This
is currently still available for sale in limited quantity
while I undergo a retail sales outlet change. There is
a version that uses the same electronic circuit that
is quicker to build. I highly recommend the Breadboard
Six Transistor Radio Kit. It is also somewhat
cheaper.
Etsy shop top level for this and other
kits and components: UsefulComponents
Etsy Shop
This is a MW AM
transistor radio which can be constructed without soldering using
3A terminal strip. It is a modern response to the 1971 Ladybird
Book Radio and works as well as some AM radios that you could buy
in a shop. It's best to read right through the construction notes
first before starting out. Why is it called a Choccy Block
Radio Kit? Choccy-Block is the nickname for the terminal
strips on which it is constructed, and they look rather like
chocolate bars.
I supply a kit of all the electronic parts, wire, terminal blocks,
knobs, mounting screws and a cardboard enclosure for this design
with wires soldered on to some parts where necessary. I've
assumed that you have a small screwdriver for the terminal screws,
a fine point marker pen, some small wire cutters for trimming the
component leads and some means of stripping the insulation from
the wire. A craft knife is good for that. You will
also need some paper, glue, scissors and sticky tape. It is
best to mount up the radio in an enclosure when it is all
working. The cardboard container supplied is pretty good to
start with, or you might choose to use some other method.
What do you need to know before
you start?
The components list has links to photographs that should allow you
to make sense of the different values and to identify which leg is
which. You need to know about resistor colour codes and you
can find that information in many places. It should be
possible to build the radio just by looking at the pictures, but
later on I've included full electronic explanations. If
those don't make any sense, don't let them put you off just
building the circuit.
The schematic diagram can look a bit daunting to the novice.
Don't Panic. You don't need to understand it all to make it
work. The idea of this design is that if you follow the plan
it should work without too much fiddling. Experienced
builders will find the schematic diagram helpful as well.
How To Build It: Follow The
Pictures and Schematic Diagram Assemble the Wire Links
Start by cutting to length, stripping the ends and assembling the
single core link wires into the terminal strips as shown.
Each wire must have the end stripped of insulation by about 6mm so
that the terminal strip screw touches the bare wire. It is
best if each wire end is only trapped under one screw on the side
which it enters the strip. For now, tighten the screws only
very gently, just enough to hold the wires in while you put
everything else together. As long as they are in the correct
holes, the wires don’t have to be absolutely in the exact shapes
shown, but it will make it easier to check everything later on if
it’s made just like in the picture. You can click on the
pictures to see a bigger version.
Assemble The Transistors and
Diodes
Identify the various different transistors and bend their legs as
shown in the BOM pictures so that they will fit into the
strips. The transistor legs will only bend a few times
before dropping off, so it’s best to get it right first
time. Assemble the two diodes in the positions shown, so
that the glass bodies are in contact with the transistors but so
that the wires are not touching the metal part. The diodes
and transistors have to be connected the correct way round as
shown by the identification letters next to the legs. Note
that Q4 is connected with the curved side facing upwards and the
flat side facing down. Q5 and Q6 may be supplied as fully
insulated parts without any metal showing to help avoid
shorts. Where this is the case, the side with the lettering
on always defines the upper side. They should be fitted with
the lettering facing down.
Assemble The Resistors
You can fit the resistors either way round, but it will be easier
to check later if the gold coloured bands are pointing down or to
the right. R8 requires an extra loose terminal block to
connect it to VR1.
Assemble The Capacitors
In the next stage, fit both the ceramic disc and electrolytic
capacitors. With the exception of the bipolar one, the
electrolytic capacitors have to be connected the correct way
round. Now that things are getting a bit crowded, you might
find that keeping the capacitor wires from touching wires that go
into other holes is quite difficult. Where this happens,
take some discarded wire stripping and slide it onto the capacitor
wires to avoid short-circuits. I needed to do this on C7 and
C5, and sleeving the wires of C8 may be a good idea too. If
the stripped sleeving doesn’t fit on very easily, try putting a
small amount of vegetable oil on the wire first. C11
requires an extra terminal block to connect it to the loudspeaker.
The second picture shows an example of covering one of the leads
of C5 where it crosses one end of R16.
Connect up VR1, the Battery Clip,
the Loudspeaker and VC1
VR1 comes with wires attached for both the volume control and
on-off function. They are longer than needed for the
standard construction in case you want to use a different
box. The pictures show them fitted and cut to the right
length for the cardboard enclosure. Do the same with the
battery clips and the loudspeaker. The connection strips of
tuning control VC1 can be bent and connected in so that the shaft
points downwards below the level of the terminal strip bottom
face. At the end of construction, this will protrude out of
the front control panel.
