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Vintage Radio
By RODNEY CHAMPNESS, VK3UG
The Healing “Scales” 403E receiver
In the late 1940s, the main household
receiver was usually located in the lounge
or dining room – and usually controlled
by the “man of the house” in line with the
rather chauvinistic attitudes of the time.
However, small 4-valve mantel receivers
designed for use in the kitchen were also
becoming quite common.
O
NE LIMITATION ON the use of
multiple sets in a household was
the radio receiver licensing regulations. Initially, they required a licence
for each individual “wireless” and in
fact, back in the sealed receiver era in
the 1920s, a separate licence or fee was
payable for each station that a person
wished to listen to. Fortunately, this
idiotic idea was overturned within a
rather short time.
However, with one set per licence,
there was no incentive to buy more
than one set and receivers were expensive anyway. Retailers had to submit
a return to the PMG on what sets they
sold and to whom, so it was not too
hard for the authorities to find out who
had unlicensed sets. The mandatory
outside aerial was also a dead give
away. Of course, many home-built sets
were never licensed.
Eventually, the licensing regulations
were eased and the household licence
came into being, allowing several sets
to be used. Ultimately, domestic radio
and television licensing was abolished
on 1st September, 1974. To read more
about this subject, go to:
http://wireless.iserv.com.au/default.
asp?m=main&id=age3
The Healing “Scales”
The Healing 403E – it looked rather like a set of kitchen scales and was
often affectionately called the Healing “Scales”.
98 Silicon Chip
To meet the need for more than
one set in the household, many
manufacturers produced small lowperformance 4-valve sets. Healing was
one of those manufacturers and produced many sets. One that has taken
the fancy of many collectors is the
Healing 403E or as it is affectionately
called by some, the Healing “Scales”,
supposedly because it looks like a set
of kitchen scales.
The set came onto the market
around 1950. It would appear that it is
designed to be placed on the kitchen
bench as the dial scale is on the slopsiliconchip.com.au
Fig.1: the circuit for the Healing “Scales” 403E
receiver. It uses a 6AN7 frequency converter, a
6AR7-GT IF amplifier & detector, a 6M5 audio
output stage and a 6X5-GT rectifier.
ing top of the set. It is then really a
bench top or table set and not a mantel
receiver. It was quite an innovative
concept although in the kitchen environment it may have been splashed
with water or food. However, despite
the potential abuse, the sets survived
quite well.
The receiver is separated quite
easily from its cabinet, requiring the
removal of three screws in the Bakelite
base, four screws attaching the chassis
and the cabinet top together and the
two control knobs.
Warning: once the set is removed
from the cabinet, do not tip it upside
down onto the dial scale, as it will
break. To make sure the dial scale is
not damaged, remove it and put it
aside until alignment or reassembly
is to take place.
This receiver had not been restored
at the time of writing. Access under the
chassis is quite good and any component there can be adjusted or replaced
readily. However, caution is needed
when operating this receiver out of
its case as the mains terminations are
exposed on the bottom of the power
transformer. If the set were mine, I
would put a protective insulated cover
over these terminals. It would be all too
easy to touch these exposed terminals
while concentrating on other aspects
of the restoration process.
siliconchip.com.au
A rear view of the chassis out of its cabinet – note the very tight layout. The
receiver uses just four valves.
Of course, the twin flex should be
replaced with a 3-core mains lead,
so that the chassis can be earthed.
Suitable replacement power leads are
available as extension cables of various
lengths at most supermarkets. Just cut
off the socket and you will have a lead
ready to wire into the chassis.
Better still, you can often salvage
perfectly good 3-core power cords
during council cleanups, as people
often discard computers and other
appliances.
Above the chassis, the components
are tightly packed in and some dismantling would be required to work
March 2006 99
This view shows the chassis from the front. The parts on top of the chassis
were rather tightly packed.
Another view of the chassis from the front, this time with repaired (left)
and replacement dial scales.
on a few items. Fortunately, work here
is not required as often as underneath
the chassis.
Circuit highlights
The Healing 403E is really a 3-valve
plus rectifier receiver, similar to many
100 Silicon Chip
sets produced by other manufacturers
of the era.
A 6AN7 is used as a frequency
changer with an output of 455 kilohertz (kHz) to an IF amplifier using
the pentode 6AR7-GT as the amplifier.
The two diodes in the 6AR7-GT enve-
lope act as the detector and automatic
volume (gain) control (AGC). The AGC
is simple and only a third of the AGC
voltage is applied to the 6AN7 and
the 6AR7-GT via the voltage divider
of R2 (1MW) and R5 (2MW). The audio
(and AGC) voltage is applied across R6
which is then applied to the grid of the
6M5 audio output valve.
The converter and IF amplifier are
run with no bias on their grids but
because their screen voltage is only
around 50V, the current drain is not excessive. There will be a small amount
of bias provided from the AGC line,
even off-station, as any circuit noise
and external interference will generate an AGC voltage, small though it
may be. Tuned to a station, the bias
is significantly higher. By only applying a third of the developed AGC
voltage to the two controlled valves,
more signal is able to be applied to
the 6M5 to achieve a reasonable audio
output level.
As a purist, I find only applying a
third of the available AGC voltage is
a poor design feature. However, when
it is considered where the set was
intended to be used – suburban areas
not far from radio stations – it works
quite satisfactorily. The 6M5 has the
common cathode bias method, which
provides around 6V of bias with a
current drain of around 25mA in this
receiver. The total current drain of
the receiver is around 37mA from the
6X5GT rectifier.
