This is only a preview of the November 2002 issue of Silicon Chip. You can view 29 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. Articles in this series:
Items relevant to "A Windows-Based EPROM Programmer":
Articles in this series:
Items relevant to "SuperCharger For NiCd & NiMH Batteries; Pt.1":
Purchase a printed copy of this issue for $10.00. |
VINTAGE RADIO
By RODNEY CHAMPNESS, VK3UG
The AWA 532MF 32V Table Receiver
In the January 2001 issue, we described the
Operatic Mignon 32V receiver. This month, we
cover another 32V set, AWA’s high performing
532MF, also known as the Hotpoint-Bandmaster
V55DD. This used a line-up of high-gain valves
to give it very good performance on broadcast
and shortwave bands.
The 532MF was produced from
around 1950-1955 and is a 5-valve
receiver. The valves used are 6BA6 RF
stage, 6BE6 converter, 6BA6 455kHz
IF stage, 6AV6 detector, AGC and 1st
audio stage, followed by a 6AQ5 pentode audio output. It covers the normal
AM broadcast band and the popular
international 6-18MHz shortwave
band. Due to the use of these high gain
valves the performance is extremely
good – as you would expect.
The set is virtually a high performing 240VAC receiver that has been
designed to work off 32V DC, drawing
a total current of 1.3A (see Fig.1). The
heaters are wired in series to operate
directly off 32V. As the 6AQ5 draws
450mA of heater current and the other
valves only draw 300mA, the equalis-
As was the case with all Australian sets of that era, the AWA 532F has all the
major broadcast station markings for every state.
82 Silicon Chip
ing resistor R21 ensures that the extra
150mA drawn by the 6AQ5 is shunted
away from the other valves. The 200V
DC of HT (high tension) is provided
by a synchronous vibrator power pack
running directly from the 32V.
The voltage supplied by 32V lighting plants varies considerably, so a
series resistor (R23, 5Ω) is switched
in series with the supply to drop the
voltage applied to the set by around
6.5V when the batteries are on charge.
With 16 fully-charged cells the
nominal voltage supplied is 33.6V
(2.1V per cell). When the batteries
are flat the voltage drops to 28.8V and
when fully charged and gassing the
voltage rises to 40V, hence the dropping resistor R23. Some users of 32V
lighting plants put an additional cell
or two in series with the battery bank,
which makes the supply either 34V or
36V.
Under these circumstances, the
battery voltage could easily rise above
40V on charge – way in excess of 32V.
This was done to overcome the voltage
drop on the power cables from the
batteries to wherever the electricity
was being used. But it gave some of
the appliances (including radios) a
bit of a hard time. Globes burnt really
brightly!
In the May 2000 article, I spoke of
making obsolete 6V and 32V sets useful when AC power became available
in farming communities. One of the
sets I did convert was a 532MF and it
performed well. A secondhand shop
asked me if I’d fix up a set that they
had got in that continually blew fuses.
It was an unmodified 532MF and they
were trying to get it to work on 240V
AC! I said that I would be interested in
doing a swap as I had a converted set
that worked well and I really wanted
an unmodified receiver.
We did a deal – I just hoped that
www.siliconchip.com.au
Looking for an old valve?
or a new valve?
BUYING - SELLING - TRADING
Australasia's biggest selection
Also valve audio & guitar amp. books
SSAE DL size for CATALOGUE
This view inside vibrator power supply box shows the second shielded box. This
two-box construction was used to suppress vibrator hash and noise.
the damage to the set was only to the
fuse. Fortunately the fuse was the only
damage done – but if the fuse had been
larger than the 3A fitted, the set could
easily have been a write-off.
Restoring the 532MF
Removing the set from the cabinet
first involves pulling off of the four
push-on knobs and laying them aside
with the celluloid sheets that have
the control functions marked on them
(that is, if they are still with the set;
mine weren’t). You then remove the
four screws from the back, withdraw
the four bolts under the cabinet and
slide the chassis out.
