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France WWII SBO
Clandestine 6L6 transmitter
The BCRA Transmitter 1 was a valve-based short wave (SW)
clandestine transmitter,
developed in the 1940s for the
French intelligence service
BCRA
(later: DGSS).
The device was intended for use as part of a
clandestine base station, and can only be operated from the local AC mains.
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The image on the right shows the bare device, which measures just 260
× 185 × 95 mm and weights less than 4.5 kg. As it has no provisions for
powering it from an alternative source, it was probably
meant for use in urban environments, where it was used aside
a regular short wave receiver,
such as the wartime MCR-1
or HRO-5.
The transmitter has a built-in morse key,
but also allows an external key to be connected at the
front panel.
In addition it was possible to
connect a numeric slide pad,
which allows an untrained operator to send
numeric morse code.
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It is currently unknown when the BCRA Transmitter was made, but there are
indications that it was manufactured in the last year of WWII, when the
area around Paris had already been liberated, whilst the eastern part of France
was still occupied by the Germans. The device is largely built with
French parts
(all from the area around Paris), complemented by British and
American parts.
It is also possible that the device was made shortly after WWII,
for the newly established French
Stay-Behind Organisation.
Neither of these theories
can be confirmed, as all components inside the device were already available
in the 1920s and 1930s, and none of them carry a date code (which was
common from the 1950s onwards). Any additional information would be
welcome.
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As the actual designator is currently unknown, we have
nicknamed it 'BCRA Transmitter', referring to the fact that it is
attributed to the BCRA/DGSS,
which was the main
intelligence service of Free France
during WWII.
It was later renamed DGER (1944)
and then SDECE (1946). Nevertheless, the
name BCRA remained in use for many years, well into the
Cold War.
in 1982, the SDECE was succeeded by the DGSE.
➤ More
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The diagram below provides a quick overview of the controls and connections
of the transmitter. At the left side is a two-pin socket for connection of
the mains AC voltage. Note that the correct mains voltage should be selected
with the plug on top of the transformer,
prior to connecting it to the mains. Also at the left is the
cathode-current meter
that is activated by the morse key.
All other controls and connections are at the front panel. At the bottom
left is the power switch, which has two positions: A (arrêt, off) and
M (marche, on). At the top are two screw terminals – (A1) and (A2) – for
connection of the antenna wires, and one (T) for connection to
ground (terra).
An 11-position rotary switch
(ANT) is used for matching the tank coil to the antenna impedance.
The other two knobs are for adjusting the crystal
(OSC) and the tuned anode circuit (PLA, plate).
A suitable crystal must be installed in a
raised valve socket
inside the device, for which the top panel
has to be removed.
Finding the correct settings for the (OSC) and (PLA) knobs is rather
cumbersome, so it is assumed that a table with default settings was
supplied with the crystals. A former user has
written the settings for two crystals
inside the case of the device featured here.
At the right edge is a small built-in morse key.
Although it has a long internal arm and performs reasonably well,
most users would probably have connected an
external morse key
(of their own choice) to the
(MAN) terminals at the left
(manipulateur), especially when sending long messages.
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The actual transmitter measures 260 x 185 x 95 mm and weights less than
4.5 kg. It is housed in a welded metal black enclosure with a removable bottom
panel (screws) and a loose top panel that gives access to the valves and the
crystal socket.
It is powered from the AC mains and does not have any provisions for an
external power supply or power inverter, indicating that it was intended for
use in urban areas. Its layout suggests that it was usually placed to
the left of a receiver, such as a HRO-5
or the MCR-1, both of which were
widely available in liberated France.
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The transmission frequency is determined by means of quartz crystals that
should be installed in the raised socket
inside the enclosure. It can be
accessed by removing the top panel.
The three crystals shown in the image on the right were found with the
transmitter featured here. As the frequency and the preferred setup for
two of these crystals is
written inside the enclosure,
it is likely that they were originally distributed with the transmitter.
They are designed to withstand relatively high currents.
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The image on the right shows a suitable rubber
(reconstructed) power cable for the
transmitter. It has a two-pin female Bulgin plug at one end, that mates with
the mains socket on the left side of the transmitter.
The cable has been fitted with a modern power plug at the other end, so that
it can safely be connected to the mains wall socket in mainland Europe. In
reality it would probably have had banana-plugs, so that it could also be
connected to a lamp fitting adapter as well.
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The transmitter could be used with the
built-in morse key that is located
towards the right edge of the front panel, or with any regular morse
key that was available, by
connecting it to the MAN terminals
at the left edge of the front panel.
In practice, most operators would choose (or bring in)
the key of their preference. The image on the the right shows
Key Assembly No. 9, 1
made by WESTCLOX in Canada. These keys were supplied with
the famous Wireless Set No. 19 (WS19), and were widely available during
WWII.
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The transmitter was supplied with the simple numeric morse encoder shown in
the image on the right. The device measures just 100 x 60 x 9 mm, and was
connected to the
screw terminals marked MAN
(manipulator) on the front panel,
instead of the regular (external) morse key.
