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Vintage Radio
By Rodney Champness, VK3UG
Breville 730 Dual-Wave
5-Valve Receiver
Manufactured in 1948, the Breville 730 tabletop receiver was housed in an attractive
timber cabinet and covered both the broadcast
and shortwave bands. It featured a wide
audio response and this, coupled with a large
loudspeaker, gave very good performance.
T
HIS 1948 BREVILLE 730 receiver
was obtained by a friend of mine
(Marc) in almost original condition.
In fact, it’s quite rare to come across a
set as original as this one, as most sets
have had some routine servicing and
parts replacement during their life.
Hopefully, any work that has been
done on a set will have been carried
out by a competent serviceman. An
incompetent servicemen or hobbyist
can leave a set with more faults than
it started out with and can sometimes
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even destroy hard-to-get parts.
In the case of Marc’s Breville 730,
the only evidence of any service work
was on the band-selection indicator. In
fact, the condition of this 65-year-old
set is so good that it has obviously
been used in a lounge room for most of
its life. It had eventually failed when
the ECH35 converter developed a
short circuit (as confirmed by a valve
tester), after which it had been carefully stored away.
As a result, virtually no damage
has occurred to either the cabinet or
chassis, other than the normal ravages
of time.
Circuit details
The Breville 730 has a conventional
superheterodyne circuit that’s similar
to many other receivers of the era.
However, it does have some features
which, although not unique, are not
seen in many other receivers.
Fig.1 shows the circuit details. The
signal from the antenna is fed to an
input tuned circuit and the position
of the band-change switch determines
whether shortwave (6-18MHz) or
broadcast band tuning is selected.
As shown, the primary of the
shortwave antenna tuned circuit (top)
is in series with the primary of the
broadcast-band antenna tuned circuit.
Capacitor C2, a 100pF capacitor across
the broadcast-band coil, performs two
tasks: (1) it acts as a low impedance
to earth for the bottom end of the
shortwave antenna primary and (2)
it tunes the primary winding of the
broadcast-band coil to below the lowest frequency on this band.
This technique enhances the performance at the low-frequency end of
the broadcast band.
Note that the primary of the shortwave antenna coil has little effect on
the performance of the broadcast-band
antenna tuned circuit and may even
boost its performance slightly. This
circuit works well and simplifies the
band switching.
Converter stage
The selected output from the antenna tuned circuit stage is applied
to the signal grid of an ECH33/35.
This functions as a converter or local
oscillator stage.
In operation, the local oscillator
tuned circuits are also switched to suit
the selected band (either broadcast or
shortwave). Note, however, that there
is an error in the circuit diagram regarding the oscillator switch position
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Fig.1: the circuit is a fairly conventional 5-valve superhet design,
although the 3-coil first IF transformer is somewhat unusual. The
IF stage is tuned to 446kHz and has switch-selectable bandwidth.
– the antenna switch is shown in the
broadcast position, while the oscillator
switch (immediately following C5) is
shown in the shortwave position.
Capacitor C6 acts as both a padder
and a phase change network when the
broadcast band is selected, to provide
positive feedback for the oscillator. It
also provides an earth return for the
shortwave oscillator primary feedback
winding.
By contrast, the shortwave oscillator
tuned circuit does not have a padder
capacitor attached to its tuned winding. Because of the relatively small difference between the oscillator and the
signal frequencies on shortwave, some
manufacturers left this component
out. Note also that double-spotting
or image reception is quite common
on shortwave receivers having no RF
stage and a 455kHz IF (intermediate
frequency).
Once the ECH35 has converted the
tuned RF signal to a 446kHz IF (not
455kHz as we normally expect), it
is applied to the first IF transformer.
This transformer is different to most
as it has three windings. The primary
is tuned to 446kHz and so is the secondary when the tone control (immesiliconchip.com.au
diately below it on the circuit) is in
its centre (normal) and bass positions.
By contrast, when the tone control
is switched to the wide range position, the third coil is switched into
circuit, in series with the secondary
tuned circuit.
In practice, I suspect that the secondary of this IF transformer is detuned to give a broad response through
the IF strip. In addition, I suspect that
the third coil is coupled to the first
tuned circuit so that the combination
of the primary and secondary tuned
circuits also broadens the response
(with a dip in the centre), so that the
receiver has an audio bandwidth of
up to 10kHz. This, combined with the
set’s large loudspeaker, would result in
good quality audio although it should
be noted that AM broadcast stations
later restricted their audio bandwidth.
Following the first IF transformer,
the signal is fed to a 6U7G IF amplifier stage and the resulting signal then
applied to the second IF transformer.
The IF signal is then fed to the detector diode in a 6G8G detector, AGC and
audio amplifier valve.
From there, the detected audio
signal is fed via R7 and C13 to vol-
ume control R12 and then to the grid
of the 6G8G. This circuit technique
enabled Breville to overcome the oftexperienced problem of “scratchy”
volume controls, caused when DC
from the detector is applied directly
across them.
