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VINTAGE RADIO
By RODNEY CHAMPNESS, VK3UG
The Airzone 500 series receivers
Airzone’s 500 series radio receivers were
typical of the 1930s era. This month, we take
a look at the 505/515 models which were
5-valve superhets employing 455kHz IF
stages but with no automatic volume control.
During the early 1930s, Airzone
(1931) Ltd produced a series of progressive receiver designs. The particular chassis featured here is a 500
(which has been modified to 500P
standard), while the cabinet is a 555.
And just to add to the confusion, the
circuit diagram is for receiver models
505 and 515.
However, in those days, manufacturers often built a chassis which
was fitted to different cabinets (table,
mantel or console). The chassis had
one number and the cabinet another,
while yet another number was often
given to the completed assembly.
1930s design philosophies
The early to mid-1930s was a time
when superhet receiver design was
really taking off. Before that, until
about the end of the 1920s, consumers
had to be content with tuned radio
frequency (TRF) receivers which had
reached the zenith of their design. But
as good as many of these sets were, a
new direction in design was needed to
make radio receivers both economical
to buy and easy to use.
Initially, superhets were even more
cumbersome than TRF receivers, until
tetrode and pentode valves became
common. What’s more, purpose-designed converter valves such as the
2A7 and later clones had not appeared
commercially on the scene at the beginning of the 30s. To get around this
problem, ingenious circuit designers
developed the autodyne converter.
This provided a local oscillator and
achieved radio frequency (RF) amplification and conversion to the intermediate frequency (IF) all in the one
pentode valve.
The IF amplifier design was well
established early in the 30s and the
generic design remained with us well
into the solid state era. However, the
design of the detector stage was in a
state of flux at the time and the diode
detector had yet to establish itself in
the role it would come to dominate
within a few years. Instead, during
this period, many different types of
detectors were used in radio receivers.
By contrast, audio amplifiers had
also reached a reasonable degree of
sophistication. Indeed, no further
major design advances subsequently
took place in domestic receivers while
the valve remained king.
The Airzone 500/505/515
The Airzone 500 came in a stylish cabinet and has just two controls: volume
and tuning. The control settings are visible through “peep hole” escutcheons.
80 Silicon Chip
Fig.1 shows the circuit of the Airzone 505/515. The anten
na/aerial
circuit is quite standard for the era,
with a 10kΩ potentiometer (R3)
connected across the primary of the
aerial coil. The potentiometer not
www.siliconchip.com.au
Fig.1: the Airzone 505/515 series used a superheterodyne circuit with 455kHz IF stages but no AGC. R3 (at the antenna input) functioned as the volume control.
only attenuated the incoming signals
but also increased the effective value
of the 58 valve’s cathode resistor (R4)
from 220Ω up to a maximum of 10,220
ohms. Hence, R3 had the dual role of
controlling the gain of the 58 and the
amount of signal being fed to the 57
autodyne converter.
Local/DX switch
A number of sets also included
a “Local/DX” switch. This allowed
a further reduction of receiver gain
when strong stations were being received. On Fig.1, the switch is shown
in series with R2 (155Ω) – ie, the 155Ω
resistor was switched into circuit in
the “Local” position, when strong
signals were present. However, the
receiver featured in this article does
not have this facility.
Careful inspection of the circuit
shows that the receiver has no volume
control apart from R3. This meant that
the set could still have had some audio
output when R3 was set for maximum
attenuation unless further measures
were taken.
In fact, Airzone got around this
problem rather nicely by including
a voltage divider consisting of R11,
R5, R4 and R3 across the high tension
(HT) line. When R3 is at maximum
attenuation (ie, the wiper is at the far
lefthand end position), the voltage at
the junction of R5 and R4 could be as
high as 60V positive with respect to the
chassis. As shown, the 58’s cathode is
attached to this junction, while its grid
is at chassis potential, so in effect the
bias can be up to -60V.
This is more than enough to comwww.siliconchip.com.au
pletely cut off the 58 valve. And that
meant that no signal could get through
to the detector and so there was no
audio output.
The converter is the common auto
dyne arrangement from the early 30s.
