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Vintage Radio
By Associate Professor Graham Parslow
AWA 1946 Fisk Radiola
Model 92 Egg Crate
Despite having modest performance and
cut-price circuitry, many vintage radios
have become collectors’ items due to their
distinctive cabinet styles. The AWA Fisk
Radiola Model 92 “egg crate” radio from
1946 is one such set.
M
ANY ICONIC radios from the
golden age of radio have been
given nicknames by collectors due to
their appearance. These radio nicknames include “scales”, “beehive”
and the “plum pudding”. In the case
of the AWA Model 92 radio, its “egg
crate” nickname was derived from the
distinctive style of its speaker grille.
Often, the same cabinet style was
used for a number of different circuits,
so dating this egg crate radio isn’t easy.
I discovered this when my curiosity
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was aroused about the date of a photo
that was on the July 2015 cover of the
HRSA journal “Radio Waves”. It was
of a J. P. Aarons store in Melbourne
and showed an egg crate radio in the
window.
After some discussion with Kevin
Poulter who had acquired the image,
a date of 1946 emerged as the most
probable for the “Radio Waves” photo.
The egg crate cabinet definitely existed
in 1939 but it may well have been
released earlier.
The dial on the radio featured in this
article is clearly labelled “The Fisk
Radiola” and this is also shown in the
Model 92 advertisement reproduced
with this article. However, Ernest Fisk
left AWA in 1944 and this ended “The
Fisk” series of radios, the exception
being the Model 92 that continued
with his name. By contrast, other
post-war models that were housed in
the egg crate cabinet bear only the title
“Radiola”, which was proprietary to
AWA and RCA America.
The hyperbole of the AWA advertisement for the Model 92 really is over
the top for what I have long-considered
to be an ugly duckling. Part of the
promotion reads: “The strikingly
beautiful cabinet, designed by an
artist of distinction, has exquisitely
graceful lines and is a masterpiece of
streamlined simplicity”. However, despite my initial reservations, I became
much more favourably inclined to
this unique package as the restoration
proceeded.
That said, when you cast an eye
over the minimalist circuit used in
the Model 92, AWA’s claim that it was
“Australia’s finest broadcast receiver”
was ludicrous.
As can be seen from the photos,
there were just two controls on the
front of the cabinet (one on either side
of the dial) and these were for volume
and tone. The tone switch has only two
positions: treble-cut on and treble-cut
off. The tuning control is situated on
the righthand side of the cabinet.
Circuit details
The Model 92 circuit appears in
AORSM (Australian Official Radio
Service Manual) Volume 4, which
covers 1940-41 radio receivers. It is
a 4-valve superhet and uses common
valves from the late 1930s.
Fig.1 shows the circuit details as
they appeared in the AORSM manual.
It’s rather unconventional in appearance because the valves are shown
“upside down”, with the plates tosiliconchip.com.au
Fig.1: the circuit uses a 6A8 converter stage,
a 6G8G IF amplifier & detector, a 6V6 audio
output stage and a 5Y3 rectifier.
wards the bottom and the heaters at
top.
As shown, a 6A8 mixer oscillator
stage is followed by a 6G8 which functions as an IF amplifier and detector
(there’s no AGC). A single 6V6 pentode
is used as the audio output stage,
while the rectifier is a 5Y3 which has
its heater powered from a separate 5V
power transformer winding.
Interestingly, the speaker is an
electrodynamic type and its 1kΩ
field coil also filters the HT rail from
the rectifier. It’s very much a pre-war
design, although the ARTS&P label on
the featured radio indicates that this
particular radio was manufactured
post-war, ie, in 1946. This date is also
consistent with the plastic figure-8
twin-core flex that was used for the
240VAC power lead.
This view shows the chassis
before it was cleaned. It was
covered in dust and grime
but was otherwise in good
condition.
Cleaning up
This radio had waited on a shelf
for over five years before the “Radio
Waves” cover finally motivated me
to restore it. Its initial appearance
was quite untidy, due mainly to a
torn speaker grille cloth (styled with
coarse mesh fabric), a loose dial window, dirty knobs and a faded Bakelite
cabinet.
Internally, the radio was covered in
grime and that meant that the chassis
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would have to be thoroughly cleaned
before I could work on the circuit.
I tackled the cabinet first. The original speaker grille fabric was beyond
repair and that presented a problem
because its coarse pattern is part of the
character of this radio. Fortunately, I
soon discovered that I had some similar fabric on hand, the only problem
being that it was pale blue. That problem was quickly solved by spraying
the fabric with ivory-coloured paint, a
technique that works quite well.
