This is only a preview of the March 2007 issue of Silicon Chip. You can view 32 of the 104 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 "Programmable Ignition System For Cars; Pt.1":
Items relevant to "Remote Volume Control & Preamplifier Module; Pt.2":
Items relevant to "GPS-Based Frequency Reference; Pt.1":
Purchase a printed copy of this issue for $10.00. |
Vintage Radio
By RODNEY CHAMPNESS, VK3UG
The EILCO 6104 lunch-box RFDS radio
This view shows the set with
the front cover removed,
ready to be connected to a 12V
battery, antenna and earth.
Radio transceivers designed for use with
the Royal Flying Doctor Service (RFDS)
are now scarce but well worth collecting.
My EILCO RFDS radio transceiver is one
recent acquisition that I’ve been able to fully
restore.
T
HE AERIAL MEDICAL SERVICE
(AMS) commenced operation in
1928 from Cloncurry in north-west
Queensland, providing medical assistance to people in the outback. Before
then, with no telephones or good roads
in areas remote from Cloncurry, it was
extremely difficult for people in those
areas to access medical services – even
though Cloncurry boasted a wellequipped hospital.
By contrast, the AMS had a doctor
88 Silicon Chip
who could fly out to visit people in
need of medical attention. Subsequently, the AMS became much more
effective when, in 1929, the first radio
link in what was to become the Royal
Flying Doctor Service (RFDS) commenced at Cloncurry.
The radios in use at station homesteads at that time were extremely
simple, consisting of a 1.5W singlevalve Morse code transmitter (crystal
controlled) and a 2-valve regenerative
high-frequency (HF) TRF receiver. The
base station was much more complex,
as it transmitted voice with a power
of 50W and used a high-performance
receiver in order to receive the lowpowered homestead transmissions
(see “Outback Radio from Flynn to
Satellites” by Rodney Champness
for more details on the early days of
the RFDS and the radios used in the
outback).
The RFDS radios evolved over the
years from sets made (initially) almost
exclusively by Traeger Transceivers
to those made by a number of other
manufacturers. Traeger produced
many fine transceivers over the years
but their methods of construction and
the designs used eventually became
outdated.
Traeger had pioneered the use of
plug-in modules for the transmitters
and receivers. This technique worked
siliconchip.com.au
extremely well when only three channels were allocated for a particular
flying doctor network. It meant that
if people were transferred to another
network or if a frequency change for
a network was necessary, then all that
had to be done was to send a plug-in
module out to the affected stations.
This module could be installed and
correctly adjusted by relatively nontechnical people.
Unfortunately, this approach subsequently became cumbersome and
expensive when more than five channels were needed.
The birth of EILCO
One of the companies that rose to the
challenge of producing transceivers
suitable for use in the outback was the
Electronics, Instrument and Lighting
Company Pty Ltd (EILCO), which is
known these days as Codan. EILCO
was started by three University of
Adelaide graduates – Ian Wall, Alistair
Wood and Jim Bettison – in 1959. As
a sideline, before the establishment
of EILCO and before graduating, they
often repaired equipment for the university and in some cases built better
replacement equipment.
In 1961, they were asked to complete the construction of some HF
radio equipment for the Anglican Bush
Church Aid Society at Ceduna. They
looked at the design and the partly
constructed transceivers and decided
that a different approach to the job
would be better for all concerned. The
idea was accepted and the EILCO 6104
transceiver was born.
The set itself is about the size of a
lunch box. It has five crystal-locked
transmit and receive frequencies and
an 8W HF AM transmitter which covers a nominal frequency range from
2-7MHz.
The EILCO 6104 subsequently prov
ed to be very popular as a portable
transceiver with the RFDS networks,
mineral exploration teams, government departments and many other
groups that were just realising the value of communications in the outback.
