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Vintage Radio
By Rodney Champness, VK3UG
More Philips Twins – the Dutch 209U
and the Australian 112A
Continuing on with our series of Philips twins, this month we take a
look at the Dutch Philips 209U multi-band receiver and its Australian
“twin”, the model 112A. Under the skin though, these are two very
different receivers.
L
IKE OTHER Dutch/Australian
Philips twins, the 209U and 112A
receivers look the same at first glance
but on closer inspection, are as different as chalk and cheese. In this case
though, there is a slight difference in
cabinet size, so the Dutch parent company obviously produced more than
one variant of this particular cabinet
style with only minor differences
between them.
As before, the cabinet moulds for
the Australian-built receiver were
obtained from the parent company,
probably after the parent company
had finished with them. The Dutch
209U receiver was manufactured from
1946-1947 while its Australian look82 Silicon Chip
alike was produced somewhat later,
from 1948-1949.
Model 112A circuit details
Fig.1 shows the circuit details of
the Australian Model 112A. It’s a
fairly conventional 4-valve superhet
receiver with 455kHz IF stages.
As shown, the antenna signal is fed
to a tuned circuit consisting of L1 and
C1 and resonates at a frequency just
below the broadcast band. This boosts
the sensitivity of the receiver at the
low-frequency end of the band, while
trimmer capacitor C2 boosts the performance at the high-frequency end by
feeding signal into coil L2. The signal
in L1 also inductively couples into L2.
Basically, the tuned antenna circuits
in sets of this era and later were designed to extract the maximum amount
of signal from relatively short antennas. In effect, this was done by partially tuning the antenna using fixed
value components. This technique
significantly improved receiver performance compared to sets using the
antenna-tuned circuits of the 1920s.
As an aside, to get the best performance from crystal sets, additional
tuned circuits for the antenna are
used. These must be capable of tuning the antenna right across the band,
as in the crystal set described in the
April 2007 issue. In addition, highfrequency 2-way radios must also
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Fig.1: the Australian model 112A receiver
is a conventional 4-valve superhet with
455kHz IF stages. Note the tuned antenna
circuit. L1 & C1 boost the sensitivity at
the low-frequency end of the band, while
trimmer capacitor C2 boosts the sensitivity
at the high-frequency end.
have fully tunable antenna circuits
(ie, the antenna must be tuned to the
operating frequency) if they are to
work efficiently.
Getting back to Fig.1, the RF signal
from the antenna circuit is tuned using C3 (one section of the dual tuning
gang). The resulting signal, in the range
from 530-1620kHz, is then applied
to the signal grid of V1, an ECH35
converter.
Note that no provision has been
made to adjust the inductance of L2,
so it cannot be peaked at the lowfrequency end of the tuning range.
However, by adjusting the position
of the dial pointer and adjusting the
oscillator padder capacitors to suit,
some peaking the set’s low-frequency
performance is possible. It’s a fiddly
process though and doesn’t always
achieve perfect results (the article on
alignment in the February 2003 issue
described the techniques necessary for
good results).
At the other end of the band, C2 is
adjusted to peak the performance at
around 1500kHz.
Local oscillator
The local oscillator is based around
V1, coils L3 and L4, the other section of the tuning gang (C4) and their
associated components. The values
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The view inside the model 112A receiver. The chassis is well laid out and
all parts are readily accessible.
of padder capacitors C8 and C9 are
adjusted so that stations appear at
the correct position of the dial at the
low-frequency end of the tuning range,
while C10 is used to do the same thing
at the high-frequency end.
V1’s output appears at its plate
and this is coupled to a double-tuned
455kHz IF (intermediate frequency)
transformer. The resulting 455kHz IF
signal is fed to V2, an EBF35, where it
is further amplified and then applied
to another double-tuned 455kHz IF
transformer. Its output is in turn fed
to a detector diode in V2.
