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Vintage Radio
UDISCO L6 TRF Radio from 1926 or 27
By Dennis Jackson
There was a least one Australian company that developed the batterypowered triode valve TRF wireless receiver of the early 1920s to its limit:
UDISCO, the United Distributing Company. When advertising, UDISCO
would often draw attention to the single dial control they used.
According to the 1993 Electronics
Australia publication “The Dawn of
Australia’s Radio Broadcasting”, wireless telephony in Australia kicked off
after the end of The Great War (also
called World War 1) in 1918. The first
direct wireless telegraphic messages
between England and Australia were
received on the 22nd of September
1918 by Ernest Fisk (later Sir), one of
the founders of AWA.
On the 13th of August 1919, Ernest
Fisk demonstrated audible sound
reception across Sydney without
wires. Various professional experimenters and amateurs were to add
their talents to the development of
wireless transmission, until licensed
broadcasting became available to the
general public in early 1923, after the
government enacted the necessary
regulations.
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As one who witnessed the advent of
television around 1960, I can imagine
the excitement of the times. Peoples’
daily lives were taken up by manual
effort, and there was little opportunity to understand the broader world.
The cost of owning a wireless was far
higher then compared to today.
I have memories of conversations
with those who lived during that era.
One uncle told of his excitement when
receiving his first feeble signals after
months of experimenting with a simple single-valve regenerative receiving
while living on the family farm.
But not all were wholly in favour.
A common belief was that wireless
sets could distract women from their
domestic duties during the day or
affect peoples’ social lives in the evenings.
The simple crystal wireless might
Australia's electronics magazine
suit a boy lying in bed listening to
his favourite cowboy show, but was
not of much use in a family situation.
Once tuned radio frequency (TRF) sets
became available, they were soon the
instrument of choice.
Two stages of tuned radio frequency
amplification selected and amplified
the station of choice, followed by a
detector to separate the audio signal from its carrier frequency. Two
stages of transformer-coupled audio
frequency amplification were used to
power a loudspeaker, giving a somewhat distorted output of less than 1W,
still sufficient to amaze all listeners in
an average room.
UDISCO’s history
UDISCO was founded in Australia in 1911, selling household goods
and importing electrical components.
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Fig.1: this circuit was initially traced by hand with sparse few known values filled in. As we couldn’t find a circuit
diagram online for the UDISCO L6, this should be the next best thing.
The company produced a wide range
of sets between 1925 and 1929, ranging from kit sets sold under the brand
name UMAKIT to advanced receivers
like the UDISCO Super Six, a TRF set
selectively tuning over six wavebands
from 2000m to 20m with no gaps.
Early sound technology and valveera radio, particularly from the 1920s
and 1930s, have fired up my imagination as far back as I can remember.
Around 30 years ago, I came by one
of my more valued wireless finds at
auction. The name engraved in the
bottom corner of the front panel reads
“UDISCO Model L6 Made in Australia by United Distributors Ltd Patent
No. 20643 US No. 1610918 No.176”.
The only reference I can find to my
UDISCO receiver on the internet is one
photo. My first task was to gain some
understanding of how it worked. The
circuit is more elaborate than the usual
simple five-valve TRF receiver. I admit
to at first being puzzled, so I began to
sketch a rough circuit. Three attempts
later, it began to make sense.
It’s an upmarket TRF set with single-
point cable-ganged tuning, housed in
a heavy 300mm-high 819 × 380mm
solid oak stained cabinet. The cabinet is meant to last for a generation or
two and is typical of its time. The circuit has four RF stages with unusual
choke-capacitive coupling (including
the detector), plus the usual two stages
of transformer-coupled audio.
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The four RF coils are of the binocular type, resembling a single coil cut in
half and bent over on itself, each half
having an equal number of windings
that are effectively wound in opposite
directions.
Both confined RF fields are intended
to oppose each other, preventing interaction between adjacent coils and
unwanted incoming signals making
neutralising, shielding and angular
placement unnecessary. The four sets
of binocular tuning coils are mounted
out of sight under one of the four
sub-panels fixed to the baseboard.
As can be seen from the circuit diagram (Fig.1), the first half of the first
binocular tuning coil (L1) is tapped
and switched to facilitate aerial matching. The binocular tuning coils seem
to work, making this set very stable in
operation with clear reproduction. It
tunes stations without the howling or
whistling common in TRF sets using
simple triode valves.