The pad serves two purposes: (1) is can be used by an agent who has
no knowledge of the morse alphabet,
and (2) it hides the characteristics of
the 'hand' of a regular morse operator. The latter could often be recognised
by an experienced interceptor, in a similar way as handwriting.
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The keying pad
came with a short 2-wire cable
that connects it to the
MAN terminals
on the transmitter's front panel. The cable has a plug that can be
inserted at the rear end of the keying pad, and a thinner plug that can be
used as a sliding contact, by inserting it into one of the five lanes,
and swiping it from front to back (or vice versa). Note that the lanes for
the numbers 1, 2, 3 and 4 are also used (in reverse direction) to produce
the numbers 9, 8, 7 and 6 respectively.
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The transmitter is housed in a welded metal enclosure that has many
perforated ventilation holes. The interior can be accessed from the top
as well as from the bottom. At approx. one third of the bottom,
is a horizontal frame that holds three
octal valve sockets,
one of which is for the crystal.
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Accessing the interior from the top is simple and only requires the top
panel to be shifted towards the front panel.
This reveals the transformer, the valves, a large cylindrical metal crystal,
and the antenna coil, as shown in the image on the right.
The transformer is mounted towards the rear and extends to the bottom
section of the device. At the top is a
removable shorting plug that is used
to select the appropriate mains voltage. It is suitable for 110, 120, 130,
220 and 250V AC. By default it was set to 220V. Note that it is now set to 250V,
to match the European 230V mains.
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Towards the front – mounted to the rear of the 11-position (ANT) selector –
is the antenna coil,
which consists of two separate concentric set of
windings that are isolated by means of a plastic layer.
The inner coil is part of the tuned circuit that is connected in series
with the anode of the 6L6 valve. The outer coil has many taps that are
connected to the 11-position (ANT) selector.
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The crystal is seated in a raised octal socket,
so that it can easily be removed
and swapped for one with a different frequency. It is housed in a
rather heavy cylindrical metal enclosure that is attached to the bakelite
base at the bottom. The image on the right shows the quartz crystal after the
metal enclosure has carefully been removed.
The bakelite base has 5 pins and can be seated in a regular
octal valve socket. The actual quartz plate is held in a ceramic
frame, with two thick metal contact plates at either side, pressed firmly
together by a spring-loaded metal frame.
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The spring-loaded frame is connected to ground and ensures that the metal
enclosure (shield) is also properly grounded. This type of crystal is clearly
meant to withstand rather high currents.
Also accessible from the top are the 5Z4 rectifier valve
and the 6L6 oscillator/PA valve.
The holes in the side of the enclosure and in the removable top panel,
ensure that they are properly cooled.
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To access the bottom section of the interior, the base panel of the enclosure
has to be removed. This requires four screws (two at either side) to
be removed, as shown in the image on the right.
This section gives access to the solder terminals of the valve sockets
and the mains transformer. It also holds most of the passive components –
such as resistors and capacitors – most of which are manufactured by
V. ALTER in France. At the rear end, is a large cubical 2µF capacitor –
made by Wireless-Thomas in France – that
is used to smoothen the HT voltage from the rectifier valve.
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This 2µF oil-paper capacitor is non-inductive and unipolar,
and is likely to be broken after more than 70 years. This was also the
case with the transmitter featured here, which probably had not been turned
on in the past 30 years. Luckily it is large and accessible, and can
easily be restored.
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The BCRA transmitter featured on this page was found with a morse swipe pad,
that is described in more detail above. The device allows an agent
to send numeric message in morse code, without
knowledge of the morse alphabet.
The image on the right shows the interior of the device. It consists of five
rods that are made of bakelite and brass, held together by two brass brackets
at the ends. One end-unit has a hole for a plug,
whilst a swipe contact can be
moved over the brass pieces.
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The transmitter is built around a single (metal) 6L6 valve that acts as
the oscillator as well as the PA. It is driven by a quartz crystal that is
connected between the grid (g1) and the cathode (k) of the valve.
The morse key is connected between the cathode (k) and ground.
A meter is
present
(on the left side of the device)
to show the cathode current.
The tuned anode circuit consists of two concentric coils, which means that
the antenna is galvanically separated from the HT voltage.
At the bottom right is the internal mains power supply unit (PSU).
It consists of
a conventional transformer that is suitable for a wide range of mains AC
voltages. It produces three secondary voltages: two 6.3V (LT) outputs for the
filaments of the valves, and a double 500V (HT) output for the anode voltage
that is rectified with a 5Z4, to supply the anode voltage (plate) for the 6L6.
Note that the 2A fuse is
hidden inside the voltage selector plug
(S2) on top of the transformer.
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When we received the transmitter featured on this page, it was not in a
very good condition. Although cosmetically it was in a displayable state,
there were many electrical problems that had to be sorted prior to connecting
the device to the mains.
See below for a list of all issues.
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First of all, all valves (tubes) inside the unit were missing, and had been
replaced by cylindrically shaped metal objects that happened to fit into the
valve sockets. Secondly, it came with a same-era power inverter that clearly
does not belong to the transmitter as there is no connection for it.