The final picture shows the main circuit from underneath, with the
shaft of VC1 pointing upwards.
Wind and Connect the Antenna
Coils
The input coupling coil is the small one which takes the signal
from the ferrite rod and feeds it into the radio. This is
constructed as follows. First make a paper tube about 2.5cm
long. Use a strip of some normal printer or magazine paper
about 2.5cm X 10cm. Put stick glue on about 8 of the 10cm
length then wrap this around the ferrite rod so that the glue
sticks the paper and forms the tube.
After this has dried, using single core PVC insulated copper wire,
wrap 6 turns onto the tube. Secure the start end with sticky
tape first and then the far end when it is complete. You can
follow the photographs which show this process. Don't wrap
the paper, the wire or the tape too tightly or you won't be able
to slide the coil up and down the rod. I recommend winding the wire by
rotating the rod and former rather than wrapping the wire
around. Wrapping it around tends to put a twist in the
length of the wire which makes it tangle up more easily.
The main MW tuning coil is the large one which is connected to the
tuning capacitor.The
paper tube for this is made from paper 6cm X 10cm. First
wind the coil in one direction across the tube for about 25 turns
and then 20 turns over the original turns coming back in the other
direction. When
this is done, connect the large coil to the tuning capacitor
connections and the coupling coil into the radio input.
That’s It, but Check and Secure
If you've gone stage-by-stage and everything is in the right
place, it should be ready to go. It's worth double checking
the direction of the two diodes, the transistors and the
electrolytic capacitors. Then go down both sides of each
terminal strip checking that they are tightened up enough to hold
the wires in but still not over-tightened at this stage.
None of the wires should be loose enough to pull out without
forcing it. Check that there are no shorts between wires
going into different holes.
C13 Retrofit Capacitor
When I first designed this and had taken all the photographs, I
had made a mistake. I'd forgotten to put a supply rail
decoupling capacitor across the main 9V battery rail, but it made
no difference when I added one so I went ahead and published
anyway. This is completely fine with alkaline batteries, but
when using the older cheap zinc-carbon types you need C13, a 100uF
electrolytic capacitor connected as shown in the picture below to
act as a reservoir to stabilise the battery voltage. This
should always be fitted.
Starting Up and Initial Testing
Connect the battery and rotate the volume control up to about
halfway. Turning the tuning control should immediately allow you
to hear the main MW stations. Finishing Off
The following pictures show how to put the radio into the
cardboard box supplied. The box should be marked out as
shown and holes made in the appropriate places for the volume
control, tuning control and loudspeaker. Applying tape to
the holes around the volume and tuning controls strengthens the
box and the two controls can be fixed quite securely. It is good
if you can use a pillar drill to make the circular holes neatly,
but as it's cardboard it is possible to do it with a screwdriver
and scissors as shown. The loudspeaker slats can be cut out
with a craft knife or scissors. Craft knives are razor
sharp, so use an appropriate level of care.
When all the holes are ready, the radio circuit can be offered up
to the board so that the shaft of VC1 goes through the correct
hole. It may be easier to put the 4mm bolts and washers in
position first then the position of VC1 can be adjusted to meet up
with them. Tighten up the bolts into VC1 just enough to
squeeze the board a little. The front face of VC1 and the
bottom of the terminal strips should all be flat against the
surface. The knobs can be attached now, by putting them over
the shafts and tightening the grub screw in the side. The loudspeaker can be held against
the sound holes with sticky tape or more permanently attached with
sparing use of PVA glue around the edge. The battery can also be taped down, or
when the box is closed it will hold in quite steady if it is just
wrapped in some paper
When everything is fitting well, providing everything is still
working you can give each of the terminal screws a final
tightening, fold up the box and put a couple of elastic bands
around it. The side flaps fold over first, then the longer
top and bottom flaps fold down over them to meet in the
middle. If you tape up the box more permanently, the
internal part of the side flap next to the battery can be cut
shorter and left untaped to allow that to be opened for changing
the battery. I stayed with just the elastic bands, so that
it was easier to show the inner workings.