Next, the knobs were removed and
cleaned using a brush and warm, soapy
water. I then cleaned the cabinet and
wiped it over with “Armor All” and
it came up looking almost like new.
October 2015 87
AWA’s claim that the Fisk Radiola Model 92 was “Australia’s finest broadcast
receiver” was a bit over the top, considering the modest circuit it employed.
Once the outside was looking good,
the chassis was brushed out to remove
any fluff and then liberated from its
grime using a turpentine wash. It was
then blown out with compressed air
to thoroughly dry it.
That done, the broken dial cord was
removed and replaced. I also discovered that the 5-inch speaker cone had a
small tear and this was repaired using
PVA glue.
Electrical restoration
The original fly leads to the top-cap
grids of the 6A8 and 6G8 valve were
sheathed in cotton-covered rubber
insulation. Over the years, this covering had frayed and now looked tatty.
As a result, the corroded grid caps at
the ends of these leads were removed
and cleaned so that they would later
make good connections to the valve
grids. The valve-cap wiring was then
sleeved with yellow heatshrink tubing, both for appearance and to ensure
good insulation, and the valve caps
reattached.
Next, the two dial globes were
removed and checked. Blown dial
globes are a common problem in old
radios but these both tested OK, so they
were simply cleaned and reinstalled
in their positions behind the colourful
dial glass.
Under the chassis
At first glance, the under-chassis
wiring and parts were all original
except for the electrolytic capacitor
(25µF 25V) used as a cathode bypass
on the 6V6 output pentode.
A close inspection indicated that
The cabinet came up
looking like new but the
torn speaker cloth had
yet to be replaced when
this photo was taken.
It was later swapped
out for a similar coarseweave fabric that had
been sprayed with
ivory-coloured paint
to match the original
colour.
88 Silicon Chip
the two chassis-mounted HT filter
electrolytics (C25 & C26) had dried out,
with perished red rubber at the base
of the cans. Rather than remove them,
these capacitors were left in place on
the chassis in order to maintain the
original appearance. It was just a matter of cutting the appropriate leads to
disconnect them and then installing
two 33µF 450V electrolytics under
the chassis to serve as the HT filters.
In addition, paper capacitor C22
(0.05µF), which couples the detected
signal from the 6G8 to the 6V6 audio
output stage, was replaced with a
0.047µF 630V Mylar unit. Based on
long experience, I always replace this
usually leaky capacitor as a matter of
routine. This prevents leaked HT from
overloading the output pentode with
positive grid bias, which can quickly
destroy both the output valve and the
output transformer.
The identical 0.05µF tone control
capacitor was also replaced as a matter of course.
The last modification before switchon was to remove the old 2-core mains
flex and install a 3-core cable. This
allowed the chassis to be securely
earthed, making the unit much safer
to work on. In addition, the new cable
was securely clamped into position.
The original cable had been secured
by tying a knot in the lead just inside
the chassis (see photo), which is illegal
these days. It was common practice
back then, though.
Applying power
Power was first applied with all
the valves removed. This resulted in
glowing dial globes and a steady power
consumption of about 10W (as expected), so I was optimistic that the set
would work as soon as the valves were
installed. Unfortunately, my optimism
was misplaced, as I quickly found out.
To test the set, I reinstalled the
valves, set the volume control to about
one third, applied power and allowed
time for the valves to warm up. The
power consumption settled down to
56W (AWA quote 60W) but not a sound
could be heard.
Tuning across the dial gave no result
but when I turned the volume up to
full, the set gave a sudden “crackle”
and multiple stations became audible.
It sounded rather like a poor crystal set.
I again tuned the set across the dial
and the result was much the same, although the mix of stations did change.
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This is the fully restored chassis, ready for re-installation into its cabinet.
In addition to the electrical repairs, the dial cord required restringing and a
small tear in the speaker cone was repaired with PVA glue.
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This view shows the underside of the chassis prior to restoration. Note the knot
(now illegal) tied in the twin-flex mains cord that was originally fitted to the set.
The volume control and several faulty electrolytic and paper capacitors had to
be replaced to restore the set to operation, along with various other repairs.
That indicated a fault somewhere in
the mixer-oscillator circuitry.
Checking under the chassis did not
reveal any problems so I moved on
to the above-chassis components. It
didn’t take long to spot the problem
– the lead to one section of the tuning
capacitor had come adrift. This was
quickly resoldered and the radio could
then be correctly tuned to individual
stations.
