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VINTAGE RADIO
By RODNEY CHAMPNESS, VK3UG
The AWA FS6 army transceiver
A significant number of vintage radio buffs
collect and restore amateur, military and
commercial radio equipment. Along with
the more familiar domestic radio gear, this
equipment is also a part of our radio
heritage and an interesting part of it at that.
The front panel layouts of military
radios are purely utilitarian and they
don’t look a million dollars in matt
khaki or some other unexciting colour.
Aesthetics was not one of the design
principles and there are no elegant
timber consoles here. However, it is
interesting to compare them in many
ways with the domestic radios of the
same era.
During WWII and on the military
surplus market in the late 1940s and
early 1950s, the FS6 high frequency
(HF) portable army transceiver was
common. Many amateur radio operators modified them for use on amateur
radio bands and they were also used
by bushfire brigades, the Flying Doctor
Service (WA), fishermen and other
organisations.
However, the FS6 is not the sort of
gear that the majority of vintage radio
buffs collect. There are various reasons
for this, the most common being that
there are now very few such units
around.
General description
The 1943 provisional training man
ual on the FS6 gives us a few pertinent
facts about the equipment, which I
have paraphrased. It is quoted as a
medium range, portable combination
sender-receiver, suitable for pointto-point communication. It has a frequency range of 4.2-6.8MHz and will
operate on both W/T (CW telegraphy,
Morse) or R/T (radio telephony).
For transporting over short distances, the set can be classed as a 2-man
pack. The weight of the station is listed
at 157lb, or about 71.4kg (I’m glad they
said “short distances”) and it can be
used as a ground station or as a mobile
truck station.
The receiver has two headphone
outlets so that two opera
tors can
operate the set at the same time. The
receiver is a 5-valve superheterodyne
with AVC, while the sender (transmitter) has three valves consisting of a
master oscillator, a modulator and an
RF power amplifier.
The power source is a 6V, 75Ah
battery. A 6V vibrator power supply
provides all of the HT and bias voltages and the set draws 3.2A on receive
and 6.0A on transmit W/T. If a pedal
generator is used, only a 25Ah battery
is needed to power the set.
Background to the FS6
This view shows the FS6 transceiver with the valve access panel open. Military
equipment was designed so that all valves were accessible via a single panel.
80 Silicon Chip
Prior to WWII, the military had a
variety of specifications that had to
be met when supplying equipment for
their particular needs. One particular
specification for radio equipment
specified that the valves should all be
www.siliconchip.com.au
This is the complete FS6
station, with the power
supply unit at right.
accessible through a hatch in the front
of the case. This is clearly shown in
one of the photographs of the transceiver – here, the hatch (or cover) has
been left open and all the valves are
clearly visible.
This made valve replacement an
easy task. However, there was a real
disadvantage in that the valves could
not be placed in their optimum positions for stable operation, maximum
gain or highest frequency operation.
I suspect that this requirement came
into being around the end of WWI
when valves were very unreliable and
hadn’t been rescinded by the outbreak
of WWII.
In the late 1930s, AWA produced
a predecessor for the FS6 designated
the 101. I don’t know exactly when
it was produced but I suspect that,
at the time, it was the most modern
set of its type, using the recently produced octal battery valves. It was a
very low-powered set, mounted in the
same cabinet as the FS6 and supplied
with power from a slightly smaller
power supply.
The 101 struggled to provide an RF
output of 0.4W on voice and 0.8W on
CW, whereas its successor, the FS6,
put out 4W and 8W respectively – a
big improvement. However, to the
untrained eye, the 101 looks the same
as the FS6.
I suspect that AWA was asked to
provide an updated, more powerful
version of the 101, that looked virtuwww.siliconchip.com.au
ally the same and could be used in the
same way as the 101 (although I have
no firm evidence for this). For example, the microphone, headphones,
connectors and the like would have
to be interchangeable between the
two types.
