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Vintage Radio
By RODNEY CHAMPNESS, VK3UG
The AWA B25/6 Stereogram
Towards the end of the valve era, the local
radio industry began producing stereo versions
of what used to be (mono) radiograms. These
usually had a power output of several watts
per channel and their bass response was often
curtailed to stop acoustic feedback via the
cabinet to the stereo pickup cartridge.
T
HIS AWA STEREOGRAM is similar in concept to most others of the
era. Normal AM radio reception was in
mono, with a standard converter and
one stage of intermediate frequency
(IF) amplification and detection.
From here on, the difference between a mono radiogram and a stereo
radiogram is apparent. The audio system is split into two identical amplifiers feeding speakers at the left and
104 Silicon Chip
righthand ends of the cabinet to give
the stereo effect. Some stereograms
used the normal mono radiogram
cabinet and put the second speaker
into a satellite speaker box. This
meant that a better stereo effect could
be achieved.
The AWA B25/6 is a single-cabinet
stereogram which stands on four
splayed legs. There is a 6 x 9-inch oval
speaker at either end of the cabinet.
The radio and amplifier chassis is in
the lefthand end and the controls are
accessed under the lift-up lid above
the 4-speed record changer.
To the right of the changer is a small
area for storing a few 12-inch records.
The righthand end is largely empty
space with the second oval speaker
situated in it. This was a relatively
simple stereogram, designed to cater for the middle to low end of the
market.
Removing the chassis
We all hope that the removal and
reinstallation of the “works” from a
cabinet will be easy and straightforward. While cleaning the dirt and
muck off the cabinet, I looked carefully
to see how the receiver chassis could
be removed from the cabinet.
It looked like it could be a challenge.
I’d seen the data on how to dismantle
some of the slightly earlier models that
appeared to be the same, as shown on
the technical data sheets. However,
close inspection revealed that the
layout inside the record changer section was quite different to the ones in
the data sheets.
I removed the knobs and could see
that the chassis would drop down
inside the cabinet if two mounting
screws were removed. Before doing
this, I placed the cabinet on its
lefthand end and looked underneath again to see if access was
available from the underside
of the cabinet. Well it was.
I removed three screws and
the cover came off. I looked
more carefully to see why only
three screws were used on the
underside of the cabinet to secure
the cover.
The reason soon became quite
obvious. The cover originally had
been stapled to the underside of the
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Despite the age of the unit, the chassis was in quite good condition. Note the
mounting method for the pots, selector switch and tuning gang.
cabinet and when it had to be serviced
in the past, the serviceman had to lever
the staples out to remove the base.
I could now see the chassis and
observed that if I removed the two retaining screws, the chassis would fall
into the cabinet and probably break
some valves. I then got the bright idea
that if I removed the record changer I
could gain access through the changer
cutout and be able to remove the
screws and support the chassis at the
same time.
It was still a menace to disconnect
the cables, as the connections were
tight on some miniature spade connectors. With some difficulty, I managed to
get the chassis out without damaging
anything. After I had overhauled it,
there was the job of putting the chassis back into the cabinet. I had to get
extra help to do this but I got it back
together.
Thinking there had to be a better
way of doing this, I noticed four screw
heads on the front of the cabinet I
hadn’t seen before – part of a decorative trim. I removed these and the
front baffle with the two speakers on
it came away from the cabinet. This
would make it much easier to remove
the chassis.
Further investigation showed an
even better method: lay the cabinet
on its right hand end and remove the
base panel and the front speaker baffle. Once this is done, the chassis is
reasonably easy to get at.
The moral of the story is to explore
all possibilities of how to remove the
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innards of a set before jumping to the
conclusion that the manufacturers had
a fiendish delight in making it extremely difficult for servicemen or restorers. I
still believe most manufacturers could
have used more lateral thinking and
come up with a much better means to
gain access to the works.
While the chassis and record changer were out of the cabinet, the inside of
the cabinet, the chassis and the record
changer were cleaned with a brush and
later with a kerosene-dampened rag.
