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VINTAGE RADIO
By JOHN HILL
Look Ma, no tuning gang!
Generally, vintage radios have tuning gangs
but that doesn’t apply to all sets. Coming
across a set without a tuning gang often
throws the restorer into a bit of a tizz. That
such sets work quite well is acknowledged
but understanding how the circuits work is
something else.
In the early days of radio, variable
inductance tuning was common until
good single gang variable capacitors
became available. Subsequently, the
introduction of multi-ganged variable
capacitors almost spelt the death knell
of variable inductance tuning. It never
died completely, however, being used
extensively in transmitters and a few
special purpose receivers. It was also
often used to tune aerials to resonance.
Gradually, iron dust and ferrite
cores for radio frequency (RF) coils,
aerial coils, oscillator coils and
intermediate frequency (IF) transformers became more common for
fine adjustment of tuned circuits for
best performance. It was found that a
considerable variation in the inductance of a coil could be achieved by
sliding an iron dust or ferrite core in
and out of a coil. In fact, a tuned cir-
The Astor GPM receiver was housed in a pink plastic cabinet and featured a
large tuning dial. This unit had been knocked around somewhat during its
history and someone had carried out some rough and ready service work on it.
68 Silicon Chip
cuit consisting of a variable inductor
with a fixed capacitor across it could
be easily made to tune the broadcast
band – and other bands as well.
The Astor GPM and BNQ
In the early 1950s, Radio Corporation started to bring out inductance
tuned radios under the brandnames
of Astor, Airchief and several other
labels. The inductance tuning system
really suited car radios as dust tended
to block the ganged tuning capacitors
previously used.
On the domestic front, the Astor GPM and BNQ mains-powered
models were produced in 1955 and
1956 respectively. The circuit shown
is for the GPM (Fig.1) and I have a
BNQ – but no circuit for it. However,
the circuits are almost identical, the
differences being that the BNQ used
a 6X4 rectifier in lieu of the 6V4 and
the cathode resistor on the 6BH5 is
47Ω. The BNQ model also has a tone
control.
One of the unusual features of these
sets is the variable inductance tuning.
Radio Corporation was one of very few
manufacturers that used inductance
tuning in 240V domestic receivers. A
number of manufacturers used it in
their car radio ranges, however.
The circuitry around the 6BE6 in
the schematic diagram can be seen to
be unusual compared to what is considered to be the norm. Instead of two
windings on each coil or a tapping,
both coils consist of a single winding.
Fig.2 shows the aerial input circuit
redrawn to make it a little easier to
understand. Amateur radio operators
will be quite familiar with this circuit
as it is called a pi-coupler. It is commonly used to match the impedance
of the transmitter output valve to the
aerial circuit and to tune the stage.
Fig.1: the circuit for the
Astor GPM radio receiver.
An unusual feature is the
use of variable inductance
tuning.
In the average set, the aerial connects to a coupling coil of about 400Ω
impedance and wound on the same
former as the tuned winding. This impedance is the RF “resistance” of the
“average” aerial within the broadcast
band. The coil is inductively coupled
to the tuned winding. The signal voltage in the tuned winding is increased
due to transformer action and the Q
of the tuned circuit.
In the pi-coupler tuned circuit, the
low impedance input from the aerial
is matched by the 650pF capacitor
(C63), while the high-impedance
input to the grid of the 6BE6 is
matched by the combination of C64,
C13 and C14. This tuned circuit also
increases the signal voltage applied
to the grid of the 6BE6 by the action
of the circuit’s Q and by the ratio of
the values of the capacitors at each
end of the coil.
So it does exactly the same job as
a circuit with a fixed inductance (or
fixed inductances) and a variable
capacitor. The advantage is that only
one winding is used.
In addition, the circuit in Fig.2 also
acts as a low-pass filter. This means
that it lets all frequencies below the
design frequency through but pro-
gressively blocks higher frequencies.
This is a handy feature for broadcast
receivers, as it helps to reduce the
image response without the need for
an RF stage.
