This is only a preview of the January 2006 issue of Silicon Chip. You can view 41 of the 120 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "Pocket TENS Unit For Pain Relief":
Items relevant to "“Little Jim” AM Radio Transmitter":
Items relevant to "Universal High-Energy Ignition System; Pt.2":
Items relevant to "Building The Ultimate Jukebox; Pt.2":
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A F lea - P o w er
The A M Broad
Little Jim
AM Transmitter
Dubbed “Little Jim”, this low-power “baby” AM radio transmitter
has a range of just a few metres, to keep it nice and legal. It’s ideal
for sending the output of your MP3 player or personal CD player to
your car radio, or for feeding recordings of vintage radio shows to
vintage radios, so they sound really authentic.
By JIM ROWE
W
HY WOULD YOU want a broadcast band AM transmitter with a
power output so low that it can only
be received inside a radius of about
four metres? Well, let’s say you’ve just
finished building a replica of a classic
1940s’ era AM radio, which you’re
entering into a club competition.
Wouldn’t it be great if you could tune
it into an “authentic” old time radio
32 Silicon Chip
program, to recreate the way it might
have sounded back then?
With this little transmitter you’ll be
able to do just that, by rebroadcasting
historic radio programs like those
available on CD from Screensound
Australia (see sidebar).
Alternatively, you might want to
play the music from your personal
MP3 or CD player through your car
radio when you’re driving to and from
work – but the radio lacks direct audio
inputs. With this little transmitter,
that’s no problem – although you will
need to modify it slightly so that it runs
from the 12V car battery.
In short, the whole idea of this
project is allow any line-level audio
signal to modulate an RF carrier in the
AM broadcast band, so that it can be
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Un i t F or
c as t Ban d
Fig.1: this is the block diagram for “Little Jim”. A tunable RF oscillator sets
the carrier frequency and this is them amplitude-modulated by the audio
signal. The modulator’s output is then amplified and fed to an antenna.
transmitter uses just a handful of parts
(including two transistors and the
modulator IC) and fits inside a standard UB3 sized plastic jiffy box. It’s also
low in cost and easy to build, as all
the parts fit on a small PC board. And
it’s run from either a plugpack power
supply or a 12V battery, so safety isn’t
a problem, even for beginners.
How it works
played through a nearby conventional
AM radio.
The carrier frequency of the transmitter can be tuned to virtually anywhere in the lower half of the broadcast band – ie, from 550kHz to about
980kHz. This allows you to choose
a frequency that’s away from any of
the broadcasting stations operating in
your area, to ensure interference-free
reception.
The audio quality from the transmitter’s signal is very close to that of
the program material you feed into
it, because it uses a special balanced
modulator IC. There’s also a modulation level control, so you can easily adjust the transmitter for the best
balance between audio volume and
minimum distortion.
But the best part is that the whole
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Although it’s extremely simple and
designed for very low output power,
“Little Jim” uses exactly the same
“building blocks” as a full-sized AM
radio transmitter. Fig.1 shows the
details – it consists of an RF (radio
frequency) oscillator, a modulator and
an RF output amplifier or “buffer”.
The job of the RF oscillator is to
generate a continuous sinewave signal of constant amplitude and with
a frequency in the range from 5501650kHz – ie, in the AM broadcast
band. This provides the transmitter’s
“RF carrier”, which is the frequency
you tune to with your AM radio to
receive the signal.
In most full-size AM transmitters,
the RF oscillator uses a quartz crystal and is fixed in frequency, so the
station concerned is always found at
exactly the same place on your radio’s
tuning dial. However, in this case, the
oscillator is tunable over the range
from 550kHz to about 980kHz, so you
can set the transmitter’s frequency
to a part of the band that’s currently
unoccupied in your area, for clear
reception.
The signal produced by the RF oscillator is fed into the modulator, which
is the “heart” of the transmitter. As
shown in Fig.1, this also receives the
audio signals from your MP3 or CD
player. In this case, the stereo signals
from the player are fed in via a simple
mixing circuit, to convert the signals
into mono. The resulting mono signal is then fed to the modulator via a
modulation (volume) control, which
sets the modulation level.
