD-IC, the TDA7088 with 16
connections. Instead of a rotary capacitator as in older receivers, the radio uses
the capacity diode D1. The higher the voltage at the diode, the lower its capacity
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and the higher the reception frequency. The only tuning point is coil L1 to set the
lower threshold of the oscillator frequency. The low frequency end stage is a
simple class-A amplifier with the two transistors T1 and T2. Quiescent current is
approx. 20 mA. The circuit still works at a good sound as of an operating voltage
of 2.2 V.
The PCB is designed so that all components are equipped in SMD build around
the actual receiver TDA7088. This makes set-up easy. Some of the wired compo-
nents soldered in by you can be replaced to change certain radio features. R1 de-
termines the frequency range that can be tuned. A smaller resistor increases the
tuning range. This is a good idea, e.g. if you want to operate the radio with NiMH
batteries at 2.4 V. R2 determines the width of the AFC capture range. If you want
to be able to listen to, e.g., weak stations close to stronger stations, it may be a
good idea to increase R2 up to MOto reduce the capture range. The two connec-
tions RE2 and SC1 on the PCB are initially not used. They are reserved for later
expansions. The TDA7088 was originally developed for button tuning. The two
buttons for „reset“ and „scan“ are included in the circuit diagram. If you want to
convert the receiver accordingly, the PT2_2 connection to the frequency con-
troller slider must be separated. You can install a switch here, so that the re-
ceiver can be tuned either by buttons or the poti. For more information on
possible conversion, see the online magazine ELO (www.elo-web.de).
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