For example, if the set is tuned to
693kHz, the image for this set (which
has a 455kHz IF) is 1603kHz. In this
set, I measured the image response as
being 35dB down on the wanted signal. This means that a 2mV 1603kHz
signal would be required at the aerial
to have the same effect in the set as a
30µV signal at 693kHz.
Oscillator circuit
The oscillator circuit is also con
figured as a pi-coupler but is really
being used as a Colpitts oscillator. The
6BE6 is normally used as a Hartley
Fig.2: this diagram shows the aerial
input circuit of the GPM receiver,
redrawn to make it easier to under-stand.
oscillator, whereby the tuning coil
is tapped and the cathode goes to
that tap. However, in this case there
is no physical tap, as can be seen
on the circuit diagram. Instead, it
is capacitively tapped by capacitors
C10, C62 and C65, with the tapping
point towards the end going to pin 6
of the 6BE6. The oscillator feedback
ratio is controlled by the ratio of the
values of C10:(C62 + C65) and remains
constant across the broadcast band.
As a result, this type of oscillator is
more reliable than some others used
in vintage radios.
As an aside, some sets which use
6A7 converter valves and the like are
rather unreliable and may drop out of
oscillation on the lower frequencies.
This is often due to the actual circuit
used for the oscillator, where the actual amount of feedback varies significantly across the band. I’ll talk about
this in a later article and describe how
it can be largely overcome.
The circuit shown in Fig.1 uses no
padder. So how did the manufacturer
obtain good tracking across the broadcast band, with the resonant frequencies of the aerial and oscillator coils
remaining 455kHz apart at all times?
Elementary my dear Watson!
June 1998 69
The oscillator coil is wound as a
solenoid, with each turn right alongside the other. The aerial coil, on
the other hand, has its turns wound
side-by-side to begin with and then
they are variably spaced over the rest
of the winding. This can be seen on
the photograph of the rear of the unit,
which clearly shows the tuning mechanism (the aerial coil is the smaller
diameter coil).
This is a simple way to do it but no
doubt it initially took some experimentation to get the tracking right. A
specially made cam would then have
been fitted to the winding equipment
so that it could easily wind this coil.
There were no computers then to
make the job easier.
Philips on occasions made inductance tuning mechanisms too and
two views of a typical Philips unit
can be seen in the photographs. It is
much smaller than the Astor unit and
is shielded, being built into one of
their small IF can-sized assemblies.
The Philips unit is also gear-driven
compared to the metal belt drive on
the Astor. A quite compact set could
be made with a Philips variable inductance tuner.
Restoring the BNQ receiver
SILICON
CHIP
This advertisment
is out of date and
has been removed
to prevent
confusion.
70 Silicon Chip
I got my BNQ at an auction for a
nominal price of $3. The cabinet, like
many plastic-cased sets, had faded
from its original pink where it had
been exposed to sunlight, so that it
now looks a bit mottled. It was a bit
knocked around and someone had
carried some rough and ready service
work on it at some stage during its
history.
Care is needed when removing
this set from its case. The front of the
case consists of three plastic sections
which are held together by plastic
spigots. These go through holes in the
mating section and are then melted
into one another to hold the sections
together. However, this is a very weak
system and the front plastic escutcheon plate will break away from the
main front section of the case with
very little pressure.
To make matters worse, the dial
lamps are attached to the escutcheon
plate with short wires and it is very
easy to withdraw the chassis and rip
the escutcheon out at the same time.
I extended the leads going to the dial
lamps to overcome this problem.
Another problem occurs if the chas-
sis-mounting clamps are not tightened
correctly when the set is reinstalled
in the cabinet. Unfortunately, it isn’t
easy to tighten these clamps as it’s not
possible to bear straight down on the
screws which are slightly in from the
back edge of the cabinet front. If the
clamps are loose and you push the
knobs on, the chassis slides back and
again causes the escutcheon to break
away from the front section of the
cabinet. It’s great fun having to repair
the cabinet for the second time. In
this case, it was broken before I even
worked on the set, so others had had
problems too.
