This is only a preview of the June 2009 issue of Silicon Chip. You can view 31 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 "High-Current, High-Voltage Battery Capacity Meter, Pt.1":
Items relevant to "GPS Driver Module For The 6-Digit Clock, Pt.2":
Articles in this series:
Items relevant to "Build A Beam-Break Flash Trigger":
Items relevant to "PICAXE Humidity Measuring Using The HopeRF HH10D":
Articles in this series:
Purchase a printed copy of this issue for $10.00. |
Vintage Radio
By RODNEY CHAMPNESS, VK3UG
The AWA 693P 3-Band
8-Transistor Portable
. . . the transition from valves to transistors
T
RANSISTORS WERE introduced
into domestic radios in Australia
around 1958. I can actually remember
the first transistor set that I owned, a
pocket Sony.
By today’s standards, this set was
a poor performer and was only suitable for receiving stations in the near
vicinity. Its main drawback was that it
generated a fair amount of noise due to
the germanium transistors used.
By about 1960, Australian manufacturers were producing quite reasonable transistor radios. However,
although Japanese sets were by then
using PC boards (of greatly varying
quality), Australian manufacturers
took longer to make the transition. In
fact, some Australian manufacturers
didn’t use PC boards until well into
the early 1960s.
As a result, some early Australianmade transistor sets were built just
like valve sets, with components
wired point-to-point. Some even used
sockets for the transistors, just as valve
sets used sockets. However, Australian
manufacturers did eventually move
over to PC boards – the benefits of using PC boards were simply too great
to ignore, especially in terms of cost
and ease of assembly.
The AWA 693P
The AWA 693P is an impressive 3-band
8-transistor set that was manufactured by
AWA in Australia during the early 1960s.
It doesn’t use a PC board but instead
employs point-to-point wiring, just like
the valve radios of that era.
86 Silicon Chip
The AWA 693P 8-transistor radio
featured here is one such Australianmade set that used valve-radio construction techniques. It is a well-made
3-band receiver that was manufactured
some time around 1960.
In keeping with the era, it is housed
in a wooden cabinet covered with
leather and leatherette. It is similar
in size to the valve receivers it was
intended to replace and was no lightweight either, tipping the scales at a
hefty 7.2kg (ie, about the same as a
valve set).
Inside, the circuit used two secondgeneration PNP germanium transistors for the critical RF and autodyne
converter stages, while all the other
siliconchip.com.au
Fig.1: the circuit is a conventional 8-transistor superhet design, with transistor VT1 functioning as an RF amplifier
stage, VT2 as the converter, VT3 & VT4 as IF amplifiers and VT5-VT8 as the audio stages. Diode MR4 is the detector.
stages used first-generation germanium transistors. It also used an
internal telescopic antenna but there
are also terminals on the rear so that
an external antenna and earth can be
connected for improved performance.
In addition, terminals are provided
to allow a portable turntable to be
connected to the audio input of the
receiver.
The latter feature was probably rarely used. Battery powered turntables of
that era were thin on the ground and
their quality left much to be desired.
Lantern batteries
Power for the set is derived from
two 509 lantern batteries connected in
series to give a 12V supply. In addition,
the junction of the two batteries provides a centre-tap for the audio output
stage, so that it could be used without
an output transformer. However, this
meant that the speaker had to be a nonstandard type with a 45-ohm voice coil
to match the transistor output stage.
Because of the size of the batteries,
siliconchip.com.au
their life is quite good in this set and is
somewhere in the region of hundreds
of hours.
Tuning range
The tuning range of this set is quite
useful, particularly for those living
in more remote areas. As well as the
broadcast band ((530-1650kHz), there
are also two shortwave bands covering
2-6MHz and 6-18MHz.