The choke-capacitive coupling
between the four RF valves is via four
large circular honeycomb-wound RF
chokes mounted between the sub-
panels and the front panel. They
block RF from the B+ 90V supply and
divert the RF signal through a 4nF
mica capacitor to the tuned grid of
the next stage.
Valve lineup
Most Australian-built radios from
the 1920s that I have seen used Philips
Bakelite-based triode valves with
dumpy glass envelopes. This set initially used Philips A609 6V triode
valves, designed for use with a 6V
accumulator for the A filament supply.
The A609 was first manufactured
An example photograph of the ‘binocular’ type RF coils.
Australia's electronics magazine
January 2023 91
in 1926, the same year as this set.
Its oxide-coated filament drew only
60mA and used the same four-pin base
as the popular USA-manufactured
UX201A. The UX201A had a thoriated
filament drawing 250mA at 5V, making them interchangeable with some
adjustment of the filament rheostats,
while Philips had a sales advantage
due to reduced battery drain.
Mounted along the top edges of
sub-panels two, three and four are six
metallic tubular adjustable capacitors
of a few picofarads each. My interpretation of their purpose is that C14,
C16 & C18 provide a small measure
of feedback to their respective valves
giving some regeneration. C15, C17 &
C19 also appear to be part of this network. This is just an educated guess,
they could actually be for balancing
out inter-electrode capacitance within
the RF valves.
Possibly confirming my determination, removing the adjustable sliding
rods inside the insulated tubes gives
a modest reduction in sound volume.
The grid bias to V1, V2 & V3 is from
the negative filament line via taps on
tuning coils L1, L6 & L9.
Controls
The front panel looks uninteresting, with only two controls. There
is a reduction dial for tuning and
directly under it, a smaller knob for
adjusting the rheostat (R1) controlling
the plate-anode current to the four RF
valves for volume control.
R1 is bridged by capacitors marked
C21 and C22, which are in series and
centre tapped, going to the positive filament line. Choke L13 in the anode circuit of V4 has the primary of L14 (the
first audio transformer) taken from its
more positive side instead of directly
from the anode, as one might imagine,
but it works better that way.
A long, narrow sub-panel just under
the hinged lid holds additional knobs.
The first on the right controls a vaned
trimmer capacitor (C1) across the first
of the four tuning capacitors, to adjust
for any misalignment as stations are
tuned across the bands.
The second knob controls a rheostat (R2) in series with the A+ battery
supply to adjust the valve heater current according to the battery voltage.
It also affects the volume (along with
the external knob mentioned earlier).
Knobs three (C2), four (C3) and five
(C4) perform similar functions as knob
one, tuning capacitor trimmers. Knob
six (C9), marked “control”, adjusts the
positive feedback from the plate of V5
to the grid of V4 (the detector) to provide regeneration.
Restoration
Opening up the lid reveals a series of aditional knobs connected to the chassis.
The knob at the top of this photo is connected to trimmer capacitor C1.
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Silicon Chip
Australia's electronics magazine
At least one of the audio coupling
transformers was replaced sometime
during the history of this set. I also
noticed that, at some point, radio-
frequency choke L3 had been added
across choke L4. It seemed unnecessary, so I removed it.
I inspected the set and couldn’t
spot any more apparent problems,
so I decided to switch it on and see
if it worked. I connected to an aerial
and Earth plus my most trusted horn
speaker before wiring in my special
battery eliminator power supply and
making voltage adjustments. As is typical, there was not even a buzz, and no
amount of knob twiddling could coax
this set into the faintest whisper.
I should have performed a closer
inspection by checking the voltages on
the valve pin sockets. Using a signal
tracer, I found a signal at the grid of
V1 but none at the plate. Also, detector
V4 lacked HT on the plate, indicating
there were open-circuit anode chokes.
The very fine-gauge cotton-covered
wire used in these large honeycomb-wound coils was adrift from
the respective terminals. Worse, both
siliconchip.com.au
C1
R2
C2
C5-C8
C4
Reaction
Feedback
Bias
C22
C21
Adjustable
Capacitors
L14
L15
Anode choke coils
I removed the chassis from the cabinet to effect some repairs (the 9V battery was used for testing and is not part of the set).