Mr Radio Corporation certainly
didn’t get this part of the receiver’s
design right!
Circuit problems
Now onto the electronic restoration.
There were quite a few minor problems with the set which stopped it
from working.
First, I discovered that a chassis-mounted electrolytic capacitor
had lost its capacitance and someone had simply connected another
capacitor across it. This is not really
a good move as the faulty unit may
have gone short circuit later on. In this
case, both capacitors had lost almost
all their capacitance, as indicated on
a capacitance meter.
Further investigations revealed
that several resistors had either gone
high or open circuit and so these
were replaced. I usually check the
resistors in circuit with a multimeter
and make allowance for any parallel
resistances in my assessment. If there
is any doubt, one end of the resistor is
unsoldered so that it can be checked
by itself, with any effect from other
parallel components.
Paper capacitors in old receivers
are quite often leaky, sometimes with
a leakage resistance as low as a megohm. I use a high voltage tester across
the paper capacitors to see how much
leakage there is. Murphy has a great
time with paper capacitors! The leakiest ones always seem to be in positions
where no discernible leakage can be
tolerated, such as the audio coupler
between the plate of the first audio
stage and the grid of the audio output,
or the AGC/AVC bypass.
These nominated capacitors were
replaced because they were quite
leaky, along with several others. The
cathode bypass on the 6BH5 was left
These two photos show the top and bottom views of a typical Philips inductance tuning
mechanism. This unit is much smaller than the unit fitted to the Astor receiver.
in position, as it would have to be
very leaky to cause a problem. As a
general rule, it’s a good idea to replace
all paper capacitors with polyester or
similar types where leakages under
about 100MΩ can cause a noticeable
alteration in the operating conditions of the valves in the set. Paper
capacitors become more leaky as the
temperature of the set rises.
I “rescued” a bagful of paper capacitors from defunct TVs many years ago
and decided to test them at about 50°C
in the kitchen oven. Before going into
the oven, they tested OK on a multimeter but after heating, the multimeter
showed almost all of them to be leaky.
As a result, they were all consigned
to the rubbish bin. 50°C is not an
unreasonable temperature, as sets
that are running can easily develop a
temperature this high or higher inside
them. On the other hand, polyester
and polystyrene capacitors came out
of this test smelling of roses.
Radio Corporation had a habit of using a combination of single conductor
rubber-insulated hookup wire as well
as plastic-covered wire in their sets. I
don’t know why they did that as the
rubber-covered wiring often has to be
replaced – the rubber goes hard and
cracks off or goes gooey and behaves a
bit like a resistor rather than an insulator. Perhaps plastic-covered wire was
more expensive than rubber-covered
wire in the early 1950s.
Anyway quite a bit of the wiring
in critical areas had to be replaced. If
there are any doubts about the safety
of perished rubber wiring, it should
always be replaced.
Switching on
Before applying power to the set,
the insulation of the power transformer to the set chassis was checked with
the high voltage tester. I also checked
to make sure no shorts existed from
high tension to chassis and tested the
speaker transformer to make sure its
primary winding was OK. In this case,
the speaker transformer was OK although this component had obviously
been replaced at some stage in the
past, probably because the primary
had gone open circuit.
Once these checks had been completed, the set was plugged in, power
applied and the high tension (HT)
voltage checked using a multimeter.
This looked OK and so voltages elsewhere in the set were measured to see
if they were as expected. Most were
but one wasn’t, so a bit more sleuthing
was needed.
At this stage, the set was actually
working but seemed very low on
output and was not very sensitive. I
checked the voltages around the set
and found that the 6BH5 wasn’t drawing any current. The reason for this
was that there was no screen voltage,
due in turn to the fact that one end of
the resistor from the HT to the screen
had become detached. I resoldered it
and that fixed the problem.
