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Vintage Radio
By RODNEY CHAMPNESS, VK3UG
The little 1934 Astor Mickey
From the 1920s onwards, Astor produced
many fine receivers, the Astor Mickey being
one of their early mantel receivers. It was a
very compact 5-valve receiver and the “OZ”
model number that was used to denote the
Australian model now seems quite relevant.
It’s probably just coincidence that
Astor used “OZ” to denote the Australian version of this receiver. The
term “OZ” didn’t become slang for
Australia until much later on, so it’s
impossible to say just why the “OZ”
model number was used.
The Astor Mickey “OZ” was a modi-
fied version of an American receiver
that was designed to run off 110V
mains. In the US, Radio Corporation
must have thought that they had it
made with the “Mickey Mouse” name,
since it reminded people of the Walt
Disney character of the same name.
However, the people at Walt Disney
The Astor Mickey model OZ was built into an attractive walnut cabinet. Note
the very small elementary dial scale. It consists of a reduction drive and uses a
gramophone pick-up needle(!) as the dial pointer.
82 Silicon Chip
were not amused and legal action
eventually resulted in the name being
altered to just plain “Mickey”.
US designs
Quite a few of the receivers sold in
Australia during the 1920s and 1930s
were close copies of American sets
of the era, often being built under a
licence agreement. Australia’s manufacturing base for radio receivers was
not as advanced as America’s at that
time and so the use of American designs made good business sense for
manufacturers looking to steal a march
on their rivals.
The Astor Mickey “OZ” was quite
a compact receiver for its time, despite the fact that it included a power
transformer, a couple of intermediate
frequency (IF) transformers, a tuning
gang, various coils, a loudspeaker, an
output transformer, miscellaneous
passive components and, last but not
least, five large valves. In fact, Astor
did a marvellous job of shoe-horning
them all into such a small space.
A side effect of this “shoe-horning”
was that the audio output and rectifier
valves cause other components in their
near vicinity to get quite hot as well.
For example, the tops of the valve
envelopes are quite close to the top of
the cabinet and this inevitably became
heat-damaged. To minimise this, vertical ventilation slots were cut into the
lefthand end of the cabinet to assist
airflow, while a sheet of asbestos(!)
was fitted above the valves to reduce
heat transfer to the cabinet.
Most of these sets will still have
the asbestos fitted, so be careful if you
are working on one of these receivers.
Asbestos is a carcinogen and should
be treated with great caution. To prevent fibres of asbestos coming off the
sheet, it could perhaps be sprayed
with clear Estapol which should seal
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This front view shows the
chassis after it has been
removed from the cabinet.
Notice how closely the
components have been
packed together. There’s no
wasted space here!
Below: a rear view of Astor
OZ. This shows the very
compact nature of the set,
considering that it uses
full-sized components.
Be aware that a sheet of
asbestos is used above the
two valves at the right of
the photograph.
its surface and thus prevent any loss
of material. How you deal with it is
up to your own good commonsense.
I’m certainly not an expert on dealing
with asbestos safely.
Front-panel controls
The front panel of the receiver carries the volume and tuning controls,
with the volume control to the left
and the tuning to the right. A brass
plate behind each knob identifies its
function and these plates are attached
to the wooden cabinet via escutcheon
pins. The loudspeaker is fitted behind
a fret in the front of the cabinet, which
is covered with speaker cloth.
The tuning control is similar to that
used in many other early sets and features a small circular dial-scale that’s
located behind the knob – in fact, it’s
hardly worth calling a “dial scale”.
There is a reduction drive to the gang
and the pointer for the tuning consists
of a gramophone pick-up needle that’s
inserted into the reduction drive brass
ring. There are no frequency calibrations or station callsigns on the dial
scale – just a 0-100 scale.
There were certainly some big improvements made to dial-scales in the
years following 1934, when this set
was manufactured.
Removing the chassis
The set is reasonably easy to dismantle. First, the knobs are unscrewed
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and the brass dial-scale is levered off.
That done, four bolts are removed from
the base of the cabinet, after which the
chassis is can be slid out of the case.
This has to be done carefully, as it is
a tight fit.
With the chassis exposed, it quickly
becomes clear that there is a lot of
radio packed into a small space!
There is very little space between the
chassis-mounted items and you need
nimble fingers to remove the detectorcum-first-audio valve. That said, I’ve
seen more awkward layouts than this.
However, it is just as well that tuning
capacitors are usually trouble free, as
the gang is completely covered by the
oscillator and antenna coils and their
associated components.
