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VINTAGE RADIO
By RODNEY CHAMPNESS, VK3UG
The Armstrong C5 dual-wave
console receiver
For those familiar with vintage radio, the main
brand names of the domestic receivers – AWA,
HMV, Astor and Stromberg-Carlson, etc – all
roll easily off the tongue. But when the name
“Armstrong” is mentioned to someone, their
most likely response is “never heard of it”.
Recently, I was visiting a friend
(Laurie) and he showed me an Armstrong console receiver. He had never
heard of the brand before it came into
his possession and neither had I.
As can be seen from the photos, the
console cabinet housing the receiver is
quite an attractive piece of furniture.
The timber used is lighter than that
used on some of the HMV consoles of
the same era but this has one advantage
– it makes the set much easier to lift!
Another difference is that the dial
slopes back at a slightly greater angle
than on most other consoles, which
makes the set easier to operate when
the operator is standing up. It also
means that the chassis is mounted on
a sloping shelf, although (fortunately)
the angle is not high enough so that
the chassis slides out by itself when
the retaining screws and knobs are
removed.
Large tuning dial
A dominant feature of this set is the
larger than average circular tuning
dial. This is very attractive in appearance and makes the set easy to tune.
As can be seen in the photos, there
is a small red circle at the bottom of
the dial. This is marked “Short Wave”
and has a dial lamp behind it which
illuminates when the set is receiving
shortwave signals. Certainly, console
manufacturers set out to make their
sets look special in what ever way they
could to impress people.
A glance in the back of the set reveals that the dial drive system and
the tuning gang are mounted several
centimetres proud of the chassis. This
was necessary because the large dial
would have otherwise interfered
with the chassis. But despite being
84 Silicon Chip
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There is plenty of space inside the cabinet for the 5-valve chassis and the large
electrodynamic loudspeaker. The loudspeaker’s field coil also acts as the filter
choke for the HT supply.
on “stilts”, the tuning system is quite
stable in operation and has minimal
backlash.
On shortwave, the mechanical
stability of the dial system may not
have been particularly good but then
shortwave was usually only used on an
occasional basis. In fact, the shortwave
bands could have been left off 99% of
domestic radio sets, as they were rarely
used by the general listening public.
By contrast, listeners who genuinely
wanted to listen to shortwave could
purchase more upmarket sets, such
as one of the AWA “seven banders”,
which were serious shortwave receivers.
valve line-up is quite conventional
and includes a 6A8G converter, a
6U7G 455kHz intermediate frequency
(IF) amplifier stage, a 6B6G diode detector and first audio amplifier and a
6F6G audio output valve. The power
supply uses the ubiquitous 5Y3GT as
the rectifier.
Note that the 6B6G is not shielded
and it would appear that the metal
plate mounted between it and the
output valve provides enough shielding to prevent audio feedback. The
audio output stage drives a 305mmdiameter (12-inch) electrodynamic
loudspeaker.
Circuit details
A somewhat unusual feature of this
receiver is that it doesn’t have AGC.
Instead, it relies on manual control of
the converter and IF amplifier gains,
via a wirewound potentiometer.
The moving arm of this pot is con-
The Armstrong C5 console is a conventional 5-valve dual-wave receiver
covering the broadcast band from
550-1600kHz and the shortwave band
from 7-22MHz (42 to 13 metres). Its
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Vintage Radio – continued
keep in mind if you come across sets
with two trimmers on the IF transformers and it’s this: one trimmer is
usually at HT voltage! Damage can be
done to the transformer winding if it
is shorted to chassis while being adjusted. What’s more, you could receive
a very nasty (and possibly lethal) shock
if you are careless enough to come into
contact with this HT voltage!
Detector & audio amplifier
The two diodes in the 6B6G valve
are strapped together to act as the
detector. The resulting audio signal appearing across the diode load resistor
is then applied to the grid of the 6B6G
and the signal from this stage then fed
to the 6F6G audio output valve. This
then drives an output transformer and
a 305mm (12-inch) Rola electrodynamic speaker.
The tone control circuit consists of
a capacitor between the plate of the
6F6G and a potentiometer which connects to earth.
Power supply
This close up view shows the C5’s chassis from the back. Note how high the
tuning gang sits so that the large tuning dial can easily be accommodated. The
holes in the tops of the IF cans provide access to the alignment trimmers.
nected to earth, while one end of the
resistance track is connected to the top
of the selected antenna coil. The other
end of the track goes to the cathodes
of both the 6A8G and the 6U7G via a
cathode resistor. This system works
well but it would have been simple to
arrange delayed AGC to the controlled
valves. Back bias is used for the 6F6G
and this could have been used on the
AGC system as well.
Another unusual feature of this set
is that the antenna lead-in is routed
right across the underside of the chassis to the wave-change switch. Other
manufacturers usually route this lead
so that it is at least “semi-shielded”
from the rest of the circuit, to avoid
any possibility of unwanted feedback.
However, Armstrong obviously didn’t
have a problem with this, as no signs
of any instability were evident when
the set was tested.
On a similar theme, both the antenna and oscillator coils for the broadcast
band are mounted on the chassis. By
contrast, the shortwave coils, which
are more critical in their placement,
are mounted on the wave-change
switch itself.
All coils are air-cored with no pro86 Silicon Chip
vision for adjusting their inductances
and this includes the IF transformers
as well. By contrast, the broadcast
band has trimmers on both the antenna and oscillator coils for tuning
adjustment at the high-frequency end
of the dial. In addition, the oscillator
coil has a padder for adjusting the lowfrequency dial calibration.
