Click for homepage
← France
Spy
WWII
  
BCRA transmitter
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, as it can only be operated from the local AC mains.

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.
  
BCRA transmitter

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 appreciated.

  1. 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

BCRA transmitter seen from the left BCRA transmitter Three crystals found with the radio Crystal Morse key connected to the BCRA transmitter Using the swipe contact Operating the internal morse key Close-up of the cathode current meter
A
×
A
1 / 8
BCRA transmitter seen from the left
A
2 / 8
BCRA transmitter
A
3 / 8
Three crystals found with the radio
A
4 / 8
Crystal
A
5 / 8
Morse key connected to the BCRA transmitter
A
6 / 8
Using the swipe contact
A
7 / 8
Operating the internal morse key
A
8 / 8
Close-up of the cathode current meter

Features
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.

BCRA transmitter seen from the left Left side Top panel removed Antenna coil tap selection switch and antenna terminals PA tuning (PLA) (anode) and morse key Oscillator tuning (OSC) BCRA transmitter seen from the right Internal morse key
B
×
B
1 / 8
BCRA transmitter seen from the left
B
2 / 8
Left side
B
3 / 8
Top panel removed
B
4 / 8
Antenna coil tap selection switch and antenna terminals
B
5 / 8
PA tuning (PLA) (anode) and morse key
B
6 / 8
Oscillator tuning (OSC)
B
7 / 8
BCRA transmitter seen from the right
B
8 / 8
Internal morse key

Parts
Transmitter
TX
Quarz crystals Mains power cable Morse key
Key
Numeric morse keying pad
Transmitter
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.

  
BCRA transmitter seen from the left

Crystals
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.

  
Three crystals found with the radio

Mains power cable
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.

  
Mains power cable

Morse key
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.

  
Morse Key No.9 wired for BCRA transmitter

Numeric keying pad
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.
  
Morse keying pad with cable

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.

  1. Morse Key No. 9 kindly donated by Museum Jan Corver [2].

Three crystals found with the radio Crystal 45.17 MHz crystal (open) Morse keying pad Cable between keying pad and transmitter Morse Key No.9 wired for BCRA transmitter Morse key connected to the BCRA transmitter Morse key wired to the MAN-terminals on the transmitter
C
×
C
1 / 8
Three crystals found with the radio
C
2 / 8
Crystal
C
3 / 8
45.17 MHz crystal (open)
C
4 / 8
Morse keying pad
C
5 / 8
Cable between keying pad and transmitter
C
6 / 8
Morse Key No.9 wired for BCRA transmitter
C
7 / 8
Morse key connected to the BCRA transmitter
C
8 / 8
Morse key wired to the MAN-terminals on the transmitter

Interior
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.

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.
  
Interior (top) seen from the left

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.

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.
  
45.17 MHz crystal (open)

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.

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.
  
Interior - bottom

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.

Top lid partly removed, revealing the mains transformer and the quartz crystal Interior - top Interior (top) seen from the left Main 6L6 valve (front), 5Z4 rectifier valve (right) and crystal Removing the crystal Valve and crystal sockets Taps on the mains transformer Voltage selector plug with built-in fuse
Coils with taps (and morse key arm) Written instruction inside case Interior - bottom Interior (bottom) - Mains switch, meter, OSC capacitor and choke Interior (bottom) - meter and choke Interior (bottom) - 2F capacitor and mains power socket Antenna coil 45.17 MHz crystal (open)
D
×
D
1 / 16
Top lid partly removed, revealing the mains transformer and the quartz crystal
D
2 / 16
Interior - top
D
3 / 16
Interior (top) seen from the left
D
4 / 16
Main 6L6 valve (front), 5Z4 rectifier valve (right) and crystal
D
5 / 16
Removing the crystal
D
6 / 16
Valve and crystal sockets
D
7 / 16
Taps on the mains transformer
D
8 / 16
Voltage selector plug with built-in fuse
D
9 / 16
Coils with taps (and morse key arm)
D
10 / 16
Written instruction inside case
D
11 / 16
Interior - bottom
D
12 / 16
Interior (bottom) - Mains switch, meter, OSC capacitor and choke
D
13 / 16
Interior (bottom) - meter and choke
D
14 / 16
Interior (bottom) - 2F capacitor and mains power socket
D
15 / 16
Antenna coil
D
16 / 16
45.17 MHz crystal (open)

Morse keying pad
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.
  
