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Vintage Radio
By RODNEY CHAMPNESS, VK3UG
The Mullard Meteor 600
4-Valve Mantel Receiver
A great variety of valve receivers were designed and produced
for the domestic market in Australia. These ranged from
complex, multiband receivers to relatively simple sets designed
for the bottom end of the market. This little receiver falls into
the latter category and although it’s a reasonable performer, it
could have been much better.
T
HE MULLARD Meteor 600 came
onto the market in 1947, at a time
when Australia was still recovering
from the restrictions and scarcity of
raw materials due to WWII. It is housed
in a relatively small brown bakelite
cabinet but the components are not
90 Silicon Chip
squeezed in, as they were in some
small sets of the era.
As can be seen from the photos, the
patterning on the front of the cabinet
is rather unusual and this accentuates
the round dial scale and its escutcheon. It is an interesting feature and
helps make the set reasonably attractive in appearance.
Another feature is that the chassis is
easily removed from the cabinet. This
simply involves removing the two control knobs plus four screws from the
bottom of the cabinet that secure the
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Fig.1: the circuit of the Mullard Meteor 600. It’s basically a 4-valve superhet with the IF amplifier stage operating at
455kHz. The volume control is unconventional & works by varying the back bias to the first two valves in the line-up.
chassis in place. The chassis can then
be slid out of the cabinet, complete
with its loudspeaker and dial scale.
The dial scale is circular and has a
red background with yellow station
markings and a yellow dial pointer.
There are a few stations from NSW,
Victoria, Queensland, Tasmania and
South Australia shown on the scale
but none from Western Australia. This
initially made me wonder if the set was
even marketed in WA, although I now
think it probably was. That’s because
the ratings shown for the power transformer indicate that it can be used at
40Hz, which was the mains frequency
used in Perth at that time.
The Mullard Meteor 600 covers the
broadcast band from 540-1620kHz and
has a fairly simple tuning system. This
uses a control shaft to drive a drum
attached to the tuning gang via a dial
cord. The dial cord is simply wrapped
around the control shaft three times
and the ends attached to the dial drum
so that it can be rotated one way or the
other. However, although the mechanism is simple, replacing the dial cord
requires removal of the dial scale to
gain access to the drum.
One interesting feature is the way
in which the volume control works. It
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doesn’t work in the conventional manner which is to vary the audio level
that’s fed to the audio output stage.
Instead, in this set (and a number of
others of the same era), it varies the
back bias to control the gain of the first
two valves in the receiver and hence
the audio output volume.
The volume and tuning controls
are symmetrically located beneath the
dial, with the volume control on the
left and the tuning control on the right.
As with many other receivers made at
that time, there is no on-off switch and
the set has to be switched on and off
at the wall socket.
Circuit details
Fig.1 shows the circuit details of the
Mullard Meteor 600. It’s basically a
4-valve superhet with an IF amplifying
stage operating at 455kHz.
There are not many circuit variations that can be implemented in such
a simple 4-valve broadcast mantel
receiver. There is nothing that can
be considered unusual and similar
circuits are found in other 4-valve
mains-powered radios from the late
40s and early 50s.
As shown in Fig.1, the antenna is
connected to the first tuned circuit via
a coupled winding and a top-coupled
trimmer capacitor. The antenna could
be either a separate external antenna
or it could use the mains wiring as an
antenna!
In greater detail, small kitchenmodel receivers often had provision
to use the mains as the antenna and
in this case C5, a 100pF high-voltage
capacitor, was used to couple one
side of the mains to the antenna tuned
circuit. This method did have some
serious drawbacks, however. It might
have been convenient way of eliminating the need for an external antenna
but the RF signal from mains would
have been quite noisy. Additionally,
should the capacitor short, both the
antenna and the chassis could become
live and dangerous (ie, it would be at
mains potential), as the latter wasn’t
earthed via the mains plug.
The converter stage (V1) is based on
an ECH35 triode-hexode. The oscillator components consist of capacitors
C3, C7, C8 & C9, coils L3 & L4, and oscillator grid resistor R2. This produces
the sum and difference frequencies
which are then fed to an IF tuned circuit consisting of coil L5 and capacitor
C10. This tuned circuit is unshielded
(see photo) and is initially adjusted
February 2010 91
so effectively the volume will be zero
even before the volume control is at
its minimum setting.
V2 amplifies the 455kHz signal and
this is applied to the IF transformer
which consists of L6, L7, C13 and
C16. The output of this transformer
is then applied to the detector diodes
in V2 and the detected audio signal
is developed across resistor R6. The
signal is then coupled to the grid of
V3 (6V6GT) and this stage then drives
the speaker transformer and a 5-inch
(125mm) speaker.
Bias for the 6V6GT is obtained from
another back-bias voltage divider consisting of resistors R8 and R12. This
divider supplies -5.5V to the grid of
the 6V6GT.
Power for the receiver is derived
via a mains transformer which has a
tapped primary to suit mains voltages
between 220V and 260V AC (40-60Hz).