Its operating conditions had to be carefully selected in order for it to work
reliably. First, the cathode resistor is
a rather high value compared to that
used in a straight RF amplifier. Second,
the padder is wired to the top of the
oscillator tuned winding to ensure
more reliable operation. This also
keeps HT off the tuning gang, which
is much safer for the user.
Note: having HT on the gang
could also pose other problems. For
example, if the gang plates shorted,
there could be quite a “melt down”.
It would only be a matter of whether
the oscillator coil burnt out before either the rectifier or power transformer
succumbed!
Autodyne problems
Early on, there was a problem with
autodyne circuits get
t ing enough
This view shows the rear of the cabinet, with the chassis in place. Loosening two
screws underneath the cabinet allowed the chassis to slide out for servicing.
July 2002 81
HRSA 20th Birthday Celebrations
Recently the Historical Radio Society of
Australia (HRSA), celebrated its 20th birthday
over the weekend of the 20th and 21st of April
2002. Founded in 1982, it now has over 900
members.
Vintage radio collecting in Australia had been going on for many
years prior to the inaugural meeting
of the HRSA on the 17th April, 1982.
I commenced my collecting way
back in the early 1970s, collecting
military radio equipment from
WWII. However, there were others
before that who were collecting and
restoring old radio equipment. Often, they were looked upon as rather
odd people: “collecting old radios,
you’ve got to be mad!”
In Alice Springs, Len Davenport
had established a radio museum,
called the “Magic Spark Radio Museum”. Len believed that there was
a need for an organisation to promote the preservation of our radio
heritage. He spoke with Ray Kelly
(in Melbourne) at length about the
establishment of a national vintage
radio club or society.
Ray organised a meeting at his
home on April 17th, 1982. The idea
was enthusiastically embraced and
it was decided to form a national
vintage radio society, to be called
the Historical Radio Society of
Australia. Starting with only 25
members, the society got under
way immediately and their first
newsletter was produced in July
1982, consisting of just a few pages
of duplicated sheets.
From those very early days the
society has expanded greatly to over
900, with members in every state and
overseas. “Radio Waves” is now a
quality magazine of 30-44 pages on
all aspects of vintage radio and is
published every three months.
Members can obtain advice on
restoration, information on where to
obtain bits and pieces, advertise for
parts or sets that they are interested
in, obtain circuits of most radios and
in some cases identify that odd-ball
set. Recently, the club established
a “Valve Bank” and members can
obtain most valves at reasonable
prices from this source.
In the middle of 2001, the HRSA
committee commenced their planning of the 20th birthday celebrations, to be held in the Brentwood
Community Centre Hall, Mulgrave,
Victoria.
Celebrations started on the Saturday at 9AM with a “Flea Market”
– members buying and selling all
sorts of vintage bits and pieces. At
Valve radio receivers in coloured Bakelite cabinets are
now highly sought after (especially blue).
82 Silicon Chip
12.30PM, the “Class Auction” got
underway with over 100 registered
bidders and quite a number more
who came to see the valuable and
not so valuable go under the hammer. Some pieces of rarer equipment
brought prices well over the $1000
mark while other less sought-after
items brought as low as $5.
Radio displays
While the flea market and auction
were on, a “Radio Display & Concourse” was also taking place. There
were displays of early equipment
from the Marconi spark era; 1920s,
30s & 40s receivers; coloured plastic/
Bakelite radios; Australian battery
portables; military radio equipment from WWII; posters; a display
exclusively of the up-market Zenith
(USA) portable receivers; transistor
sets; and various other interesting
items from our radio heritage.
The Bakelite cases of most radios
were brown or occasionally cream.
Some manufacturers did produce a
variety of cabinet colours, either as
mixes in the Bakelite or as a painted
cover. These coloured sets are highly
sought after, particularly blue ones
which tend to sell for up to three
times the price of a brown set. A
number of these can be seen in one
of the photographs.
