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Vintage Radio
By Rodney Champness, VK3UG
The AWA 157P 7-transistor
portable radio
Built in Australia more than 50 years ago,
this AWA 157P 7-transistor radio is still
in good condition and required only a few
minor repairs to restore it to working order.
It’s built like a valve receiver, with point-topoint wiring and no printed circuit board.
T
RANSISTOR RADIOS were wellestablished as a consumer item
by about 1960, the year the AWA
157P was first manufactured. In fact,
electronics hobbyists had been introduced to transistors as components as
far back as 1954. “Radio & Hobbies”
often carried ads for the Philips OC44,
OC45, OC70, OC71 and OC72 series
germanium transistors. These usually
sold for around a pound to thirty shillings ($2 to $3).
90 Silicon Chip
By 1958, quite a few transistor receivers were coming into the country
from Japan and Australia was also starting to produce sets at that time. These
sets were quite a practical proposition
if you lived in a city where one or more
reasonably powerful radio stations
were located.
Some of the early Japanese-manufactured receivers used a phenolic
board that had holes punched through
it, with the pigtails of the components
wired to each other as required by the
circuit. These sets were quickly followed by designs using true printed
circuit boards (PCBs). However, it was
necessary to be quite careful when installing or replacing parts in such early
sets, as too much heat easily lifted the
tracks off the board.
Australian manufacturers were
slower off the mark when it came to
using PCBs and the AWA 157P 7-transistor set featured here retained the
point-to-point wiring techniques of
the valve era, despite being circa 1960.
And although the transistors were not
mounted in sockets (as some manufacturers did), several are mounted
through rubber grommets that are in
turn fitted to the chassis.
These transistor mounting grommets are roughly located where valve
sockets would be otherwise be fitted
in an “equivalent” valve set. So the
157P was very conventional for its
time. Compared to Japanese sets of the
same era, they would have been more
costly to produce.
Main features
As shown in the photos, the AWA
157P portable is built into a goodquality black leatherette and thick
card case. The case front features an
attractive perforated aluminium mesh,
behind which is mounted a 5 x 7-inch
(125 x 175mm) loudspeaker.
A hand-span direct-drive system is
used for the tuning dial. This simple
but reliable method was used by many
manufacturers to keep prices down
and is quite adequate for broadcastband portables and other low-priced
receivers.
The case itself is reasonably large.
As a result, the parts are quite well
spread out and access to the components is quite reasonable, which makes
restoration easier. However, like most
portables of the era, this set had a few
problems with its case. In some places,
the leatherette had become detached
from the thick cardboard sections and
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Fig.1: the circuit uses seven transistors in a fairly conventional superhet arrangement. VT1 is the converter, VT2 &
VT3 are IF amplifier stages, VT4 is a preamp stage and VT5-VT7 form the audio amplifier. Output pair VT6 & VT7
are wired in push-pull configuration and are driven by VT5 via centre-tapped transformer T6.
some of the stitching around the edges
had worn through.
By contrast, the set is very clean
internally for its age with no evidence of corrosion. It weighs in at
3.1kg complete with battery, so it’s no
lightweight.
Circuit details
Take a look now at Fig.1 for the
circuit details. It’s fairly typical of the
era, with an autodyne converter stage
(VT1), a 2-stage IF amplifier (VT2 &
VT3), a diode detector (MR3) and
three audio stages (VT4-VT7). The
output stage uses of a pair of transistors (VT6 & VT7) wired in push-pull
configuration.
Power is supplied from a type 276P
9V battery. The current drain with the
volume turned down is 18mA, which
is slightly more than the current drain
from the 90V battery of a valve portable. However, because the supply
to this transistor set is only 9V it is
around six times more efficient and
that’s before we even consider the
filament current in a valve set.
It’s no wonder that transistor receivers became so popular when
both battery cost and weight were so
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dramatically reduced. Of course, the
current drain did rise considerably
when the volume control was turned
up and could reach 45mA on peaks.
