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Vintage Radio
By Associate Professor Graham Parslow
The 1948 AWA model
517M mantel radio
other foreign material on the cabinet
which further detracted from its appearance.
As can be imagined, the bidding
wasn’t highly competitive and I was
able to obtain the radio at a moderate cost. In spite of its damaged and
dilapidated appearance, I was confident that it could be restored to full
working order.
Circuit details
Designed for the budget end of the market,
the AWA model 517M is a conventional
4-valve radio that was produced just a few
years after the end of WW2. Restoring this
one proved to be quite a challenge.
In 2008, I acquired a 1946 AWA
model 500M (the prelude to the 517M)
which I regard as a classic of simple
design. It’s the same size as the later
model 517M and its components and
layout are almost identical.
The older 500M is a 4-valve mantel
set with two conventional front control knobs for tuning and volume. A
problem with this design is that dial
string drive between the tuning capacitor and the dial indicator is unreliable. In use, the dial indicator has a
tendency to slip out of alignment with
the tuned station.
This was corrected for the model
517M which has concentric control
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knobs at the centre of the dial. In this
model, the outer dial knob is solidly
geared to the tuning capacitor, so that
sweeping the dial through 300° reliably rotates the vanes of the tuning
capacitor through the required 180°.
The centre knob controls the volume.
In addition, the 517M’s case differs
from the 500M’s by having a domed
top and a more elaborate speaker grille
pattern.
The radio featured here was acquired at an auction. It was sold as
damaged and had a long crack in the
top of the cabinet. In addition, a rather
large chunk of the cabinet was missing.
There were also smears of paint and
Fig.1 shows the circuit details of
the AWA Radiola 517M. It’s a conventional superhet with a 6A8 mixeroscillator, a 6G8 IF amplifier/detector/
AGC stage, a 6V6 output pentode and
a 5Y3 which provides full wave rectification to produce the HT rail. The design provides grid bias by connecting
the mains transformer’s HT secondary
centre-tap to earth via a resistor network, a technique which eliminates
the need for bypass capacitors on the
valve cathodes.
One feature omitted from the circuit
diagram is the simple “top-cut” tone
control that’s located at the rear of the
chassis. This consists of a 0.02µF capacitor and 500kΩ potentiometer connected in series between the plate of
the 6V6 and earth.
Restoring the case
The crack in the top of the case had
previously been glued but the bond
had completely failed. It was re-glued,
this time using PVA glue. PVA is not
an intuitive choice for this type of repair but experience has shown that the
bond is more enduring than for other
glues. A possible explanation for this is
that aqueous PVA penetrates the Bakelite filler (often sawdust) more completely and as a bonus, it leaves only
a slight amount of external residue.
PVA adhesive works by tangling
polymer molecules together to link the
materials being bonded. No chemical
change is involved; it simply dries out,
so the gluing is reversible.
The glued crack was subsequently
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Fig.1: the AWA Model 517M is a fairly conventional
4-valve superhet receiver. The 6A8 functions as a mixer/
oscillator (or converter) stage, the 6G8 as an IF amplifier,
detector and AGC stage, the 6V6 as an audio output stage
and the 5Y3 as a full-wave rectifier. The simple tone
control circuit on the plate of the 6V6 audio output valve
has been omitted from this diagram.
covered with grey car filler putty and
sanded back to a smooth surface. I initially made the mistake of using wet
and dry abrasive with water, as I had
wrongly assumed that the grey putty
would provide an impervious barrier.
Unfortunately, water seeped into the
crack and dissolved the PVA glue,
causing the crack to open up again.
As a result, I had to redo this repair.
It was not immediately obvious how
the large section missing from the side
and base of the cabinet was going to be
replaced. The base has two moulded
strips running from front to back that
act as feet under the case. One of these
feet only had the front half remaining,
so something had to be done to provide
a serviceable base.
In the end, the solution was to cut
out a base of 3-ply that would look like
an extra moulded layer under the radio. Contact glue was used to fix this
new base in place, after which 2-part
car-filler (bog) was used to fill the gaps
between the ply and the original feet.
Once the new base was in place,
filling the hole left in the side of the
case was straightforward. A section cut
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SC
The AWA 517M’s cabinet was in poor condition when received, with a large
crack in the top and a large piece missing from the bottom and one side.
from a plastic cylinder was taped to the
inside of the gap and bog applied from
the outside using a spatula. This bog
was roughly crafted to the final shape
but well proud of the wanted profile.
