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VINTAGE RADIO
By RODNEY CHAMPNESS, VK3UG
Philips Philetta & the Titan Tiny
This month, we publish a few more details
on the intriguing Philips Philetta and take
a close look at the Titan Tiny. The latter is a
compact, hot-chassis set and is a real death
trap for the unwary.
The article on the Philips Philetta
in the November 2000 issue sparked
quite a bit of interest. In that article,
I mentioned that I had not seen the
circuit of this intriguing little set and
Ross Paton of Auckland, New Zealand
kindly sent a copy of the circuit and
much of the technical data as originally supplied by Philips. The circuit
diagram covers both the B3D32A and
B3D33A models – the only apparent
difference is the cabinet style.
It is interesting to see how the set
achieved its various functions. As it
turned out, my “guestimations” on
how the set was engineered were quite
close to the mark. It still pays to be
cautious when making assumptions,
though. For example, a number of
radio manufacturers built generic sets
for other companies. However, with a
little experience, even an unlabelled
set can often be identified – whether
by the type of cabinet housing the set
or how the chassis is wired or labelled.
Getting back to the Philetta, Ross reports that this set is typical of the many
small table radios made by Philips
from the late 1950s through into the
1960s. In fact, many were brought to
Australia and New Zealand during this
period by migrants.
According to Ross, the ECC85/6AQ8
RF amplifier valve is not particularly
reliable in this set, as the valve is
run with no high-tension voltage for
lengthy periods when FM is not being
received. This causes the cathode
to become poisoned. If the 6AQ8 is
unavailable, the 6BQ7A/ECC180 is
worth considering as a replacement.
It is pin-for-pin compatible but its
inter-electrode capacitance is different
and this upsets the alignment which
has to be tweaked.
The various functions in the set are
achieved by a large number of switch
contacts and it is not a circuit that
you can find your way around easily.
On the other hand, it showed how a
good-quality European set was put
together.
The Titan Tiny
The Titan Tiny was a basic compact 4-valve receiver designed for the bottom
end of the market. It was housed in a white bakelite cabinet and has just two
controls: a handspan tuning knob and a combined on-off/volume control.
86 Silicon Chip
Now we go from discussing a
well-designed and reasonably sophisticated receiver to a set that was
designed for the bottom end of the
market.
I was visiting Brian Lackie at Urunga
on the north coast of NSW some time
back and he showed me an intriguing
little set called the Titan “Tiny” –
and “tiny” it is. This set has yet to
be restored, as can be seen from the
various photographs. The front view
shows a very plain small white Bake
lite cabinet, with what appears to be
a direct drive tuning control and an
on/off switch/volume control (with its
www.siliconchip.com.au
Fig.1: the Titan Tiny is a 4-valve superhet design with inductance tuning. This is a “hot-chassis” set with one side
of the mains directly connected to chassis (via a fuse) and the other side connected to chassis via a transformer
winding. Hot-chassis sets are real deathtraps, since the metal chassis (and anything connected to it) can operate at
240VAC.
knob missing) underneath. The tuning
control is quite small, so tuning the
set with its “hand span” type dial is
a little tricky.
The rear view of the set reveals
a tightly-packed chassis. However,
there is sufficient room to remove and
replace the valves if necessary. The
rear view also shows that the cabinet
has a series of “slits” (near the power
transformer) – these ensure that there
is enough airflow to keep the temperature inside the set at a reasonable level.
Also shown are the four mounting
screw-holes (one in each corner) that
are used to secure the back of the set.
This particular set is missing its back
cover and that’s an extremely serious
safety issue, as will be explained later.
What intrigued me was the size
of the set and this can be gauged by
comparing the chassis size with a
ballpoint pen (see photo). However,
despite its small size and the number
of components used, the chassis isn’t
too crowded and this makes it rela
tively easy to service.
Circuit details
Fig.1 shows the circuit details of the
Titan Tiny. It’s a conventional superhet
www.siliconchip.com.au
This front view of the Titan Tiny
chassis clearly shows the induct
ance tuning coils at the top.
which uses a 6AN7 converter, a 6N8
IF amplifier and detector, and a 6M5
audio stage. The HT is rectified by the
ubiquitous 6X4.
