Franzis Do-it-yourself User manual

Do-it-yourself
VHF Retro Radio
65228-5 RADIO 160x110_HB+Titelei_Layout 1 11.11.13 11:42 Seite 1

Dear Customers,
This product was developed in compliance with the applicable European directives
and therefore carries the CE mark. Its authorized use is described in the instructions
enclosed with it. In the event of non-conforming use or modification of the product,
you will be solely responsible for complying with the applicable regulations. You should therefore
take care to assemble the circuits as described in the instructions. The product may only be passed
on along with the instruction and this note.
Waste electrical products should not be disposed of with household waste. Please recy-
cle where facilities exist. Check with your local authority or retailer for recycling advice.
© 2014 Franzis Verlag GmbH, Richard-Reitzner-Allee 2, 85540 Haar, Germany
All circuits and programs depicted in this book are developed and tested with utmost care.
Nonetheless, it is not possible to rule out all errors in the book or in the software. Publisher and author
are only liable in case of intent or gross negligence according to legal regulation. Beyond that, publis-
her and author are only liable according to the law on product liability concerning hazards to life,
body, and health and the culpable violation of essential contractual obligations. The damage claim
for the violation of essential contractual obligations is limited to the contract-specific, predictable
damage, unless in cases of mandatory liability according to the law on product liability.
All rights reserved, including those of reprinting, reproduction and storage in electronical media. No
part may be reproduced and distributed on paper, on storage media, or in the Internet, especially as
PDF, without the publisher‘s prior written permission. Any attempt may be prosecuted. Hardware and
software product names, company names, and company logos mentioned in this book are generally
registered trademarks and have to be considered as such. For product names, the publisher uses
mainly the spelling of the manufacturer.
Translation and DTP: G&U Language & Publishing Services GmbH
Author: Burkhard Kainka
Art & Design Cover: www.ideehoch2.de
ISBN 978-3-645-65228-5
65228-5 RADIO 160x110_HB+Titelei_Layout 1 11.11.13 11:42 Seite 2

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The VHF Retro Radio
This modern VHF radio in vintage style receives FM stations
in the 87.5 MHz to 108 MHz band with good reception per-
formance. You will mainly hear the powerful local stations in
high sound quality. However, the sensitivity of the receiver also
allows you to listen to remote stations at times.

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Large-scale introduction of VHF broadcasting began only after
1945. Initially there were still many radios that were able to re-
ceive only the AM ranges – long wave, medium wave and short
wave. Many devices, however, could be upgraded with VHF ret-
rofit kits. During the 1950s the tube superhet with VHF range
became widely accepted.
This radio looks very much like a typical portable radio of the
1960s. The invention of the transistor allowed the construc-
tion of radios that consumed less power than valve receivers
and therefore could be operated with batteries. Apart from this,
they technically still resembled the older tube radios.
Thanks to the highly integrated receiver IC TDA7088 building
your own VHF radio has become so easy that anybody will be
successful in soldering this radio together. The single-ended
low frequency amplifier functions pretty much like the his-
torical tube radio. Your nostalgic radio works with a dual-stage
transistor amplifier with medium volume at low battery volt-
age. Now you only need two 1.5-V alkaline batteries for up to
100 hours of radio reception. With your DIY-radio listening to
the radio will become even more fun. Enjoy the diversity of the
VHF stations.

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Components
• pre-assembled PCB with TDA7088
• rod antenna
• speakers 8 Ω, 0.5 W
• volume control 22 kΩ with switch
• tuning control 22 kΩ
• insulated wire
• battery compartment with connection wires
• T1 PNP transistor BC557B
• T2 NPN transistor BC547B

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• D1 varactor diode 1SV101
• R1 4.7 kΩ (yellow, violet, red)
• R2 220 kΩ (red, red, yellow)
• R3 1 kΩ (brown, black, red)
• R5 330 kΩ (orange, orange, yellow)
• R6 33 Ω (orange, orange, black)
• C15 electrolytic capacitor 100 µF
• C17 100 µF ceramic (104)
Assembly of the control elements
The radio has two rotary controls – one for frequency and one
for volume. The three-port volume controller is equipped ad-
ditionally with the two-port on/off switch. If you turn the axis
all the way to the left, the switch opens. Insert the volume con-
troller into the left mounting hole. A small tab secures correct
insertion. Fix the controller with the ring nut and do not forget
the washer.
The second potentiometer with 22 kΩ is used for frequency
tuning and is mounted on the right. The connections of both
potentiometers should face inwards so that the board can be in-
stalled between them later. Then screw both rotary knobs onto
the axes so that the end stops are aligned with the printed scales.