Tony Maher, the owner of many
of the battery portables on display,
has been acutely aware that it is
not possible or practical to operate
battery portables from the batteries
A display of receivers from the 1920s. Finding parts for
some old sets can be a real challenge.
www.siliconchip.com.au
feedback to sustain oscillation in
the oscillator section. This problem
was ultimately solved by the late Lay
Cranch. He found that the primary of
the first IF transformer impeded the
feedback circuit.
In early circuits, the inductance
acted as a choke and the capacitor was
too small to allow sufficient feedback.
This problem was solved by increasing
the value of the capacitor and reducing
the inductance.
IF amplifier
These early radios all have one thing in common: attractive wooden cabinets.
These have all been restored to “as-new” condition.
that were used in the past. Hence, he
decided to design a DC-DC inverter
to power these receivers.
He produced it as a kit and he
has been besieged with requests for
them. In this way, Tony is making our
old valve portables useable as well as
being display items. I applaud this
as I believe that wherever possible
our vintage radio equipment should
be heard as well as seen.
The display was the best I’ve
ever seen of this nature. The
equipment was in immaculate
condition and must have impressed the general public as
well. The military equipment
naturally didn’t look anywhere
near as “pretty” as the domestic
radios, being more in keeping
with its intended role. There were
people around who could answer
the questions of the visitors so
that all knew more about our radio
history than before they came to
the display.
Those interested in finding out
more can contact the HRSA at
PO Box 2283, Mount Waverley,
Victoria 3149. New Zealand enthu
siasts can contact the New Zealand
Vintage Radio Society (NZVRS)
secretary at 2 Levy Rd, Glen Eden,
Auckland, NZ. The NZVRS is older
than the HRSA as it was established
in 1979.
Both organisations have web sites.
The HRSA web site is at www.hrsa.
asn.au, while the NZ-VRS site is
www.nzvrs.pl.net
The IF amplifier is quite a standard
circuit. The main difference between
it and later circuits is that it does not
have automatic volume control (AVC/
AGC). It relied instead on manual volume (gain) control, as provided by R3.
Most manufacturers at that time
were using 175kHz IF (intermediate
frequency) amplifiers, whereas Air
zone used 455kHz IF amplifiers in
this design. This meant lower gain
than from 175kHz amplifiers but
the image response was decidedly
superior (which is why 455kHz later
became the standard for domestic
receivers).
The detector is an “anode bend” or
plate detection type. This involves
operating the 57 towards cut-off by
using a higher than normal cathode
resistor (R6). For best fidelity, the
cathode should be bypassed only for
RF (IF) frequencies but this reduces the
overall gain. As a result, Airzone opted
for higher gain but at the expense of
increased distortion.
Electrolytic capacitor C4 should
have had a 500pF mica capacitor
across it to filter out any remaining
IF signals. That’s because electrolytic
capacitors of that era had poor performance at both IF and RF frequencies
after a short time in use. Filtering
the IF energy at the plate of the 57 is
standard practice with this design,
to keep IF signals out of the audio
output stage.
Phono terminals
Early portable transistor radios are now very much collector’s items. These
have all been fully restored.
www.siliconchip.com.au
The 505 and 515 both have a pair of
terminals to allow the use of a record
player turntable to be connected to
the receiver. The input is connected
across the terminals marked “Phono”
at the bottom of the second IF transformer secondary. However, notice
able distortion would be evident at
the audio output with the circuit
values used. In addition, the receiver’s
July 2002 83
The component layout under the chassis is generally uncluttered but note that
the coils sit over the top of three of the valve sockets. This makes it difficult to
access components around these sockets for servicing. The IF adjustments are
accessed through holes in the rear apron of the chassis.
volume control would need to be set
to minimum, to avoid radio stations
coming in over the top of the record
being played.
There is no volume control when
playing records. For normal operation
of the set, the phono terminals are
shorted. The model 500 doesn’t have
this facility which is of doubtful value
anyway.
Audio output
The audio output stage is quite
conventional. The electro
dynamic
The components on the top of the chassis are all easy to access. The two 8µF
electrolytic capacitors near the power transformer were replaced but left in-situ
on the chassis to keep the set looking as authentic as possible.
84 Silicon Chip
speaker and speaker transformer are
plugged into a socket which sensibly
disconnects the HT voltage from the
set proper when removed.