Note that most radios use a triode
as the first audio amplifier but this set
uses a 6G8G pentode for additional
audio gain. The output from this stage
appears at the anode and is applied
to the grid of a 6V6GT audio output
valve. This in turn drives a speaker
transformer and an 8-inch (200mm)
loudspeaker.
In addition, the audio on the plate
of the 6V6GT is sampled via an RC
network and fed to the 6G8G’s cathode
to provide tone control and negative
feedback.
Record player terminals
The receiver is equipped with terminals which allow a record player (PU)
to be connected. However, this really
doesn’t work that well because there’s
no way of turning off the RF section of
the set when records are being played.
A combination of the latest broadcast
episode of “Biggles” and a recording
May 2012 99
The chassis of the Breville 730 was in quite good order although some corrosion was evident, especially on
the power transformer cover and at the top of the tuning gang.
of Tommy Dorsey playing over the top
of each other would hardly have been
satisfactory!
A simple switch would have solved
this problem, with one pole used to
switch the HT (high-tension) rail to
the RF stages on or off and another
pole to switch the input to the audio
amplifier between radio and turntable.
AGC & power supply
The automatic gain control (AGC)
signal is obtained from the plate of the
6U7G and is applied to the AGC diode
in the 6G8G. This diode is normally
biased off, as its anode is 1.5V negative with respect to the cathode of this
valve. As a result, it will not generate
any AGC voltage until the incoming
signal exceeds 1.5V.
This delayed AGC signal is applied
to both the converter (ECH35) and IF
amplifier (6U7G) stages. Both these
valves share a common cathode resistor (R2) and 2.3V of bias is obtained
before the AGC voltage is applied.
The power supply is standard for the
era and uses a power transformer plus
a 5Y3GT rectifier. The transformer’s
primary is tapped at 220V, 240V and
260V, while the secondaries consist of
a 6.3V winding for the heaters and dial
lamps, a 5V winding for the 5Y3GT’s
filament and a centre-tapped 270V
per side winding for the high tension
(HT) supply.
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The output of the rectifier is filtered
using an 8µF electrolytic capacitor
(C27), a 12H (Henry) choke and a following 16µF electrolytic (C26).
Cabinet restoration
As mentioned earlier, the cabinet
was in quite reasonable condition.
However, as antique dealers have often
pointed out, timber items stored in
very dry environments can develop
cracks and this cabinet was no exception.
These cracks and splits were carefully repaired using an epoxy adhesive
(Araldite). And because the timber
was so dry, Marc applied linseed oil
to the inside of the cabinet using a
paintbrush. The outside of the cabinet
also received attention, with linseed
oil applied sparingly using a cloth.
The revitalised cabinet now looks
quite good despite the minimal attention paid to it. Further restoration
was not considered desirable in the
interests of originality.
The original speaker cloth was in
poor condition and so this was replaced with some open-weave brown
cloth obtained at a haberdashery. It
looks authentic even though it isn’t
genuine speaker cloth. In addition,
new rubber buffers were fitted to the
bottom of the cabinet, replacing the
old ones which had either perished,
become hard or had gone missing.
Finally, the cabinet features a celluloid strip which is mounted behind
the various controls and which carries the control legends. Although
yellowed with time, it is still original
and quite legible. These strips usually
deteriorate and fall to pieces over time
but this one is good for many years yet.
The control markings on the strip
are (left to right): On-Off-Volume; Tone
– Bass, Normal, Wide Range; Station
Selection; and Wave Change SW/BC.
Circuit restoration
The chassis is quite easily removed
from the cabinet. This involves removing the four control knobs and the
dial-light assembly, followed by the
four screws underneath the cabinet
which secure the chassis in place. One
of the dial lamps had to be replaced,
after which the inside of this assembly
was repainted white to ensure good
reflectivity.
One problem with many sets is that
the dial-scale is left behind (ie, still
attached inside the cabinet) when the
chassis is removed. The Breville 730
is no different in this regard but the
redeeming feature of this set is that the
alignment frequencies are marked on
the edge of the dial scale, along with
the position of the dial pointer when
the tuning gang is closed.
That certainly makes it easier to
get the dial pointer lined up with the
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station markings correctly when the
chassis is reinstalled.
Once the chassis had been removed,
Marc could immediately see that some
of the wiring was in need of replacement as the rubber insulation had
perished. This particularly applied to
the dial-lamp leads as the insulation
had actually fallen off and the wires
were shorting.
Closer inspection of the wiring revealed several other leads that were
shorting due to perished rubber insulation. These leads were all replaced,
after which the dial-light supply line
was isolated and the valves removed.
This was necessary to allow highvoltage tests on the power supply, to
confirm that it was in a safe condition.
First, the insulation resistance between the mains and chassis and other
windings of the transformer was tested
using a high-voltage insulation tester.
These were all found to be in good
order, with over 200MΩ of resistance
in each case.
That done, the old power cord was
replaced with a new 3-core cable. This
was securely anchored to the chassis
using a cord clamp.