The view underneath the chassis is
a bit more frightening, with a mass of
components and various leads going
March 2004 83
The under-chassis view of the Astor Mickey OZ reveals a real dog’s breakfast,
with a mass of components and leads going everywhere! It is a difficult receiver
to service because the components are so crowded together.
all over the place. This particular set
had been serviced on several occasions
in the past and this has only added
to the confusion with the layout. Replacement components appear to have
been tacked in wherever possible and,
over the years, a significant number of
the capacitors and resistors have been
replaced. However, they were not all
replaced at the same time, as components from several eras are evident.
Circuit details
Redrawing the circuit diagram of
this radio using circuit symbols from
the end of the valve era would quickly
disguise the fact that it was designed
in 1933. In fact, if the valve type numbers were unknown and if the field
coil on the speaker is ignored, this
circuit could easily be mistaken for
one of many dozens produced during
the 1960s. Even by the mid-1930s,
the superhet receiver had been almost
fully developed.
Of course, there are differences between this set and later sets but these
84 Silicon Chip
are purely refinements of what had already been produced. For example, the
quality of the coils improved with the
advent of iron dust and ferrite cores, as
well as then being able to make them
much smaller. In addition, the valves
became much smaller with the introduction of 7-pin and 9-pin units, but
their characteristics remained similar
to the octal and pre-octal valves that
they replaced.
For example, the 6D6 (in this set)
later became the 6U7G, which has
almost the same characteristics as the
later miniature 6BH5.
Another difference is that electrodynamic loudspeakers gave way to
permanent magnet units, which saved
power because they didn’t require a
field coil to produce a magnetic field.
And over the years, the electrolytic
and paper capacitors gradually became
smaller for the same capacitance, with
the unreliable paper types ultimately
replaced by polyester capacitors.
Finally, towards the end of the
valve era, thermionic power recti-
fiers were replaced by more efficient
silicon power diodes. So while there
were significant improvements in the
components used, the circuit designs
of common domestic radio receivers
remained much the same.
Australian modifications
This receiver was, as mentioned earlier, an “Australianised” version of an
American radio. The American design
was for a transformerless set which ran
directly off the 110V mains. In this design, the valve heaters would have all
been connected in series, which meant
that 69V was needed across them for
best performance (possibly achieved
by using a dropping resistor).
As a result, the circuitry of the
receiver were designed to operate
efficiently off 110-140V DC. At this
voltage, the 43 output stage gives quite
reasonable audio output.
Modifying the set for Australia
involved adding a mains transformer
to supply the voltages required. This
transformer allowed the set to be used
with the Australian 240V mains and
featured three heater windings to cater
for the various heater voltages. In adwww.siliconchip.com.au
Fig.1: the circuit for Astor Mickey OZ is a fairly conventional 5-valve superhet.
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dition, the rectifier circuit was modified to function as a full-wave unit,
instead of the half-wave unit used in
the original design.
However, some later versions of this
radio used valve heaters that were
wired in series and a half-wave rectifier was used to supply the HT voltage
for the set. These later receivers were
very much an American design, with
a power transformer “hung” on the
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mains to give the right voltages. As
before, it was no longer necessary to
use a dropping resistor to reduce the
110V to 69V as the heater winding
on the transformer provided just the
right voltage.
The power transformer probably fitted in the space vacated by the heater
dropping resistor in the American
sets. And as well as providing the
correct voltages, it certainly makes the
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March 2004 85
Vintage Radio – continued
stage and a pentode audio output stage.
Provision is made for an extension
speaker, as shown in the lower right
of the circuit diagram.
All stages use cathode bias except
for the audio output stage, which uses
back bias. The field coil is in the negative lead of the power supply and 1/6th
of the voltage across this is applied as
the back bias.
The power supply is conventional
and uses a mains transformer and
full-wave rectifier (V5) to produce
the high-tension (HT) voltage (135V).
Lower voltages for various sections
of the receiver are obtained from a
voltage divider network across the
HT rail, consisting of resistors R14,
R15 & R16.
Alignment
This top rear view of the chassis again shows how close the major components
(valves, IF transformers, etc) are together. Note the side adjustments on the aircored IF transformers at the rear of the chassis
set safer to work on. Indeed, Australians have always had a dislike of live
chassis equipment, in contrast to the
Europeans and Americans.
Circuit details
As mentioned earlier, the circuit
layout is quite standard, although the
tuned input circuit does require some
comment. This tuned circuit consists
of L3, L4, C4 and C5, with tuning
capacitor C4 being adjusted to tune
to the desired station. In addition,
these components, together with the
remainder of the parts in the input
circuit, form a complex network that’s
designed to have a broad response
across the broadcast band but with the
response dropping off rapidly outside
this band.