The resulting audio signal from the
detector appears across R8 and volume control potentiometer R9. From
February 2013 83
Fig.2: the Dutch 209U receiver is also a 4-valve superhet design but with performance
equivalent to a 5-valve circuit due to the dual role performed by valve B2 (see text).
It’s a multi-band design and is more complicated than the Australian model 112.
there, the signal is fed via C19 to the
grid of V3, a 6V6GT audio amplifier
stage. This stage in turn drives the
loudspeaker via an output transformer
(L12/L13). No negative feedback is
used in the audio amplifier.
Simple AGC
The IF signal level at V2’s plate is
greater than it is at the detector diode
and this signal is also applied to the
AGC diode via C14. This set has simple
AGC and as soon as there is any signal
(including any interference or other
noise), a small AGC bias appears on
the AGC diode. This is a good design
feature as V2 has no standing bias
and the set will normally be tuned to
a station. However, it would not be
considered good design practice in a
communications receiver.
By contrast, the converter stage
(V2) does have standing bias due to
the voltage across R2. Note that the
converter receives just 20% of the
AGC voltage applied to the IF valve
due to the voltage divider formed by
resistors R1 and R5.
The power supply is designed to
operate off voltages from 220-260VAC
84 Silicon Chip
at 40-60Hz. Australia now has 50Hz
mains but 40Hz was used in Western
Australia for some time and 60Hz is
used in the USA (although it’s doubtful that Philips exported this set to the
USA). However, it was a good selling
point and it’s possible that some of
the home-lighting plants of the era
ran at 60Hz.
Power transformer
The power transformer has two secondaries: (1) 6.3V for the valve heaters
and dial lamps and (2) a 376V centretapped secondary for the HT (high
tension). Note resistor R13 (250Ω)
between the centre tap and the chassis.
The voltage developed across this is
fed to V3’s grid via R10 and R11 and
provides a bias of -9V for this stage.
Finally, resistor R12 decouples
the HT supply at V3’s plate from the
HT line fed to other sections of the
receiver. This is good design practice
as it minimises IF and audio feedback
between the stages.
The Dutch 209U circuit
Now let’s take a look at the circuit
for the Dutch 209U receiver – see Fig.2.
This is a very different circuit to the
one used in the 112A. It’s also a 4-valve
superhet receiver but in this case, the
first stage uses a UCH21 triode heptode (B2). This valve can be used as a
separate pentode and triode and/or as
a converter. In this receiver, it is used
as both and so it has the performance
of a 5-valve set.
Unlike the 112A, which tunes the
broadcast band only, the 209U is a
multi-band receiver. The tuning ranges
cover three bands: 157-400kHz long
wave, 530-1600kHz medium wave
(broadcast band) and 5.88-18.2MHz
shortwave. In addition, this receiver
is designed to work on both AC and
DC mains and as such, can have a live
or “hot” chassis.
Hot-chassis sets are considered by
most vintage radio collectors to be
dangerous to work on, as any carelessness can lead to a severe electric
shock or even death. In fact, they are
often shunned because of this and only
those who know what they are doing
and are aware of the dangers should
work on them
That said, a receiver that uses a
power transformer to isolate the
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mains from the chassis can be quite
dangerous too, as these may have an
HT rail voltage in excess of 500V DC
(ie, across the main electrolytic filter
capacitors) soon after switch-on. This
HT rail is also capable of delivering a
fatal electric shock and so all sets need
to be treated with respect, not just the
hot-chassis AC/DC types.
Valve heater voltages
The valves used in the Dutch 209U
receiver have 20V, 50V or 55V heaters, each rated at 100mA. As a result,
when these are connected in series,
a heater supply rail of 20 + 20 + 55 +
50 = 145V is required. Connecting a
resistor or resistors in series with these
heaters allows the set to be operated
from 220V.
If the set is to operate from 125V, the
heaters are instead switched into two
strings with three heaters in series on
one string and the rectifier heater on
the other. Series resistors are then used
to reduce the voltage drop across each
string to the required 125V.
