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Vintage Radio
By Rodney Champness, VK3UG
The Philips Twins – the Australian
Model 138 & the Dutch BX221-U
Not quite Philips twins: the Australian model 138 (left) and the Dutch BX221-U (right) look almost the same from the
outside (apart from their dial scales) but are very different on the inside.
Back in October, we took a look at the Dutch BX462A and the
Australian model 115 receivers, two sets that look the same on
the outside but are very different on the inside. This month, we
look at another pair of Philips “twins”, the Dutch BX221-U and
the Australian model 138.
A
S BEFORE, THESE new Philips
twins are very different on the
inside, despite being housed in identical Bakeliute cabinets. And once
again, the Dutch set is a triple-band
receiver, whereas the Australian 138
is a broadcast-band only set.
The Dutch set also has five valves
versus four for the Australian set, and
it can cater for mains voltages from
110-220V whereas the model 138 is
limited to 200-260V. But perhaps the
most important difference is that, unlike the Australian set, the BX221-U
doesn’t use a mains transformer. In
fact, one side of the mains is directly
98 Silicon Chip
connected to chassis, so this “hot-chassis” set can be dangerous to work on
if you don’t know what you are doing.
The Dutch BX221-U and the Australian 138 models are smaller than the
sets described in October. However,
the reason for their identical appearance is the same. The parent company
in Holland developed moulds for
Bakelite cabinets for various receivers
after World War II and several of these
cabinets were also used for sets built
by Philips in Australia.
Usually, the Australian lookalike
didn’t appear until 1-2 years after the
European set. However, the BX221-U
and 138 models featured here both appeared around the same time, in 1953.
The 138 circuit
Take a look now at Fig.1 for the
circuit details of the Australian model
138. It’s a typical 4-valve superhet
mantel receiver intended for suburban
use. The valve line-up is 6AN7 (converter), 6N8 (IF amplifier/detector/
AGC), 6M5 (audio output) and an EZ82
rectifier, the latter similar to a 6V4.
As shown on Fig.1, an external antenna is connected to the junction of
L1 and C1, an input tuned circuit that’s
resonant just below the broadcast
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Fig.1: the Australian model 138 receiver is a 4-valve superhet
design with a 455kHz IF stage. It uses a 6AN7 converter (V1), a
6N8 IF amplifier/detector/AGC stage (V2), a 6M5 audio output
stage and an EZ82 rectifier (V4).
band. As with many receivers of this
vintage, the antennas used were usually quite short (typically 6-7 metres)
and were either run around a picture
rail or along the floor beside the skirting board. As a result, the signal pickup was nowhere near as good as from
an external antenna around 30 metres
long and 10 metres above ground.
Being resonant just below the broadcast band, the input tuned circuit
boosted the pick-up of signals at the
low-frequency end of the tuning range.
By contrast, a small capacitor is used
to boost the performance at the highfrequency end. It’s shown on Fig.1 as a
small hook that’s adjacent to terminal
3 on the antenna coil.
In practice, this capacitor is just a
short length of wire that runs from
terminal 2 and finishes near the top
of tuned winding L2. In addition, the
antenna input signal is inductively
coupled between L1 (the primary of
the antenna coil) and the L2 secondary.
From there, the signal is tuned using
variable capacitor C2 (and trimmer C4)
and fed to the grid of V1, the 6AN7
converter valve. The local oscillator
consists of V1 itself plus inductors L3
& L4 (the oscillator coil), capacitors C3,
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This is the view inside the model 138 receiver. Unlike the BX221-U, it uses a
power transformer which makes it a lot safer to work on.
C5 & C6 and resistor R1. The tuning
range of the receiver is 530-1620kHz.
Among other things, this circuit
produces a 455kHz IF (intermediate
frequency) at V1’s plate, due to mixing
the tuned input signal with the local
oscillator signal (ie, the difference
frequency). This is then fed to the first
IF transformer which consists of two
tuned windings, ie, L5 & C7 and L6 &
December 2012 99
The BX221-U chassis is more complicated than the model 138’s and includes an extra valve plus a ferrite rod antenna
for the long-wave & broadcast bands. There’s no power transformer, so caution is required when working on this set.
The underside of the BX221-U’s chassis is quite crowded although access to
individual parts is generally quite good.
C8. The filtered 455kHz IF signal is
then fed to the signal grid of V2, a 6N8
duo-diode-pentode valve.
V2 amplifies the IF signal and its
plate circuit drives the primary (L8)
of the second 455kHz IF transformer.
The tuned secondary (L7) then feeds
the detector/AGC diode in V2.
100 Silicon Chip
The detected audio signal appears
across resistor R6 and volume control
R7. It’s then taken from R7’s wiper and
fed to the grid of audio amplifier V3
(6M5) via C14 and R11. R11 is typically
included in the grid circuit of audio
output valves, particularly those with
high gain. It has two purposes: (1) to
minimise any parasitic oscillation in
high-gain valves and (2) to attenuate
any IF signal that may be present in
the audio signal.