Normally, portable receivers which
cover these latter ranges have rather
direct tuning. This can make tuning
to shortwave stations rather difficult
but this problem has been solved in the
AWA 693P. In this set, the dial-drive
is a 2-speed type. Once the station has
been roughly tuned, it is then tuned
in the opposite direction. This engages
the “slow-motion” reduction tuning
mechanism, allowing the station to be
easily fine-tuned.
It’s interesting to note that lifting the
front cover of the receiver to access
the controls reveals the Royal Flying
Doctor Service shortwave frequencies.
These are attached to the inside of the
panel along with a map of the world
with the various time zones listed.
The set is quite a good performer on
shortwave too, thanks to the inclusion
of a radio frequency (RF) stage in the
front-end. In fact, this receiver can be
considered a serious replacement for
the much earlier 7-band AWA valve
receivers.
Circuit details
The circuit (see Fig.1) is really quite
conventional for a set of that era. As
shown, the antenna input consists
of a network that allows the use of
either the in-built telescopic antenna
or an external long-wire antenna on
all bands.
On the broadcast band, a loop-stick
antenna is used most of the time but
connecting a long-wire antenna can
boost the performance of the loop-stick
in difficult reception areas. Note that
the broadcast band input also has a
series tuned intermediate frequency
(IF) rejection circuit (L1, C6) across
June 2009 87
This is the view inside the back of the set with the two 6V lantern batteries
in place. Note the point-to-point wiring and the components around the
band-change switch at top left.
the tuned antenna loop-stick winding.
This is intended to prevent maritime
radio transmissions close to 455kHz
from breaking through into the IF
amplifier stage.
RF transistor VT1 (2N370) amplifies
the input signal from the antenna and
passes it via further tuned circuits to an
autodyne converter stage based on VT2
(2N370). The output from this stage is
then applied to the first IF transformer
and thence to a 2-stage IF amplifier
consisting of transistors VT3 & VT4
(2N218). Both stages are neutralised
by C33 & C40 respectively.
From there, the signal passes to a
detector and AGC diode (MR4), after
which the detected signal is fed to
VT5 (2N218). This stage acts as both
an AGC amplifier and first audio am-
plifier. The resulting AGC is directly
applied to VT3 and VT1 and also from
VT3 to the first two IF transformers
via MR3 to control the gain through
the amplifier.
The detected audio signal at the
emitter of VT5 is fed through an RF
choke (L2) to filter out any remaining
IF signals in the audio. The audio is
then applied via volume control R27
and tone control R28 to audio driver
stage VT6 (2N408).
VT6 is turn feeds a driver transformer with three windings. One goes
to the collector lead of VT6, while the
other two drive the base leads of output
transistor pair VT7 and VT8 (2N270).
As shown, these two output transistors are wired in series and each has
a thermistor in its base-bias circuit to
stabilise the quiescent current against
variations in temperature. Germanium
transistors are particularly sensitive to
heat and will draw considerably more
current as the transistor junction temperature rises unless precautions are
taken. If the current rises to any extent,
a runaway situation can occur where
more and more current is drawn until
the transistor finally destroys itself.
The emitter-collector junction of
VT7 & VT8 drive the speaker’s voice
coil, the other side of which goes to
the centre tap of the power supply. Assuming that the output transistors and
their base bias networks are matched,
then there will be no current through
the loudspeaker when no audio signal
is present.
At least, that’s the ideal situation.
In practice, there will always be some
current through the speaker’s voice
coil but this will be very small if there
are no faults in the class-B amplifier
output stage.
Conversely, when audio is applied
to the transistors, one will draw more
current while the other will be cut off
and will draw no current. This means
that the DC voltage at the junction of
the transistors and the speaker can
vary between nearly 0V and -12V with
reference to the chassis with the volume control set for maximum output.
This in turn means that the amplifier side of the speaker coil can vary
between -6V and +6V with reference to
the other side of the speaker voice coil
(which is connected to the midpoint of
the battery pack). Of course, at normal
listening levels, the voltage excursions
are much less severe.