In the service notes, there is a comment that the transformer lamination
stack may be one inch (25.4mm) or
1.5 inches (38mm) high. I believe that
for 40Hz mains, the stack would have
been 1.5 inches high. The transformer
in this particular set is designed for
50Hz and 60Hz mains, as it only has a
one-inch high lamination stack.
The HT secondary winding of the
transformer drives a 6X5GT rectifier
(V4) and the filter capacitors have
common positive terminals, while
the filtering resistors are wired into
the negative supply line and consist
of R8 and R12.
Back bias
The old Mullard Meteor 600 cleaned up quite well, as is evident from these
above-chassis views. The parts on the top of the chassis are easy to access.
using a Philips-type fixed-wire trimmer. Once adjusted, it was expected
to retain its tuning almost indefinitely
although this doesn’t always work out
in practice as we shall see.
Both the antenna coil and the oscillator coil are housed in the same metal
can, located on the back edge of the
chassis. This is convenient for access
but means that both would have to be
replaced if a fault developed in either
section.
The signal from V1 is applied via
92 Silicon Chip
the tuned circuit to V2, an EBF35 IF
(intermediate frequency) amplifier.
Bias is applied to both V2 and V1 via
resistors R5 and R15. R14 and C23 act
as a filter for any hum that may be on
the back bias line, which consists of
R7, R4 and R3. R4 varies this bias from
-1.2V up to -26V.
At -26V, the EBF35 will still have
some gain, as its plate current is not
cut off until the bias reaches around
-38V. However, the ECH35 will be
cut off, as its cut-off voltage is -17V,
The voltage drop across the back
bias line of R8 and R12 is around 75V.
I question why such a high voltage is
dropped across the network. It seems
to me that more effective use of the
available high-tension (HT) voltage
could have been made. The voltage
across C17 is of the order of 185V and
with 75V dropped across the back bias
network, only 110V is available for the
valves. The total HT current drain is
around 28 milliamps.
6V6GT valves really don’t work all
that well until they have 150V or more
on their plate and screen elements.
By reducing the voltage drop across
the back bias network and re-jigging
the power supply circuit, around
150V could have been supplied to
the 6V6GT. This could have been
obtained without changing the power
transformer or many other small parts.
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This under-chassis view shows the set before it was rewired. Note the incorrect use of green/yellow mains earth wire for
some of the connections (this wiring was later replaced).
Because of the low HT voltage, the
audio output is quite limited and it
becomes distorted if pressed hard on
strong stations.
Restoration
The owner of this set kindly loaned
it to me so that I could complete the
restoration and write this article.
When he bought it, it had largely
been restored. However, a common
problem is that although sets are often
advertised as having been restored, the
restoration is often not complete or has
not been done correctly. Such was the
case with this radio.
As mentioned earlier, the set is easily removed from the cabinet and the
works readily accessed. The cabinet
is in good condition and required no
attention from me. However, a quick
look at the chassis showed that the frequency converter had been changed,
along with the paper and electrolytic
capacitors. The wiring had also been
changed.
The chassis layout should have been
better that it is. In some cases, the
inputs and outputs are close together
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and this has made it necessary to fit a
metal shield in the centre of the chassis
to ensure stability. The coils connected
to the converter valve are located quite
some distance apart too. In addition,
placing the rectifier valve next to the
IF/detector valve is just asking for hum
to show up in the audio output.
In fact, the audio coupling capacitor
is only 1nF, which does indeed suggest
there was a problem with hum in the
audio. By using a low-value capacitor here, the low-frequency output is
restricted, reducing the hum problem
in the process.
Only one lead of the original wiring
was left in the set. That was the earth
lead and the insulation on it had perished. No doubt, the previous restorer
had replaced the wiring because the
insulation had perished.
Although the replacement wiring
had been installed neatly, the type of
wire used was incorrect. In particular,
scraps of mains wire had been used
in various places, including green/
yellow mains wire. The latter should
only be used for mains earth wiring.
As a result, much of the wiring was
redone, not only to conform to the
necessary standards but also to make
the restoration look more original.
The replacement capacitors had all
been wired in correctly and the two
replacement electrolytic capacitors
even had C17 and C21 marked on
them. However, they had been transposed in the circuit, which could
prove confusing. There was no sign of
the mains antenna capacitor, which
I would have removed anyway for
safety reasons.
Finally, the original 2-core mains
lead had been replaced with a 3-core
lead and the chassis earthed. However,
this lead had not been anchored correctly and so had to be secured using
a cordgrip grommet.
Does it work?
Everything else appeared to be in
good order so the next step was to test
the mains transformer with a highvoltage insulation tester. This showed
no signs of excessive leakage between
its windings or to the chassis, so it was
now time to try the set out.
The first step was to apply power
February 2010 93
This photo shows the three trimmer
capacitors around the oscillator &
antenna coils. Two are semi-fixed
tubular types while the third is a
Philips “beehive” type.
The first 455kHz IF tuned circuit
consists of coil L5 & its parallel semifixed trimmer C10.
with the chassis resting upside down
and check the voltages at various
points in the circuit. These were all
found to be quite close to those listed
in the service sheet and the power
consumption was a quite reasonable
24W. I then connected an external
antenna to the set but found that the
performance was nothing to get excited about.