From my experience, attenuation
of the IF signal is dramatically improved by connecting a 47pF capacitor
between the grid of the audio output
valve and earth. This also improves
the stability of the IF amplifier stage
in many receivers, as it prevents (or
greatly reduces) radiated IF signals
from the audio output stage from getting into the input of the IF amplifier.
The audio signal from R7 is amplified in V3 and then fed to the loudspeaker via an output transformer
(L9). There is no negative feedback in
this receiver since it doesn’t have a lot
of audio gain (there’s only one audio
amplifying stage).
Power supply
The power supply is quite conventional and is based on a power
transformer and a full-wave rectifier
(V4). The power transformer has a
tapped primary winding that allows
connection to voltages between 200V
and 250V AC with mains frequencies
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Fig.2: the Dutch BX221-U 3-band receiver is also a superhet design but has five valves, giving it better performance
than the model 138. Note that this is a “hot-chassis” set, with one side of the mains connecting directly to the circuit
earth (ie, there’s no mains transformer).
between 40Hz and 60Hz. In addition,
there are three secondary windings:
two 6.3V heater windings and a centretapped high-tension (HT) winding.
The HT secondary voltage is rectified by V4 (EZ82). The resulting HT rail
is filtered by C17 for the plate circuit of
V3, while R12 and C18 provide extra
filtering and decoupling for the plate
circuits of V1 and V2. The screen circuits of V1 and V2 also get additional
filtering using R2, R3 and C9. As a
result, this set has good decoupling
between the various stages.
A back bias voltage of -6.5V for the
6M5 is developed across resistor R13,
while extra resistors provide a bias of
-1V to V1 & V2 and around -0.2V to
the detector/AGC diode in V2.
This means that under very weak or
no-signal conditions, the receiver will
be quiet as the detector/AGC diode
will be back biased (ie, -0.2V). However, good radio signals (and noise)
will quickly cause this cut-off bias to
be exceeded and the AGC voltage to
rise. Only around one fifth of the possible AGC voltage developed is used,
so the converter and IF valves do not
have their gain cut back as much as in
a typical 5-valve receiver.
In suburban locations, this results
in a higher output from the detector
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than is typical for 5-valve sets, which
largely compensates for the lower gain
of a single-stage audio amplifier.
The BX221-U circuit
Now let’s take a look at the circuit
for the Dutch BX221-U – see Fig.2. It’s
a very different receiver to the 138,
although externally it looks almost
identical. However, unlike the 138, the
BX221-U has little lever arms behind
each of its front-panel controls.
The lefthand end of the cabinet carries the on-off-volume control and this
has a 2-position tone switch behind
it. Similarly, the righthand end of the
cabinet carries the tuning control and
an associated wave-change switch
lever.
As stated, the BX221-U is a 3-band
design and tunes 150-261kHz (longwave), 517-1622kHz (broadcast band)
and 5.94-18.2MHz (shortwave). Unfortunately, it’s not easy to work out
The model 138’s chassis layout is much simpler than the BX221-U’s layout,
with no complicated band switching.
December 2012 101
Fig.3: these diagrams show how the converter stage (V1) is wired for each of
the three bands on the BX221-U. On shortwave (top), an internal foil-plate
antenna is switched in but this is switched out for the broadcast (centre) and
long-wave (bottom) bands and a ferrite-rod antenna used instead.
the switching arrangement for these
three bands from the circuit diagram
and this must have proved a problem
for many servicemen without access
to comprehensive service data.
Fortunately, the detailed service
manual shows the way each frontend section is wired when each band
is selected – see Fig.3. This makes it
much easier to understand the set’s
operation on each band and makes
servicing much easier.
On the shortwave band, an internal
foil-plate antenna glued to the inside
of the cabinet is used and an external antenna can also be attached. By
contrast, on the other two bands, the
foil-plate antenna is switched out and
a ferrite-rod antenna rod is selected
instead. This ferrite-rod antenna (see
photo) is 250mm in length and is the
longest I have seen. As before, an external wire antenna can also be used with
the long-wave and broadcast bands.
Oscillator arrangements
Separate antenna coils are used for
each band, with the long-wave and
medium wave coils both wound on
the ferrite rod antenna. The oscillator
arrangements are also different.
Only one oscillator coil is used for
the broadcast and long-wave bands and
the circuit is typical for a broadcastband oscillator. However, on the longwave band, C11 and C15 are switched
into circuit to lower the oscillator
frequency and restrict its tuning range.
For long-wave reception, the oscillator
only tunes from 602-713kHz, whereas
on the broadcast band it tunes from
969kHz to 2074kHz.