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Volume control (potentiometer) with switch
Insert the speaker by pushing it into the corresponding slot. The
connectors should face upwards to make sure that the connec-
tions to the board will be short. The speaker sits securely in its
slot. You can, however, also add a drop of adhesive or hot glue.
Loudspeaker

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Screw the solder lug to the base of the telescopic antenna. Then
slide the antenna from the inside through the housing opening
and push the end into the flat holder which you have to secure
correctly in the housing with the enclosed adhesive tape. Now
the antenna is mounted sufficiently firmly but can still be fixed
with some additional adhesive tape later.
The antenna

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Soldering
The board contains a large number of SMD components (sur-
face-mounted device, wireless components) already soldered
on: the receiver IC TDA7088, 15 condensers and one resistor.
You will only need to solder in some components with con-
necting wires. This includes all components of the AF amplifier,
the coils and all components needed for the diode tuning of
the radio.
The SMD components
Now the PCB is to be assembled, please refer to the circuit dia-
gram on the last page of the manual for orientation.

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Components on the circuit board
Equip the PCB with the electronic components according to
the layout diagram. First install the coils SP1 and SP2. Both
coils have to be built of the included connecting wire. They
each have three turns with an inner diameter of 5 mm and a
length of approximately 7 mm. Use the electrolytic capacitor
100-µF as winding mandrel. First wind three turns closely ad-
jacent to one another. Then pull the turns apart to reach a total
length of approximately 7 mm. Absolute accuracy is not crucial
here because the coil can still be slightly modified after instal-
lation. Remove the insulation from the wire ends. Only then
pull off the coil from the winding mandrel. Now solder on the
two wires at the bottom. Then cut off the protruding wires with
sharp pliers approximately 2 mm above the circuit board.

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Winding the coils
Installation of the coils
Insert the resistors R1 (4.7 kΩ, yellow, violet, red), R2 (220 kΩ,
red, red, yellow) close to the connections to potentiometer PT2.
The resistors R3 (1 kΩ, brown, black, red), R5 (330 kΩ, orange,
orange, yellow) and R6 (33 Ω, orange, orange, black) belong to

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the AF amplifier on the other side of the board. The resistor
R4 (5.6 kΩ) is already soldered as an SMD component. Now
bend the connecting wires so that they are suited for vertical
assembly.
The resistors
Solder the D1 varactor diode (1SV101 in the flat transistor
housing with two connections, imprint V101), following the
direction shown on the PCB marking. The flat, printed side
points towards the coil.
The D1 varactor diode

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Now solder in the electrolytic capacitor C15 (100 µF). Observe
the mounting direction. The positive terminal is marked on the
board. The negative terminal – which is marked with a white
line – points towards the IC. Afterwards install the ceramic disk
capacitor C17 with 100 nF (print 104). The mounting direction
is not important.
The capacitors
Finally you have to build in the transistors. Take care not to mix
the types up. T1 is a PNP transistor BC557B, and T2 is an NPN
transistor BC547B.
The fully equipped PCB

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Next, solder on the appropriate wire pieces to the two potenti-
ometers. For length refer to the wiring diagram. Now you can
either pass the wire ends through the holes and solder them like
the other components or, alternatively, you can solder them flat
on the solder pad. The PCB has to be mounted in a suspended
position between the two potentiometers; it is held in place by
six wires. In addition, there is one wire leading to the switch
of the volume potentiometer, while two more wires go to the
speaker. The black wire of the battery compartment has to be
connected to the GND connection. The red connector goes to
the volume control switch. The remaining wire is to be soldered
to the antenna connection. If this connecting wire is longer
than is necessary it can still be used to increase the effective
length of the antenna and thus contribute to a good reception.
Now the radio is fully assembled and waits to be tested. Sign off
the assembly with your name and date in the wiring diagram
on the last page of your instruction manual. Then you should
copy or tear out the page and glue it into the radio case. In this
way you will be able to understand everything even after years
and carry out repairs if necessary. This is also the case with old
radios: You can repair them even after many years because you
will always find the wiring diagram inside.

