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Vintage Radio
By RODNEY CHAMPNESS, VK3UG
The Astor Football GR/GRP
3-Valve TRF Mantel Receiver
Manufactured around 1948, the Astor GR/
GRP receiver was nicknamed “Football”
because of its cabinet shape. It’s a low-cost
3-valve TRF set designed for tough times.
“WIRELESS” RECEIVERS were initially all tuned radio frequency (TRF)
types. The superheterodyne circuit
was not invented until Major Edwin
Armstrong developed the concept during World War 1. Superhet receivers
are more complex than TRF receivers
but have many advantages where high
performance is required.
By contrast, TRFs were traditionally
used where cost, non-critical performance and simple circuit design were
important. Many TRF radios are easy
to operate but those using regeneration require operating skills that many
non-technical listeners find hard to
acquire. Although TRFs are rarely
92 Silicon Chip
seen these days as domestic receivers, they are still used in the form of
super-regenerative receivers for such
things as garage door openers.
Most receivers manufactured from
the mid-1930s onwards were superhets but manufacturers occasionally
produced a simple, cheap TRF set to
satisfy the low-cost end of the market. The Astor GR/GRP is one such
example.
The Astor Football (GR, GRP)
The Astor “Football”, as it is affectionately known, is a small, 3-valve,
economy mantel broadcast receiver
produced around 1948. “Football”
wasn’t its official name but the cabinet
is around the same size and shape as
an Australian Rules football, hence the
unofficial nickname given to the set by
users and collectors. It was intended
for use as a kitchen or bedroom radio;
anywhere radio signals were strong.
The Football’s bakelite cabinet came
in a few colours, with brown and
cream being the most common. The
cabinet is made in two parts, which
are separated by undoing three screws,
two underneath the cabinet through
rubber buffers and the other through
the back. However, the design leaves
something to be desired, as the thread
that goes through the cabinet to the rear
retaining nut fouls the 6G8G valve and
makes it difficult to replace the back.
To transport the set, four fingers
are inserted through the back section
of the case and it is then carried that
way. However, an antenna is required
for decent reception and the 5.2metre permanently connected antenna
doesn’t lend itself to easy portability.
The set can have either of two type
numbers: GR or GRP. GR refers to a set
that uses a 50Hz power transformer
and a 5Y3GT rectifier, whilst GRP
refers to a set that uses a 40Hz transformer and a 6X5GT rectifier. Apart
from that, the two sets are identical.
Circuit details
Because it is a TRF set, the Astor
Football’s circuit is quite simple.
However, in order to obtain reasonable performance, reflexing is used to
gain extra sensitivity. Fig.1 shows the
circuit details.
The input tuned circuit (29, 13, 33,
34) is quite conventional, with no
automatic gain control (AGC) voltage
applied to a 6G86 RF valve. As shown,
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the input signal from the antenna is
amplified by the 6G8G and passed on
to another tuned circuit (30, 12, 33 &
35). It then goes via an RC network to
a detector diode in the 6G8G. The end
of the untuned winding not connected
to the 6G8G’s plate is bypassed to earth
via capacitor 6.
The detected audio signal is fed
through resistor 18, capacitor 4 and
resistor 16 to the grid of the 6G8G. The
residual RF from the detector is largely
filtered out by capacitor 9. Capacitor
8 prevents the audio and bias signals
from being shunted to earth by the
tuned circuit.
The 6G8G amplifies the audio as
well as the RF signal. In the plate
circuit, the untuned winding of the
tuned circuit does not impede the
audio signal. As far as the audio signal
is concerned, there is only the plate
load resistor (22) in circuit. The audio
is developed across this resistor and
applied via capacitor 3 to the grid of
a 6V6GT audio output valve.
The 6V6GT amplifies this audio
signal and applies it to a speaker transformer which in turn drives a 5-inch
(125mm) loudspeaker. Note that there
is no negative feedback in the audio
circuitry, in keep with the Astor’s role
as an economy receiver.
The power supply is quite conventional, with either a 6X5GT or a
5Y3GT acting as a full-wave rectifier.
The 6X5GT appears only to have been
used in the units supplied with a
power transformer intended for 40Hz
operation. At the time this set was
produced, Perth (WA) apparently had
40Hz mains while most of the rest of
Australia had 50Hz mains.