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In addition, the side and top panels of the case were straightened, a new
tuning lamp was fitted, and a replacement for the missing knurled nut of
the ground terminal (T) was found and fitted. One of the antenna terminals
was freely movable, apparently as a result of a manufacturing error.
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The terminal (A2) was dismounted, repaired and refitted. A 5Z4 rectifier
valve and a metal 6L6 M oscillator valve were obtained from our friends at
Museum Jan Corver
in Budel (Netherlands) and installed in the appropriate sockets, as shown.
Next, the strange metal objects – that had been found in the valve
sockets when the transmitter was obtained – were investigated.
They turned out to be the original quartz crystals, that had been manufactured
to withstand high currents. One is marked in kHz, whilst the other two
are marked in metres. All three are still functioning.
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After careful inspection of all components inside the transmitter, and
temporarily removing the 6L6 valve, it was decided to gracefully power up
the transmitter, in order to give the large 2µF capacitor a chance to reform
itself. Although it seemed to be alright after one hour of slowly increasing
the voltage, it broke down as soon as the 6L6 was re-installed and power was
drawn.
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It was then decided to remove the 2µF capacitor, open it, and
remove the contents,
in order to fit a modern replacement inside.
Modern capacitors are generally much smaller, so there was plenty of room
to fit a couple of them. After adding isolation to prevent short-circuits,
the case was closed again
and refitted inside the transmitter.
This time, after powering up the transmitter, everything worked as expected,
and even the original crystals appeared to be working well. Two of the crystals
are in the 4 MHz range, and only resonate at the 2nd harmonic of ~ 8 MHz.
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By adjusting the (OSC) and (PLA) settings carefully, some crystals can be made
to resonate at the 3rd harmonic (12 MHz), which suggests that the transmitter
was intended for the 6.5 - 12.5 MHz range. Although we haven't measured the
output power, it is likely to be in the 5 - 6 Watt range.
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So far, the following issues have been observed (crossed out when restored):
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All valves missing Strange objects in the valve sockets (crystals) Tuning lamp (glass) broken Bottom panel missing Case panels bended Removable top panel bended Antenna terminal A2 loose Capacitor mounting stub loose (screw missing) Resistor leg broken Mains socket broken (ceramic) → replaced by bakelite variant Knurled nut missing from ground terminal 2µF HT capacitor broken. Wiring of morse keying pad missing
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In addition, the following was done:
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- Mains voltage set to 250V (was 220V) 1
- New mains power cord made from contemporary materials
- Lettering and wiring of morse keying pad restored
- Unrelated power inverter removed
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Necessary to avoid the transformer from saturating on the
230V AC mains voltage in Europe.
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- Shield
- Filament (LT)
- Anode
- Grid 2
- Grid 1
- pin missing
- Filament (LT)'
- Cathode, gate 3
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- Shield
- Filament (LT)
- pin missing
- Anode 2
- pin missing
- Anote 1
- pin missing
- Cathode, filament (LT)'
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- not connected
- Ground
- pin missing
- Crystal (1)
- pin missing
- Ground
- pin missing
- Crystal (2)
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Frequency 6.5 - 12.5 MHz (estimated) Oscillator crystal-operated Output 5 - 6 Watts (estimated) Valves see below Mains 110, 120, 130, 220 and 250V AC (50 Hz) Dimensions 260 × 185 × 95 mm Weight < 4.5 kg
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Three large quartz crystals – housed in a heavy cylindrical metal enclosure –
were found with the transmitter featured here. One is marked in KH, which
probably means kHz. The other two are marked in M, which probably means
metres. All three crystals appear to be working, although only one of them
resonates at the fundamental frequency,
whilst the other two resonate at the 2nd harmonic (f2).
This suggests that the lower frequency limit of the transmitter is
~ 6.5 MHz.
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ID
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Frequency
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Overtone
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Actual
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4213 KH
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4.213 MHz
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×2 = 8.426 MHz
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8.433 MHz
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66 M 03
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4.533 MHz
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×2 = 9.066 MHz
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9.084 MHz
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45 M 17
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6.641 MHz
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×1 = 6.6410 MHz
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6.637 MHz
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From the table is becomes clear that for the first two crystals, the actual
frequency is a few kHz higher than the specified value. This suggest that
the crystal is running in parallel resonance mode, whilst the frequency
engraved on the enclosure is specified for series resonance.
The actual frequency can be tuned down
somewhat by increasing the (tunable) parallel capacity (OSC).
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Part
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Brand
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Country
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Mains socket
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Bulgin
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UK
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Mains switch
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Arrow
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USA
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2µF capacitor
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Wireless-Thomas
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France, Montrouge (south-west of Paris)
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Other capacitors
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V. Alter
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France, Courbevoie (north-west of Paris)
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Resistors
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V. Alter
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France (idem)
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Valves
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RCA
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USA
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Valve sockets
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Chicago
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USA
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Meter
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© Crypto Museum. Created: Sunday 21 July 2019. Last changed: Saturday, 17 December 2022 - 10:23 CET.
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