The set was easy to use – it was only
necessary to pull up along the track,
open the set up, put the battery clips
onto the 12V vehicle battery, sling up
a wire antenna into a tree (hopefully
there was one nearby, even if stunted),
attach an earth, adjust the tuning controls and call the base station.
In short, the design was a big sucsiliconchip.com.au
This above chassis view of the 6104 shows the unit with the power supply
module (bottom) swung out of the way. It normally sits above the PC board.
cess and Codan has since grown into a
well-known and respected company in
the field of HF radio communications
equipment.
The 6104
As mentioned earlier, the 6104 was
initially designed for use with the Anglican Bush Church Aid Society. However, it’s a sure bet that the founders of
EILCO saw that the set would also be
suitable for use by other organisations
with only slight modifications to the
basic transceiver.
By the way, the set was the fourth
unit that EILCO designed in 1961,
hence the 6104 type number. However, while the first two digits indicate the year of the design, they do
not necessarily indicate the year of
manufacture.
The set itself is built into a metal
“lunch box” case measuring 295mm
long (including protrusions), 210mm
high (including handle) and 105mm
wide. As shown in the photo, removing
the lid reveals a very neat and uncluttered transceiver control panel.
The far left controls are used to tune
and load the transmitter to the antenna
in use, on any particular frequency
selected. As an additional aid to tuning
“unusual” antennas, a control marked
1-2-3 is also fitted.
The two terminals along the top
of the control panel with the torch
globe between them are the antenna
and earth terminals. The torch globe
is used as an indicator to show when
the transmitter is correctly tuned – ie,
when the globe is at its brightest.
Note that when the transmitter is
correctly tuned to the antenna, the
receiver is also tuned. That’s because
March 2007 89
Fig.1: the receiver circuit uses eight germanium transistors and shares its antenna input stage with the transmitter.
TR1 is the RF amplifier, TR2 functions as the converter stage and TR5 is the local oscillator. Transistors TR3 and
TR4 are IF amplifier stages, while TR6-TR8 make up the audio amplifier.
the receiver uses both the loading/
tuning circuitry and the transmitter
output circuit as its input circuit. This
is an advantage when the transmit
and receive frequencies are the same
or do not differ greatly. Conversely, if
they do differ greatly, the receiver’s
sensitivity will be severely reduced.
The remaining control at the top of
the panel is used to switch between
transmit and receive. In the receive
position, only the fully-transistorised
receiver is operating and this draws
around 25mA with no signal input.
The set can therefore be used for
monitoring for long periods without
flattening the 12V battery (after all,
who wants to have a flat battery in
their vehicle)!
In the standby position, the transmitter’s valve heaters are turned on
(so that it is ready to operate) and the
current drain rises to around 0.8A.
And finally, in the transmit position,
the transmitter is operating and the
current drain rises to around 3A.
90 Silicon Chip
The remaining controls on the front
panel are an on/off volume control
(lower centre) and the channel-change
knob (bottom, far right).
Receiver circuit
The 6104 was built in several variants, the two main ones being the Mk.1
and the Mk.2. The main differences
between these two variants are in the
receiver.
The unit I have is the Mk.2 with
the 6415 receiver. This receiver is a
plug-in unit and can either be used in
the 6104 or used as a separate local or
remote monitoring receiver with one
or more channels. However, when
remotely controlled, only one channel
was commonly fitted.
In particular, the RFDS and many
other HF services used the 6415 and
similar units as remote receivers, locating them well away from sources of
electrical noise, such as towns.
Fig.1 shows the circuit details for
the receiver. As shown, it is an 8-tran-
sistor unit based on second-generation
germanium transistors.
As stated previously, the antenna
input circuit is shared with the trans
mitter. Following this stage in an RF
amplifier based on TR1 and its input is
protected from high-level transmitter
signals – either from its own transmitter or another nearby transmitter – using an OA5 diode connected between
base and emitter.
Transistor TR2 functions as a converter stage, while TR5 functions as
the local oscillator. Note that the local
oscillator is crystal controlled.