Note that the 6.3V dial lamp is run
from a 5V tapping on the heater winding in order to extend its life. Another
point of interest is that the receiver’s
power consumption is around 35W,
which is almost the same as most
5-valve sets of the era.
There is no conventional volume
control. Instead, the volume is controlled by the back-bias system. This
can be varied anywhere between -2V
and -22V and is applied to the variablemu 6G8G valve. The more bias, the less
amplification.
Restoring the GR
This particular receiver had been
largely restored when it was given to
me on loan. First, the bakelite cabinet
was in good order and required no atsiliconchip.com.au
Fig.1: a 3-valve TRF circuit is used in the Astor GR/GRP receiver, with the
6G8G valve functioning as a reflexed RF and audio stage.
The above-chassis parts are close together but can be easily accessed once the
valves have been removed. Note the metal shield on the 6B8G8 RF valve.
tention. Second, all the paper capacitors, one electrolytic filter capacitor
and a couple of wires with perished
insulation had been replaced. And
third, the 2-core figure eight power
lead had been replaced with a 3-core
type.
The wires that had been replaced
were bright blue and looked out of
place. As a result, I replaced them with
brown hook-up wire which looked
similar to the original wiring.
The next thing I noticed was that
the cardboard speaker cloth template
was much the worse for wear. There
were a couple of choices here – either
May 2009 93
While looking at the dial-drive
mechanism, I also noticed that all the
pulleys that the dial cord runs over
are riveted into place. This means that
none are free to turn, which places
further stress on the dial-drive mechanism but not on the fibre gear-drive,
thankfully.
This is not an isolated incident as a
previous Football that I serviced had
exactly the same riveted dial-pulley
problem. What’s more, its a problem
that cannot be easily overcome. Having said that, it’s worked for around
60 years with no real problems, so
it really isn’t worth worrying about.
Still, it really was a cheapskate method
of constructing the dial-drive mechanism.
Trying it out
This view shows the underside of the chassis with the restoration nearly
completed. Among other things, the mains cord should be rewired, so that
its earth lead is attached to a crimp eyelet lug which is then securely bolted
to the chassis (the soldered tag-strip connection to chassis shown here could
become faulty, due to the joint going “cold”).
I could make a new template out of
thick cardboard or I could patch up
the original by gluing strips of cardboard to it.
In the end, I decided to repair the
existing template although in retrospect it would have been better to
have made a new one. Of course, I only
found some suitable cardboard after
I had finished repairing the original
template but that’s always the way.
Next, the light-coloured speaker
cloth had rotted so I replaced it with
some new material. This was glued
in place using Tarzan’s Grip. Unfortunately, I only had dark-brown cloth, so
I also had to change the colour of the
dial pointer so that it could be seen.
This was changed to white using a
white-out correction pen and does not
look out of place.
Parts layout
The parts under the chassis are all
quite accessible, so the set is easy to
service. The layout however, is something of a dog’s breakfast. Because the
set has relatively low gain, parts placement is not overly critical, although
personally I like to see all components
related to the operation of a particular
section grouped together.
94 Silicon Chip
Above the chassis, things are rather
different and there is little spare space.
However, with the valves removed,
all parts can be easily accessed for
service.
This particular radio had apparently been used in a kitchen, as fat
and grime were evident on the chassis
and various components. This gunk
was removed from the chassis (after
removing the valves) using a kerosenesoaked kitchen scourer, after which the
chassis was wiped down with a clean
cloth. Kerosene is slightly oily and so
some remains on the chassis and helps
prevent further rusting.
Next, the knobs were cleaned with
soapy water and a nailbrush to remove
the finger grime that had built up over
many years of use. However, while I
was rotating the tuning control to gain
access to all sections of the knob, I
noticed that the fibre gear that drives
the tuning capacitor has several badly
damaged teeth. This means that tuning from one end of the dial to the
other will eventually strip these rather
delicate teeth.
This is an area of weakness in some
Astor receivers but fortunately, a
member of the HRSA makes replacement gears.
Having finished the repairs, I powered the set up while monitoring the
various voltage rails. Everything was
normal except for the plate voltages
which came up slightly higher than
shown on the circuit. However, this is
understandable as the voltages marked
on the circuit were obtained with a
mains voltage of 230V AC while mine
is 245V AC (despite the fact that the
official voltage these days is 230V).