The Completed Radio
Now that it's all together, you can count how many AM stations you
can pick up, bearing in mind that the antenna is directional so
you have to rotate the radio for the best signal. If you
find that you are missing the high frequency end of MW you can
move the main tuning coil closer to the end of the rod, shuffling
up the coupling coil to meet it. How good is it compared to
something bought from the shops? The three models that I
have made up are all of identical performance and I think that it
compares well with a small shop radio. The tuning is sharp
enough to distinguish the three Absolute Radio frequencies of
1215, 1242 and 1260 kHz in Farnborough, Hampshire UK which is good
going for this simpler design. There's no need for external
aerial wires as with some home constructed designs. The
audio amplifier drives the small loudspeaker to a good volume.
My Radio Isn't Working.
Have another look for shorts between components where they are
spanning many terminals and where they are close to other
wires. If you've got a multimeter, use the 10V voltage
measurement range to check around the d.c. voltages when tuned
away from a station. With a fresh 9V battery you should see
voltages that are very close to those marked on the schematic
diagram. Very close in this instance means within plus or
minus 0.3V of the figure in the diagram. The voltages marked
on the schematic diagram are measured values for direct connection
of 9.0V power with low signal input, all with the negative meter
lead connected to the negative battery terminal. If you see
a voltage that is more than about 0.3V out either way, then there
is probably something wrong in the circuit around that
transistor. If all voltages look low, the battery rail is
probably shorted out in some way. If you have intermittent
operation that comes and goes with vibration or handling, gently
prod the various parts of the circuit until you find the sensitive
part. There will usually be a wire loose or shorted in that
area.
The area around Q2 and C5 is quite sensitive to interference from
nearby wiring. If you have used a different enclosure,
ensure that the wiring in general and the two coils on the ferrite
rod in particular is kept physically distant from that area of the
circuit. If you find
that the radio is working but is very hissy or squeals, try moving
the coupling coil up the ferrite rod a small distance away from
the main coil.
End Of Construction
That's the end of the construction notes, and is as far as you
have to read if you just want to build the radio. What
follows is some more in-depth technical description and design
philosophy discussion for the electronics engineers. After
that there is a FAQ which may be useful if you're still having
problems getting going, or if you've bought all your own
components. If you fancy trying some further modifications,
there's a page with some ideas on too.
Design Philosophy
As noted at the top, this is intended to be a modern replacement
for the 1971 Ladybird Book "Learnabout: How to Make a Transistor
Radio" radio. I've described elsewhere how I didn't have
much success with this as a kid, mostly due to the problems with
1960s transistors and their substitutes in a simple design.
I have made up kits of parts for the 1971 G.C.Dobbs design but I
didn't want to encourage the use of germanium transistors and
LT700 output transformers. I propose this design as an
alternative. It is more complex and has twice as many
transistors, yes; But with the additional complexity it
works better, it is more consistent, you can get all the parts,
and it is cheaper.
Why do we need another design for a simple AM radio when there are
already so many?
Although there are many designs around, none of them seemed to
bring together all of the points below.
Uses entirely cheap modern components.
Is very insensitive to any component variation.
Has no integrated circuit black box components.
Can be built with a solderless assembly scheme.
The solderless assembly scheme is reliable and doesn't have to be
taken apart afterwards.
Has no fiddly adjustments.
Uses design principles which can be fully explained.
Uses no funny electronic tricks.
Works as near as possible to something that you might buy from a
shop.
Terminal Strip Construction
I've started from the assumption that soldering puts many people
off electronic projects. A good soldering iron isn't cheap,
burns the inexperienced quite readily and will set fire to the
house if it doesn't have a proper stand and falls off the desk if
left switched on unattended. It requires a small amount of
skill too. It's possible to make transistor projects on
plug-in breadboards, and then you can have ICs as well.
That's true. You will also generally find that the parts
fall out, the connections are terrible and you are told to take
your circuit apart at the end of the lesson so that the breadboard
can be used next time. What's the point in that?
Terminal strip isn't perfect. The components have to be quite
large, you have to get the wires under the screws, look out for
shorts, and when you finally give it a decent firm screw down at
the end, the component leads are not usually re-usable
afterwards. But you do get to take your project home and it
can be made reasonably reliable in operation.
Choccy Block Radio Detailed Circuit Description Choccy Block Radio FAQ
Should
you have any problems or
questions, my main email address
is shown below. This
address has been the same since
1997 and unless I'm on holiday
beyond mobile coverage, it is
checked daily including the spam
folder:
29-MAR-2017: Note to the effect that Q5 and Q6 may be supplied as
fully insulated parts.
24-JAN-2022: Email address image added as per other project pages.
26-AUG-2022: Removed references to my ebay shop, having been
"binned-off" from that site for telling the truth
about their business practices to
the UK vice-president.
