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VINTAGE RADIO
By RODNEY CHAMPNESS, VK3UG
Restoring a B15 mantle radio
with some interesting faults
Just when you think that you’ve seen all the
faults possible in vintage radios, another one
suddenly pops up to shatter that sense of
complacency. Such was the case with an AWA
B15 1960s “plastic” mantle set that I restored
recently. It had an interesting problem that lies
dormant in many sets.
water. After quite a bit of elbow grease,
it scrubbed up quite well, as can be
seen in the photos. Unfortunately
though, the cabinet had several broken mounting posts. This prevented
both the back and the loudspeaker
from being properly secured.
As luck would have it, I was able
to raid another scrap B15 for missing
screws and a few other minor pieces. The back of the set is normally
A friend recently asked me to do up dropping along the way.
fastened by four metal-thread screws
a rather dilapidated AWA B15 5-valve
Obviously, the first step in the – two at the bottom into the chassis
mantle radio that he had acquired. restoration process was to give the
and two into threaded metal sleeves
Apparently, the set once belonged to
set a thorough clean-up. This meant embedded in plastic posts. It was one
a farmer and by the look of it, had
removing the chassis and stripping of these posts that had broken.
spent many years in a shed gathering all other parts from the set, so that the
To fix this problem, I fitted the metal
dust, grease, grime and the odd mouse
cabinet could be scrubbed in soapy sleeve into the base of the broken post
and glued it in place using
a small amount of 5-minute
Araldite. I then made a larger
sleeve out of paper and fitted it
over the metal sleeve. Another
batch of 5-minute Araldite
was then made up and poured
between the two sleeves. Some
of this oozed out the bottom,
making a wider area for the
glue to grip.
Once this batch had set, another small amount of Araldite
was mixed up and poured
between the sleeves to build
the post up further. It was a
bit of a messy job but the end
result was quite satisfactory.
The owner wasn’t after a meticulous restoration job – just
a set that looked good and
worked well. And of course,
cost comes into it too.
The mice had been hungry
and had devoured a section of
The AWA B15 is a 5-valve mantle set from the 1960s. The plastic cabinet caused a few
the speaker cone. Despite this,
restoration problems, as some of the internal mounting posts had broken.
74 Silicon Chip
Fig.1: the circuit of the AWA B15 mantle radio set. The two hand-drawn 68pF capacitors were added to improve
filtering of the 455kHz IF.
it still sounded OK after it had been
cleaned, with no audible poling of the
voice coil. It was repaired by “gluing”
a piece of writing paper over the 25
x 25mm hole, using clear nail polish.
Nail polish easily soaks into porous
materials and is quite a good “glue”
for this type of repair.
The repaired speaker can been seen
in one of the photos. It may not be a
joy to behold but it works well – particularly as price was an important
part of the whole deal.
Unfortunately, two of the loudspeaker mounting posts had also
been broken – a common problem
with these sets. This meant that the
loudspeaker had to be secured using
just two retaining clips and mounting
posts. The other two support posts
were then attached using Tarzans
Grip. There was just no other economical way of attaching the loudspeaker
to the front of the cabinet.
Paper capacitors
UCC brand paper capacitors had
been used throughout the set and
these were all found to have significant leakage. All were replaced except
for C14, C16, C21 and C27, as these
four capacitors could have significant
leakage without affecting the opera
tion of the set. Once this work had
been completed, the set “worked”
but its performance was rather poor.
Clearly, it needed more work to bring
it up to scratch.
A close examination of the chassis
soon revealed that someone in the
past had replaced R11 (220kΩ) with
a 100kΩ resistor. This resistor had
altered the operating conditions for
the 6AV6 (V3), so it was replaced with
the correct value.
Next, I discovered that the HT
voltages from the power supply were
rather low. A replacement 6X4 rectifier valve soon livened things up by
increasing the HT voltage, after which
the set was beginning to show some
promise.
Alignment
My next step was to carefully check
the set’s alignment. The IF (intermediate frequency) stages were found
to be close enough for all practical
purposes, although a tweak did give
a bit of a lift in performance. The oscillator stage was a different story. It
was about right at the low frequency
end of the dial but was miles out at the
top end. This meant that the oscillator
trimmer had to be wound in, so that
the stations appeared at the correct
places on the dial.
In order to detect the peak output
(and thus the correct alignment point),
I attached a digital voltmeter across
the AGC filter capacitor (C3 in this
case). This allowed me to measure
the AGC voltage developed when a
reasonably strong input signal was
applied to the set.