I tried replacing the EBF35 IF amplifier and the 6V6GT audio output valves
but this made no difference. I didn’t
try changing the ECH35 converter, as
it appeared to be new.
So what was causing the performance of the receiver to be so poor?
The voltages were close to what they
should be, the necessary capacitors
had been replaced and the defective
wiring had been replaced.
This set (and some Philips sets) uses
custom-made semi-fixed capacitors
which act as trimmers in the tuned
circuits. A couple of these trimmers
can be seen in the photo of the oscillator/antenna coil assembly, with one
towards the top of the picture. It consists of a thick wire inside a ceramic
tube, with many turns of fine wire on
the outside of the tube. The number
of turns of wire wound on the ceramic
tube determines the capacitor’s value.
To adjust these trimmers (or semifixed capacitors), it is necessary to
initially wind on more turns than
required, then gradually remove turns
until the circuit is tuned to the correct
frequency (it’s too bad if you take too
many turns off). This set has five of
these trimmers and the only trimmer
not of this type is C9 which is a Philips
beehive-type trimmer.
As mentioned previously, the semifixed capacitors are meant to retain
their values and supposedly never
need readjustment after the set leaves
the factory. However, that was wishful thinking as some certainly needed
tweaking in this set and they are difficult to deal with.
The previous restorer had decided
to leave these “one-time” adjustable
trimmers well alone. However, I decided I just had to bite the bullet and
adjust some of them to improve the
set’s lacklustre performance. They
are hard to adjust and extreme care is
needed when doing this as two of them
operate at the HT voltage! However,
it needed to be done if the set was to
operate correctly.
First, I adjusted C9 so that the set
tuned to the local Italian station on
1629kHz, then checked the alignment
of the IF stage.
The circuit seemed to be roughly
tuned to 455kHz but the response to
nearby frequencies was greater than
expected, indicating that the IF stage
needed alignment. I assumed that the
capacitors had been adjusted correctly
when the set was made so to make
the adjustments a bit easier, I placed
small 10pF mica capacitors across
each tuned winding. I then set the
signal generator to 455kHz and gradually removed turns of wire from each
trimmer until the peak response had
just been reached and was beginning
to dip again.
When I went too far, I just rewound
several of the turns back onto the ceramic tube and glued them in place. It’s
not a very elegant method but it works.
Because both C10 and C16 are live
to around 110V (with respect to the
chassis), extreme care is needed to ensure that nothing shorts when pulling
the wire off to adjust these trimmers.
And although I thought I was being
careful, I wasn’t careful enough and
did get a “bite” off the 110V.
In retrospect I should have worn
rubber gloves when doing this task and
I certainly will in future. So be warned
– even experienced people can make
mistakes. Unfortunately though, these
adjustments cannot be made with the
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94 Silicon Chip
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set turned off otherwise I would have
done it that way.
After making these adjustments, the
set’s performance was considerably
enhanced and it performs quite well
local stations.
Photo Gallery: Tasma 585 Mantel Radio
Summary
The Mullard Meteor 600, despite
no having outstanding RF or audio
performance, is quite a reasonable
little receiver for use in the kitchen or
bedroom. It is housed in a relatively
small bakelite cabinet but the components are not so crowded that service
is difficult.
However, the layout of the components leaves a lot to be desired and
the electronic design is deficient in
a number of areas as well. For example, the set could have used AGC
(automatic gain control), as the two
RF valves are variable mu types. It is
not that it would have required extra
components.
In addition, the HT could have been
made as high as 150V which would
have meant that all valves operated
with improved performance, especially the 6V6GT audio output stage.
This could have been achieved with
only relatively minor changes to the
values of a few low-cost components.
It all goes to show that some makers got away with some very ordinary
design and layout techniques but only
because the sets were budget models
and were not required to be high performers. However, this set could have
been a good performer without adding
to the cost.
That said, it’s a nice-looking set
and if its limitations are accepted it
is a worthwhile receiver to have in a
collection.
Finally, as a reflection on design,
T
he Tasma 585 was a large-size mantel
radio, with a vibrator power supply pack nearly half the
size of the radio itself and powered by a 6V accumulator. The valve line-up was as
follows: IM5G RF stage, IC7G mixer, IM5G IF amplifier, IK7G detector/first audio
amplifier and IL5G audio output. The intermediate frequency (IF) was 458kHz.
Features included dual-wave tuning, a permanent magnet speaker and a handsome
wooden case. Photo by Kevin Poulter for the Historical Radio Society of Australia
(HRSA). Phone (03) 9539 1117. www.hrsa.net.au
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ordinary.
I distinctly remember two transceivers I had to test from different
manufacturers. One barely met the
test criteria and disappeared from
use within a few years, while the
other was so good that it is still used
by some services some 35 years later.
The same thing has happened in the
design and construction of domestic
valve radios – the large manufacturers
didn’t always get it right, while many
small manufacturers produced excellent equipment.
Basically, a lot depended on who
the designer was at the time when it
came to the quality and performance
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February 2010 95
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