The following IF stage is based on
transformers T1 and T2, with a ceramic
filter between the two transformers. It
operates at 455kHz and the IF input
circuitry establishes the shape of the
IF amplifier response curve.
Transistors TR3 and TR4 function as
RC-coupled IF amplifier stages. These
in turn drive transformer T3 and the
detector/AGC diode (D2). The resulting AGC voltage is applied to the base
siliconchip.com.au
Fig.2: the transmitter circuit is a hybrid design, with valve V1 functioning as a Pierce oscillator and crystals X1-X5
setting the output frequency. V2 is the power amplifier output stage, while the modulator makes use of the receiver’s
audio amplifier stage to amplify the microphone signal. This signal is then fed to he modulator’s output stage
which is based on TR3 and TR4
of TR1 which in turn controls the gain
of transistors TR2 and TR3 in the RF
and IF amplifier stages.
In addition, the audio signal from
the detector is applied via a volume
control to a 2-stage audio amplifier
based on TR6-TR8. The output from
this amplifier is fed to a 50mm loudspeaker on the front panel.
By the way, the symbol used for the
transistors in Fig.1 may seem unusual,
particularly for younger readers. In
fact, it is one of the first symbols used
for transistors and “Electronics Australia” magazine used it regularly in
the 1960s.
Transmitter circuit
The RF section of the transmitter is
siliconchip.com.au
This is the fully
restored Eilco
6104 transceiver
in its “lunch box”
metal case. The
case was restored
by powder
coating it (cost
$40) and it now
looks like new.
March 2007 91
and TR2 at the left of the transmitter
circuit or transistors TR5 and TR6 at
the lower left of the transmitter circuit.
The choice here depends on whether
NPN or PNP power transistors are used
in the inverter.
The modulator makes use of the
receiver’s audio amplifier stage, so
this stage does double duty. In practice, this involves switching the audio
amplifier’s input from the receiver’s
detector to the microphone’s output
instead. The amplified microphone
signal is then fed to the modulator’s
output stage which consists of TR3
and TR4.
This stage provides about eight watts
of audio to fully modulate V2 which is
the PA (power amplifier) valve.
Restoring the 6104
These two photos show the top and bottom chassis views of the EILCO 6104
transceiver (taken from the rear). The two valves are used in the oscillator
and power amplifier (PA) stages of the transmitter section (see Fig.2).
based on conventional valve circuitry.
As shown, it uses a Pierce oscillator
circuit based on a 6CK6, with crystals
X1-X5 setting the frequency. This stage
controls the grid of the power amplifier
(PA) which is built around a 6CW5.
Coil L2 and its associated parallel
capacitors are used to tune the transmitter output. Final output tuning and
antenna matching is then performed
by the tappings on the righthand side
92 Silicon Chip
of coil L2 in conjunction with C12,
L3 and the components connected to
switch SW2.
A concise set of operating instructions is glued to the inside of the
removable lid on the top of the carry
case. Note that the valve stages in
the transmitter require a high voltage
supply of 300V DC. This is obtained
from a transistor-based DC-DC inverter
consisting either of transistors TR1
The transceiver I obtained was in
quite good order internally but externally it was a different matter. It had
had a hard life in the outback and still
had Dymo labels around the channel
change knob, indicating that the RFDS
frequencies were installed.
Dismantling the set is not difficult
and simply involves removing two
screws at the righthand end of the
case and two at the bottom. Once this
is done, the set simply slides out of
the case.
The chassis was quite clean and it
was easy to access the various sections.
Despite a thorough examination, I
found no problems with the circuit and
it’s nice to have a restoration project
once in a while that requires relatively
little work.
I decided to tune the set up on
3565kHz, as I had crystals that suited
that frequency which I had removed
from another transceiver. Initially, I
installed a 4020kHz crystal into the
channel 1 position of the receiver
(4000kHz - 455kHz gives a receive
frequency of 3565kHz). That done, I
was able to tune up the receiver by
selecting various coil tappings and
adjusting the trimmer capacitors for
optimum performance.