Having set up the voltmeter, the
antenna coil was slid along the ferrite
rod and adjusted for peak output with
the set tuned to 621kHz. The antenna
trimmer was then adjusted for peak
output at around 1450kHz.
After all this effort, the performance
was still not really all that marvellous.
What else could be checked? First, I
tried replacing the 6BE6 and 6BA6 RF
& IF amplifier valves (V1 & V2) and
found that this lifted the performance
noticeably. I also found that the back
bias across R15 was low, so a fresh
6AQ5 (V4) was tried. This noticeably
increased the back bias, along with
the audio output level.
The set was now better but still had
a couple of problems. The first was
an intermittent fault, the set working
quite nor
mally and then abruptly
JUNE 1999 75
Restoring a B15 mantle radio – continued
The loudspeaker cone was patched up by “gluing” a piece of writing paper over
the 25 x 25mm hole, using clear nail polish. It might look a bit rough but the
speaker still works well.
changing volume. It appeared to be
slightly sensitive to movement and
eventually, after some judicious prodding around with the insulated handle
of a small screwdriver, I discovered
that C16 was the culprit. One tap
dropped the volume; another brought
it up again.
I should have known – another
UCC capacitor. Not only did the UCC
capacitors of that era go leaky, they
also went intermittently open-circuit.
Once this was replaced, the volume
remained constant but the set still
wasn’t right – it tended to oscillate
at the low frequency end of the dial
and the sensitivity didn’t seem to be
what it should be.
I was puzzled about this, as
everything in the set appeared to be
correct. However, I then remembered
that I’d had problems with several
of these sets in the past, due to the
455kHz IF signal radiating back into
the input (and also causing interfer
ence in other sets). I had previously
cured this problem by connecting a
68pF ceramic or mica capacitor between pin 7 of the 6AQ5 and earth and
adding a similar capacitor between
the top of the volume control (RV1)
and earth.
76 Silicon Chip
Once again, this approach did the
trick – in fact, the instability at the low
frequency end of the dial disappeared
as soon as I connected a 68pF capacitor from pin 7 of the 6AQ5 to chassis.
This also improved the sensitivity to
the extent that a signal of just 1µV was
useable. Readers should note, however, that the amount of improvement
achieved by this minor modification
may vary from set to set.
After that, it was simply a matter of
reinstalling the chassis in the cabinet
and giving the set a final soak test. The
set ran very well and the owner was
happy with the result.
Insufficient filtering
Why did the extra 68pF capacitors
make such a difference? A glance at
the circuit reveals that very little filtering of the 455kHz IF signal takes
place after the detector. In fact, only
C19 and C24 provide any degree of
filtering and they don’t do much. In
effect, the unwanted 455kHz IF signal
being fed to the audio amplifier was
de-sensitising the whole set!
C24 (100pF) is relatively ineffective
where it is and would have achieved
a better result if it was connected
between the grid of the 6AQ5 and
chassis. C22 provides some filtering
when the tone control (RV2) is in the
“bass” position but has no effect when
the tone control is wound towards the
“treble” position.
You might think that V3 and V4
are only audio amplifiers and so they
won’t amplify 455kHz IF signals very
much. However, that’s not the case.
Circuits almost identical to this were
used as video amplifiers in valve TV
sets and were capable of amplifying
frequencies up to 5MHz or more, so
amplifying a signal at 455kHz is no
problem at all!
This means that the 455kHz IF
signal should be filtered out. In fact,
quite a few receivers benefit from better IF filtering, so fitting a capacitor to
the grid of the audio output valve is
often worthwhile. Without this modification, the AWA B15 mantle radio
also causes interference to other sets
in the near vicinity if they only use a
loop antenna.
Summary
Some vintage radio enthusiasts
would not consider a “plastic” set like
this worthy of restoration. However,
they are part of our radio heritage and
so many find them quite collectable.
That point aside, this particular set
was a real mess when it arrived and
it certainly had had a long, hard life.
It needed a thorough clean-up, the replacement of most of its paper capaci
tors and four replacement valves (all
low in emission) to get it up to scratch
– and even then it required extra work
to fix the remaining problems.
Fortunately, the coils and transformers in these sets are almost
always OK, the exception being the
speaker transformer which can occasionally have an open circuit primary
winding.
Was it worth the effort? In terms
of time and money, it was a doubtful
proposition but for the old gent who
owned it, the sentimental value made
it worthwhile. It certainly looks a lot
better than it did when it came in and
it now performs very well – better
than new, given the improved IF signal filtering. It is amazing how many
of our manufacturers didn’t quite get
things right.
I have several such sets in my collection, all with the IF filter modification. They are not my favourite sets
but I am quite happy to have them
SC
on display.
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