The necessary switching to do this
is achieved via a plug-and-socket arrangement on the rear apron of the
chassis. By changing the wiring to a
couple of resistors in these strings, it’s
also possible to run the set on either
110V or 200V. The wiring is such that
when used on AC, the HT voltage is
of the order of 150V. When operated
from 220V, the current drain is around
100mA for the heaters plus a further
70mA for the plate and screen loads
of the valves.
Because this is an AC/DC receiver
with a “hot” chassis, the antenna lead
has a capacitor in series with it, so that
no voltage appears on the antenna.
There is no earth on this set; instead,
it relies on the mains to effectively act
as the earth.
Foil plate antenna
In addition to the external antenna,
there is also a foil-plate antenna on the
inside of the back panel of the receiver.
This is also isolated by a capacitor to
make sure no mains voltage appears
on it. This antenna is shown at the
top-left of the circuit diagram and is
connected to the signal grid of the
UCH21 triode-heptode converter valve
(B2) via C120 and C101.
When aligning the receiver, this
plate antenna must be attached and the
back panel fitted in place as it affects
the antenna input tuning adjustments
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This view of the model 112A shows just how few parts there are underneath the
chassis. Note that this photo was taken before the power cord was rewired and
properly clamped into position (the Earth connection was also later improved).
The Dutch 209U’s chassis is more crowded than the model 112A’s and is further
complicated by the band-switching arrangement at lower left. As a result, it’s
the more difficult of the two sets to service.
at the high-frequency end of each tuning range.
As shown in Fig.2, all the antenna
input coils are wired in series and
various sections shorted to earth as
required. At the same time, the secondaries are switched to valve B2’s
signal grid, depending on the selected
frequency band. The oscillator circuits
are also switched as required to the
triode section of the B2 converter
valve.
The mixed signals appear at the
plate of B2 and are fed to a doubletuned 452kHz IF transformer (S51S54). However, one version of the
receiver has an IF of 468kHz.
Next, the resulting 452kHz IF signal
is fed to the signal grid of the heptode
section of valve B3 (UCH21). B3 amplifies this IF signal and it is then fed
through a second double-tuned IF
transformer. It then goes to the detector
and AGC diodes in B5, a UBL21 duodiode-power pentode valve.
The detected audio appears across
resistor R12 and volume control potentiometer R11, with the signal at
R11’s wiper then fed to the grid of the
triode section in valve B3. The amplified signal from B3 is then fed via C83
to the pentode section of audio output
February 2013 85
The Dutch 209U receiver is a hot-chassis design (ie, no power transformer),
so care is required when working on this receiver – see text.
stage B5. B5 in turn drives the speaker
via a transformer.
As with the model 112A, there is
no negative feedback or tone control
circuit in this receiver.
Power supply
The power supply is a typical AC/
DC configuration. As stated above, a
number of valve heaters are wired in
series and a resistor or resistors are
wired in series with this heater string
to bring the total voltage drop up to
the applied mains voltage.
Also as stated, around 145V is
dropped across the heaters, so a further 75V is dropped across the series
resistor for 220V operation.
The rectifier valve (B6) is a UY1N,
which is a half-wave type. This has
C110 (22nF) wired across it to filter
out mains interference and artefacts
generated by the rectifier itself. The
HT rail appears on B6’s cathode and
is filtered by C1 (the first filter capacitor). This HT rail is fed to valve B5’s
plate via the primary winding of the
speaker transformer.
The remainder of the receiver derives its HT supply via a HT filter
made up of R1 and filter capacitor C2.
Back-bias for the amplifying valves is
derived from the voltage across resistor
R75 (below B5). B5 receives around
-8.5V but B1 and B3 receive only a
portion of this due to R34, R35, R11 &
R12. The bias voltage applied to valves
86 Silicon Chip
B1 and B3 before AGC is applied is
around -1.5V.