The receiver remained the same in
each set (101 or FS6) as it had proved
to be quite satisfactory. The transmitter though was a complete redesign
but was still be capable of fitting into
the original space allocated for it in
the 101.
the equipment assembled and almost
ready to operate, needing only a battery and an antenna and earth to complete the installation. The microphone
rubber mouth piece had perished, so it
is no longer fitted. Normally the power
Looking for an old valve?
or a new valve?
Valve lineup
The 101 used all battery valves.
A 1K5G acted as a master oscillator
with two parallel 1K5Gs in the RF
output stage. Voice modulation of the
1K5G output stage was achieved by
impressing the voice signal onto its
grid. Carbon microphones are high
output units and there was sufficient
audio energy put out to modulate the
transmitter output stage when fed
through a microphone transformer.
The transmitter was redesigned to
use a 1L5G as the master oscillator and
the ubiquitous 807 small transmitting
valve as the output stage. However,
it was more difficult to modulate the
grid of the 807. As a result, another
1L5G was added after the microphone
transformer to boost the audio to a
sufficient level to modulate the transmitter effectively.
Another of the photographs shows
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This view shows the FS6 power supply with the access cover removed. The
vibrator is at bottom left.
supply is stood to the left of the set and
the headphones should be low impedance. The cabling between the power
supply and the transceiver is made
in such a way that it is impossible to
interconnect them incorrectly.
The receiver is located in the
righthand section of the cabinet while
the transmitter is on the left. The receiver has relatively few controls and
they don’t follow the general layout
used on domestic receivers.
Starting at the top left, the control
here is the aerial tuning control, which
is used to peak the performance of
the receiver on different aerials. Once
set, it can be locked in position by its
central knob. At the far right is the
volume control, with the earth terminal nearby.
The tuning control is in the centre,
with the vernier tuning control just to
the right of it. On the far right are the
parallel headphone sockets. At the
Photo Gallery: Tasma Model 22
TASMA MODEL 22:
manufactured by Thom
& Smith, Sydney in 1931,
the Tasma Model 22 is a
3-valve TRF receiver with
the following valves: 224
detector, 247 output and
a 280 rectifier. (Photo and
information courtesy of
Historical Radio Society
of Australia).
82 Silicon Chip
bottom left is a toggle switch which
changes the operation of the receiver
from voice (radio-telephone) to Morse
code (wireless-telegraphy) – ie, this is
the mode switch.
Above the mode switch is a “mystery” circular black bakel
ite holder
of some sort. These devices were a
mystery to me for some time and
are mounted on many military radio
transceivers. They are a holder for a
fob watch. It was necessary to have
a watch in a secure spot so that radio
schedule (skeds) times could be kept.
The transmitter front panel is a
little more crowded than the receiver.
At the top left is the earth terminal,
with the mode switch (speech-key)
alongside it. In the centre top is the
aerial current meter which is used
when tuning the transmitter for
maximum output power. Alongside
it to the right is the aerial terminal
and just below that is the “send/off/
receive/net” switch which controls
the switching between the transmitter
and the receiver.
On the bottom left are the variable
frequency oscillator controls. There
is a locking knob in the centre of the
frequency control knob and a rubber
drive vernier onto the edge of the
main control knob. The two controls
at bottom right are used together to
tune and match the transmitter to the
aerial. Alongside these controls is the
microphone jack.
The Morse key is set into the valve
access cover and is slid out when it
is to be used. Alongside it is a jack
which is used for remote control of the
set. However, remote control of sets
like these is rather limited in scope
compared with what is expected from
modern remote controls.
The power supply is in the separate
case at left and has only one control
which is the on-off switch. It also has
three plug/socket connections to go to
the transceiver and the battery.
The receiver
The receiver has a conventional
superhet circuit typical of the era. The
converter uses a 1C7G, followed by a
1K5G first IF amplifier on 460kHz, then
a 1K7G as a second IF amplifier, detec
tor, AGC detector and reflexed first
audio. Following the 1K7G, audio is
applied to another 1K7G which feeds
a pair of low-impedance headphones
via a transformer.