Interior (upper side)

Morse keying pad with cable Using the swipe contact Interior (lower side) Interior (upper side) Soldered at one end Cable between keying pad and transmitter Swipe contact Connection to transmitter
E
×
E
1 / 8
Morse keying pad with cable
E
2 / 8
Using the swipe contact
E
3 / 8
Interior (lower side)
E
4 / 8
Interior (upper side)
E
5 / 8
Soldered at one end
E
6 / 8
Cable between keying pad and transmitter
E
7 / 8
Swipe contact
E
8 / 8
Connection to transmitter

Circuit diagram
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.


Restoration
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.

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.

Before power could be connected to the device, the metal objects in the valve sockets had to be removed, and an appropriate power plug had to be found for the mains socket on the left side.

Sadly, the ceramic mains socket appeared to be ruptured and was beyond repair, so we replaced it with a contemporary bakelite variant from the same manufacturer (Bulgin). A matching power plug was found and a suitable mains power cord was created. Next, the loose mounting stub of a large capacitor was refitted to the side of the case, and a broken resistor leg was repaired.
  
Mains socket broken (ceramic)

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.

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.
  
Main 6L6 valve (front), 5Z4 rectifier valve (right) and crystal

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.

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.
  
Broken 2F capacitor

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.

So far, the following issues have been observed (crossed out when restored):

  • 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
  • Unrelated power inverter
In addition, the following was done:

  • Mains voltage set to 250V (was 220V) 1
  • New mains power cord made from contemporary materials
  • Lettering and wiring of morse keying pad restored
  1. Necessary to avoid the transformer from going into saturation on the 230V AC mains voltage in Europe.

Radio in original state Mains socket broken (ceramic) Capacitor mounting stub broken Broken lamp and missing ground terminal screw Case panel bended Three crystals (fitted in the valve sockets) Bakelite Bulgin mains sockets, seen from the inside. Left side panel (with replaced Bulgin mains socket)
Broken 2F capacitor Fitting a modern alternative Restored capacitor Capacitor and resistor refitted
F
×
F
1 / 12
Radio in original state
F
2 / 12
Mains socket broken (ceramic)
F
3 / 12
Capacitor mounting stub broken
F
4 / 12
Broken lamp and missing ground terminal screw
F
5 / 12
Case panel bended
F
6 / 12
Three crystals (fitted in the valve sockets)
F
7 / 12
Bakelite Bulgin mains sockets, seen from the inside.
F
8 / 12
Left side panel (with replaced Bulgin mains socket)
F
9 / 12
Broken 2F capacitor
F
10 / 12
Fitting a modern alternative
F
11 / 12
Restored capacitor
F
12 / 12
Capacitor and resistor refitted

Connections
6L6 Valve
  1. Shield
  2. Filament (LT)
  3. Anode
  4. Grid 2
  5. Grid 1
  6. pin missing
  7. Filament (LT)'
  8. Cathode, gate 3
5Z4 Valve
  1. Shield
  2. Filament (LT)
  3. pin missing
  4. Anode 2
  5. pin missing
  6. Anote 1
  7. pin missing
  8. Cathode, filament (LT)'
Crystal
  1. not connected
  2. Ground
  3. pin missing
  4. Crystal (1)
  5. pin missing
  6. Ground
  7. pin missing
  8. Crystal (2)
Specifications
  • 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
Valves
Crystals
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.

ID Frequency Overtone Actual
4213 KH 4.213 MHz ×2 = 8.426 MHz 8.433 MHz
66 M 03 4.533 MHz ×2 = 9.066 MHz 9.084 MHz
45 M 17 6.641 MHz ×1 = 6.6410 MHz 6.637 MHz
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).

Components
Part Country Brand
Mains socket Bulgin UK
Mains switch Arrow USA
2µF capacitor Wireless-Thomas France, Montrouge (south-west of Paris)
Other capacitors V. Alter France, Courbevoie (north-west of Paris)
Resistors V. Alter France (idem)
Valves RCA USA
Valve sockets Chicago USA
Meter ? ?
References
  1. Wikipedia, French Resistance
    Retrieved July 2019.

  2. Museum Jan Corver, Morse key and spare parts — THANKS !
    Retrieved August 2019.
Further information
Any links shown in red are currently unavailable. If you like the information on this website, why not make a donation?
Crypto Museum. Created: Sunday 21 July 2019. Last changed: Monday, 09 September 2019 - 21:04 CET.
Click for homepage