However, that wasn’t the end of
the set’s problems because the sound
could only be heard when the volume
control was close to its maximum setting. Below this setting, there was complete silence (no hiss at all) and this
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indicated that either the oscillator or
the IF amplifier wasn’t working. This
seemed weird until a look at the circuit diagram revealed that the volume
control was in an unusual location.
In this circuit, the volume control
(R5) is a 4kΩ wirewound pot. It was
easily disassembled by removing the
backplate and a quick check with a
multimeter located an open circuit
in the middle of the resistance wire.
As a result, a replacement pot was
substituted with the expectation that
this would fix the problem but it made
no difference. Even with the good pot
in place, the radio still had only two
modes of operation – either complete
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October 2015 89
The ARTS&P label on the back of the chassis indicates that this Model 92 set
was manufactured in 1946. It’s in quite good condition considering its age.
silence or overload at high volume.
It was time to take a closer look at the
circuit. As shown on Fig.1, the wirewound volume pot varies the voltage
at the cathodes of both the 6A8 mixeroscillator and the 6G8 IF amplifier. As
a result, the pot varies the negative bias
on the control grid of each valve and
thus varies its amplification. What’s
more, because this gain control works
partly in the RF section, it explains the
lack of AGC.
Modifying the volume control
Unlike other volume control circuits, this arrangement means that
the set’s volume increases as the
The July 2015 cover of Radio Waves carried
a photo of a J. P. Aarons radio store in Melbourne. Note the Model 92 egg crate radio in
the window display.
90 Silicon Chip
wiper moves towards earth. In other
words, the volume increases as the
grids become less negatively biased
by the voltage developed across the
cathode resistance. This meant that
even with an open circuit in the middle of the original pot’s winding, the
high volume end still worked because
the wiper was still connected to earth
via the intact section.
The problem was that this radio had
a critical cut-off bias that prevented
the front-end from working at low
volume settings. As a result, I decided
to modify the circuit and install a more
conventional volume control circuit.
After some experimentation, I ended
up permanently installing a 100Ω
resistor in place of the wirewound
pot. This gave a sufficient reduction
in front-end gain to avoid overload
on strong local stations while still
preserving enough gain for weaker stations. A conventional volume control
was then installed by replacing R12
(500kΩ) in the grid circuit of the 6V6
output stage with a pot and connecting
C22 to its wiper.
One surprise discovery was the
existence of a 25µF electrolytic capacitor between the original volume pot’s
wiper and earth. This capacitor isn’t
shown on the circuit diagram (but is
in the parts list) and was apparently
installed as a cathode bypass. As a
result, a new 22µF capacitor was paralleled with the fixed 100Ω bias resistor
I’d installed in place of the pot.
The end result was a reasonably
standard performance from the radio.
However, I got one more surprise when
I checked the voltages around the 6V6
output pentode. Its plate was at 247V,
the screen at 272V and the grid bias, as
set by the cathode resistor, was -14.3V.
This bias is too high for a 6V6 to give
undistorted amplification; instead, it
normally needs -7V to -9V.
Despite this, the audio was quite
clean. So why wasn’t there any evident
distortion?
One possible answer was that the
valve fitted to the set wasn’t really a
6V6. Unfortunately though, this valve
had no identifying markings on it, even
though I had been careful not to rub
any markings off the valves during the
cleaning process.
As it turned out, an experienced
member of the HRSA knew the answer.
AWA immediately post-war may have
substituted a 6F6 for the 6V6, since
the 6F6 was sometimes more readily
available. It’s also possible that a repair
technician with a spare 6F6 made the
substitution at a later time. However,
the clincher that this radio has a 6F6
is that its cathode resistor (R10) has a
value of 325Ω, not 250Ω as specified
in the parts list for a 6V6.
The higher-value resistor generates
the higher bias required for a 6F6, so
that explains the mystery.
More egg crates
There is more to the egg crate story
than the Model 92 because AWA also
housed the R84, R86 509M and 174
models in this cabinet. A publicity
release from AWA dated December
1939 states that the new Radiola
Model 174 is “housed in a strikingly
beautiful cabinet of moulded Radelec
available in a variety of attractive
colours.” Those colours were ebony,
walnut, jade, blue, pink and ivory. This
model also featured a loop aerial that
“obviates the need for both aerial and
earth connections for local reception”.
In summary, the egg crate radios
might be electrically unremarkable
but their distinctive style and variant
colours make them collectable. Provided they are obtained in reasonable
condition, they are generally easy to
work on and repair.
Footnote: a special thank you to HRSA
President Mike Osborne for reviewing
this article. His suggestion of adding
AGC, as in the AWA model 500MY, will
SC
be a future project.
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