In operation, the modulator uses the
audio signal to vary the amplitude of
the RF signal (ie, it varies the amplitude of the carrier). When the audio
signal swings positive, the amplitude
Keeping It Legal
The AM transmitter described in
this article has very low RF power
output (a tiny fraction of a watt) and
is designed to have a range of no
more than about four metres.
Do not attempt to modify the
circuit with the aim of increasing
its power output or to increase its
range by feeding its output into a
much larger antenna, because this
would greatly increase the risk of
interfering with the reception of
licensed broadcasting stations.
It would also make you liable to
prosecution by the broadcasting
and spectrum management authorities and probable confiscation
of your equipment as well.
January 2006 33
Fig.2: the final circuit uses a Colpitts oscillator based on transistor Q1 to generate the carrier frequency which is
then modulated by the audio signal fed into pin 1 of IC1 (MC1496). The modulated RF signal is then amplified by
common-emitter amplifier stage Q2 and fed to the antenna. Potentiometer VR2 sets the modulation depth.
of the carrier is increased and when it
swings negative, the carrier’s amplitude is reduced.
As a result the RF output signal from
the modulator is varied up and down
in amplitude, directly in step with
the audio signals. In other words, the
RF carrier is “amplitude modulated”.
The waveforms in Fig.1 show the
basic idea.
Amplitude modulation or “AM”
is just one way of using an RF signal
to carry audio or other kinds of information from one place to another.
Another approach is to frequency
modulate the carrier and this transmission standard is called “FM” (for
frequency modulation).
The amplitude-modulated RF
output from the modulator is very
weak, so before it can be fed to our
transmitting antenna (which is just
a short length of wire) we have to
increase its level slightly by passing
it through the third building block:
the RF buffer amplifier. This stage
amplifies the modulated RF signal to
34 Silicon Chip
a level that’s just high enough to cause
weak radio signals to be radiated from
the antenna.
Circuit details
OK, so that’s the basic operational
details of the transmitter. Now let’s
take a look at the circuit diagram –
see Fig.2
The RF oscillator (which generates
the carrier signal) is a simple Colpitts circuit, based on transistor Q1.
This uses the primary winding of RF
transformer T1 as the inductive arm of
its resonant circuit, along with fixed
470pF and 100pF capacitors and a
miniature tuning capacitor (VC1). T1
is a miniature local oscillator coil from
a low-cost AM receiver coil kit.
The output of the oscillator is taken
from the secondary winding of T1.
This is then fed through a 4.7nF DC
blocking capacitor and a series 10kW
resistor to one of the two carrier inputs
(pin 10) of IC1, an MC1496 balanced
modulator specially designed for this
kind of use.
The second carrier input of IC1 is
pin 8 and is tied to ground potential
as far as RF signals are concerned
using a 10nF capacitor. However,
the IC needs both its carrier inputs
held at a DC bias level of about +6V
and that’s the purpose of the voltage
divider network involving the 1.5kW,
560W and 1kW resistors between +12V
and ground.
The 1kW resistor between pins 8 and
10 ensures that both carrier inputs are
biased at the +6V level. It also forms a
voltage divider with the 10kW resistor
from T1, to reduce the unmodulated
carrier level at IC1’s inputs to below
60mV RMS – the maximum level
which can be applied to its carrier
inputs for undistorted output.
IC1’s audio modulating signal inputs are at pins 1 and 4 and these have
to be biased lower than the carrier
inputs, to about +4V DC. This voltage
is provided across the lowest 1kW
resistor in the main bias divider and
fed to the two audio inputs (pins 1 &
4) via two 1.5kW resistors. In addition,
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the audio inputs are connected via
10kW resistors to trimpot VR1, which
allows fine adjustment of their relative
bias – and hence the modulator IC’s
operation.
The stereo audio input signal is fed
into the unit via jack socket CON2 and
mixed together via two 10kW resistors
to form a mono signal. This signal is
then fed to modulation depth control
VR2.