That done, the set can be turned
upside down as it will largely rest on
the vibrator power supply box. The
set is full of black “moulded mud”
paper capacitors. If they have splits
anywhere on them it is advisable to
replace them. The audio couplers
(C28, C33) and the AGC bypasses (C7,
C9, C18) should be replaced as a matter
ELECTRONIC
VALVE & TUBE
COMPANY
PO Box 487 Drysdale, Victoria 3222.
Tel: (03) 5257 2297; Fax: (03) 5257 1773
Mob: 0417 143 167;
Email: evatco<at>mira.net
Premises at: 76 Bluff Road,
St Leonards, Vic 3223
www.evatco.com.au
ELAN Audio
The Leading Australian
Manufacturer of Professional
Broadcast Audio Equipment
Featured Product of the Month
PC-BAL
PCI Format
Balancing
Board
Interface
PC Sound
Cards to
Professional
Systems
Not only do we make the best range of
Specialised Broadcast "On-Air" Mixers
in Australia. . .
We also make a range of General Audio
Products for use by Radio Broadcasters,
Recording Studios, Institutions etc.
And we sell AKG and Denon Professional
Audio Products
For Technical Details and Professional Pricing Contact
This under chassis view shows the right angle drive to the wave change switch
(the large white arrow points to the mechanism).
www.siliconchip.com.au
Elan Audio 2 Steel Crt
South Guildford WA 6055
Phone 08 9277 3500
08 9478 2266
Fax
email sales<at>elan.com.au
WWW elan.com.au
November 2002 83
on any sets that don’t work on 240VAC
mains. Instead, I fitted a 2-pin polarised plug so that the set can not be
accidentally plugged into the mains
(as the secondhand dealer had done
earlier).
I coupled the set to my 32V DC
power supply to see how it performed.
I made sure that the chassis was attached to the negative terminal of the
supply. Many 32V sets were connected
up with positive to the chassis – and
they don’t work with a negative HT
voltage! Surprisingly the electrolytic capacitors didn’t seem to suffer
provided the sets were not left with
reversed voltage on them for more than
30 seconds or so. Usually customers
who bought a 32V set were told to
connect it to power and if it didn’t
operate within around a minute, to
turn it off and reverse the connections
to the power plug! The set would then
operate.
Mine performed quite satisfactorily
and the valves were all in good order.
However, the HT voltage was down
a bit so a check of the power supply
was called for.
The circuit does not show it but
the set has provision to use it with a
32V turntable so that records could be
played through it. I wonder how many
sets with this provision were actually
used with a record player?
Vibrator power supply
The under chassis layout is quite good for component accessibility. Note that
the original black “moulded mud” paper capacitors have been replaced with
dipped polyester capacitors.
of course, by polyester or greencap
capacitors.
The resistors can be checked to be
sure that they are within tolerance but
I’ve found they’re rarely at fault.
Cleaning up
The set was relatively clean and
only required a dust out and a light
scrub with a Nylon scouring pad,
dampened in kero
sene, to get it in
quite good nick. The cabinet got the
usual scrub in soapy water in the
laundry trough. It is always necessary
to be careful not to wet the speaker
cloth, unless it is being taken out to
84 Silicon Chip
give it a really good clean.
The dial lamps were OK and were
not showing darkening in the envelope, so they were left alone.
Even the dial cord was in quite
reasonable condition. The dial drive
system is not the easiest to re-string
and requires nearly two metres of cord
to do the job. It is desirable to remove
the dial pointer and the dial scale
before endeavouring to re-string the
mechanism. It is just so much easier
to do once the dial scale has been
removed.
I replaced the power cord and the
power plug. I don’t leave 3-pin plugs
Vibrator power supplies are not as
easy to service as AC mains supplies.
For a start, the supply is shielded.
In fact, it consists of a shielded box
with another shielded box inside it, as
shown in one of the photographs. The
shielding is indicated on the circuit
diagram by the dashed lines around
the vibrator portion of the circuit.
