This is only a preview of the October 1996 issue of Silicon Chip. You can view 24 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "Send Video Signals Over Twister Pair Cable":
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VINTAGE RADIO
By JOHN HILL
A new life for an old Hotpoint
My first commercial radio was a 1940s
4-valve AWA Radiola. Recently, I had the
chance to restore an almost identical model
and that’s what this month’s story is about.
So how did an old Hotpoint get into the act?
I have mentioned before my early
interest in radio and how my spare
time as a lad was spent building crystal
sets and simple regenerative receivers.
This was an exciting time of my life
and I have fond memories of those
distant days. But although this period
spanned many years, it came to a very
abrupt end.
My tinkering with home-made radios finished the day my father bought
me a new receiver for my bedroom.
Actually, I think my mother was the
main instigator behind this move because she had tired of the perpetual
mess that graced the top of a chest of
drawers. For years, this area had been
strewn with a variety of radios, mountains of batteries, including a smelly
rechargeable lead-acid B battery, and
other miscellaneous accessories such
as headphones, with their long dangling cords.
From my mother’s viewpoint, that
was untidiness of the worst kind and
it had to go!
However, in order to remove the
junk without fuss or ill feeling, there
had to be a satisfactory replacement.
Enter one new radio in the form of
a late 1940s 4-valve AWA Radiola
mantel model with a brown Bakelite
cabinet.
It must have been Mum’s idea because it wasn’t even Christmas or my
birthday – it just happened!
The little Radiola was in regular
use for about 10 years up until the
time I left home for the big smoke.
Sometime after that it strangely disappeared. Presumably it developed
some terminal complaint and was
gently laid to rest. At the time I never
bothered to ask what happened to it.
Now that I would like to know, no-one
can remember.
Different styles
The Hotpoint receiver after restoration. This particular model with the ovalshaped Bakelite dial escutcheon (part of the cabinet moulding) survives better
than the model with the separate moulded plastic escutcheon.
88 Silicon Chip
My old Radiola had a cabinet style
that was not as common as a similar
and slightly larger model of that era.
As a result, I had, for quite a while,
been looking for one to add to my
collection – not that postwar 4-valve
Radiolas are highly sought after collectables. I just wanted one the same
as the one I had back in the 1950s for
sentimental reasons.
Just why there were two distinct
cabinet styles is something of a mystery. However, the smaller one had an
oval shaped dial while the other had a
rectangular dial. Otherwise they were
much the same inside and the dial
shape was about the only noticeable
difference between models.
It was the oval version that I was
seeking. This cabinet style is far more
durable than the rectangular model.
The reason for this difference is that
the oval dial has a Bakelite escutcheon
whereas the other is white plastic. The
latter warps and cracks with age and,
after 40 years or so, is inclined to fall
to pieces.
Many other Radiolas of similar
vintage have the same lousy plastic
in their speaker grilles and these too
can look terrible due to the distortion
that takes place over the years. When
it comes to plastics, some are far more
stable than others.
Bakelite vs. plastic
Before going further, let’s briefly
digress and examine the differences
between Bakelite and plastic, just to
clarify that last paragraph.
Although they are both plastics,
Bakelite is a thermosetting plastic
which is very stable and holds its form
extremely well, even over time spans
exceeding 60 years.
Thermoplastics, on the other hand,
have quite different characteristics and
many early thermoplastics virtually
self-destruct after 40 years or so. However, thermoplastics can be re-melted
and recycled, whereas thermosetting
plastics cannot!
From a collector’s point of view,
I’m not particularly interested in restoring any receiver that has a badly
deteriorated cabinet due to the use of
poor quality plastics. A restoration
job should result in a receiver that
both looks and performs as new (or
close to it).
If the cabinet or cabinet fittings have
cracked or warped out of shape, then
the set is not worth restoring. Well,
that’s how I see the situation!
A Radiola with the rectangular dial escutcheon. This 4-valve model is unusual
in that it is a dual-wave receiver. Very few 4-valve sets have a shortwave band.
The Hotpoint substitute
Anyway, the little 4-valve Radiola
I was seeking finally came my way in
the form of a Hotpoint! This was, in
fact, exactly the same as a Radiola but
marketed under another name. Both
sets were made by AWA and there were
sometimes minor cabinet differences
to distinguish the two but not in this
instance.
Unfortunately, the Hotpoint had a
white cabinet with numerous cracks
which showed up as black lines. Although the set was working, it was in
terrible condition with an intermittent contact in the on/off switch and
crackles in both the volume and tone
controls.
But for the miserable sum of $10, it
was worth buying, even if it wasn’t a
Radiola in a Bakelite cabinet.
The old Hotpoint receiver used a couple of unusual valve types: an N78 (6BJ5)
and a 6AR7 GT. Note the shield on the 6AR7, a peculiar characteristic of this
Australian-designed and manufactured valve.
What I didn’t realise at the time was
that there was a suitable Bakelite cabinet stowed away in the shed, which
I had completely forgotten about. I
have no recollection as to where it
came from or how it was acquired.
The Radiola cabinet was discovered
quite by accident while I was looking
for a valve tester which, I might add,
could not be found. No doubt it will
be unearthed while I am looking for
something else some other time.
So it was only a matter of combining
the Hotpoint chassis with the Radiola
cabinet and I would have a working
model of my original little 4-valve
radio.
Cleaning up
The Bakelite cabinet had seen better
October 1996 89
A rear view of Hotpoint chassis. This chassis uses a 5-inch (125mm) permag
speaker whereas many radios of this era still used electrodynamic speakers.
The chassis cleaned up quite well.
days. The dirty front half looked so
different to the reasonably clean back
half that I initially assumed they may
not have been a matched pair.