The basic circuit is similar in many
www.siliconchip.com.au
FS6: RECEIVER UNIT
Fig.1: the FS6 receiver unit employed a conventional superheterodyne circuit with five valves.
ways to sets such as the Fisk Radiola
184 of 1939.
This set had a different job to do
compared to the domestic receivers
of the era and so there are noticeable
differences in some areas of design.
Portable military radios very rarely
had loudspeaker output, for several
reasons: (1) it was much easier to
hear clearly what was being sent to
you if headphones were used and
this avoided mistakes; (2) quite a bit
of radio traffic was for the officer in
command only and no-one else (operator excluded); and (3) when the
sets were used on the front line, you
didn’t want the enemy hearing what
was going on or being able to pinpoint
where you were.
The other major difference is in
the use of a beat frequency oscillator,
which is V5 (1C7G) in the circuit.
When receiving Morse code messages, this oscillator – which operates
at around 460 kHz in the FS6 – beats
with the incoming Morse signal to
give an audio tone. AGC systems of
the era didn’t work well with Morse
code signals, so the AGC system is
disabled and a variable bias is applied
to the AGC line via a ganged volume
control (R12 and R16 are ganged via
a belt).
www.siliconchip.com.au
Inside the FS6 receiver (rear of chassis view).
May 2002 83
un
soldered and then the mounting
screws are removed. The transformer
is then withdrawn through the underside of the chassis (plate) – nifty.
It is not possible to operate the
receiver out of its case unless jumper
leads are attached to the 4-pin plug
shown in the top left of the circuit
(see Fig.1). The connections are: (1)
HT +175V <at> 27mA (maximum); (2)
filaments +6V <at> 240mA; (3) bias -6V
<at> < 1mA; and (4) aerial/antenna. The
chassis is the common return circuit
for the HT and filament negative, the
bias and the antenna system earth.
The sender (transmitter)
Inside the FS6 transmitter (rear
of chassis view).
By increasing the volume control,
the volume setting is increased and
the bias applied to the AGC line (via
R16) is decreased, thus increasing the
gain of the RF section of the set.
In operation, the receiver works
quite well and is reasonably sensitive.
The dual-speed tuning is easy to operate and, using the vernier dial drive,
shortwave stations are tuned as easily
as broadcast band stations on typical
domestic radios.
This particular receiver needed no
replacement components when it was
refurbished back in 1973, although
that situation may have changed by
now of course. The receiver circuitry
is removed by undoing four screws at
the front of the set and pulling it out
of the case.
Most military sets aren’t particularly
easy to service and the FS6 receiver
appears to be no different. That said, a
close inspection of the chassis reveals
84 Silicon Chip
that much thought has gone into making the components relatively easy to
access, despite the compact nature of
the radio.
For example, the bottom adjustments for the IF transformers are easily
accessed at the back of the set. On
the other hand, the top adjustments
are close to the front panel and at
first sight, appear to be inaccessible.
However, although not obvious in the
photographs, there is a small panel
on the front of the set that is removed
to gain access to the IF transformer
adjustments.
Should an IF transformer become
defective, it is surpris
ingly easy to
remove. Conventional wisdom has
it that IF transformers are removed
through the top of the chassis (plate)
but this clearly impossible with the
FS6, as the transformer top is hard
up against the front panel. In this
case, the transformer connections are
In this section I’ve gone into more
detail than usual, as many vintage
radio buffs are unfamiliar with the
operation of transmitters. Generally
an AM or CW transmitter has a simpler circuit than a superhet receiver
but they are noticeably different from
each other.
As with the receiver, the transmitter
is removed from the case by undoing
four screws and pulling it out. A rear
view of the chassis is shown in one
of the photos.
This is a reasonably simple transmitter, having just two stages in the RF
section and one in the audio section.
A 1L5G (V2) acts as the variable frequency oscillator (master oscillator or
VFO) and determines the actual transmission frequency. It has temperature
compensating capacitors (top left in
circuit) to ensure that the transmitter’s
output frequency does not drift unduly
when operating.