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The wiring diagram
The complete wiring

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Initial testing and settings
Insert two 1.5V mignon alkaline batteries. Turn on the radio
and set the volume control to maximum. Turn the frequency
knob, and you will quickly find a station coming loud and clear
from the speaker.
Now you have to adjust the frequency. Use a radio at hand for
comparison. The station at the very bottom of the FM frequen-
cy range should appear at the left stop. You can shift the recep-
tion range by modifying the SP1 coil. Push the turns tighter to-
gether to receive lower frequencies. If there is still a large empty
area at the left to the bottommost station, pull the turns apart
a little again. For fine-tuning insert a screwdriver between two
turns to slightly bend them apart.
Once the lower band edge is adjusted correctly, you can also
adjust the upper 108 MHz edge. However, the frequency is de-
pendent on the battery voltage. If you realise during operation
that you cannot adjust the topmost station any more, just re-
place the batteries.

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Reception practice
When fine-tuning a station, the receiver’s AFC (Automatic Fre-
quency Control) will lock on to the exact frequency. Then the
station can be heard in a certain range of the frequency control-
ler. Position the controller as exact as possible in the middle of
this range. If battery voltage drops sharply during operation,
the frequency may change and you will have to re-tune into the
radio station. Depending on the modulation of the transmitter
there may be sound distortions due to over-modulation of the
power amplifier. If this happens, turn down the volume control
a little. With its moderate volume level the FM retro radio is
ideally suited for a relaxed listening experience in the evening.
If you extend the telescopic antenna to its full length, the radio
can receive every strong local station. Connecting a longer an-
tenna wire will allow you to receive also weaker stations (for in-
stance the local stations of the neighbouring towns). To achieve
an even more powerful reception performance you can con-
nect a second, additional antenna wire to the GND connection.
Thus you will have a dipole antenna. The optimum length is 75
cm per wire. You can improve the reception of a weak station by
carefully aligning both wires.

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Explanations on the wiring diagram
Most VHF superhet receivers use an intermediate frequency of
10.7 MHz. The receiving frequency is first converted to the in-
termediate frequency and then filtered, amplified and demodu-
lated. The VHF retro radio is a superhet, too, which converts
the received signal to an intermediate frequency. But its inter-
mediate frequency is much lower, at 70 kHz. For this reason the
intermediate frequency filters are able to function without bal-
anced coils. The FM demodulator becomes simpler and better
protected against distortion. All essential stages fit into a single
SMD-IC, the TDA7088 with 16 pins. Instead of a variable ca-
pacitor, as used in older receivers, the radio works with the D1
varactor diode. The higher the voltage at the diode, the lower
is its capacity and the higher the receiving frequency. The only
balance point is the L1 coil which can be used to adjust the
lower edge of the oscillator frequency.
The low frequency power amplifier is a simple class-A ampli-
fier with the two T1 and T2 transistors. The quiescent current
is approximately 20 mA. The circuit will deliver a good sound
quality at an operating voltage as of 2.2V.

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The board is designed in a way that its components are all as-
sembled in SMD technique around the actual TDA7088 re-
ceiver. That makes construction easy. You can exchange some
of the wired components you have soldered yourself to modify
certain properties of the radio. R1 defines the tunable frequen-
cy range. A smaller resistor will enlarge the tuning range. This
makes sense, for instance, if you want to operate the radio with
NiMH batteries at 2.4V. R2 determines the width of the AFC
catch range. If you want, for instance, to receive weak stations in
the vicinity of stronger stations it may be useful to decrease R2
down to 100 kΩ or less to get a smaller capture range. The two
RE1 and SC1 connections on the board remain initially free.
They are intended for subsequent extensions. The TDA7088
was originally designed for button tuning. You can find the
reset and scan buttons in the wiring diagram. If you want to
adapt the receiver accordingly, you have to separate the PT2_2
connection to the slider of the frequency control. You can also
insert a switch here so the receiver can be tuned optionally with
the buttons or the potentiometer.

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