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Hagelin BC-38
Motor-driven pin-wheel cipher machine with keyboard

The BC-38 was an electrically-driven mechanical cipher machine designed and built by A.B. Cryptoteknik in Sweden (later: Hagelin). As the name suggests, it was derived from the purely mechanical C-38. The 'B' was added as a prefix to the model number, to show that the device has a keyboard. So, the BC-38 is in fact a C-38 with a motor and a keyboard.

The machine measures approx. 34 x 32 x 12 cm and is therefore significantly larger that its manually operated counterparts. It also needs an external power source. The advantage of having a keyboard and motor-driven operation however, is that it greatly speeds up communication, e.g. in large command centres.

Furthermore, the BC-38 is equipped with a double printer at the left; one for the plain text and one for the cipher text. The output is printed directly onto a narrow paper tape, and both printers can be used simultaneously.
  
Typical view of the BC-38 with the lever down

Although the machine is intended for operation from the mains power network (110 or 220V), it can still be operated manually, which is very useful in case of a power loss. For this, a retractable handle is present on the right. It is usually folded down and locked in a neutral storage position, but can be released easily when necessary.

The BC-38 was popular with the the US Army during WWII, where it was used to communicate with M-209 machines in the field. It was also used by the governments of other countries during and after WWII, such as Sweden and Norway. Production of the BC-38 started in the late 1030s and continued throughout the 1940s, until it was replaced with the slightly modified and improved BC-543. The latter is nearly identical to the BC-38 and is backwards compatible.

The leather grip BC-38 with the top lid present and closed Typical view of the BC-38 with the lever down MODE selector BC-38 with the lid open Interior Printer The cipher discs and the cage

Interior
At the heart of the machine is an assembly of cipher (pin) wheels and a 'cage' with bars and lugs, very similar to the interior of an M-209. The only difference with the latter is that it is mounted the other way around, as shown in the close-up image below.

The six pin-wheel cipher discs are located towards the back of the machine, with the wheels protruding the top lid through a series of slots. A small locking lever to the right of the wheels can be used to disengage the wheels in order to (re)set them.

Each wheel has a different number of letters on its circumfere. In front of the wheels is an axle with a series of differently spaced cogwheels that is used to move the individual wheels during encipherment. The character counter on the right is connected to this axle as well.
  
The cipher discs and the cage

The cage, with the bars and movable lugs, is mounted in front of the cipher wheels, on the main motor-driven shaft. To the right of the cage is a coupling mechanism that allows the machine to be operated without rotating the cage in plain text mode (shown in uncoupled state here).

Mechanism
The BC-38 is a very complex mechanical machine. Although it is driven by an electric motor (see below), the entire enciphering/deciphering is mechanical. Basic operation of the machine is identical its non-electric equivalent, the C-38 and the M-209. On the M-209, the user has to enter a letter by setting a letter wheel at the left side of the machine and then engaging the mechanism manually, whereas the BC-38 allows a letter to be typed directly on a keyboard, whilst the mechanism is moved by an electric motor.

Each letter of the keyboard is converted into a rotation (angle) by means of a large notched axle that is mounted just behind the keyboard. It is clearly visible in the image on the right (top left to bottom right). Each notch is mounted at a different angle that corresponds to the angle of the same letter on an M-209.

When the machine is turned on, the motor starts spinning at a constant speed, whilst the rest of the mechanism is in rest. Pressing a key on the keyboard initiates a series of actions, which are described in detail below.
  
The notched axle, just behind the keyboard, that converts a key-press into an angle on the print wheel.

First of all, when a key is pressed, the rest of the keyboard is blocked, whilst the notched axle, or alphabet shaft, moves to the desired angle. This involves the activation of two mechanisms. One mechanism, located at the left (image #2), releases the wound-up alphabet shaft, whilst the second one blocks the keyboard. The latter is located behind the alphabet shaft, close to the main spindle. It is the part with the large spring, clearly visible in image #3.

When this has happened, the mechanism is coupled to the main motor-driven spindle at the heart of the machine. As a result, the cage makes a full revolution. At the end of this revolution, the result is printed onto a paper-tape (#4 and #5). At the same time, the cipher wheels are moved to the next position, by a rounded notch on the circumfere of the cage (#6).

When the cage makes its revolution, the drive gear at the right hand side (#7) is rotated as well. It winds-up a small spring-loaded mechanism to the right of the alphabet shaft (#8), so that the shaft is ready to encode the next letter. The gear mechanism at the right is also used by the retractable handle at the right of the machine.