The VFO is similar to the local
oscillator in a superhet receiver but
works at a higher power level. The
signal from the VFO is applied to the
807 where it is amplified. The output
signal is then applied to the tuning
and matching circuits consisting of
C14, L2 and S3 which ensure efficient transfer of power to the antenna
system.
The output circuit of the transmitter
is not unlike the circuitry of crystal
sets, where the antenna and diode detector are matched to the tuned circuit
for best performance. It is necessary to
have a transmitter working at optimum
efficiency for several reasons: (1) to
provide the maximum output power
transfer possible (for maximum efficiency); (2) to keep the valve dissipa
tion down; and (3) to economise on
the use of electrical power (important
www.siliconchip.com.au
when using batteries).
In the Morse code mode (CW), the
transmitter has grid block keying. This
involves placing a bias of -20V onto
the grids of the 1L5G (V2) and the 807
(V3), which effectively prevents the
1L5G from conducting and in the case
of the 807, reduces the current drain to
a safe level when the key is up. When
the key is down, the bias is removed
and the transmitter operates.
When the transmitter is switched
to voice operation, the VFO (V2)
runs continuously with no blocking
bias. The 807 power amplifier operates using both fixed bias (-20V)
and self-bias, the latter derived from
half-wave rectifying the signal from
the VFO at its grid. The voice signal
from the carbon microphone is amplified by V1 (1L5G) and fed to the
grid of the 807 via T2, the modulation
transformer.
The voice signal is impressed onto
the grid of the 807 in series with the
-20V bias. The resulting variation in
the bias causes the 807 circuit to act
as a high-power mixer. Mixers have
several signals in their outputs and, in
this case, we have the VFO frequency,
the audio frequency and the products
VFO + audio frequency and VFO - audio frequency.
The circuit only passes the RF signals. The VFO signal is the carrier and
the + and - audio signals form the upper and lower sidebands. These three
components form the composite AM
signal we are all familiar with.
The power supply
The power supply is quite large for
the amount of power it provides to the
transmitter and receiver.
As shown in one of the photos, there
isn’t a lot of spare space inside the
case. The front cover is just clipped
on by two catches, one on each end.
But despite this being an imperfect
fit around the edges, there is very
little interference to reception from
the supply.
This is also quite a reliable power
supply. The vibrator had a long life
and rarely required replacement. This
was probably due to the design of the
vibrator transformer (T1) and the buffer arrangement (C6, C7, R7 & R8). In
addition, the vibrator was sealed and
filled with nitrogen gas.
Summary
Commercially made HF radio transwww.siliconchip.com.au
FS6: SENDER UNIT
Fig.2: the sender (or transmitter) circuit sender used three valves: a
modulator (V1), a VFO (V2) and an RF power amplifier (V3).
ceivers were rare and expensive after
WWII. However, many of these WWII
transceivers came onto the surplus
market at a relatively low price.
They were often extensively modified to provide crystal control of the
transmitter and to provide different
and extended frequency ranges. Loudspeaker output was also sometimes
added. For example, amateur radio
operators modified them extensively
to operate on the 3.5MHz and 7.0MHz
bands.
Unfortunately, the FS6 wasn’t very
suitable for use where mud, water,
humidity and dust were common –
and that included many areas where
the set was expected to be used. The
set had no sealing gaskets around the
cabinet edges, along control shafts or
over sockets to prevent the ingress of
water, mud and the like. Additionally,
it did not have “tropic proofing” or
desiccator crystals to keep the moisture out of the set.
It also drew more current from the
battery than later sets and its tuning
range was quite restrictive – 4.26.8MHz, whereas later sets intended
for the same job tuned 2-8MHz.
However, despite its many faults I
believe that in the right environment,
it would have proved an easy set to
operate and keep in working order.
As an aside, the later 122 portable
transceiver cost around 1000 pounds
during WWII – about the same as a
Holden car soon after the war.
Military equipment wasn’t cheap
to produce!
SC
May 2002 85
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