Converter stage B1, a UCH42 triodehexode, amplifies the incoming tuned
RF signal and mixes it with the oscillator signal to produce a number of
frequencies. These are fed to the first
IF transformer (S15 and S16 on Fig.2)
which is tuned to the 452kHz IF (intermediate frequency) – ie, the difference
between the tuned RF signal and the
oscillator frequency.
The signal from this doubled-tuned
IF transformer is fed to B2, a UF41
remote cut-off RF pentode, where it is
amplified and then fed to the second
IF transformer. From there, the signal
is fed to a detector diode in valve B3,
a UBC41 duo-diode-triode. The audio
output from the detector then goes via
the wave-change switch to B3’s grid
which provides the first stage of audio
amplification.
As well as selecting bands, the
wave-change switch also includes
provision to select an external record
player. However, because this is an
AC/DC set (ie, hot chassis), both sides
of the record player input are isolated
by capacitors (C35 & C36) to prevent
electric shock (ie, these capacitors are
in series with both the active signal
input and the earth side of the signal
input).
Note that the external antenna and
earth inputs are similarly isolated using capacitors.
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Following amplification in B3, the
signal is fed to B4, a UL41 high-gain
power output pentode. This then
drives an audio output transformer
and a 5Ω loudspeaker. A simple
switched negative feedback/tone
control circuit is wired between the
speaker’s voice coil and B3’s grid.
There is also some negative feedback
due to the cathode resistor not having
a filter capacitor wired across it.
Power supply
As stated above, this is a hot-chassis
set, with one side of the mains directly
connected to the circuit earth. There is
no mains transformer, so you’ve got to
really know what you are doing when
servicing such sets. Electrocution can
be fatal so the best advice is, “if in
doubt, don’t touch them”.
The power supply is typical for an
AC/DC set, with the heaters of the
valves all wired in series and drawing
around 0.1A. The voltages across the
heaters are as follows: B1 = 14V, B2 =
12.6V, B3 = 14V, B4 = 45V and B5 =
31V, giving a total of 116.6V.
When powered from 110V AC (or
thereabouts), the heater circuit is wir
ed directly across the mains. However,
for 220V AC, resistors are connected
in series with the heater string.
B5, a UY41, is a half-wave rectifier.
It produces a HT voltage of about 185V
on its cathode when the set is connected to 220V AC mains. For 110V
AC mains, the HT voltage could be
as high as 154V but is more likely to
be around 130V DC. And if the set is
connected to 110V DC mains, the HT
voltage is likely to be just 90V volts, so
its performance will be limited.
In short, while the BX221-U can
work on DC mains, it will perform
much better on AC mains.
No back-bias network
The HT filter consists of the usual
two electrolytic capacitors and a decoupling resistor. There is no back-bias
network. Instead, B3 relies on contact
potential bias at its grid, while B4
has cathode bias due to R17, a 150Ω
resistor. Similarly, valve B1 is biased
via its cathode resistor but B2 has no
standing bias.
Instead, bias for B2 is obtained only
when there is sufficient signal at the
detector/AGC diode to cause it to conduct. The AGC system is a quite simple
whereas other Philips sets usually
have some form of delayed AGC and
siliconchip.com.au
even quiet AGC (QAGC). Because the
set will usually be tuned to a station,
an AGC voltage (and hence bias) will
normally be applied to B2. This may
be satisfactory for domestic radio receivers but it would not be used with
communications receivers.
Comparing the two receivers
As already pointed out, the Australian model 138 is a 4-valve receiver
whereas the Dutch BX221-U is a
5-valve set. As a result, the BX221-U
provides better performance in outlying areas away from radio stations.
Because it’s a 3-band set, the dialscale of the Dutch set is more complex
than the 138. The service manual for
the BX221-U runs to 12 pages, which is
just as well because as with other European Philips sets, it’s quite complex
both electronically and physically.
By contrast, the service data for the
model 138 runs to just three pages.
Restoration
The owner of these sets, John de
Haas, has done some work on these
receivers but there is still more to
do. While the cabinets are generally
in good condition, the bottoms have
been scratched, although some of these
scratches can be removed by polishing.
The wiring in both sets has been
run using plastic-coated wire, so the
insulation is still in good condition.
However, any components that have
deteriorated and/or are critical to safe
and proper operation of the sets have
been replaced.
Despite the age of these sets, alignment of the tuned circuits was not
found to be necessary except for a few
minor tweaks. And while the model
138 would be relatively easy to align,
aligning the front end of the BX221-U
is not a job to be taken lightly.
Summary
It’s interesting to observe the different approaches taken by the European
and Australian manufacturers of these
two Philips sets. The Australian set
is simpler, less costly to build and
easier to service, while the Dutch set
is more complex but is ultimately a
better performer.
In addition, the BX221-U has three
bands which makes it more versatile.
So which one would I like in my collection? The answer is “both”, because
they are twins and we shouldn’t sepaSC
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December 2012 103
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