In addition, two 10kW resistors have
been connected between the audio
inputs of CON2 and ground. These
are used to provide suitable loads for
your CD or MP3 player line/headphone
outputs. If your particular player needs
loads of less than 10kW, these two resistors can be reduced in value.
As shown in Fig.2, the modulating
signal from VR2 is fed to just one of the
modulator’s audio input pins – in this
case, to pin 1 via a 4.7mF DC blocking
capacitor. The second input (pin 4) is
tied to ground via a 100mF capacitor,
so the full audio (AC) voltage from VR2
is effectively applied between the two
input pins.
The 1kW resistor connected between
pins 2 & 3 of IC1 is used to set the
internal gain of the modulator, while
the 10kW resistor from pin 5 to +12V
sets the IC’s internal bias and operating
current level.
or four metres, despite its very low RF
power output.
Modulated carrier outputs
Construction
The modulated carrier outputs from
IC1 appear at pins 6 & 12, which are
both connected to the +12V rail via
3.3kW load resistors. In this circuit,
we only use the output from pin 12
and this drives the base of RF amplifier transistor Q2 via a 12kW resistor.
Transistor Q2 is connected as a simple common-emitter amplifier stage,
with an unbypassed emitter resistor
to ensure low gain and stability. Its
amplified output is developed across
the collector load formed by L1, a
broadcast-band antenna coil wound
on a very small ferrite rod.
As well as forming Q2’s collector
load, L1 actually forms part of the
transmitter’s antenna, because the
ferrite rod inevitably radiates some
RF energy. However, its very small
size makes it a rather poor radiator,
so an external wire antenna (about
two metres long) is also connected
to Q2’s collector via a 10nF coupling
capacitor.
This “dual antenna” system gives
the transmitter a range of about three
Construction is easy, with all the
parts mounted on a small PC board
measuring 122 x 57.5mm. This board
has cutouts in each corner, so it can
fit snugly inside a standard UB3 size
jiffy box.
Note that there are actually two
slightly different versions of the PC
board, to suit the two different 3.5mm
stereo jacks sold by kit suppliers. The
board coded 06101061 suits the jack
sold by Dick Smith Electronics, while
the version coded 06101062 suits the
jack sold by both Jaycar Electronics
and Altronics.
There are no other differences – apart
from the provisions for mounting the
different 3.5mm jacks (CON2), both
board versions are identical.
Fig.3 shows the assembly details.
Begin the by fitting the PC board
terminal pin for the antenna wire
connection, located just to the right
of the antenna rod, then fit DC input
connector CON1 and the audio input
jack CON2.
That done, you can install the re-
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Power supply
The circuit is powered from a regulated 12V rail and this is derived from
the mains via a 12V DC plugpack supply, diode D1 and 3-terminal regulator
REG1.
A 12V DC plugpack supply is specified, since these typically deliver 1516V when only lightly loaded. The
transmitter circuitry draws less than
40mA in operation, which means
that REG1 has quite enough “head
room” to provide a well-regulated
+12V output.
Diode D1 provides reverse polarity
protection, to prevent the circuit from
damage if the supply is connected the
wrong way around.
Alternatively, for use in situations
where no mains power is available,
the transmitter can be powered from
a 12V battery (eg, a car battery). This
involves removing REG1 and replacing
it with a wire link between its input
and output connection pads. More
about this later.
Finally, LED1 is used to provide
power-on indication. It’s connected
across the 12V supply in series with
a 1kW current-limiting resistor. (ie, the
current through the LED is 10mA).