On shortwave, it is another matter
entirely. It is simply a case of “forget
it”, as there are no adjustments for the
shortwave band at all! This seems a
bit crude at first glance but when you
look at the dial-scale calibrations on
shortwave and consider how poorly
calibrated most shortwave sets were
at that time, it probably didn’t matter
all that much.
The IF amplifier stages are tuned to
455kHz which is quite conventional.
However, the IF transformers are different from most others, as they are
tuned using two trimmer capacitors at
the top of each unit. At that time, most
manufacturers had changed over to
fixed capacitors, with slugs (iron dust
core) used to alter the inductance to
tune the IF transformers to the correct
frequency.
There’s one very important point to
The power supply is quite conventional, with the loudspeaker’s field
coil also acting as the filter choke. The
centre tap of the power transformer
goes to earth via a low-value resistor
which provides bias for the 6F6G. Why
this couldn’t have also been used to
provide bias for the RF-stage valves
and to provide delayed AGC is a puzzle. Perhaps the designer didn’t feel
confident that he could get it right and
stayed with a “tried and true” method
from previous designs.
General comments
Although the receiver has an “ARTS
& P” transfer on the chassis, the letter
which designates the year of manufacture is missing. However, it’s likely
that the set was manufactured during
the 1937-39 period.
(Editor’s note: according to the Historical Radio Society of Australia, the
“ARTS & P” system was a licensing
system that was used in Australia and
New Zealand between 1934 and the
1960s. The system was introduced to
verify that radio manufacturers paid
royalties for items that were covered
by patents. Each licensed radio was
fitted with a small sticker attached to
the back of the chassis and the colour
of the sticker is a useful way of determining the age of manufacture).
The year indicators for these transwww.siliconchip.com.au
Photo Gallery: Astor “Mickey
Mouse” Model BE
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Produced in 1936 by Radio Corporation (Melbourne), the BE is another example of a
small mantel set carrying the “Mickey Mouse” name. An interesting feature of the
set was the provision of a 7-pin socket which enabled a shortwave converter (dubbed
the “Oversea-er”) to be connected. This unit contained its own 6A7 frequency
changer valve and enabled the receiver to tune the 6-19MHz shortwave band. The
receiver was fitted with the following valve line-up: 6A7 frequency changer, 6D6
IF amplifier, 6B7 detector & first audio amplifier, 41 audio output and 80 rectifier.
(Photo courtesy Historical Radio Society Of Australia (Inc.).
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fers are as follows: A = 1934, B = 1935,
C = 1936, D = 37, E = 1938, F = 39,
and G = 1940 (it’s possible that “G”
may have been used for several of the
war years).
The set’s controls are mounted underneath the dial and along the front
edge of the chassis. From left to right
they are: tone, tuning, wave change
and volume. This differs from most
sets, which have the tuning control to
the right of the volume control, to suit
righthanded people.
The painted (green) chassis was
in good condition, with the chassismounted components arranged logically and neatly. The chassis is easily
extracted from the cabinet by first
removing the control knobs and the
screws holding it to the mounting
shelf. The loudspeaker plug is then
disconnected from the chassis, after
which the assembly can be removed
from the cabinet.
A close inspection of the chassis
reveals that the mechanical and wiring
layouts are quite logical, with good acwww.siliconchip.com.au
cess to most parts. This makes it easier
to work on than many other sets from
the same era. In fact, Laurie found that
there wasn’t much to do to get the set
up and running.
Over its life, the set has only had
one paper capacitor changed and this
was to a Ducon brand capacitor as
commonly used in the 1950s. These
capacitors were not particularly good
and became leaky after only a few
years, so it may have to be replaced
again soon.
In fact, the set has been sitting
around for some time since it was
initially restored, so it will need to be
completely rechecked before being
used. In particular, it will be necessary to check that none of the critical
capacitors have become excessively
leaky.
The one I routinely replace in a set
without AGC is the audio coupler
between the two audio stages. Other
than that, most of the capacitors in
this design can be quite leaky without causing any harm to the set or
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January 2004 87
The C9 chassis has a simple layout, with easy access to all parts. Note the knot
used to restrain the power cord, which is unacceptable by today’s standards.
having any discernible effect on its
performance.
Testing capacitors
I use both 500V and 1000V testers to check capacitors for excessive
leakage. These high-voltage testers
are able to detect leakage resistance
in capacitors that’s not evident on a
normal ohmmeter.
Another very effective method that
can be used is to heat the capacitor and
then measure its resistance. To do this,
it’s necessary to first disconnect one
A close up view of the Armstrong C5’s impressive dial. The small red circle at
the bottom of the dial illuminates when the set is receiving shortwave signals.
88 Silicon Chip
lead of the capacitor from the circuit.
That done, you connect an ohmmeter
(preferably digital) across the capacitor using a couple of clip leads and set
the meter to a very high ohms range
(over a hundred megohms if possible).
Finally, you use a hair drier to gently
heat the capacitor until it’s quite warm
(85°C to 100°C).
If the capacitor is defective, its leakage current will increase significantly
and the meter reading will decrease.
Finally, despite its age, this receiver
was obtained in quite good condition.
The cabinet required some touch-up
work in a few places but generally it
had been very well looked after during
its life. What’s more, the chassis was in
very good condition and needed little
more than dusting.
This is one set that was obviously
not stored in a damp garage or shed
after being retired from service!
Although not a “top of the line”
receiver, the Armstrong C9 is a wellmade set that would have given reliable and impressive service over many
years. It’s obvious that a lot of thought
went into the design of the set and it
is a worthwhile receiver to have in a
collection – provided you have enough
SC
room for a console!
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