After allowing it to set for 20 minutes, a rasp was then used to further
shape the profile, so that it was very
close to what was wanted. It was then
just a matter of using some spray putty
May 2016 91
At left is another view of the damaged cabinet while the view at right shows the unit with repairs well under way. The
bottom of the case was repaired by gluing a new base made from 3-ply under the existing base, while the hole in the
side was filled using a section cut from a plastic cylinder. Car-filler “bog” was then used to fill the gaps.
and sanding to give the final finish.
Unfortunately, restoration to the
original mahogany Bakelite finish
wasn’t a practical option. However, the
model 517M came in a number of other
attractive colours, so spray-painting
the case was the obvious answer.
I wanted a light colour and previous experience has shown the value
of starting with a light undercoat. The
case was therefore sprayed with white
undercoat inside and out. I was keen
to try vivid yellow and I applied an
experimental coat to the inside of the
case. My wife said “yuck” to yellow
so I reached for a spray can of Heritage Cream and this turned out to be a
good choice.
The rusty-red speaker grille cloth
This tattered manufacturing label was
attached to the inside of the cabinet. It
clearly shows the connection between
the 6A8’s top grid and the tuning gang.
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that came with the radio had faded
strips and had also frayed at the edges.
Fortunately, some speaker grille fabric
I’d purchased from Mack’s Electronics in Rundle St, Adelaide during the
1960s had an appropriate yellow and
brown pattern combination and, in
fact, was similar to some of the patterns used by AWA.
A piece of this cloth was cut to size
and glued to the inside of the case,
giving a flush finish. This gave a better
appearance than the original mounting
method, which involved attaching the
cloth to a cardboard baffle that was riveted to the frame of the 5-inch speaker.
Chassis restoration
The chassis on the old model 517M
was rusted and dusty, so it wasn’t only
the case that had suffered with age.
My first step was to remove the
valves and here I got a surprise. Instead
of a 6V6 output pentode, this radio
had a 6AU4 installed. So was this an
equivalent? The answer is a resounding “no”. It’s not even close because
the 6AU4 is a single plate, half-wave
rectifier that was used in high-current
applications in early TV sets. It had
simply been plugged into the 6V6’s
socket to give the appearance of a full
complement of valves!
Another strange “modification” involved a connection between the 6A8’s
top grid and the antenna post. Fortunately, establishing the correct connection to the tuning capacitor was easy.
It was just a matter of referring to the
tattered layout diagram that had been
glued to the inside of the case.
The top of the chassis was thorough-
ly cleaned and any corrosion rubbed
back with abrasive paper. The rusted
sections were then covered with metallic silver paint. This sacrificed the
original stencilled chassis lettering
and the ARTS&P label. As a result, reproductions were computer-generated
and printed onto acetate transparencies before being attached to the rear
of the chassis.
The earlier model 500M had two
narrow straps of metal running under
the chassis from front to back to reinforce the structure. These also acted
as anchor points for the screws which
fastened the chassis to the case.
By contrast, in the 517M, a pair of
wrap-around end sections are attached
to the chassis to serve the same functions. This more substantial metalwork also provides extra shielding,
the only disadvantage being that a significant number of components cannot
be accessed without removing these
brackets (not too difficult, fortunately).
Component replacement
All but one of the components
looked original, the exception being
the second HT electrolytic filter capacitor. This had obviously been replaced at some stage.
The low-value capacitors were each
sheathed in a one-piece moulded pitch
case. This is arguably superior to the
earlier style end-filled pitch case with
a cardboard cylinder as a cover. Even
so, after many years, the pitch contracts and splits and a number of cracks
were visible in some of the capacitor
cases (and in coil cases).
Because it has a high voltage applied
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across it, capacitor C22 was replaced
as a matter of routine. This capacitor
couples the signal output from the 6G8
to the 6V6 audio output stage.
The model 517M has a rear-mounted
DPDT switch for the mains and this
was used to switch both strands of the
original twin-core mains flex. This is
clearly superior to single-pole switching which could result in 240VAC
mains Active being connected directly
to the transformer (the conventions
for connecting Active and Neutral
to sockets were not introduced until
the 1950s). This DPDT switch was
retained when the original twin-core
mains flex was replaced with a 3-core
mains lead. This new lead was firmly
clamped inside the chassis and allow
ed the chassis to be securely earthed,
in the interests of safety.