A large ferrite rod (200mm-long x
13mm-diameter) is used for the signal
pick-up. In addition, the AWA 157P
has provision for an external antenna
and earth to boost the performance on
distant stations and this scheme works
very effectively.
Transistor VT1, a 2N219, is wired
as an autodyne frequency converter.
Its 455kHz output is fed to the base
of VT2 (2N218), the first IF amplifier,
via double-tuned IF (intermediate
frequency) transformer T3.
From there, the signal is applied via
another double-tuned IF transformer
(T4) to transistor VT3, the second IF
amplifier. VT3’s output is then fed to
single-tuned IF transformer T5 and
then to detector diode MR3.
As an aside, triode valves have considerable capacitance between their
grid and plate elements and will often
oscillate in RF and IF circuits if they
are not neutralised. Similarly, transistors have considerable capacitance
between the base and the collector
and may also oscillate if not neutral-
ised. As a result, the two IF amplifier
stages are both neutralised using 6.8pF
capacitors C16 & C22 to make sure this
doesn’t occur.
The detected audio signal is fed to
the base of VT4 (another 2N218) which
serves as a preamp stage. Its output is
taken from the emitter and fed via a
10kΩ volume control pot to the second
audio amplifier VT5 (2N408). The signal on the collector of this transistor is
then fed to audio transformer T6 which
in turn drives output pair VT6 & VT7
(2N270) which operate in push-pull
configuration.
PNP transistors
As is typical of the era, the transistors used in the AWA 157P are all
PNP germanium types. As a result, the
positive terminal of the battery is connected to the chassis and all voltages
are negative with respect to the chassis (ie, the supply rail is at -9V). This
“positive earth” is the opposite to what
we normally expect in a set and must
be kept in mind when servicing some
early transistor radios.
Complex AGC
As with many other transistor reJuly 2012 91
diode MR2 conducts and shunts the
signal that’s fed to VT2, thereby further
reducing the receiver’s gain.
Audio amplifier design
The circuit is built on a metal chassis, similar to a valve receiver. Note that
several of the transistors are mounted in rubber grommets which are in turn
mounted on the chassis.
There’s no printed circuit board here – just good old-fashioned point-topoint wiring that mimics valve receiver construction techniques. Despite its
age (over 50 years), the chassis is still in excellent condition.
ceivers, the AGC system in the AWA
157P is more complex than is usually
the case with valve receivers. The output from the detector not only has an
audio component but also a DC component which increases (ie, becomes
more negative) as the signal strength
increases. This DC voltage (along with
the audio signal) is applied to VT4
and as a result, the emitter voltage
increases with stronger signals.
VT4 acts as a low-impedance DC
amplifier for the AGC system as well
as an audio preamplifier. A third of
the DC voltage at VT4’s emitter is
92 Silicon Chip
applied to the emitter of VT2 (via a
voltage divider). As a result, VT2’s
emitter voltage increases (from around
1V) with increasing signal strength
and this in turn reduces the gain of
this stage (note: VT2’s base voltage is
biased to 1.25V by the voltage divider
consisting of R5 & R6).
In addition, the supply rail to VT2
is decoupled using R10 and C19. With
no signal input to the set, the voltage
across C19 is around -5.5V but this
increases to around -8.2V with a strong
signal as VT2 draws less current. If the
incoming signal is extremely strong,
The audio amplifier is bound to look
quite foreign to an audio enthusiast
today. It has only two stages and three
transistors, to give sufficient audio and
gain from the signal at the collector of
VT4, a germanium 2N218. This was
devised long before the days of complementary transistors, direct-coupled
amplifiers, high negative feedback and
so on. Indeed, look at the circuit and
you will find that there is no negative
feedback around the audio amplifier.
None. So how does it work?
The signal from the volume control
is AC-coupled to the base of VT5, a
2N408 transistor which is operating
in class-A. It drives an interstage
transformer, T6. Why would you need
an interstage transformer in an audio
amplifier? At the time, designers had
not figured out a simpler way to generate two out-of-phase signals to drive
a push-pull class-B output stage. In a
valve amplifier, they would have used
a “phase splitter” but trying to couple
such out-of-phase signals had yet to be
worked out.