Par t s Lis t
1 PC board, code 06101061
(DSE version) or 06101062
(Altronics and Jaycar versions),
122 x 57.5mm
1 UB3-size jiffy box (130 x 67 x
44mm)
4 M3 x 10mm tapped spacers
9 M3 x 6mm machine screws,
round head
1 M3 hex nut
1 mini RF oscillator coil in can
(T1 – red slug)
1 Ferrite rod, 55mm long, with
BC band coil (L1)
1 Mini tuning capacitor 60160pF, with disc-type knob
and mounting screws (VC1)
1 2.5mm concentric DC socket,
PC-mount (CON1)
1 3.5mm stereo jack, PC-mount
(CON2)
1 mini control knob (to suit VR2)
2 cable ties, 100mm
1 PC terminal pin, 1mm diameter
1 2m length of insulated hookup
wire
1 50kW horizontal trimpot (VR1)
1 50kW log pot, 16mm PC-mount
(VR2)
Semiconductors
1 MC1496 balanced modulator
(IC1)
1 7812 +12V regulator (REG1)
2 PN100 NPN transistor (Q1,Q2)
1 3mm green LED (LED1)
1 1N4004 silicon diode (D1)
Capacitors
1 220mF 25V RB electrolytic
1 100mF 16V RB electrolytic
1 22mF 16V RB electrolytic
1 10mF 16V RB electrolytic
1 4.7mF 16V tantalum
2 100nF monolithic
1 10nF metallised polyester
1 4.7nF metallised polyester
2 470pF NPO disc ceramic
1 100pF NPO disc ceramic
Resistors (0.25W 1%)
2 15kW
3 1.5kW
1 12kW
4 1kW
8 10kW
1 560W
2 3.3kW
1 470W
sistors. These are not polarised, so
you can fit them either way around
although it’s best to have their colour
codes all running in the same direcJanuary 2006 35
The PC board is mounted on the lid of the case using four
M3 x 10mm tapped spacers and eight M3 x 6mm machine
screws. Note how the antenna rod is secured using plastic
cable ties.
tion to aid checking later on. Table 2
shows the resistor colour codes but
you should also check each unit with
a digital multimeter before installing
it, just to make sure.
The non-polarised ceramic, monolithic and metallised polyester capacitors can go in next. Again, these can
again go either way around but be
sure to fit the correct value in each
position. Once they’re in, install the
larger polarised capacitors. These include the 4.7mF tantalum unit which
goes just below VR2 and the four RB
electrolytics. Note that these must all
be fitted with the correct polarity, as
shown on the layout diagram.
The final capacitor to fit is tuning capacitor VC1. This fits on the top of the
board, with its spindle stub shaft and
three connection tabs passing down
through matching holes in the board.
The board is then turned over and the
capacitor body attached to the board
using two of the M2.5 x 4mm screws
supplied with it. Don’t lose the third
screw, though – you’ll need it later to
attach the disc knob to VC1’s spindle.
Now solder VC1’s three connection
tabs to their board pads.
The oscillator coil T1 is next on
the list. This is effectively polarised,
because there are three connection
pins on one side of its base and only
two on the other – be sure to orient
36 Silicon Chip
it correctly before pushing it all the
way down onto the board. There are
seven solder connections to make in
all – five pin connections plus two for
the can lugs.
Trimpot VR1 and modulation control pot VR2 can now go in, after which
you can fit the semiconductors – diode
D1, transistors Q1 and Q2, IC1 and
LED1. These parts are all polarised
so be sure to install them as shown
in Fig.3.
LED1 should initially be installed
with its body about 20mm above the
board (this can be done by sliding a
20mm-wide cardboard spacer between
its leads and pushing the LED down
onto this spacer). Its leads should then
be bent down through 90° at a point
about 14mm above the board, so that
the LED faces away from the board and
will later protrude through a matching hole in the side of the case during
final assembly.
Mains or battery power
If you intend running the transmitter
from a mains plugpack, install regulator REG1 in the position indicated. As
shown in Fig.3, this is mounted horizontally on the board, with its metal
tab secured by an M3 x 6mm machine
screw and nut.
To do this, first bend its leads down
by 90° at a point 6mm from its body,
then fit it to the board and secure its tab
using the screw and nut. That done, its
leads can be soldered to their respec-
tive pads. Don’t solder its leads before
securing the tab. If you do, the solder
joints could fracture due to stress as
the screw is tightened.
Alternatively, if you intend running
the transmitter from a 12V battery,
REG1 is left out and a small wire link
fitted instead. This link should be
fashioned from a short piece of tinned
copper wire (or a resistor lead offcut),
bent in an inverted-U shape with its
centre section just over 5mm long. This
is then fitted between the two outer
connection lead holes for REG1 and
soldered to the pads underneath.