I had expected the radio to work
properly from the outset but my confidence was misplaced. Instead, its
performance was extremely poor and
there was noticeable hum in the audio.
So I had some troubleshooting to do.
First, I connected an electrolytic
capacitor across each of the electros in
the power supply in turn, to determine
if one of these was faulty. This quickly
showed that capacitor 10 was indeed
faulty and this was confirmed when I
checked it on my capacitance meter.
I replaced it with a 22μF 350V electrolytic capacitor, which I hid under
the chassis. The old capacitor was
simply disconnected but left in place
to keep the set looking as authentic
as possible.
With the new capacitor, the performance improved markedly, although
it still wasn’t brilliant. This isn’t
exactly a high-performance set but
the performance was still well down
compared to the one I had previously
restored.
With the various voltages being
relatively close to specification, I knew
that the 5Y3GT rectifier valve was in
good order. As a result, I tried replacing the 6V6GT audio output valve
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but this gave no change. However,
when I replaced the 6G8G, the set’s
performance improved dramatically.
The original 6G8G was obviously well
past its use-by date!
Alignment
All that was left now was to touch
up the alignment. Alignment is a task
that many newcomers to vintage radio
endeavour to steer clear of and indeed,
aligning some receivers is quite a task.
Sets like the AWA 7-band series or a
Barlow Wadley XCR-30, for example,
can be quite a challenge.
By contrast, the Astor Football is
extremely easy to align, with only
three adjustments involved.
There is a standard procedure for
aligning the receiver but for the average collector, a somewhat simpler
method also works extremely well.
First, connect the antenna that’s
normally used to the receiver, then
adjust the dial pointer position for
equal overshoot at each extremity of
the dial. That done, tune to the lowfrequency end of the dial and use a
signal generator to check that the set
is tuned to about 535kHz.
If you don’t have a signal generator,
just tune to a station that’s close to the
low-frequency end of the dial.
Because there is no adjustment to set
the lowest frequency received, some
sets may not tune down to 535kHz
due to tolerances in the tuned circuit
components. If so, there’s not much
you can do about it – just accept it.
The tuning should now be adjusted
to 1400kHz or to a station that’s close to
this frequency. The dial is not marked
with frequency indications so it will
be necessary to check the station frequencies in order to do this.
Regardless as to whether a signal
generator or a radio station is used as
the signal source, the alignment procedure is the same. If the signal is received at a lower indicated frequency
on the dial than where it should be, it
will be necessary to screw both trimmer capacitors in to increase their
capacitance. This will gradually move
the tuning further up the dial, towards
the correct spot.
Conversely, if the station appears
higher up the dial that it should, reduce the trimmer capacitances. In both
cases, it’s only a matter of adjusting
the trimmers to peak the signal at its
correct dial location.
The selectivity of a TRF set mainly
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These views show the front of the chassis without the speaker cloth (top)
and with the new speaker cloth attached (bottom).
depends on the frequency that’s tuned.
In the case of the Football, the selectivity is about 10kHz at the low-frequency
end (ie, at around 550kHz) and about
30kHz at the high-frequency end (ie,
around 1400kHz). This is one of the
deficiencies of TRF sets; ie, the higher
the frequency tuned, the worse the
selectivity.
Summary
The Astor GR /GRP football is very
much an economy set and corners
were cut in its design, as shown by
the dial-pulley arrangement. Basically,
it is an uncomplicated TRF set with
reflexing around the 6G8G. However,
don’t be put off by the idea of reflexing, as it is not a troublesome area in
this set.
If there are problems, it is only
necessary to measure the values of the
various components in this section to
determine where the problem lies.
The set is easy to work on below
the chassis, although not quite so easy
on top. It is dead simple to align but
not very sensitive. However, for its
intended use as a kitchen or bedroom
set in a suburban environment, its
performance is quite adequate.
There are few, if any, problems with
this set that cannot be easily fixed. For
this reason alone, it would make an
ideal first restoration project.
In short, the Astor football is a fav
ourite with many collectors and is
well-worth having in a collection, as it
was one of the last domestic TRF valve
SC
designs to be sold in Australia.
May 2009 95
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