TRF RADIOS - Profe Saul

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2 Νοε 2013 (πριν από 3 χρόνια και 10 μήνες)

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TRF RADIOS

In the very early days of wireless a TRF (Tuned Radio Frequency) radio was the next
step up from a crystal set.


It offered amplification of both the radio frequency and the
audio so that more stations could be received and the sounds would be
loud enough to
power a loudspeaker.


TRF sets are quite simple and require battery or mains power.


In the 1920' intil the
1940's the glass valve was the component used for amplification.


A valve is quite a
large device, about the size of an eggcup, the f
iliaments glow red hot and the radios
would get quite warm consequently they also use quite a lot of power.


A TRF radio has its limitations and was superceeded by the
Superhet
, which is a
principle used in all modern receivers.


It was not until the intro
duction of the transistor
in the mid 1950's that radio sets could be made smaller and truly portable and consume
much less power.


The TRF radios described here all use transistors, though the circuit layouts are fairly
similar to some simple valve TRF rad
ios.


THE LADYBIRD

I was introduced to the construction of TRF radios as a child with the Ladybird book
'Making A Transistor Radio' written by Rev. G.C. Dobbs and published in 1972.


The
book describes how to make a very simple receiver and then progresses

in stages up to a
3 transistor TRF radio that uses a technique called
Regeneration

(also called
Reaction
)
which uses the first Radio Frequency transistor 3 times by implementing regeneration.


Regenaration is
positive feedback

, similar to the howl
-
around

produced when putting a
public address microphone too close to the loudspeaker.


In the radio the amount of this
feedback has to be very carefully adjusted to just the right level, just before it begins to
howl, in this state the radio is at its most sens
itive and is very selective.


Unfortunately the design uses old Mullard OC71 and OC45 germanium PNP transistors
which are no longer made.


I am sure that equivalent silicon NPN could be found with
some research and used in the circuit with some adjustment

to component values.


I still
have my original radio that I built in the 1970's and it still works a treat, in fact I use it
to listen to the football in the garage.




The Ladybird book decribes a novel solderless technique for construction, using a
wood
en 'breadboard' and brass screws and screw cups to trap the leads of the
components.


I have since rebuilt the radio into a smaller case and soldered the
components to tagstrip.


Here are some illustrations of the radio.


Photo
showing the
'Breadboard'
method of
construction.


The
regeneration
amount is
controlled by
the 10pF
trammer
capacitor.


The OC45
transistor
amplifies the
tuned radio
signal once,
and then
again.


The
diodes
change the
radio signal
into an audio
signal
(detect) and
the transisor
then
amplifies
this too.


The two
OC71
transistors
add further
audio
amplification
to drive a

This is complete working circuit digram.

loudspeaker.



The Completed

Radio






Ab
ove: My own re
-
housed TRF radio,
with components soldered together
onto a tag strip.



RADIO IN A MATCHBOX


A Matchbox Radio

(This one use
s the ZN415 IC
-

see further down)

In 1975 the idea of being able to
build a complete working radio
that would fit inside an ordinary
matchbox seemed absolutely
amazing.


Today it is run of the
mill, but this article from
Everyday Electronics of Sept
1975
is really interesting.


It uses
the Ferranti ZN414 integrated
circuit which is no longer
available, but the direct
replacement MK484 can be used
with confidence and is available
from Bowood Electronics.


I have built this radio and
another one using the ZN
415 IC
which includes an additional
stage of audio amplification.


Have a look at the ancient article
below, as stated the MK484 IC
can be used in place of the
ZN414, the 500pF trimmer may
be still be available from
Bowood Electronics or you may
have somet
hing similar in your
junk box.


The circuit could still
be made up using a miniature
tuning capacitor salvaged from a
discarded Chinese pocket radio.


(I have drawn in a pair of
IN4148 diodes which are not
absolutely necessary, but act to
stabilise the vol
tage.


Disregard
my other pencil jottings.)












Even simpler
would be to use
the circuit on the
right which
omits the BC107
transistor and
uses

a simple
Crystal earpiece
to listen to the
radio.


It would
make it even
easier to
assemble into a
matchbox.


Also have a look
at the circuit
below, which
uses the ZN415
integrated circuit
which includes
additional
amplification,
enough to
directly drive
a

pair of walkman
headphones.


The two 32 ohm
earpieces must
be arranged so
that they are
wired in series to
give the
necessary 64
ohm load.
The
ZN415 makes
assembley even
easier.






Circuit digram of the Ferranti ZN415 single chip radio.


This circuit
can also be used for
the later ZN416 and ZN416E integrated circuits
-

if you can find one.


The ZN415 / ZN416 / ZN416E is, like the ZN414,
also discontinued by F
erranti, but you may be able
to find one from somewhere, there may even be a
replacement IC, but I have not come across one.