Antenna rod & coil
The final component to fit to the
transmitter board is the antenna rod
and coil assembly (L1). This is secured
using two small cable ties, each of
which loops around under the board
through the pairs of 3mm holes provided for this purpose.
(Note: do not replace the cable
ties with wire or any other metal
bands. A metal loop would form a
“shorted turn” and this would absorb
RF energy and seriously degrade the
performance).
Unfortunately, making the coil’s
connections to the board can be a bit
tricky. In most cases, there are four
leads and it’s not easy to work out
which are the correct two to use – ie,
the actual start and finish of the coil.
In fact, the only reliable way to
identify the start and finish leads is to
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TUNING
Fig.3: install the parts on the PC board as shown here, taking care to keep all
component leads as short as possible. Note that board has been designed to accept
both 16mm and 24mm pots for VR2 (although a 24mm pot would not allow the
board to fit inside the specified UB3 case.
check all lead combinations with an
ohmmeter and go with the combination that gives the highest reading –
typically around 11W.
Another little trap is that with
many of these coils, the intermediate leads actually consist of two fine
gauge insulated wires, twisted tightly
and soldered together at their outer
ends. This means that if you decide
to cut these leads short, they must be
bared and soldered together again –
otherwise you’ll find that the coil has
become an open circuit between start
and finish. And of course, the transmitter won’t function very well with L1
open circuit, as this prevents Q2 from
drawing current!
A word of advice: if you do shorten
any of the coil leads, it’s a good idea
to check the coil continuity with your
multimeter before you solder the start
and finish leads to the board.
The last step in wiring up the board
is to solder the end of a 2-metre length
of insulated hookup wire to the “ANT
WIRE” terminal pin at the end of the
antenna rod. That done, it’s time to fit
the tuning “disc knob” to VC1’s shaft
and fasten it in place using the remaining M2.5 x 4mm screw.
The board assembly is now ready to
attach to the box lid (used here as the
transmitter’s base). Before doing this,
however, you may need to drill and
cut the various holes in both the lid
and the box itself, if you’re building
the project from scratch. The location,
size and shape of each of the holes is
shown in Fig.5.
Alternatively, if you’ve purchased
a complete kit, the box will probably
be supplied predrilled, with screened
graphics for the front panel.
The PC board assembly is secured to
the lid using four M3 x 10mm tapped
Table 1: Capacitor Codes
Value
100nF
10nF
4.7nF
470pF
100pF
μF Code
0.1µF
.01µF
.0047µF
NA
NA
EIA Code
104
103
472
470
100
IEC Code
100n
10n
4n7
470p
100p
spacers and eight M3 x 6mm machine
screws (see photo). Once that’s been
done, it’s time to check the transmitter’s operation.
Checkout & adjustment
It’s easy to check and adjust the
transmitter’s operation using a frequency counter, an oscilloscope and an
audio signal generator. However, these
are not essential and you can do the
Table 2: Resistor Colour Codes
o
o
o
o
o
o
o
o
o
siliconchip.com.au
No.
2
1
8
2
3
4
1
1
Value
15kW
12kW
10kW
3.3kW
1.5kW
1kW
560W
470W
4-Band Code (1%)
brown green orange brown
brown red orange brown
brown black orange brown
orange orange red brown
brown green red brown
brown black red brown
green blue brown brown
yellow violet brown brown
5-Band Code (1%)
brown green black red brown
brown red black red brown
brown black black red brown
orange orange black brown brown
brown green black brown brown
brown black black brown brown
green blue black black brown
yellow violet black black brown
January 2006 37
Fig.4: this is the full-size front-panel artwork. It can be cut out and used directly
if required and can be protected using wide strips of adhesive tape.
job quite well using just a multimeter
(preferably a DMM) and a reasonably
sensitive AM radio receiver.
The step-by-step adjustment procedure is as follows:
(1) Switch the radio on and tune it to
a convenient frequency in the lower
section of the broadcast band, away
from any of the local broadcasting
stations (in Sydney, you can tune to
about 820kHz).