Finally, to reduce the distortion
in the class-B output stage, negative
feedback is applied to the base of VT6
via a 100kΩ resistor (R40).
The overhaul
A label on the inside-back of the set gives some basic service information,
including the transistor types and locations plus the dial-cord arrangement.
88 Silicon Chip
My first impressions of this set
when it was loaned to me was that it
had had a hard life. The leatherette,
leather surfaces and the metal grill of
the cabinet all showed signs of wear.
The first job in the restoration was
to remove the chassis from the cabinet.
This proved to be quite easy. First, the
back of the set is removed by undoing
a single screw. The batteries are then
removed, after which you undo three
screws from the base of the cabinet
and unplug the internal whip antenna.
The chassis and battery holder are
then simply slid out of the cabinet.
siliconchip.com.au
The set’s owner was uncertain as
to whether or not it was working, so
I obtained a couple of 6V 509 lantern
batteries, slipped them into their holders and switched the set on. Well, the
set did work but its performance was
woeful. It was quite insensitive, picking up very few stations, and its audio
output was quite distorted. In short, it
sounded rather sick so it was time for
some troubleshooting
I began by measuring the voltages
around the front-end of the set but
could only get 5-6V where I should
have been measuring close to 12V. As
a result, I checked the batteries and
found that one was open circuit, even
though it was brand new!
With a good battery fitted in its
place, the set sounded much more like
it should. The distortion had gone but
it was still not performing at all well
in terms of sensitivity. As a result, I
checked the alignment of the IF amplifier stages but found that they were
near enough to spot on.
RF alignment
It was a different story with the RF
and converter stages and the oscillator
tuned circuits. A cursory examination
revealed that one Philips trimmer was
missing its adjustment cap, an extra
capacitor had been fitted across the
broadcast-band oscillator coil and the
core of the oscillator coil was sitting
much further out of the coil than I
would have expected.
This all indicated that the front-end
had been tweaked by someone who
clearly didn’t know what they were
doing.
The oscillator tuned circuit on each
of the shortwave bands appeared to be
reasonably accurate, so I first concentrated on aligning the RF and antenna
circuits. First, a Leader (LSG11) signal
generator was pressed into service to
determine the tuning range on each
band. That done, I then adjusted
each coil and trimmer capacitor for
best signal (or maximum noise). And
what an improvement that made – the
performance on the shortwave bands
was now quite good.
According to the service information for this set, the broadcast band is
normally aligned first. However, I left
it until last as it appeared to have more
problems than the shortwave bands.
The tuning range was well out and
stations were appearing in the wrong
locations on the dial.
siliconchip.com.au
These two photos show the front and rear views of the chassis after it has
been removed from the cabinet. The oval-shaped speaker is a special type
with a 45-ohm voice coil.
My first suspect here was padder
capacitor C16. Perhaps its value had
changed or maybe the wrong one had
been fitted. As a result, I disconnected
one leg of C16 and tested it on the
capacitance range of my multimeter.
It gave the correct value so that theory
bit the dust.
Next, I removed the extra capacitor
that had been installed across C14. I
then adjusted the oscillator circuit so
that it covered the correct range. This
is done by adjusting the coil at the lowfrequency end of the dial and the trimmer capacitor at the high-frequency
end (this has to be done several times,
as each adjustment interacts with the
other). However, because the rest of
the RF and antenna circuits were so
badly out of tune, I had to use a very
high output from the signal generator
in order to force signals through the
set during this procedure.
Now that the oscillator was tuning
correctly, it was time to look at the
other tuned circuits for the broadcast
band. The location of the antenna coil
on the loop-stick antenna had not been
altered since the set was manufactured
but I decided to check it all the same.
June 2009 89
Photo Gallery: Astor JN Dual-Wave Receiver
up by the loop-stick antenna. However, when the shield that normally
sits between the IF amplifier and the
loop-stick was put back in place, this
instability disappeared. This shield
piece is held in place with three metal
thread screws and is bonded with flexible straps to adjoining metalwork to
ensure effective shielding.