MORE REACTION

The ZN414, ZN415 and ZN416 radios described above are straight TRF radios that use
no reaction (regeneration).


A
popular design pre
-
dating the ZN' circuits was the HAC
circuit.


HAC stands for Heard All Continents, and when used as a short wave radio that
is certainly possible.


It is very sensitive and quite selective too.


I have built one and
recieved Japan, Ameri
ca, China, Australia, Poland and Turkey, amongst many others.


This circuit was very kindly se
nt to me by a Short Wave Magazine Reader.


As with many such circuits of its day it used plug
-
in coils that were made by Denco Ltd
of Clacton.


The coils are no l
onger available, but it is perfectly possible to wind your
own.


I have provided the coil winding details below.



The Trusty HAC Circuit




The HAC TRF Radio



Internal view of the HAC TRF Radio


MULTI
-
BAND COIL WINDING DETAILS

The old HAC circuit detailed above is very
easy to construct, however the design relied
on the use of the DENCO (Clacton Ltd.)
Green Range of coils which are no longer
ava
ilable.


It is quite straightforward to
wind your own coils and the diagram on
the right shows how and the table below
details the number of turns and what guage
of enamelled copper wire to use.


Take a short piece of 10mm diameter
ferrite rod (about 50 to

100mm long) and
make a paper sleeve to wrap around the rod
(for easy repositioning of the coil assembly
once in use).


Over this paper sleeve wind
the first coil (L1) and secure the wires 2
and 5 in place using some Sellotape.


Over
the top of L1 wind the

second coil (L2) and
secure the wires 3 and 4 in place.


Finally
alongside

the first windings wind the third
coil (L3) again securing the wires 8 and 9
in place with a little bit of Sellotape.


When wiring the coil into the circuit ensure
that the wires a
re connected in the correct
place according to the numbers shown.


Be
especially careful not to get the wires of the

The diagram above (top) shows how the coil is
regeneration coil L2 swapped over as the
regeneration (reaction) effect will not
occur.




With a cursory glance at the schematic diagram a
nd
the drawing of the coil opposite you might expect
that wire 3 from the regeneration winding (L2)
would be physically next to wire 5 of the main
tuning coil (L1) however wires 3 & 4 are effectively
swapped over because the feedback from the Drain
MPF102
field effect transistor is out of phase with
its Gate and so would be out of phase with the radio
signals in L1 if applied to the 'top' of the coil near
the wire 5 end of L1.


Since regeneration is
effectively controlled positive feedback it must be
applie
d In
-
Phase with the received radio signal.


COIL WINDING CHART:

BAND

Range

(Approx)

L1

(turns)

L2

(turns)

L3

(turns)

Wire

s.w.g.

SW1

4
-

14MHz

15

8

4

28

SW2

10
-

30
MHz

6

4

2

22

MW

5.2
-
1.6MHz

50

18

15

32


physically wound onto the ferrire rod former while
the circuit


diagram shows the importance of the
lead connection numbering and how the
coil is
wired into the tuning circuit.

The number of turns for each winding on the coil and the guage of enamelled copper wire used
are just suggestions that happen to work on my radios quite well.


Other guages of wire may be
used and the number of turn
s adjusted until the required range is obtained.


In any event some
experimentation with the number of turns will always be required.




L3 is not too critical as it is just a coupling winding for the aerial.


L1 determines the range of
frequencies covered

by the set and the number of turns can be increased to tune lower
ferquencies or decreased to tune higher frequenciess.


The most critical winding is L2 and some
experimentation will almost always be required here.


You may find that you can obtain
reacti
on across most of the band but not all of it, in which case add a turn or two.


Alternatively
you may find that reaction cannot be reduced sufficiently at some frequenciess, in which case
remove a turn or two.


I found the Medium Wave coil the most difficu
lt to get just right and had
to adjust the position of the windings relative to each other to obtain satisfactory results






MY FIRST SW TRF RADIO
:

Here is the circuit for my first Short Wave t.r.f. radio.


As you will see it is very similar to
the HAC

described above and uses the same type of coils, but with just two transistors it
will only power an earphone.


I spent many hours with this set as a kid listening to short
wave stations from all over the world on a crystal earphone.


Most listened to bro
adcasts
on this simple little radio were Swiss Radio International, Radio Netherlands, Radio
Sweden, Radio Canada, Radio Moscow and The Voice Of Turkey
-

and many, many
more.


I called it the
TWO TANSISTOR WONDER

-

because it was!




TWO TRANSISTOR WONDER




The very

old and rather battered but trusty little
Two Transistor Wonder
.


The case is
original but the knobs have been replaced with more modern ones.



(Note the several holes remaining in the front panel from previous modifications).



The 'gubbins' inside the Two Transistor Wonder