(2) Turn the volume up (you’ll just
hear static at this stage) and position the radio near the transmitter,
orientated so that its internal ferrite
rod antenna is roughly parallel to the
transmitter’s ferrite rod.
(3) Turn the transmitter’s tuning control (VC1) to one end of its range, set
trimpot VR1 well away from its centre
position (this is important) and set VR2
(modulation depth) to its midrange
position.
(4) Turn the adjustment slug in T1
anticlockwise a couple of turns using
a small screwdriver or alignment tool.
(5) Feed a stereo audio signal from
your MP3 or CD player into the trans-
mitter by plugging the audio cable
into CON2.
(6) Apply power to the transmitter
and check that the power LED (LED1)
lights. If it doesn’t, unplug the power
lead and look for your wiring mistake.
You’ve probably fitted either D1 or
LED1 with reversed polarity.
(7) Use your DMM to check the supply voltage at the output pin of REG1,
relative to board earth; it should be
very close to +12V. Check that the
voltage at pin 8 of IC1 is close to +6V
(if these voltages check out correctly,
your transmitter is very likely to be
working correctly).
(8) Listen carefully to the radio while
you turn the transmitter’s tuning knob
very slowly towards the other end of its
range. At some point, you should start
to hear the music from your MP3 or CD
player, after which you should be able
to tune the transmitter so that its signal
is received at a good strength.
Troubleshooting
Can’t find the signal? The first thing
to do is to try tuning the transmitter
Vintage Australian Radio Programs On CD
If you’d like to rebroadcast genuine old time Aussie radio programs
through your “Little” Jim AM Transmitter, you should know that many of the
programs are now available on CD from
ScreenSound Australia (the National
Screen and Sound Archive).
Currently they have some 11 different CDs available, with classic “golden
age of radio” programs, including quiz
shows, serials like Dad & Dave and
38 Silicon Chip
Mrs ’Obbs, comedies like The Bunkhouse Show and McCackie Mansion,
and so on. All CDs are currently available for $24.95 each, including GST
(but not postage).
For more information on what’s
available, visit the ScreenSound
website at shop.screensound.gov.
au. You can even buy the CDs direct
via their secure online purchasing
system.
back the other way but even more
slowly and carefully than before. If this
still doesn’t bring success, try turning
the adjustment slug in oscillator coil
T1 anticlockwise another half-turn
(or even a full turn if this later proves
necessary). This will shift the oscillator’s tuning range up in frequency and
should allow you to correctly adjust
the transmitter when you tune VC1
over its range again.
If you still can’t find the transmitter’s signal, it may be that its output
is a little too weak to be picked up by
the receiver. In that case, try draping
the transmitter’s antenna wire over
the receiver, or twist it around the
receiver’s telescopic FM antenna if it
has one, just to couple in a bit more
of the transmitter’s output.
Once you’ve found the signal and
adjusted the transmitter’s tuning con
trol for the best reception, try turning up the transmitter’s modulation
control (VR2). This should make the
reception even louder and clearer
but if you turn the control up too far,
the music will become distorted. Just
back it off again until the distortion
disappears.
You can also try adjusting trimpot
VR1, because a small amount of adjustment one way or the other can
also improve transmission clarity.
That said, you’ll find that its optimum
position is about halfway between the
centre and one of the end positions of
the rotor (on either side).
Don’t set this trimpot (VR1) too close
to its midway (centre) position, because this balances out the RF carrier
altogether and gives double sideband
(DSB) suppressed carrier modulation.
And that gives and quite high distortion when you’re using a normal AM
receiver.
Once all the adjustments have been
made, your Little Jim AM Transmitter
is working correctly and you’re ready
for the final assembly.
Final assembly
If your UB3 box has vertical PC
board mounting ribs inside, you’ll
also have to cut some of these away.
That’s because the transmitter board
assembly is a fairly tight fit inside the
box and the ribs foul the ferrite rod
and its coil.