At a quick glance, the converter
stage appears normal and apparently
uses capacitive tuning. However, close
inspection reveals that this is not
January 2002 87
Photo Gallery: AWA Radiola 80 TRF Receiver
Manufactured by AWA in Sydney in 1930, the Radiola 80 featured a “coffin
top” style cabinet and a matching loudspeaker in a separate cabinet. This
set is a 6-valve TRF receiver and employed the following valve line-up: 3
x type 22 (1st RF amplifier, 2nd RF amplifier & detector), 2 x L410 (1st and
2nd audio) and P410 (audio output). (Photo and information courtesy Historical Radio Society of Australia).
true. T1 and T2 are not tuning capaci
tors but are instead high-frequency
(1600kHz) trimmers for the aerial and
oscillator tuned circuits. However,
the circuit symbols used here are for
a normal tuning gang and would fool
anyone who casually glanced at the
circuit (other Titan circuits use rec
ognised symbols for trimmer capacitors).
Actually, this set is inductance
tuned by sliding iron-dust cores inside
the aerial and oscillator coils. If you
carefully examine the photographs of
the chassis, you can see the dial cord
mechanism that’s used for sliding the
cores in and out.
Dial cord
Behind the tuning knob, the dial
cord is attached to a shaft in much the
same way as in many Astor sets. One
end then comes up to the top of the set
and does a right angle turn around a
dial cord pulley so that it runs parallel
to a horizontal metal plate. As it progresses across the plate, an iron-dust
core is attached to it at a strategic point
and this is then threaded through one
of the coils. The cord then continues
WARNING!
The Titan Tiny set featured in this article is a hot-chassis set, with one
side of the 240VAC mains directly connected to chassis via a fuse and
the other side connected to chassis via a transformer winding. This
means that the metal chassis itself and any parts connected to it
could easily be operating at 240V AC and this applies even if the fuse
(which is actually a dial lamp) blows.
Under no circumstances should any part of a hot-chassis set be touched
while it is plugged into the mains. Hot-chassis sets are extremely dangerous
and should be left strictly alone unless you are very experienced and know
exactly what you are doing.
88 Silicon Chip
around another two pulleys and then
traverses back across the plate and
through the second coil (via another
iron-dust core).
Finally, the cord goes over another
pulley and disappears down to the
other side of the dial drive mechanism.
It is a very simple version of the
Astor tuning system but I don’t believe
it is as good. Some other sets, such as
the Barlow Wadley XCR30, also used
ferrite or iron-dust cores attached to a
dial cord for tuning. However, the Barlow Wadley uses just one core, a cam
switch and three coils in line to tune
the RF stage from 0.5-30MHz with 300°
of control rotation. There is certainly
nothing wrong with inductance tuning
but getting it just right can sometimes
be a problem.
Adjusting the two tuned circuits
does not appear to be a particularly
easy job in the Titan Tiny. Theoretically, it would be possible to shift
a slug along the dial cord or to shift
the actual location of the coil for best
alignment of the circuit. However, it
appears to me that it was a once in
a lifetime alignment job although a
determined restorer should be able to
wring the last little bit of performance
out of the set.
Padder coil
Most vintage radio restorers have
become used to padders being either fixed or adjustable capacitors of
around 400-450pF. There is no such
thing in this set. If you believe the
circuit notations implicitly, it has a
padder coil across the oscillator coil.
So how does this work?
The inductance of the oscillator coil
is less than the aerial coil in most domestic receivers. In this set, however,
it appears that the oscillator and aerial
coils are the same – both electrically
and physically. By paralleling another
inductance, the effective inductance
of the oscillator coil is reduced to a
value equivalent to what other sets
use in this position. It also includes a
conventional 350pF padder capacitor
as well.
The aerial coil has a capacitor to
couple it to the aerial proper. This is a
low value “gimmick” capacitor, which
ensures that the aerial does not load
the aerial tuned circuit to any degree.
This receiver also includes bias and
delayed AGC for the 6AN7 and 6N8
valves. This bias (about 0.8V) and
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The parts were all tightly packed in, as this under-chassis view reveals. The
lamp in the bottom lefthand corner served as the mains fuse – crude and potentially lethal for the inexperienced serviceman!
delayed AGC is provided by a voltage
divider consisting of 50kΩ and 4kΩ
resistors from the oscillator’s grid.
The audio output stage is a little
strange in that the screen of the 6M5
appears to be operating almost in a
starvation mode. Most receivers have
the screen coming directly off the HT
line at the junction of the 2kΩ resistor
and the HT supply to the receiver RF
stages.
A real deathtrap
We now come to the power supply.
At first glance, and without the benefit
of a circuit diagram, the Titan Tiny
appears to be a conventional mains-operated set with a power transformer.