Four of the six knobs adjust trimmer capacitors across the tuning caps.
wires on L4 had broken close to the
coil. The outer was easy enough to
pick up, but the inner close to the coil
former had only a couple of millimetres of stub left.
With no second chances, several
careful scrapes with a razor blade
exposed a streak of clean copper and
I gently added a dab of solder to join
another thin wire. I then added a small
blob of Blu Tack to keep it rigid. But
there was still no continuity.
With fading hopes, I decided on a
closer inspection under a large magnifying glass with good light. A tiny
green spot of verdigris was visible. I
poked it with a needle to expose two
short, stubby wire ends, which I then
bridged and set in place with another
blob of Blu Tack. It then had continuity which was a considerable relief.
A tiny drop of acid solder flux, probably splattering during manufacture,
had corroded the wire through in subsequent years. That explained why RF
choke L3 had been soldered across it.
Now that I’d fixed L4, it was no longer necessary.
With both anode choke coils now
repaired, there should have been some
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response from the horn speaker, but it
is never that easy. I re-checked everything twice more; all seemed good,
but there was still no response. One
or more of the six A609 valves must
be low on emission, so I would have
to set up my Paton valve tester, which
has a four-pin UX socket to suit these
early triodes.
All valves displayed less than 50%
emission, with one being a total dud,
probably resulting in this set’s retirement. These old Philips 6V triodes
are very seldom for sale now. I keep a
few known-good UX201A 5V triodes
for replacements, so I fitted them after
re-adjusting the filament ‘A’ supply.
This brought forth a hint of croaky
reception from the ancient horn
speaker. Some careful adjustment of
the single tuning control on the front
panel, together with the anode voltage
rheostat and then all six knobs along
the under-lid sub-panel, resulted in
surprisingly ample sound.
A rocking armature speaker gave a
less strident output; no doubt, a further improvement could be obtained
by fitting a moving-coil unit through
its output transformer.
Australia's electronics magazine
The similar AWA Radiola C54
At this stage, I remembered that I
had previously purchased an AWA
Radiola Battery Six model C54 from
around 1928. Electronically, it is a
similar set but probably as basic as a
six-valve TRF wireless could be. The
point of interest was that the model
C54 also used four sets of binocular
RF tuning coils.
In that set, the more typical inductive coupling was used between
all four RF stages instead of choke-
capacitive coupling. I decided to try
to get both TRF sets working so I could
compare their performance.
Unfortunately, both coils in each
AWA audio coupling transformer
had gone open circuit. Someone had
worked around that by inserting the
high impedance speaker in the HT circuit of the first audio valve and feeding
its grid through a 100nF capacitor from
the detector anode. The final audio
stage had simply been disconnected,
making this a five-valve set instead of
the original six.
I have successfully rewound open
circuit windings on audio coupling
transformers using very fine enamelled
January 2023 93
An advert from Wireless Weekly, June 1927 showing a UDISCO Neutrola which
uses a case that is very similar to the L6.
copper wire (0.1mm/4-thou diameter). Patience and a gentle touch are
required. Fortunately, I was able to
scrounge two working replacements
from the junk boxes of friends, with
the originals perhaps to be rewound
sometime in the future.
Two of the six valves were missing. I tested the remaining four British
Marconi Osram valves for emission,
and three came up good. The Philips
B406 appeared to be similar, and once
added and everything connected and
tuned in correctly, this set now gives
good reception for our two main local
stations.
Both sets are inaudible when the
aerial is removed and are free of any
sign of oscillation in everyday usage,
possibly due to the use of binocular
tuning coils. Sensitivity is limited in
TRF sets due to the low RF gain of
the front-end when compared to my
two superheterodynes from the same
period.
Still, the output volume is good
considering the meagre gain of these
early triode valves, particularly in the
output stages.
Substituting a 71A or a UX112A
power output valve (both have 5V
0.25A filaments) gives a noticeable
increase in audio volume. These
valves are compatible with UX201A
types and became available in early
1927.
In conclusion, the UDISCO model
L6 is a good user-friendly receiver,
making up for its plainness in ornamentation by its sheer bulk, complexity and exceptional performance. SC
A photograph of the AWA Radiola model C54. Like the UDISCO L6, it is also a six-valve TRF set and uses four sets of
binocular tuning coils.
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Silicon Chip
Australia's electronics magazine
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