Ultimately, when NPN and PNP
power transistors became available, the
solution was easy but this was more
than 10 years away (with complementary germanium power transistors).
The secondary of the interstage
transformer is split into two halves,
with each half driving the base of a
PNP output transistor (VT6 & VT7).
The centre tap of the secondary is connected to a resistive divider and this
provides the base bias to the two output transistors which operate in class
B, albeit with a small quiescent current
to minimise crossover distortion.
Note that each output transistor
drives only one half of the primary of
the associated output transformer, T7,
with DC flowing into the centre-tap
and out into the respective collectors
of the output transistors.
The operating conditions of the output transistors were stabilised against
thermal runaway (yes, they had it in
those days – they discovered it!) by
the negative temperature coefficient
(NTC) thermistor, TH1. It worked quite
well and again, was the solution long
before such circuit techniques as “Vbe
multipliers” were devised.
And as far as negative feedback was
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The large tuning gang (left)
was repaired by removing
it from the chassis and
then carefully bending the
rotor blades to prevent
them shorting to the stator
plates. Note the wirewound trimmer capacitor
attached to the gang.
Right: replacing these
two 25μF electrolytic
capacitors cured the noisy
volume control operation.
concerned, it was more trouble than
it was worth. With transformers in
the circuit, the resultant phase shifts
meant that only a small amount of
negative feedback could be applied
before instability became a problem;
better to do without!
Cabinet restoration
Removing the chassis from the case
is quite straightforward. The first step
is to remove the two control knobs by
pulling them off their shafts, followed
by the hand-span dial. The latter is
removed in similar fashion and was
quite tight in this set but eventually
came loose without damage.
The chassis itself is held in the case
by three screws, one at either end of
the handle and one through the bottom
of the case. Once these were removed,
the chassis could then be slid out
through the back (after unclipping
the rear flap).
That done, the case was wiped clean
with a moist cloth. The next step was
to repair the case where the leatherette
had come away from the bottom of the
rear flap. The leatherette was simply
glued back onto the cardboard using
contact adhesive and held in position
using a couple of clamps and scrap
timber until the glue dried.
The leatherette had also come away
from the top edges of the case and this
was repaired in similar fashion. Once
these repairs had been completed, the
case looked quite good. It wasn’t practical to repair the worn stitching along
the edges but this particular problem
is not particularly obvious.
Next, the plastic hand-span dial
wheel was given a polish to reduce the
scratch marks that were on it. The red
station indicator line had also worn
away in places over the years and
this was repaired by dipping a steel
nibbed-pen in red paint and running
it carefully along the old line. This
method worked well and the indicator
line now looks like new.
The dial-scale itself was also lifting
along the edges so this was glued back
into place using Tarzans Grip®. All that
remained then was to remove the years
of grime from the flutes of the knobs
and this was done by scrubbing them
with a nail brush and soapy water.
Circuit repairs
Leaky capacitors are far less critical
in transistor sets than in valve receivers and it is usually safe to turn transistor sets on before doing any component
replacement. The exceptions are when
there is a short across the battery socket
or where badly overheated (burnt)
components are obvious.
In this case, it was immediately obvious why this set had been taken out
of service – the stators and the rotors
on both sections of the tuning gang
were shorting at the low-frequency
end of the dial. In addition, the volume control was extremely noisy, with
many dead spots on the track.
In short, it was a bit of a basket case!
However, it seemed that if I could cure
both of these problems, the receiver
would probably work.
The twin-gang tuning capacitor was
hard to get at in-situ, so I removed all
the wires soldered to it and carefully
labelled them. I then removed the extension on the tuning shaft, after which
I removed the tuning gang and its small
adaptor plate which was mounted to
the chassis. I then removed the three
screws that held the tuning gang to the
mounting plate.