Brian initially thought so and so did
I but to Brian’s dismay, the mains apwww.siliconchip.com.au
peared to have a short to the chassis.
As a result, Brian cut the power lead off
so that a deadly mistake wasn’t made
later on when he was endeavouring to
restore the set.
Some time later, however, the circuit turned up in the AORSM Manual
No.11 (1952) and this showed that the
set is a “hot chassis” type. So how
come it’s got a power transformer in
it?
Yes, it does have a transformer in
it but it is still hot-chassis. In this
set, the transformer only supplies the
valve filaments and dial lamps. The
HT is achieved by wiring the 6X4 as
a half-wave rectifier, with one side of
the mains connected to the plates via
two 350Ω resistors (these limit the
peak rectifier current through the 6X4).
In addition, a .005µF capacitor (mains
rated) is fitted to each plate to suppress
interference on the mains.
The Neutral (hopefully) side of the
mains goes to the chassis via a fuse,
which is actually another dial lamp.
We now return to the missing back
on this set. Without a back, this
set would be lethal if it’s the mains
Active (and not the Neutral) that’s
connected directly via the fuse to the
chassis. In other words, the chassis
and much of the circuitry – including the dial lamps – would all be at
240VAC!
It all depends on which way around
the mains is connected.
But here’s the rub – this set is still
dangerous even if it’s the Neutral
that’s connected to the chassis via
the fuse. As shown on the circuit, the
Active is connected to the chassis
via the power transformer’s primary.
This means that if the fuse blows, the
chassis will be at nearly 240V!
If that happens, you yourself could
act as the fuse and have nearly 240V
placed across your body if you touch
ed the chassis and anything else that
is earthed. What a lovely death trap!
Back in the 1950s and earlier, the
January 2002 89
This rear view shows how the major parts fit into the
space. The dial-cord is connected to tuning slugs which
slide backwards and forwards inside the tuning coils.
safety of the user was not considered
as important as it is now. To make
matters worse, the knob was missing
from the volume control on this set
and the metal shaft was protruding
through the cabinet. Later sets used
recessed metal or plastic shafts so that
the possibility of an electric shock or
worse was eliminated.
I don’t know what sort of back was
originally attached to this set but I
suspect that it was probably a thick
cardboard type with perforations for
ventilation. The circuit shows a line
switch but Brian’s set has no such
line switch. Perhaps it was removed
A vital part is missing from this Titan Tiny and that's the
back. During operation, the chassis could be at 240V AC
and that make it potentially lethal.
at some time in the past, when the
back was lost?
Restoring the set
This little set isn’t up and running
yet. It has obviously been serviced in
the past, as some of the parts have been
re
placed with newer components.
And with the replacement of a few
critical components, there is no reason
why this set should not perform quite
reasonably.
Tuning of the IF stages should not
be a drama but the front-end circuits
may prove a bit of a challenge. The
Titan Tiny was obviously designed
Photo Gallery: Healing Model 24 TRF Receiver
The Healing Model 24 is
a small wooden 3-valve
receiver manufactured
by A. G. Healing in 1934.
The set uses the following
valves: 57 detector, 2A5
output and 80 rectifier.
(Photo and information
courtesy of Historical
Radio Society of
Australia).
90 Silicon Chip
for the lower end of the market, with
a simple box-shaped cabinet, a tiny
dial scale and no outstanding features
to get excited about. But as a kitchen
set in a metropolitan setting, it would
have been quite adequate.
The biggest black mark against it
is the fact that it is a hot-chassis set.
This makes it a real deathtrap for the
inexperienced serviceman – anyone
touching the bare metal chassis or any
other parts could easily be touching
the mains Active, even if the fuse has
blown!
Hot-chassis sets need to be designed
with great care if they are to be safe for
use by the general public. Some manufacturers were quite conscientious
in their endeavours to make their sets
safe but others made apparently little
effort by today’s standards.
Warning label
Would I be pleased to have one
of these sets in my collection? Yes, I
would – not because I think it’s marvellous but because it’s an example of
a very simple little set that achieves
some things in interesting ways.
I would restore it and make it as safe
as I could and plaster a big label on
the back warning that it is a hot-chassis set. And I would run it through a
1:1 ratio isolation transformer as an
additional precaution.
Finally, my thanks to the reader
who pointed out the correct spelling
for “mantel” in Mailbag for November
2001. Yes, mantel radios were deSC
signed to sit on a mantelpiece.
www.siliconchip.com.au
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