The ribs to remove are mainly those
at the rear side of the box, where they
interfere with the ferrite rod. However
it’s also a good idea to cut away any
siliconchip.com.au
Fig.5: this diagram shows the
drilling and cutout details for
the plastic case.
siliconchip.com.au
January 2006 39
Why Did We Call It “Little Jim”?
Now then, perhaps we should explain the “Little Jim” monicker. Why not
“Little Harry”, or “Little Jack”, or “Little
Curly” or “Little Mary”? Come to think
of it, why “Little” anything?
The answer to that question can
be found in two May 1938 issues of
“Wireless Weekly”, the forerunner to
“Radio & Hobbies” magazine which
itself later evolved into “Radio, TV &
Hobbies” and finally “Electronics Australia”. Those 1938 issues of “Wireless
Weekly” described the construction
of a 1-valve AM radio receiver which
they called – you’ve guessed it – “Little Jim”.
The headline to the article was
“Little Jim – Brings Test Play To Your
Bedside!”. Don’t get excited – they
were talking about the cricket!
“Little Jim” was pretty simple as
AM radios go, using just a single 6A6
twin-triode valve as both a regenerative detector and audio amplifier. It
generated sufficient output to drive a
pair of headphones and the original
was built into an old butter box with
an aluminium front panel. A 45V B
battery generated the high tension
(HT), while the 6.3V AC filament supply was derived from the 240V mains
via a transformer.
You could build “Little Jim” by
scrounging the parts yourself but there
was also a kit available. Yes, they had
kits back in those days and “Little Jim”
was available as a kit of parts (without
the cabinet) for the princely sum of
four pounds from a company called
Foxradio (Fox and MacGillicuddy) of
57 York St, Sydney.
Of course, we’re not too sure
ribs on the end near the holes for CON1
and CON2, because these can make
final assembly more difficult. You
should also cut away any ribs on the
front of the box, around the holes for
LED1 and VR2, as this make the final
assembly even easier.
The ribs are easy to remove. The
ABS material used in these boxes is
fairly soft and can be cut away using
a sharp hobby knife or small wood
chisel.
Once the ribs are gone, remove the
knob from modulation pot VR2 (if you
40 Silicon Chip
The original “Little
Jim” was a 1-valve
AM receiver built into
a modified butter box,
with an aluminium
front panel.
Fig.6: the circuit
used a single
6A6 twin-triode
valve as both
a regenerative
detector and
audio amplifier.
It generated
sufficient output
to drive a pair of
headphones
(actually, we have no idea) why the
“Wireless Weekly” editors called
their receiver “Little Jim” but no matter. That was the name it was given
and it proved to be very popular – so
popular, in fact, that it was republished
in the very first issue of “Radio & Hobbies” magazine, in April 1939.
That set was followed by a full
battery-powered version dubbed “Little Jim’s Mate” in the May 1939 issue.
But it didn’t end there, with lot’s more
have fitted it for the checkout) and
unscrew the nut from VR2’s ferrule.
That done, thread the free end of the
transmitter’s antenna wire through the
small hole in the rear of the box (from
the inside) and pull most of it through
the hole. You can now introduce the
box to front of the lid/board assembly
at a suitable angle, passing VC1’s disc
knob through its slot and LED1 and
VR2’s shaft through their respective
holes.
Next, swing the box down over the
board assembly, pulling the remain-
variations published in subsequent
years. In short, there were lots of “Little
Jims” and his “mates” published during
the valve era.
So that’s where we got the name
from. When Jim Rowe came in with
his new flea-powered AM transmitter, we initially struggled to come
up with a good name for it. “Why not
Little Jim?”, said the office smart-elec
and despite the groans all round, the
name stuck.
ing antenna wire through its hole as
you do so. As it comes down, slide it
slightly towards the CON1/CON2 end,
so that the ferrule of CON2 enters its
clearance hole.
That done, you can fit the nut to
VR2’s threaded ferrule. Tighten it
firmly and then refit the knob.
Finally, turn the assembled box over
and fit the four supplied self-tapping
screws supplied to fasten everything
together. Your “Little Jim” AM Transmitter is now finished and ready for
SC
action.
siliconchip.com.au
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