The power supply is conventional,
with a transformer and an 80 rectifier.
The heater winding for the majority
of the receiver is 2.5V and it is centre-tapped to reduce the amount of
hum in the audio output.
Restoring the 500
To remove the chassis from its
cabinet, it is first necessary to remove
the two knobs and the two bolts from
underneath the cabinet. One interesting feature here is the fact that
the bolts are located in slots. When
loosened, this allows the chassis to
be partly withdrawn so that valves
may be replaced, as can be seen in
one photograph.
At some stage during its history,
this set had been converted from a
500 to a 500P. This meant that the 57
autodyne converter had been changed
to a 2A7, the latter arranged in a much
more reliable pentagrid converter
circuit. The aerial and oscillator coils
had also been replaced with much
more modern units using adjustable
slug cores.
The chassis was given a good clean
up but the owner stopped short of repainting it as it was in good condition
for its age.
Getting at the underside of the three
www.siliconchip.com.au
valve sockets holding the 58, the two
57s and other associated components
below the coils and transformers is not
easy. Why did manufacturers have to
make life so difficult for service personnel when a more thoughtful layout
would have made life so much easier?
I’ve seen some radios and other equipment absolutely packed to the hilt
with parts and yet due to thoughtful
design layout are still easy to access.
On the other hand, I have seen many
chassis where access is difficult, like
this Airzone.
The electrolytic capacitors were
replaced but the 8µF chassis mount
units were left in place to keep the
set looking as authentic as possible.
Several paper capacitors in critical
positions, such as the grid capacitor
to the 2A5, were also replaced. Some
carbon resistors were out of tolerance
and these were also replaced, as was
R3 which was the worse for wear.
In sets of this age, it’s not a bad idea
to check that all the resistors are within tolerance (±20%). Some resistors
can also become noisy and should be
replaced, even if their value hasn’t
changed; eg, the plate resistor (R8) of
the 57 detector.
The power cable was replaced with
a modern 3-core fabric covered cable.
It looks the part and has the vital earth
wire which is sensible to have in sets
of this age.
The receiver tuned circuits were
then aligned without any difficulty.
The dial is calibrated from 550 to 200
metres, which equates to 545kHz to
1500kHz (ie, the frequency range of
the broadcast band at that time).
The IF adjustments are accessed
through holes in the rear apron of
the chassis. Two of the trimmers in
the IF cans are at full HT voltage and
should be adjusted using an insulat-
ed alignment tool. If you don’t have
the correct tool, you can cut down a
large-diameter plastic knitting needle
– just file a screwdriver blade on one
of the pieces.
Once aligned correctly, the receiver
had plenty of volume and reasonable
sensitivity. The audio quality is typical
of the era and type of detector used –
in other words it isn’t high fidelity but
it’s still quite listenable.
The controls are back to front to
what we’ve become used to, with the
tuning on the left and the volume on
the right. The settings of both controls are visible through “peep hole”
escutcheons. The volume control is
easy to use but the tuning control is
another matter. Due to the small size
of the knob and the direct drive to the
tuning capacitor, tuning is a finicky
job at best.
Restoring the cabinet
The cabinet was in reasonable
condition, so not a lot of work was
required here. First, paint stripper was
used to remove all existing varnish
and paint from the cabinet. The trims
were then spray painted black, as was
the inside of the cabinet (quite a lot of
cabinets during that era were painted
inside). Finally, the cabinet was finished off with satin/semi-gloss clear
pre-catalysed lacquer spray.
The end result is shown in one of
the photos – the set looks like new!
Summary
Airzone was one of many manufacturers in the early 30s that experimented with new ideas, as demonstrated
by the use of a 455kHz IF in this set.
Converting the chassis to a 500P with
a conventional purpose-designed
frequency converter was a also good
move.
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The set itself certainly look the
part, although it’s a shame that looks
took precedence over ease of tuning.
The audio quality, although not high
fidelity, is typical of the era and quite
acceptable.
It is hard to assess what part of the
market the set was aimed at, as it has
some very good features as well as
some cost-cutting measures. I suspect
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July 2002 85
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