The reason for this is that the designers were concerned about breakthrough from marine Morse code
stations in the 400-513kHz frequency
range into the intermediate frequency
(IF) amplifier. That said, it probably
would have been simpler to have put
an IF trap in the antenna circuit on
456kHz. However, this is one of the
earlier sets using a 456kHz (455kHz)
86 Silicon Chip
IF amplifier stage and, because it
uses air-cored low-Q transformers,
the frequency response was probably
sufficiently broad to allow signals well
away from 456kHz to get through.
The antenna circuit used in the Astor Mickey was obviously designed to
overcome this problem by rapidly attenuating signals outside the broadcast
band. Without this circuitry, either an
annoying thumping noise or a tonemodulated series of short and long
signals would have been evident to
the listener. Indeed, one of my receivers from the 1960s was prone to this
problem.
Of course, this is no longer an issue,
as the marine medium frequency (MF)
stations closed down at the turn of the
century.
The IF amplifier is quite conventional and uses trimmers to tune each
IF transformer winding. The adjustments are made from the side of each
transformer and as can be seen in
photograph, they can be adjusted with
the set in the cabinet.
The IF stage is followed by a diode
detector cum-AGC-diode stage, followed in turn by a pentode first audio
The alignment of the IF stage is
conventional and involves applying
a modulated signal from a signal generator (set to the IF frequency) to the
grid of the 6A7 RF stage. The audio
output level at the speaker (or the DC
voltage across the volume control) is
then measured and the tuning peaked
for a maximum reading.
The alignment of the antenna and
oscillator circuits is also conventional.
The set nominally tunes 550-1500kHz
but by carefully adjusting the two
trimmers on the tuning gang at the
high-frequency end of the band and
the padder capacitor (C9) at the lowfrequency end, the set can be made
to tune the entire broadcast band as
it is today.
The padder capacitor (C9) is accessed through the back of the chassis,
near the aerial and earth terminals.
However, it really is guess work as to
where the alignment points of 600kHz
and 1400kHz should appear on the
dial, as it is only calibrated 0-100!
The procedure for tuning the front
end is fully explained in “Vintage
Radio” for February 2003. On a cautionary note, don’t adjust C3 unless
you really know what you are doing.
This small capacitor (about 2pF) consists of two short lengths of insulated
wire twisted together and forms part
of the broadcast bandpass image and
IF rejection circuit.
Performance
It’s a bit hard to judge just how well
this set performs, since it has yet to
be restored. However, it’s doubtful
that it will be up to the standards of
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Photo Gallery: Philips Model
6506 – Medium Wave (1937)
Silicon Chip
Binders
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H 80mm internal width
With its vertical edge beading, chrome-plated grille bands and chrome-plated station pointer, the Philips Model 6505 is a classic example of art deco styling. The
set came with either “E” series or “A” series valves, the former with 6.3V heaters
and a 4V rectifier, the latter with 4V heaters and a 6.3V rectifier. Tuning was accomplished using a large disc and wedge wheel, with an anti-backlash mechanism.
(Set restored by Maxwell L. Johnson; photo by Ross Johnson).
comparable receivers from the 1950s
and 1960s, due to the low Q of many
of the coils.
nitro-cellulose lacquer. The interior
will be given a coat of matt black paint
to finish it off.
The cabinet
Summary
The cabinet is quite small for the
era, being just 305mm long, by 180mm
high and 140mm deep. It is, however,
quite attractive and is made from
walnut ply, with the front made from
a piece of figured walnut. Black paint
highlights the controls, the speaker
grille and the base of the cabinet.
As shown in one of the photos, the
cabinet style is different in that the
top is curved down in the centre –
almost like a small seat! It does look
quite effective and this set would have
looked every bit as good as many other
high-quality sets of the era.
The cabinet has been restored using
flat clear polyurethane and looks quite
impressive. However, a little later on,
its owner intends to finish the cabinet
restoration with a mixture of 60% gloss
Despite being a 1933 design, the
circuit of the Astor Mickey is similar
to many radios that appeared towards
the end of the valve era. It only suffers
in performance compared to these later
sets because of the inferior components that were available in 1933-4.
Astor managed to cram a lot into a
cabinet that is similar in size to most
mantel sets of the later valve era.
Considering this, access to the works
is quite reasonable. The cabinet is of
an eye-catching design and even today
the set would look good and sound
good in the home.
It’s no wonder that these receivers
command high prices when sold. If
you have the opportunity of obtaining one at a reasonable price, then
SC
“go for it”.
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H SILICON CHIP logo printed in
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& cover
H Buy five and get them postage
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Price: $A12.95 plus $A5.50 p&p.
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March 2004 87
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