Alignment
The next job was alignment. The
IF slugs seemed to be jammed so I
couldn’t do anything with the IF.
However, checking the IF by tuning
the signal generator between 400kHz
and 500kHz confirmed that there was
only one response peak and that was
at 455kHz. As the sensitivity of the
set was good, it was assumed the IF
was correctly aligned. I had no option
anyway!
Because it has an inductance-tuned
front end, you may wonder how the
alignment technique compares to a
normal variable-capacity version.
Well, the circuit is certainly different
but in fact the alignment procedure is
just the same as with the more familiar variable-capacitor tuned receiver
front ends.
The first thing was to check that the
oscillator tuning range was correct.
It tuned from 530-1700kHz and the
station calibrations were quite close
to what they should have been so all
was OK here. If the range had not been
correct, it may have been necessary
to adjust the oscillator iron-dust slug
(the one in the larger diameter coil;
see photograph) so that the set tuned
down to 530kHz. Conversely, at the
top end of the dial, the oscillator trimmer may have required adjustment
so that the signal generator could be
heard on 1700kHz.
A check at both ends of the dial
will show whether the stations appear where they should on the dial.
If they don’t line up, the procedure is
to first tune to a station at the bottom
end of the band; eg, where 3AR is
June 1998 71
This inside view of the Astor GPM mantel radio receiver clearly shows the
variable inductance tuning mechanism. The aerial coil is the smaller coil
(towards the rear of the unit) with the variably spaced winding.
marked (this was on 620kHz but is
now on 621kHz and renamed 3RN).
You then feed in a 620kHz signal
from the signal generator and adjust
the oscillator slug so that the signal
generator is heard.
This done, you go to the other end
of the dial and tune to the 3AK mark
which corresponds to 1500kHz. The
signal generator is then set to 1500kHz
and the oscillator trimmer adjusted
for maximum signal through the set.
Having got the oscillator tuning
correct, all that remains is to tune to
a station at about 600-700kHz and
adjust the aerial coil iron-dust slug
for best performance. You can either
monitor the output by ear on a weak
station using a typical aerial or using
instruments on a medium to strong
station. This done, you then adjust the
aerial trimmer for best performance
on a frequency between 1400kHz and
1500kHz.
Note that it may be necessary to
repeat these adjustments as they do
interact. Finally, seal the adjustments
72 Silicon Chip
with some nail polish or beeswax. I
have found that the inductance tuners
hold their initial adjustment quite
well and only rarely require more than
a minor tweak to get the best out of
them, as in this case.
Performance
So what is the set like to work on
and what about its build quality and
overall performance? The set is a
good performer, although there is a
tendency for some RF instability at the
low-frequency end of the dial. I get the
impression that the set was intended
for the lower end of the market – the
case certainly attests to that.
The works are built on a flat sheet
of metal with brackets to mount the
controls and the speaker, so it could
almost be said to have no chassis. The
chassis plate is situated half way up
the inside of the case, so there is a
lot of vacant space under the chassis,
although this may not be obvious from
the photograph of the back of the set.
The photograph of the front of the
set shows that it has a large semicircular dial scale, marked with virtually
all the stations that were available at
the time. On the other hand, a rather
small knob is used for tuning which
makes the job a little fiddly.
Summary
The cabinet is poorly designed as
previously mentioned and in my set,
it has also warped. As a result, the
plastic lugs at the top of the cabinet
don’t grip and the two sections can
easily come apart. The use of rubber-insulated wire when they were
also using good plastic-insulated wire
in the same set is a backward step and
the small direct-action tuning knob
doesn’t say much for the designer.
On the plus side, the performance of
the set is quite good and in general the
access is quite reasonable. I’m hard to
please in this area but so many sets are
spoilt just for a little more thought in
operational ease (ergonomics), layout
and accessibility.
I believe that Radio Corporation
built many superb sets but seemed to
lose the plot in some areas from time
to time. However, I am happy to have
SC
this radio in my collection.
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