Having completed the alignment, all
the trimmer adjustments were sealed
in position using clear nail polish. In
addition, the adjustment slugs inside
the various coils were secured using
a drop of bees wax (this can be easily
broken free if adjustment is needed
later on). A better method is to secure
the cores using some very thin rubbercore “string” (for want of a better
name). Unfortunately I’ve been unable
to source any of this rubber-core string
in recent years.
Final tweaks
NICKNAMED “SYDNEY HARBOUR BRIDGE” after its smooth arch shape,
the Astor JN dual-wave receiver was housed in an attractive, dark-chocolate
bakelite cabinet with a faint embedded pattern. Its copious size enabled
Astor to enclose a quality chassis with power and performance comparable
to a radiogram, so this model is on most enthusiasts’ must-have list.
An unusual feature is the roll-tuning dial. The station tuning is a normal
linear action but turn the thumbnail dial and the stations from another state
appear. Its interesting to note that the Victorian dial also has Devonport
shown, such was the performance of the chassis with an external wire
antenna (there was also lower electrical interference in the 1950s).
The valve line-up was as follows: 2x 6U7G, 6J8GA, 6B6G, 6V6GT/G and
5Y3GT/G. Photograph by Kevin Poulter for the Historical Radio Society of
Australia. www.hrsa.net.au; phone (03) 9539 1117.
To do this, I tuned to the low-frequency end of the tuning range and
moved my fingers close to the tuned
winding on the loop-stick. The set’s
performance immediately improved,
which indicated that the coil needed
to be moved towards the centre of the
loop-stick to increase its inductance.
This is easier said than done, as
you first have to remove the “gunk”
holding the coil in position. This was
done using a sharp hobby knife, after
which the coil was moved along the
loop-stick to peak the performance.
The coil was then secured in this new
position.
The RF coil was also peaked at
this time. I use an old plastic knitting
needle as an alignment tool, filed down
so that it has a screwdriver tip at one
end (a metal screwdriver would affect
the tuning).
90 Silicon Chip
First, I tuned to the high-frequency
end of the dial and peaked the trimmers on each tuned circuit. However,
I initially couldn’t peak the trimmer
on the loop-stick, as this was the one
without its adjustment cap. Fortunately, that was easily solved by substituting one from a spare trimmer in
my junk box.
With the substitute cap in place, I
was then able to peak this trimmer.
The RF stage could be peaked as well.
I then tuned from each end of the dial
to the other, readjusting the coils and
trimmers until there was virtually no
interaction between the adjustments.
Instability
The set was now working quite well
except for some instability at about
910kHz. This was caused by the second harmonic of the IF being picked
The volume and tone controls were
both noisy in operation. This is a common problem with old sets. This was
solved by giving them both a squirt of
Inox contact cleaner. In addition, the
dial-drive pulleys and the reduction
drive were each given a drop of oil
to ensure smooth operation. I do this
using an oil-filled hypodermic syringe
with a needle attached so that I can get
the oil where it needs to be (the tip of
the needle is ground square to avoid
accidental “jabs”).
Despite the set’s age, none of the
resistors or capacitors required replacement. The paper capacitors may
well have been leaky but this is not
usually a problem in transistor circuits
due to their low impedances and low
operating voltages.
Finally, the leather/leatherette cabinet was spruced up using a dark tan
shoe polish. This produced quite a reasonable finish although there was no
way to repair the damaged leatherette
Summary
In summary, this radio is easy to
work on and adjust. Its performance
is quite good and during the early
1960s, it would have been one of the
best transistor sets available.
This set was probably AWA’s first
multi-band transistor portable and
they did a really good job. It doesn’t
use a PC board but is still a well-made
set, built to valve construction standards. In short, it is well worth having
SC
in a collection.
siliconchip.com.au
|