The antenna coil can only be finally
tuned when the transmitter is aligned.
The IF was OK as the ceramic filter is a
fixed-frequency device and won’t shift
frequency unless it is faulty. Transformers T1-T3 were also checked for
alignment and were quite OK.
The transmitter was a little more
difficult to tune up, mainly because I
didn’t have the alignment instructions.
siliconchip.com.au
However, after working on many
fixed tuned transmitters and receivers
over the years, a reasonable “guestimation” of suitable taps and capacitor
values can be arrived at.
Of course, only people with an appropriate transmitting licence should
have a working transmitter. I experimented with the tappings on L2 and
the values of the parallel capacitor as
selected by switches SW1f and SW1g.
In this case, getting the right tapping
and capacitor value is a bit like experimenting with the tappings on a crystal
radio to get best performance.
Eventually, I got it operating as
it should, which meant that the receiver’s RF stage was now also aligned
correctly.
Photo Gallery: AWA R39 Battery Receiver
Repairing the case
As indicated earlier, the metal case
was rather the worse for wear. In the
end, I figured that there were others
far more capable than I when it came
to fixing the scratches and abrasions.
As a result, I removed most of the
handles and clips, although I couldn’t
remove the clips at the end of the case.
That done, I took the case to a local
powder coating firm and they did a
first class job on it – so much so that
it now looks like new. At $40, it was
a job well done.
The control panel was in reasonable
order but I did have to remove the old
Dymo labels. Unfortunately, glue had
been used around the labels and this
proved to be so difficult to remove
without damaging the front panel that
I just cleaned it as best I could.
MANUFACTURED BY AWA in 1937, the R39 is a 4-valve battery-powered
receiver requiring 120V HT, 2V for the valve filaments and bias voltages of
-1.5V and -4.5V. The filament and bias voltages were all obtained from an
internally fitted, tapped, battery.
The valve line-up was as follows: 1C6 frequency changer; 1C4 IF amplifier;
1K6 reflexed 2nd IF amplifier/1st audio amplifier/detector/AVC rectifier; and
1D4 audio output. Photo: Historical Radio Society of Australia, Inc.
Summary
The EILCO 6104 was one of the first
truly “lunch box” size portable HF
transceivers used in the outback. The
outback of Australia has been a harsh
testing ground for any equipment and
the 6104 (and most other Australiandesigned radio equipment) stood the
test of time out there.
My unit works well but unfortunately it can not be used today on
the amateur radio bands, as it is restricted to only five frequencies and
to AM-only transmissions. By today’s
standards, it is well and truly obsolete.
That said, my 6104 is a worthwhile
addition to my small collection of
RFDS radios, dating from 1948 onwards. Early RFDS radios are scarce
but some of the later ones are still
available occasionally. They are well
worth collecting, as they form part of
SC
our unique radio heritage.
Looking for real performance?
•
•
•
•
Learn how engine management systems work
160 PAGES
Build projects to control nitrous, fuel injection and turbo boost systems
23 CHAPTE
RS
Switch devices on and off on the basis of signal frequency, temperature and voltage
Build test instruments to check fuel injector duty cycle, fuel mixtures and brake & temperature
Mail order prices: Aust. $A22.50 (incl. GST & P&P); Overseas $A26.00 via airmail. Order by phoning (02) 9939
3295 & quoting your credit card number; or fax the details to (02) 9939 2648; or mail your order with cheque
or credit card details to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097.
94 Silicon Chip
From the publis
hers of
Intelligent
turbo timer
I SBN 0958522
94
-4
TURBO BO
OST
&
nitrous fuel cont
9 78095
8
5229
rollers
46
$19.80 (inc GST) NZ
$22.00 (inc GST)
How engin
e
management
works
siliconchip.com.au
|