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VINTAGE RADIO
By JOHN HILL
A new life for a battered Astor
Not all vintage radios are highly sought after
items. A mid-1950s 4-valve Astor can often be
picked up for just a few dollars and is usually
quite easy to repair and get going again.
Vintage radio receivers vary from
those rare relics from the 1920s to
the mass-produced plastic mantel
models of the 1950s and 1960s, with
a multitude of makes and models in
between. Although some collectors
specialise in a particular era or brand
name, many collect whatever comes
their way, regardless of age or whether
it is housed in a timber, bakelite or
plastic cabinet.
From a writing point of view, I like
to produce a similar variety in my
monthly column and endeavour to
give my readers a mixed bag of stories
about the many and varied aspects of
vintage radio. It appears that a restoration story on a relatively late-model
valve receiver is just as interesting, in
its way, as a similar story on an older
and rarer set that few of us are ever
likely to own. In fact, far more readers can relate to a late-model receiver
because that is what most people are
likely to collect.
It so happened that a particular
repair came my way recently and it
seemed to be a good one to write about
for the simple reason the receiver is so
ordinary and unspectacular. It was a
mid-1950s 4-valve Astor mantel with
odd control knobs and a smashed
speaker grille – the sort of wreck that
can be picked up at a garage sale for
$10 or less.
Apart from the broken cabinet, the
receiver was in poor condition and
although it supposedly “worked”
when purchased, it made no sound
other than a badly distorted whimper.
The first step with the grille repair was to make up a plastic louvre to replace
the missing one. The handle of a plastic knife was used for this purpose.
76 Silicon Chip
In fact, it was the type of set that one
would normally buy for spare parts.
In this instance, however, the
receiver was brought to me to be
repaired. As the woman who owns
it is a friend of the family, I really
couldn’t say no.
Of course, she realised when she
bought the set that it needed a lot
doing to it but thought that it would
be no trouble for me to fix it because
my magic wand can mend just about
anything. Oh – such faith!
Grille repairs
I decided to attempt the broken
grille repair first and it was fortunate
that the damage was not as bad as it
could have been. One broken piece
had already been glued back in place
while the other piece was missing.
This meant that a new section had to
be made and glued into position.
Finding something suitable from
which to make a new grille part was
the problem. Eventually, the handle
of a takeaway plastic knife supplied
the necessary material. It was shaped
with a file until it wedged firmly into
position, then it was glued in place.
A couple of smaller fragments were
then used elsewhere to fill in a few
missing chips.
Although the broken grille louvres
had been successfully replaced, the
stark white plastic replacements
stood out like a neon sign compared
to the rest of the speaker grille. The
repair area needed a touch up with
a matching paint but obtaining the
correct colour match was a near impossible task.
So instead of a match, a contrast was
used, and the whole grille area was
painted an off-white. The result was
pleasing enough and at the same time
disguised the repair area reasonably
well. With the grille reconstruction
completed, the circuitry was next on
the agenda.
Speaker repairs
In order to work on the speaker
grille, the speaker had to be removed
from the cabinet. This should have
been a simple operation requiring the
removal of four spring steel clips from
the plastic studs they fit onto. Alas,
two of the studs snapped off!
Removing the speaker revealed
that whatever smashed the grille also
damaged the speaker cone – it was torn
from rim to centre. Repairing the split
with silicone rubber (Silastic®) cured
the problem and while such cone
patch-ups aren’t always the neatest
looking repairs, they are effective and
long lasting.
The speaker, by the way, is unusual in that it is a very small oval type
measuring 125 x 75mm (5 x 3 inches),
so replacement was not an option.
As an aside, most 4-valve receivers
from the 1950s used 125mm (5-inch)
speakers so it appeared that the little
Astor might be at a disadvantage as far
as a good sound reproduction was concerned. However, after the restoration
had been completed, the midget Rola
performed really well and the set’s
tonal quality was excellent.
A close-up view of the finished grille repair. The whole louvre area was painted
off-white to disguise the repaired section. Although not a totally invisible repair,
there were no complaints from the owner.
Original parts
Checking out the chassis revealed
everything to be original and the state
of the 40-year old Ducon paper capacitors was not good. They appeared to
have been overheated, having bulged
ends and droplets of solidified wax
hanging from their undersides. Naturally, they were replaced and that,
in itself, would have au
tomatically
solved a number of problems.
The valves were checked next and
the valve tester’s neon quickly indicated an intermittent short in the 6BE6.
These valves often flash the shorts/
leakage neon on my valve tester but,
despite this, they usually function
Spring-steel clips are used to hold the
loudspeaker in place. As is often the
case, the plastic stud breaks off when
the clip is removed.
This is the miniature (125 x 75mm) Rola loudspeaker that was used in the old
Astor. Note the missing retaining clips and the repaired cone areas at 12 o’clock
and 2 o’clock. The cone repair was completely satisfactory.
quite normally. The rest of the valves
checked OK.