To remove the supply for service, it
is necessary to first unsolder the black
(earth), yellow (+32V) and red (HT)
wires which come out of the supply
(noting which tagstrip points they
come from). You then remove the top
cover by removing the self-tapping
screw at the back of the supply and
lifting it off. Inside you will see the
second box, which can now be lifted
out.
The outer box sits on several rubber
grommets and the inner box has foam
rubber glued to its sides, bottom and
top, as resilient mounts. The rubber
mounting is to make sure that the
mechanical vibration of the vibrator
www.siliconchip.com.au
www.siliconchip.com.au
November 2002 85
Fig.1: the AWA 532F was intended to be run from 32V lighting systems on farms. All the valve filaments are run in series across the 32V supply and an
equalising resistor (R21) takes care of the fact that the 6AQ5 filament current is higher than for the other valves.
some vibrator interference in the set
and being a purist, I wanted to eliminate it. However, I’ve not been able to
completely cure this small problem.
Alignment
This is the rear view of the chassis. The large metal box is the vibrator power
supply. Note the 6AQ5 valve located at the end of the chassis. This means that
the set cannot be sat on its end unless a block is put under the side of the power
supply to protect the valve.
is completely muffled. The rubber
mounting is also intended to make
sure that the supply is only earthed
at one spot, to reduce the likelihood
of the receiver picking up interference
from the supply.
Without this elaborate shielding
and the accompanying filtering, the
interference would be so bad that only
the strongest stations would be audible
above the obliterating hash. Sets of this
type are intended to operate in remote
rural areas, so the interference generated by the supply must be completely
suppressed by shielding and filtering,
if possible.
With the inner box removed, it is
then necessary to remove the top and
bottom plates which then exposes all
of the works in the supply. It is desirable to replace all the paper capacitors
and the electrolytic capacitor in the
supply. They may not necessarily be
faulty but they are hard to get at and if
one was faulty, you wouldn’t know it
until you had completely reassembled
the supply into the receiver and tried
it. C48, the buffer capacitor, is important and if it is faulty, the vibrator will
quickly be ruined.
The vibrator (V6732) is a 32V synchronous unit. The 32V rating is purely the rating of the reed drive coil. For
example, a 6V vibrator with the same
86 Silicon Chip
pin-outs can be used with a 32V set,
providing the drive to the reed coil is
reduced to 6V.
With the supply disassembled, I
decided to inspect the vibrator to see
if all was well with it. I unplugged
it from its socket and the lug on the
side of the vibrator near the plug was
unsoldered. Then the circlip holding
the unit inside its case was removed
and the vibrator withdrawn from the
case.
I plugged the vibrator back into its
socket, extended the three leads to the
set and tried the set out. The interference was terrible of course, but I was
looking at the vibrator to see how it
was performing. It seemed to be OK
with minimal sparking at the contacts. I decided to run a small points
file through the points to clean them,
being careful not to bend anything. I
couldn’t increase the HT voltage by
any significant amount so I left things
well alone and reassembled the vibrator. New vibrators are expensive if you
can get them and the voltage wasn’t
down significantly.
I reversed the procedure for dismantling the supply and threaded
the three wires back through the hole
at the bottom of the larger shielded
box. The leads were re-attached and
the set tried out again. There was still
The set is easy to align, with all
adjustments quite acces
sible. The
IF is 455kHz and the two bands are
broadcast and shortwave (6-18MHz).
There are no adjustments for the low
frequency end of the dial for the RF or
antenna coils. This makes alignment
simple but does mean that the performance may be lacking on the low
frequency end of each band. However,
with such high sensitivity it does not
appear to matter. I have tried adding
small ferrite slugs in the antenna
and RF coils and a slight improve
ment in performance is observed
– whether it is worth the trouble to
modify the set in this regard is questionable though.
The alignment procedure is quite
conventional and has been covered
in other articles. Basically, you adjust
the IF trans
formers for maximum
reading (on 455kHz), as measured with
a digital multimeter (DMM) across
R13. Adjust the oscillator coil slugs
near the low end of each band and
the oscillator trimmer near the high
frequency end of each band. The RF
and antenna coils are only adjusted for
peak performance (as shown on the
DMM) towards the high frequency end
of each band.