Mouse dung had to be cleaned out but
the only damage was some corrosion
on one edge of the chassis. The valves
were taken out and cleaned with soapy
water, taking care not to rub the valve
type numbers off.
B25/6 circuit details
I’m not sure if the set is a B25 or a
B26 as the chassis has no markings
on it to indicate the model. However,
the circuit appears to be similar to the
AWA B20 which is shown as Fig.1 of
this article.
The B25/6 is a 6-valve set and is
quite conventional. The AM tuner section uses a 6BE6 as a converter and a
6N8 as the 455 kHz IF amplifier/AGC
and detector. Each power amplifier
uses one half of a 12AX7 twin triode
and a 6AQ5 pentode as a single-ended
This view shows how the chassis is mounted vertically inside the cabinet (front
speaker panel removed).
January 2006 105
Fig.1: the circuit of the AWA B25/6 is similar to that for the AWA B20 shown above. It’s a
6-valve superhet design with a 455kHz IF stage.
class-A output stage. A 6X4 acts as the
power supply rectifier.
Before I switch a set on, I always
do a number of things to make sure
that it is safe to do so. First, I checked
that the earth pin of the mains plug
was connected via the earth wire to
the chassis, which it was. That done,
I used my 1000V tester to check that
there were no insulation breakdowns
between the mains wiring to the chassis and to the secondary winding of
the power transformer – all was well.
The speaker transformers were then
checked for continuity of the primary
winding and once again they were in
good order.
The next job was to check the resistors and capacitors. The resistors all
tested within tolerance or if not, they
were only a small amount out. The
capacitors were a different matter.
Tested at 500V, I found that all the AEE
capacitors were much too leaky at 1MW
to 5MW resistance and most of the UCC
capacitors were no better.
The other capacitors were all in
good condition. Where their leakage
resistance would have affected the
106 Silicon Chip
circuit’s operation, defective capacitors were replaced.
Close inspection of the chassis also
revealed a blue lead going through
an eyelet on a soldering tag strip.
Why this was done I don’t know but
it means that extreme care is needed
to solder anything to the top lug so
that the insulation on the blue wire
doesn’t melt.
The valve sockets and the gram/
radio selector switch were given a
squirt of Inox cleaner and the control
shafts were oiled. With the valves
installed, it was time to test the set.
With an outside antenna connected
and power applied, the set came up
very nicely with full high tension
voltage and a radio station playing in
the background. Good reception was
obtained right across the band. The
performance was so good that I decided
that the alignment was near enough and
didn’t need any tweaking.
The knobs were all in good condition
except for one with a white pointer,
which has cracks in the plastic. It appears to be a replacement and, unlike
the others, lacks a metal collar.
To strengthen it, I wound three turns
of 24-gauge tinned copper wire around
it in the same position as the collars
on the other knobs. I then twisted the
two ends together, quickly soldered
them and laid the soldered join down
along the knob so that the repaired
knob would fit down its escutcheon
and onto the control. It worked well
and the knob is now much stronger.
The record changer
The BSR Monarch record changer
is one of the simplest around but
for service it is necessary to remove
it from the cabinet. This is done by
removing the audio and power leads,
then twisting the toggles on the two
screws holding the changer in the
cabinet so that they will slip through
the mounting holes.
I’ve found this brand of record
changer very reliable, requiring little
in the way of fault-finding. This unit
was no exception. However, as it is
between 35 and 40 years old, the lubricants had dried out and needed to be
replaced. I mounted the changer onto
my servicing jig and then commenced
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Photo Gallery: AWA Radiola 240 (1934)
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PO Box 487 Drysdale, Vic 3222
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www.evatco.com.au
Released by AWA in 1934, the Radiola 240 is a 7-valve superheterodyne
console that tunes the medium-wave band and two shortwave bands. An
interesting feature of the circuit was the use of two RF amplifier stages ahead
of the mixer, with the first stage switched into use only when the higherfrequency shortwave range was selected. The valve line-up was as follows:
6D6 1st RF amplifier; 6D6 2nd RF amplifier; 6A7 frequency changer, 6D6
IF amplifier (460kHz); 6B7 audio amplifier/detector/AVC rectifier; 42 audio
output; and 80 rectifier. Photo: Historical Radio Society of Australia, Inc.
removing the platter. This is achieved
by removing the circlip in the centre
but first the turntable switch should
be in the off position (to disengage
the idler wheel) and the turntable
should rotate easily in a clockwise
direction. It should then be possible
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to ease the platter off its central shaft
but in this case, it was reluctant to
move upwards.