Numerous other items needed attention. The 200Ω back-bias resistor
had split, a noisy volume control
required cleaning, a new power cord
was needed, the dial cord was about
to let go, both dial lamps were burnt
These two spring retaining clips are
all that hold the cabinet together.
This view shows how the clips are
fitted to the underside of the cabinet.
December 1996 77
The two odd knobs at left were replaced with a pair of Radiola knobs which
matched the maroon colour of the Astor cabinet perfectly.
out, and the chassis was just floating
around loose inside the cabinet.
An ohmmeter check on other resistor values cleared them all as being
well within tolerance. The intermediate frequency, power, and output
transformers also passed inspection,
as did the aerial and oscillator coils.
Testing
After replacing all the necessary
parts it was time for a tryout. While
the receiver worked, there was very
little volume and an incredible amount
of distortion.
Distortion in a valve radio can often
be caused by a leaky coupling capacitor from the plate of the driver stage
to the grid of the output valve. This
allows the plate voltage to be applied
to the control grid of the output valve,
thus biasing the grid positive instead
of negative. As the little Astor had just
had all of its old capacitors replaced,
a faulty coupling capacitor seemed
unlikely.
However, a voltmeter check of the
output valve’s control grid revealed a
high positive potential. The coupling
capacitor was replaced but the situa-
The little Astor is of simple construction
and is a very basic 4-valve receiver. It was
straightforward to repair and get going.
78 Silicon Chip
tion remained the same – the control
grid was still positive!
Studying the circuitry more closely
revealed a 100pF silvered mica capacitor connected between the plate of
the 6AQ5 output valve and its control
grid, via a 47kΩ stopper resistor. This
capacitor is designed to apply a small
amount of negative feedback to the
control grid of the output valve, to
improve the audio frequency response
of the receiver. It was reasonable to
assume that this mica capacitor was
faulty and it was!
Removing the capacitor immediately cured the distortion problem and
the set sounded normal – but not for
long. After about a minute or two, the
distortion returned and the volume
faded to almost nothing.
At this stage, I recalled the short
indication when testing the 6BE6.
The valve was replaced and that fixed
that problem – no more distortion
and stable volume. A new 100pF capacitor was also fitted in place of the
faulty one and repairs were nearing
completion.
There was still one remaining problem with the receiver – it was full of
whistles. The 6AD8 IF (intermediate
frequency) amplifier valve was replaced and that eliminated the birdies, so the valve obviously had some
sort of an internal fault or a shielding
problem.
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The finished Astor mantel receiver looked quite presentable. It’s not the sort of
receiver that collectors would fight over but its owner was very pleased to have
it restored to working order.
An alignment session peaked the
IF transformers and aligned the aerial
and oscillator circuits. That completed
the restoration except for a few minor
details. One of these details was the
mounting of the loudspeaker. It has
already been stated that two of the
mounting lugs broke off when removing the speaker’s retaining clips. This
is not an uncommon happening with
this method of mounting and can make
remounting the speaker difficult.
Perhaps the easiest way out of
this situation is to glue the speaker
back in place but this should be
done with care. Some modern glues
can be rather tenacious, so use them
sparingly in case the speaker has to
be removed some time in the future.
Also, it is advisable to fit a grille cloth
to minimise the accumulation of dust
and fluff that builds up between the
bottom of the speaker cone and the
speaker baffle.
Checking mica capacitors
A megohmmeter test on the suspect
mica capacitor revealed a serious
leakage problem. Capacitors which
work at high voltages should be tested
at high voltages.
Perhaps some comment should
also be made regarding that leaky
mica capacitor. The faulty capacitor
was the only silvered mica capacitor
in the receiver. As time progresses,
more and more of these capacitors
give trouble and need replacing but
it is not always easy to detect faults
in mica capacitors.
When checking the suspect capacitor with a multimeter set to the ohms
x 1000 scale, the meter needle showed
not the slightest deflection. To many
vintage repairers this would indicate
that the capacitor was not leaking or
shorted and not the cause of the problem. Not necessarily so!
When the same suspect capacitor
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was checked at 500V using a megohm
meter, the meter reading was about
0.5MΩ and that sort of leakage is quite
unacceptable under the conditions in
which the capacitor operates.
Leakage and resistance might be
regarded as two different effects. A
good component will measure the
same whether checked on a multimeter at 3V or a megohmmeter at 500V.
But leakage in a faulty component
can increase with voltage. Which is
why capacitors that work under high
voltage conditions should be checked
for leakage at high voltages.
In conclusion, this somewhat undesirable wreck of a radio was brought
back from the dead and is once more
an operative and useful receiver.
With its repaired and painted speaker grille it has little appeal to serious
collectors but its owner was absolutely
thrilled with the transformation. The
little Astor now has pride of place in
her bedroom and is looked on as a
treasured possession.
This only goes to prove that beauty
is in the eye of the beholder. What
may not appeal to some can be simply
SC
wonderful to others.
December 1996 79
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