Once the tuning gang was free, I inserted a one-sided razor blade between
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July 2012 93
tions were quite close to the locations
marked on the dial-scale and it was
only necessary to remove a couple of
turns from the wire trimmer used in
the oscillator circuit to get them spot
on. Unfortunately, I was unable to free
the tuned winding on the loopstick antenna to adjust it at the low frequency
end of the dial, as the locking “gunk”
used on it had penetrated between the
inside of the coil former and the ferrite
rod itself. However, it does appear to
be quite close to optimum.
Finally, the trimmer was adjusted
at the high-frequency end of the dial
and once again little adjustment was
required. So it looked like AWA had
used good-quality components in the
tuned circuits.
Making up a battery
The chassis is secured inside the case using three screws – two at the top and
one at the bottom. The battery is no longer authentic, the case now housing six
1.5V cells connected in series in a 6-way battery holder.
the shorting plates to lever them apart.
This was only partially successful
and in the end I found that I had to
fully open the gang and drag a finger
across the rotors to bend them slightly.
Finally, after some further adjustments
using the razor blade, I was able to get
the rotor vanes to mesh with the stators
without any shorts occurring.
Having done that, I reinstalled the
tuning capacitor and wired it back into
circuit. As can be imagined, the entire
procedure was quite time consuming
but it needed to be done with care,
otherwise the tuning capacitor would
have been ruined.
Faulty electros
At this point, the set was tested
again and many stations could now be
heard but the volume control was certainly very noisy. Sometimes, spraying
a volume control with contact cleaner
will fix this problem but in this case
it didn’t work.
As many will know, volume controls
in valve radios that have DC flowing
through them can be quite noisy. In
some sets, the volume control is part
of the detector load and this was done
to reduce the component count. In
other cases, the control becomes noisy
because of leaking capacitors.
Electrolytic capacitors are used to
couple between the audio stages in
94 Silicon Chip
most transistor receivers and although
they do have some leakage, this is not
usually a problem. In this receiver
though, they were the problem and
replacing C31 and C32 (both 25µF
electrolytics) completely eliminated
the noise.
In fact, these two capacitors were so
leaky that I decided that it would be a
good idea to replace all the electrolytic
capacitors where leakage might cause
a problem. These included capacitors
C9, C19 and C29 (all 100µF).
Alignment
The IF transformers in the AWA
157P provide better selectivity than
those in many other transistor receivers. The first two IF transformers (T3 &
T4) are double-tuned, while the final IF
transformer (T5) has one tuned circuit
(and one tuning slug).
To align the set, I first tuned to a
weak station and endeavoured to
adjust all five IF transformer cores for
best performance. One core, however,
was jammed and couldn’t be adjusted,
so I had to adjust the other four around
the frequency that it was set at. Fortunately, it was very close to 455kHz
and the remaining tuned circuits were
also very close to this frequency, so not
much adjustment was needed.
The oscillator tuned circuit was
next on the list. As it stood, the sta-
All of my testing was done using a
small regulated supply to power the
receiver. However, in order to use it as
a portable, it was necessary to make up
a battery pack since the original 276P
9V battery style is no longer available.
This was done by fitting six 1.5V AA
cells to a 6-way battery holder and
inserting it inside an old 276P battery
casing to keep it looking original.
By the way, while testing the receiver on the regulated supply, I found that
it would perform quite satisfactorily
down to 6V. So the battery life should
be quite good at moderate volume.
Summary
This is a good-performing portable
transistor radio. The only thing it
doesn’t do well is handle very strong
signals. Certainly, an external antenna
and earth could only be considered in
more remote areas, away from stations.
It would appear that some modifications were done to the AGC system
between production runs as my set
has a slightly different circuit to that
shown in Fig.1. This may have been
an attempt to improve the set’s performance on strong signals.
Finally, at the time it was made,
manufacturers were still experimenting with construction techniques for
transistor radios. The Japanese had
begun using PCBs by 1960 but this
technology had not yet been fully
adopted by AWA. As a result, this
particular set was built like a valve
portable, with point-to-point wiring.
That said, it’s still a well-built set
that has lasted well and is worth havSC
ing in a collection.
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