Fortunately, I had some fine harddrawn steel wire from which I cut
two lengths about 150 mm long. I put
a hook in one end of both pieces and
used them as a puller on the platter,
twisting until it came free.
Like the rest of the mechanism, it
needed lubrication. I oiled the central
shaft and the idler pulley. The motor is
mounted on rubber grommet resilient
mounts. Three circlips were removed
from the mounts and the motor dropped
down under the platform.
Most of these motors can be dismantled by removing two screws. I did this
but found that the bearings were not
as accessible as in some other models.
With some types, you can either lever
a cover off the bearings or gain direct
access to them. Usually alongside the
bronze bearing is a felt wick which
can be filled with oil to lubricate the
bearing for many years. In this case, I
just flooded the bearing and hoped that
enough oil got to the wicks.
Oil is used on bearings and shafts
and grease is used on many of the
sliding surfaces of the changer. The
old congealed grease can be cleaned
off with a kerosene-soaked rag and
general purpose grease applied in its
place. This isn’t an easy task and it
may not always be possible to clean
and replace all of the grease. It is just
January 2006 107
Parts access under the chassis is good and the only faults found were the defective
UCC and AEE capacitors. No valves required replacement.
a matter of doing your best without
dismantling the changer.
There are three adjustments on these
mechanisms but they seldom require
attention. The first is the set-down
position of the stylus onto the record
and this is accessed under the tone-arm
near the pivot (a horizontal screw).
The second adjustment is the height
of the tone-arm lift, accessed from the
top of the tone-arm at the pivot point.
This is adjusted so that the arm lifts
high enough to clear a stack of six
records. Mind you, it is not advisable
to play a stack of six records on the
changer. The weight of the stack on the
spindle can easily damage the record
centre holes, apart from any damage
to the record surfaces from being in
contact. Take my advice and play only
one record at a time, to minimise any
wear and tear.
The third adjustment sets the stylus
tracking weight – normally around
five or six grams for a piezoelectric
cartridge of this type. This involves
This simple jig makes it much easier to service record turntables.
108 Silicon Chip
adjusting a spring on the underside of
the tone-arm and is easily done.
The only other maintenance job is
the replacement of the pick-up styli (78
and LP), as they have a fairly short life
before becoming worn. A likely source
of a suitable stylus for this and other
radiograms is WES Components in
Ashfield, NSW (phone 02 9797 9866).
Purchase a diamond stylus rather than
a sapphire one if possible, as they last
considerably longer.
Summary
The B25/6 is a fairly basic single
unit stereogram. It suited Mr and Mrs
Average’s lounge-room decor of the
era and did a creditable job of reproducing stereo records with pleasing
audio quality. The radio performance
is better than many receivers and the
handspan dial is sufficiently large for
accurate tuning. The set is reliable
except for the use of the troublesome
UCC and AEE capacitors. There was
no other fault found and no valves
required replacement.
Despite its age the receiver still had
its instruction manual. The original
instruction books, licences, repair
dockets and original carton can all
add to the value of a set, historically
as well as monetarily.
I question the use of a 6X4 valve as
the rectifier, as it is rated at a maximum
of 70mA and a single 6AQ5 can draw
around 45mA, although the 6AQ5s
are drawing well under 45mA each in
this set. Even so, I would have used a
6V4 rectifier which is rated at 90mA.
In summary, a nice set and worthy
SC
of a place in any collection.
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