Marc then tested and/or replaced
a number of parts that are known
to cause problems. In particular, all
the paper capacitors were replaced
with modern polyester types, while
the electrolytic capacitors were also
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This under-chassis view shows the receiver after restoration. The original paper
and electrolytic capacitors were all replaced, along with some of the wiring.
replaced due to their age and the fact
that they were visibly leaking. Several
resistors were also found to be out of
tolerance and were replaced.
The loudspeaker was the next on
the list. It had developed a number
of cracks along the speaker cone edge
and these were repaired with Selleys
“Quik Grip”.
Testing the valves
Marc’s next step was to use his
valve tester to check all the valves in
the receiver. All tested OK except the
ECH35 converter, the tester indicating
a short circuit inside this valve. This
would have completely stopped the
receiver from working and is probably what caused the original owner
to retire the set.
Marc had a working ECH35 which
could replace the dead ECH35 but its
conductive red paint shield (which
Philips call “metallisation”) had been
damaged. As a result, he decided to
make an experimental shield to replace the damaged one.
A little investigation showed that
the wire contacting the red shield and
the earth pin in the valve plug was
intact and accessible. A thin strand
of wire was therefore soldered to this
earth wire (without cracking the valve
envelope) and then spiral-wound
around the valve envelope. Some
“Wire Glue” (available from Jaycar,
Cat. NM-2831) was then applied to the
envelope to secure it in place.
If access to the earth wire is not
practical, a thin wire can be soldered
to the earth pin of the valve and then
extended up and wound around the
May 2012 101
This view shows the components associated with the band-change switch. The
two coils associated with the input tuned circuits are clearly visible.
This slide assembly is controlled by
the band-switch and indicates which
band has been selected.
envelope. Another possible earthing
shield can be seen at www.andersonproducts.com. The sales information
says that it is “carbon blended in a
non-toxic binder”.
Repairing the band indicator
The band-change switch has a lever
off to one side of the control shaft and
this controls a spring-loaded slide assembly via a length of dial cord. This
slide assembly has two small labels
which are alternatively visible through
a clear window on the righthand side
of the dial scale and indicate the band
selection (ie, broadcast or shortwave).
AWA used a somewhat similar idea in
their 7-band receivers of the same era.
102 Silicon Chip
This slide assembly wasn’t working
in the old Breville 730 as the control
cord had broken. It had been replaced
during the life of the set with single
conductor tie wire instead of dial cord
but this wire had eventually fractured
at the eyelet. Re-stringing the assembly
with dial cord soon got it working
again.
The tuning gang was also a little
worse for wear so it was the next job
on the list. First, a small hand blower
was used to remove the dust that had
accumulated between the plates. This
revealed that some of the plates had
corrosion on them, so these were carefully cleaned by pushing some fine
emery paper between them.
The chassis itself was in quite good
order and was simply cleaned using
the blower and a small brush.
Testing
Having fixed all the obvious faults
in the set, Marc then decided to power
the set up to see if there were any other
faults in the circuit. As it turned out,
the set started up normally and stations could be clearly heard. A quick
check then revealed that all the voltages were normal and no components
showed any obvious signs of distress.
Even at this early stage, the set’s performance was quite good and tweaking both the antenna and oscillator
circuits made it even better. In fact,
its shortwave performance is better
than average for a set of this calibre.
However, there were a couple of other
issues to be dealt with. One dial globe
was dead and more importantly, it
was obvious that the volume control
pot was worn out and would have to
be replaced.
The IF alignment were then checked
using a signal generator and a frequency counter (to adjust the signal
generator exactly to frequency). Because Marc had no information on
adjusting the first IF transformer with
its three windings, he decided to
proceed with the tone control in the
“Normal” position. Before adjusting
anything though, each IF transformer
was marked so that he could easily
return it to its original position should
his alignment technique with the
uncommon 3-winding IF transformer
go awry.
As it turned out, the alignment went
smoothly and the first IF transformer
was easy to adjust in the standard selectivity position (ie, Normal).
New volume control
With the set now performing well,
Marc decided to replace the worn-out
on/off-volume control. Unfortunately,
he was unable to obtain a direct replacement with a long shaft, so he
was forced to use one with a splined
shaft and make up an extension shaft
on a lathe.
This proved to be a complete success and the new volume control
worked smoothly, without crackles.
The chassis was then reinstalled in
its cabinet and the restoration was
complete.
Summary
Breville produced many fine radios
and the model 730 was one such set.
It performs well and the broad response of the IF amplifier stages (when
switched to “Wide Range”) means that
the set was able to reproduce a wider
range of audio frequencies than most
other similar sets.
The set is also easy to service, with
all parts readily accessible. However,
the inability to isolate the RF section
when a turntable is connected to the
audio amplifier section is a rather
puzzling omission, especially since it
would have been so easy to do. All that
would have been required is an extra
position on the band switch, which
could then have been labelled “Short
Wave”, “Broadcast” and “Gram”.
In summary, the Breville 730 is an
excellent receiver with many interesting features and is a worthy addition
to Marc’s collection.
SC
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