The original first HT electrolytic was
mounted above the chassis adjacent to
the power transformer. This was left in
position to maintain the set’s appearance but was disconnected and a new
33µF 450V electrolytic wired into place
under the chassis.
One thing that did puzzle me was
what looked like the end stub of a capacitor connected to earth. A bit of
circuit tracing showed that C3 (0.05µF)
was missing and may even have exploded. The other end of C3 connects
to the aerial coil and a short pig-tail
stub was evident on one of the aerial
coil lugs. A new 0.047µF 630V capacitor was fitted in its place.
This view shows the top of the chassis before restoration. A 6AU4 had been
fitted in place of the 6V6 output valve but it is completely unsuitable for this
role since it is a half-wave rectifier.
Powering up
Now for a smoke test. As a precaution, the set was initially powered up
without the valves and this produced
a steady power consumption of 8.2W
which was about what was expected.
This figure increased to just 13W when
the valves were subsequently installed
and there was no HT.
The problem was easy to diagnose;
the 5Y3 rectifier’s heater had gone
open circuit. Replacing this valve restored the HT and increased the set’s
power consumption to a more reasonable 48W.
The set now worked but there was a
disconcerting high background noise
in the audio due to electromagnetic
interference (EMI). This was quickly
traced to a bank of mains-powered LED
lights in an adjacent room and the solution was to simply switch them off.
However, although the set was working, there were clearly problems. It was
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The underside of the chassis was in better condition than the top but still
required work. Note the crude knot used to restrain the original twin-flex
mains cord (now illegal).
exhibiting erratic changes in volume, a
persistent background noise was still
evident and the loudspeaker rattled
at high volume. I began by checking
the voltages on the 6V6. Its plate was
at 248V, the screen was at 261V and
the grid was excessively negative at
-15.8V.
As a result, the grid voltage was
reduced to -11.7V by adding a 560kΩ
resistor in parallel with R13, the aim
being to avoid operating the 6V6 nonlinearly and thereby reduce the distor-
tion. This raised the set’s power consumption to 51W but it hardly altered
the poor sound quality.
The rattle could be controlled by
putting finger pressure on the back of
the speaker cone. Removing the riveted front baffle immediately revealed
the cause of the problem. The entire
circumference of the speaker cone had
come adrift and was rattling against
the frame when the set was operated.
Reattaching the cone to its frame with
craft glue stopped the rattle.
May 2016 93
Left: an under-chassis view of the
unit after restoration. Despite the
set’s age, only a few parts required
replacement
Below: the fully-restored chassis
after it had been fitted into the
repaired (and repainted) cabinet.
The ARTS&P label was reproduced
on a computer and printed onto an
acetate transparency before being
affixed to the rear of the chassis.
Below: a top-side view of the fullyrestored chassis. An additional
metal shield was later added
adjacent to the 6G8 IF amplifier/
detector valve to reduce the set’s
sensitivity to electromagnetic
interference.
that proved to be the case; the faulty
component was capacitor C14 which
feeds audio to the volume control’s
wiper via series resistor R8. It had a
crack around one end of its case and
prodding almost anywhere in the radio was enough to cause the erratic
volume changes.
In fact, the crack had penetrated
so deeply that the faulty end of the
capacitor broke away as it was being
removed.
Fixing the EMI problem
The erratic sound level problem appeared to come and go when I prodded R11 (16kΩ), C19 (200pF) and C21
(0.001µF). All were replaced but the erratic sound level variations continued.
Further prodding then cast suspicion
on the wiring between the antenna
post, the antenna coil and the oscilla94 Silicon Chip
tor coil. This wiring was replaced but
the set still continued to misbehave.
It was time to be to analyse the
fault a bit more carefully. The power
consumption remained constant during the set’s erratic performance so I
figured that it was probably a component in the audio signal path. And
The set’s sensitivity to EMI was annoying and the cause was poor shielding of the 6G8 IF amplifier valve. In
fact, some valve radios completely fail
to function without shielded IF stages.
Hence, a supplementary shield was
fitted around the 6G8 and soldered to
the side of the chassis. This proved to
be quite successful, as the sound quality was much improved and the set now
turned in quite a good performance.
So that was another vintage radio
rescued from the scrap-heap. Restoring it was quite a challenge but it was
SC
well worthwhile.
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