Delayed AGC
As with many other European
Philips sets, the 209U receiver features
delayed AGC. Both diodes in valve B5
are strapped together for the detector
and AGC functions and no AGC bias
is applied to the front-end of the set
until the signal exceeds 0.5V at the
AGC diode.
In addition, the detector doesn’t
function until the -0.5V bias on the
diodes is overcome. This means that
when tuning between stations, the set
will normally be silent unless there
is quite a bit of external interference.
Once a received signal generates more
than 0.5V at the detector/AGC diodes,
the receiver bursts into life.
However, if the amplified signal
strength is only just greater than 0.5V
at these diodes, considerable distortion will be present in the audio. In
practice, the signal will either be well
under 0.5V at these diodes or well
above it in normal operation, so that’s
not a problem.
Comparing the two receivers
From the circuit descriptions above,
it’s obvious that these are two very
different sets under the skin. Both sets
have four valve envelopes, with three
active stages in the 112A and four in
the 209U. As a result, the Dutch 209U
is the better performing receiver of
the two.
As previously stated, the 112A is
a broadcast-band only set whilst the
209U has three bands. Europe uses
both the long-wave band and the medium-wave band, whereas Australia
ceased using the long-wave band in
the late 1920s. And although Australia
used the shortwave band from time
to time, it has been used much more
often in Europe.
Because the 209U is an AC/DC live
chassis set, the rear panel must be kept
in place because contact with the chassis could prove fatal. It should be safe
if the Neutral lead is the one that’s connected to the chassis but don’t depend
on this. In fact, when I serviced such
sets in times gone by, I always made
sure that the mains plug was wired so
that the chassis was “cold”, ie, with the
Neutral connected to chassis.
By contrast, the 112A has a conventional power transformer but you still
have to be careful of the HT rail.
Both sets can be easily accessed for
routine serving by removing a panel
on the underside of the set. However,
the 112A has a lot fewer parts. The
209U has 35 capacitors and 18 resistors
while the 112A has just 21 capacitors
and 13 resistors.
Restoration
John de Haas, the owner of these
two sets, restored them to working
order mainly by replacing any faulty
capacitors and out-of-tolerance resistors. He also found that the wiring in
the 209U was in better condition than
in the 112A. In fact, quite a bit of the
wiring in the 112A had perished and
this will have to be replaced in the
near future.
The valves used in these sets are
as different as they can be. The valve
bases in the 209U mainly use the
American 8-pin lock-in base, with an
octal base used for the rectifier. By
contrast, the 112A receiver uses all
octal valves.
What I do like about the Dutch
209U receiver is the use of a UCH21
triode heptode for the converter valve
(B2). As stated previously, the triode
and heptode sections can be operated
independently and this makes this
valve quite versatile. When used as a
converter, the triode grid and grid three
of the heptode are strapped together
at the valve socket.
The service manual for the 209U is
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The two sets, side by side, with their distinctive “flip-up” dial scales. Because it’s a hot-chassis design, the Dutch 209U
receiver (right) must never be operated without its rear cover in place.
comprehensive and all service work on
the set is covered in detail. That’s probably just as well, particularly when
it comes to the dial-scale stringing
as this is not otherwise easy to figure
out. On the other hand, the manual for
the 112A is quite sparse but its circuit
is easy to follow and service without
much detail being provided.
The cabinets of both of these sets
cleaned up quite well and they are
prime examples of how good a Bakelite
cabinet can be made to look.
Summary
Both sets is capable of quite good
performance although the 209U has the
edge on the 112A. If you want multi-
band operation, then the 209U is the
only choice but there’s little to choose
between them on the broadcast band.
Finally, the 112A receiver is the
easier of the two to service. It has far
fewer parts than the 209U, doesn’t
have complicated band switching and
has a conventional power supply with
SC
a mains transformer.
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