Those with a sharp eye will have
picked up drafting errors in the published circuit. The secondary winding
of L4 (second IF transformer) appears
to go to the grid of the 6AR7GT but
should connect to a diode or both of
the diodes within the 6AR7-GT. The
oscillator grid is also not shown connected to the third grid. Labelling the
valve pin numbers would also help
servicemen and restorers. It is amazing the number of drafting errors that
did creep in.
Overhauling the set
Simple receivers like the 403E do
not require many paper capacitors to
be replaced when compared to more
complex 5-valve designs. Most of the
paper capacitors in this receiver can
be quite leaky and still not cause any
noticeable or dangerous problems.
The only critical one is C3, the AGC
bypass capacitor, and this should be
replaced as a matter of course. The
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The is the under-chassis view. Note that the power cord terminations
are quite exposed and that only 2-core flex has been used – it should be
replaced with a 3-core lead, so that the chassis can be earthed. The mains
transformer has also been overheating, as revealed by the congealed wax
on the bottom of the windings.
audio coupler (C11) has only a small
voltage across it (a maximum of around
20V). If this is leaky, all it will do is
place a negative voltage on the grid of
the 6M5 and progressively cut it off
as the volume control is increased or
a stronger station is tuned in. C13, if
shorted, would just cause the audio
to disappear but no damage would be
done to the set.
Electrolytic capacitors C8 and C14
should be reformed if necessary, after being tested with an ohmmeter
to ensure there are no short circuits
between the high tension (HT) line
and the chassis. My rough and ready
technique is to place a voltmeter across
C14, turn the set on and wait until the
voltage across it starts to rise. Once it
has risen to 50V or so, I switch off and
then, about 30 seconds later, I repeat
the procedure, this time letting the
voltage rise to around 100V before
switching off.
Do this over a period of several
minutes and the electrolytic capacitors should be reformed. However,
watch the voltmeter and the rectifier
when doing this, making sure the
rectifier doesn’t look as though it is
overheating (the plates will go red,
if this happens) and that the voltage
siliconchip.com.au
does increase rapidly once the rectifier
starts to conduct. After the preceding
tests, check with the power off to see
if the electrolytic capacitors are warm
to the touch. If they are, they should
be replaced, as they are defective. The
set should also be run for a while to
make sure the power transformer only
gets moderately warm.
In this set, it is obvious that the
power transformer has dripped some
wax, so it has been hotter than it
should be. Before doing too much on
this set, it would be advisable to do
checks on the transformer for shorts
and insulation integrity, as well as
checking the rectifier and electrolytic
capacitors for potential shorts or low
resistance to earth (chassis).
Initially, the set should be run with
the transformer lightly loaded, by
taking out all the valves and leaving
just the dial lamps in. If it gets other
than slightly warm, the transformer
may have a fault. If all is well, the
general tests indicated in the previous
paragraphs should be undertaken. The
problem may have been fixed sometime in the past but it does pay to be
very sure that there is no latent fault
in the power supply area.
Manufacturers often skimped on
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March 2006 101
Photo Gallery: AWA 519M (1947)
Manufactured by AWA in 1947, the 519M is another example of a popular
5-valve mantel receiver of that era. It was fitted into a bakelite cabinet, the
brown-white mottled unit shown here being one of the less common cabinet colours. The valve line-up was as follows: 6A8-G frequency changer;
6SK7-GT IF amplifier; 6SQ7-GT audio amplifier/detector/AVC rectifier; 6V6GT audio output; and 6X5-GT rectifier. Photo: Historical Radio Society of
Australia, Inc.
fitting some components. In this
case, a 100nF (0.1mF) or similar value
capacitor has not been put across C8
to effectively bypass any RF energy
on the HT line. When first installed,
capacitors like C8 are reasonably effective RF bypasses but as time goes
by, their effectiveness deteriorates
and the receiver will often develop an
instability problem.
Although this set has not been restored as yet, I would expect that very
little would have to be done for it to
function quite satisfactorily. I rarely
find that any valves need replacement,
providing the critical capacitors are
replaced before the set is turned on.
Aligning the set
This is a comparatively easy set to
align, with a total of eight adjustments.
Basically, the four tuning slugs in IF
amplifier transformers L3 and L4 are
peaked for best performance on a
relatively weak signal at 455kHz. It is
not imperative that the frequency be
exactly 455kHz as long as it is within
about 10kHz of 455kHz.
The front-end alignment is a little
more exacting, as several frequencies
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are involved. The dial scale needs to
be attached to the receiver so that the
alignment can be done with accuracy.
Take note of the warning earlier in the
article about how easy it is to damage
the dial scale.
The data I have on the receiver is
not specific about the spot frequencies
used in the front-end alignment. As
a general rule, 600kHz and 1500kHz
are usually used. If you don’t have a
signal generator, tuning to stations
near to these frequencies is quite adequate, like 621kHz for 3AR (3RN) and
1521kHz (2QN) in my area.
First, tune to around 600kHz and
adjust C6 (under the chassis) until
the particular station selected appears
at the correct spot on the dial. Then
adjust the core of L1 for best volume
on this station or a weak one nearby.
Now tune to a station near 1500kHz
and adjust C4 (on top of the gang) so
that the station tuned to appears at
the correct spot on the dial, and then
peak C1 (on top of the gang) for best
performance. Go over the adjustments
for 600kHz and 1500kHz several times,
as they do interact with one another.
For more information on these procedures, refer to my articles in the December 2002, January 2003 and February
2003 issues of SILICON CHIP.
Summary
This set is a quirky little receiver
that doesn’t appeal to all, although
highly sought after by many collectors. I believe that it would be easily
restored. The performance is on a par
with sets of a similar design. The design of the AGC system means that it
really hasn’t enough gain to produce
good performance in country areas but
it would be quite adequate in suburban
locations. I wouldn’t say no to having
SC
one in my collection.
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