To explain, each batch of Bakelite
has its own colour char
acteristics
and an unmatched pair of cabinet
halves can stand out like a neon sign.
Fortunately, this was not the case because when the cabinet was washed
and polished, the two pieces blended
together perfectly.
The Radiola dial had been cracked
in two places and this meant that the
Hotpoint dial had to be used. Because
there appears to be no difference between the Radiola and the Hotpoint
radios, I guess I can tolerate a name
change.
Valve types
This 7-pin miniature valve socket is fitted to a chassis that has obviously been
designed for octal valves. The other 4-valve chassis uses an octal 6V6GT in this
position.
90 Silicon Chip
Repairs to the receiver were relatively straightforward and started off
well when all four valves tested OK.
The valves used were: 6BE6 frequency
converter, 6AR7 IF amplifier and detector, N78 (6BJ5) audio output, and
a 5Y3 rectifier.
The valve complement in the Rad
iola 4 varies quite a bit. The other
receiver shown in one of the accompanying photographs uses a 6A8, 6AR7,
6V6 and 6X5. There receivers were
made at a time when manufacturers
often had to use whatever components
were available, not necessarily what
they wanted to use.
Getting back to the Hotpoint valve
line up, the 6AR7 is an odd type in
that it is an Australian-designed and
made valve used only in locally made
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Front view of Hotpoint chassis. The loudspeaker sits directly behind the dial.
Note that the dial setup uses approximately two metres of dial cord.
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Shown here are the volume control (left) and the combined tone control and on/
off switch (right). Both potentiometers were repaired by cleaning the resistance
track and repositioning the wiper arm. The switch responded to a flush-out with
a non-lubricating cleaning fluid.
equipment. It usually tests poorly for
some reason or other but this one was
OK. An EBF35 will work in its place
if a 6AR7 is unobtainable.
The N78 is also an unusual output
valve as far as domestic radio receivers are concerned. The only receivers
I have encountered that use this valve
have been these early postwar Radi
olas. Should a substitute valve be required, a 6AQ5 with a rewired socket
and grid bias modification should do
the trick.
Grid bias
Speaking of grid bias, it is worth
noting that many 4-valve receivers
are under biased. In fact, the output
valve’s bias voltage is often at about
half the recommended value, even
taking into account the lower plate
voltages at which some of these small
receivers work.
If the bias is changed in order to
produce the correct voltage, there is
a noticeable drop in output volume.
Presumably, the output valve is
deliberately under biased to raise the
output level of the receiver. One must
remember that a 4-valve receiver is really only a 3-valve receiver (plus rectifier)
and needs every bit of encouragement
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October 1996 91
In the case of the little Hotpoint, a
hollow had been worn through the
resistance track on the tone control.
This control was combined with an
on/off switch and had turned the set
on and off many thousands of times
during its 45-year life span. This problem was eliminated by simply bending
the wiper arm away from the damaged
area and onto an unused portion of
the track.
New capacitors
When the text refers to poor quality thermoplastics, it really means poor quality.
Shown here is a Radiola plastic escutcheon that has badly distorted with age. A
Bakelite escutcheon, on the other hand, would have held its shape, even over a
long period of time.
in the performance department it can
get. Under-biasing the output valve
helps to give a bit more gain on those
weaker signals –even at the expense
of valve life and sound fidelity, which
apparently doesn’t amount to much
anyway.
Problem areas
The worst problem areas of the
receiver were the volume control and
the combined tone control and on/off
switch. These components were very
worn and highly suspect, especially
the on/off switch which was making
such poor contact the dial lights were
flickering in unison. None of these
controls was replaced. Instead, they
were all repaired and they came up
quite OK.
Many volume and tone control potentiometers can be restored to good
working order simply by cleaning the
resistance track. However, this can be
a fairly temporary repair if the track
is worn.
A better repair results if the wiper
arm is bent away from its original
contact path and is placed on a
previously unused part of the track.
Such a simple modification can give
a worn potentiometer a completely
new lease of life.
Faulty on/off switches also respond
well to a cleanup and a flush-out with
a non-lubricating contact solvent is
a good starting point. An ohmmeter
set to the 1-ohm range will quickly
indicate the condition of the switch
contacts. Any measurable resistance
in a switch must eventually cause
trouble.
It is also a good idea to turn old receivers on and off at the power point,
as a 40-50 year old switch deserves a
rest. On cannot expect them to keep
working forever. Any potentiometer
combined with an on/off switch will
also benefit from switching at the
power point, as this will reduce the
wear on the track that would otherwise
occur each time the switch was used.
These replacement control knobs were so tight that the
flats on the control shafts had to be filed down so that
they could be fitted.
92 Silicon Chip
Replacing all of the paper capacitors
with more modern varieties raised
the high tension voltage by 20V. The
electrolyt
ics were the originals and
seemed OK but they were replaced
anyway.
After applying some Silastic® silicone rubber compound to the thin
outer rim of the loudspeaker, it was
time to find three control knobs.
Finding them was not a problem but
getting them to fit the control shafts
was another matter. They were so
tight that breaking them was a distinct
possibility.
This problem was solved by filing
the flats on the control shafts. They can
now be fitted and removed without
risk of breaking.
So there it is: a quick and relative-ly
easy restoration of a humble 4-valve receiver, with a few repair hints thrown
in for good measure.
From my point of view, it was a satisfying project because I could relate
to that particular model receiver. Of
course, it would have been better if
the set had been 100% Radiola. But I
guess a mix of Radiola and Hotpoint
isn’t a bad compromise, especially
when they were both made in the
SC
same factory.
This view shows the replacement loudspeaker cloth
around the dial escutcheon. Even the dial light windows
were removed and cleaned during the restoration process.
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