There is one interesting little quirk
with the physical design of this set.
The wave-change switch has a right
angle drive from the front panel (see
accompanying photograph). I’m not
sure why AWA did this but it does
work quite effectively.
Aesthetics
The AWA 532F receiver is quite
attractive as a large mantel radio or a
medium-sized table set. The cabinets
came in at least two colours: cream
and brown. The control knobs on each
end of the set had a celluloid sheet
with two holes in each which slipped
over each control spindle. The control
functions are printed on the celluloid
in white. Mine are missing. I painted
the control functions on the front of
the set many years ago, but they do
look unprofessional.
I am thinking of typing up some
labels on the computer and then copying them onto a transparent sheet via
www.siliconchip.com.au
Photo Gallery: Stromberg Carlson D70 & 1935 Essanay
Manufactured by Stromberg Carlson in 1939, the D70 is an example of a universal set designed to run from either AC or DC mains
supplies. Because one side of the mains was connected directly to
the chassis, extreme caution had to be exercised when servicing
these sets. Today, they are best operated via an isolation transformer. The set used the following valves, with their heaters wired in
series: EK2G frequency changer, CF2 IF amplifier, CBC1 1st audio/
detector/AVC amplifier, CL3 output, CY2 rectifier and a C1 Barretter. (Photos and information courtesy Historical Radio Society Of
Australia).
a photocopier. Whether the sheet will
be stiff enough I’ve yet to find out. The
labelling will be the wrong colour, but
will look better than my hand-painted
labels of several years ago.
Before reassembling the receiver, I
gave the cabinet a good clean with auto
cut and polish compound. It really
brings up Bakelite cabinets and gets
rid of minor scratches.
A less fortunate 532MF
Quite recently, I saw another 532MF
that had been converted to AC operation. I was rather dismayed at how it
had been done. The dial drive system
had been incorrectly strung, with the
pointer going the opposite way to
convention. Other faults included an
intermittent IF valve; twin-core power
cord joined just out the back of the set
(dangerous); on-off switch not wired
in and the cabinet was missing.
The alignment was out as well and
one IF transformer appeared to be
faulty. I gave it a very quick (aural)
alignment (I was just visiting and had
no tools) and got quite an improve
ment out of it. However, it was dirty
and generally it was a sad set. What a
shame. With a little tender loving care
www.siliconchip.com.au
this could once again be a
first class operational set. I
felt like saying “Can I have
it, please?”, just to give it a
good home.
The Essanay company was established
in South Melbourne in the 1920s, initially
as a manufacturer of radio components.
The company subsequently expanded into
the design and manufacture of domestic
radios in the mid-1930s but apparently
closed down prior to WW2. The receiver
shown here is a “Tombstone” model from
1935 and sold at the time for 17 guineas
(ie, £17-17-0). It covered both medium and
shortwave bands and used (mainly) the
following Philips “P” base series of valves:
AKZ frequency changer, AF3 IF amplifier,
ABC1 1st audio/detector/AVC amplifier,
AL3 output and a 1561 or 80 rectifier.
Summary
The AWA 532MF radio is a straightforward 5-valve dual-wave design of
quite high sensitivity. They are relatively easy to restore, with the vibrator
power supply being the most awkward
part to refurbish. From my experience,
they require more maintenance than
the Operatic equivalent. They are one
of the more pleasing Bakelite sets to
look at and well worthwhile having
in a collection even if no 32V power
SC
source is available to run it.
Silicon Chip Binders
Heavy board covers with mottled
dark green vinyl covering
REAL
VALUE
AT
$12.95
PLUS P
&
P
Each binder holds up to 12 issues
SILICON CHIP logo printed in goldcoloured lettering on spine & cover
Price: $A12.95 plus $A5.50 p&p
each (Australia only; not available
elsewhere). Buy five and get them
postage free.
Just fill in & mail the handy order
form in this issue; or fax (02) 9979
6503; or ring (02) 9979 5644 &
quote your credit card number.
November 2002 87
|