The notched axle, just behind the keyboard, that converts a key-press into an angle on the print wheel. Axle release mechanism Keyboard blocking mechanism Printer Printer control mechanism Cipher wheels stepping advance notch Drive gear Alphabet shaft wind-up mechanism

Electric circuit
The electric circuit of the BC-38 is very simple. Electric current is only used to power the motor and not for the encryption process. Only a few additional components are used to connect the motor directly to the mains, as shown in the circuit diagram below. A large wire-wound adjustable resistor in the power rail, allows the machine to be used on both 110V and 220V.


The BC-38 is driven by a so-called universal motor [1] . This is basically a 110V series-wound motor designed to be powered from both DC and AC sources. Such motors have a high torque on startup, but have the nasty side-effect that the speed (RPM) keeps increasing when it has no load. An adjustable wire-wound resistor (Ra), connected in series with the motor, allows the machine to be powered from a 220V mains network. A capacitor (C2) is connected directly to the brushes of the armature of the motor, in order to reduce sparks caused by the commutator.

Speed control
Motor speed is controlled by a so-called centrifugal switch (S2). On startup, the motor is connected directly to the mains. Once it has reached its nominal speed, the centrifugal switch (S2) is opened, which effectively turns off the motor. As a result, the rotational speed will decrease and the contact will close, after which speed increases again. And so on.

Two coils (L1 and L2) and a capacitor (C1) are used to protect the contacts of the switch (S2) and reduce sparks. A varistor (Rv), connected in parallel to the switch, is used to further reduce the extremely high voltages caused when opening and closing the contacts of S2.

Furthermore, two resistors (R1 and R2) are connected in parallel to the speed control circuit, in order to prevent the current from dropping to zero when opening the contacts of S2. They effectively reduce the quality factor (Q) of the resonance circuit (L1, L2, C1).
  
Motor of a Hagelin BC-543

The image above shows the motor of a Hagelin BC-543 which is nearly identical to the one used in the BC-38. The only difference is that the armature is mounted the other way around. The commutator is clearly visible at the centre and the armature winding is connected in series with the field windings. The construction at the front is the centrifugal switch. More images below.

Runaway
As most BC-38 machines are over 70 years old now, you may encounter problems with a leaking capacitor (C1). If this capacitor runs hot after the machine has been on for a few minutes, it is likely to start leaking shortly. If this happens, the centrifugal switch (S2) is shorted and can no longer control the motor's speed, resulting in a so-called runaway. If you suddenly hear the motor speed gradually increasing, this is exactly what is happening. In such cases you should turn the machine off immediately and replace C1 by a modern alternative.

Motor of a Hagelin BC-543 Centrifugal switch Centrifugal switch Commutator The varistor mounted below the centrifugal switch, seen from the rear of the machine, after removing a small panel. Close-up of the varistor mounted below the centrifugal switch, seen from the rear of the machine, after removing a small panel. Bottom side of the BC-38 showing the electric components Position of the motor inside the BC-38, clearly showing the commutator facing towards the front of the machine.

Storage
The BC-38 comes with a metal cover that can be placed over the entire machine, so that it is protected from dust and damage. The machine can be further protected by using an (optional) storage case, such as the one shown in the images below. It has soft padding on the inside.

The mains socket is located at the right rear corner of the machine and consists of 2 pins. The power cable - usually made of rubber - was stored in a hidden compartment inside the top lid of the storage case. If this cable is missing, you may have a hard time finding a suitable replacement.

The correct voltage has to be adjusted with a wire-wound slide resistor, mounted at the left rear of the machine. A black knob on top of the machine can be shifted from left (110V) to right (220V).
  
Mains cable

BC-38 with the top lid present and closed BC-38 storage case A compartment for the mains cable and mantenance tools, hidden in the top lid of the store case. The switch at the rear, and the mains socket just below it. Voltage setting and code wheels Mains cable The Hagelin-side of the mains cable (rubber) A compartment for the mains cable and mantenance tools, hidden in the top lid of the store case.

An eye witness of WWII
The image on the right shows another example of a BC-38. This one is also of WWII vintage and from the images below it becomes clear that this machine has seen some action. Although it has signs of heavy use, the machine is complete and in working condition.

It was used by the American Armed Forces during WWII and was found in Norway shortly after WWII. It came with the lid of a BC-389 (a slightly modified or improved version).
  

Clear view at the keyboard of the BC-38 BC-38 with the lid closed Another view of the BC-38 The printer paper guides Coding wheels of the BC-38 Keyboard of the BC-38 Serial number tag on the machine Serial number tag on the lid

Cryptanalysis
The document below describes the Hagelin M-209 and the C-446A in great detail. Not only is the working principle of the machines explained, it also discusses the machine's cryptanalysis and methods for its attack. The document is in Dutch and was released for publication by the Dutch school for Military Intelligence (DIVI) in 2011.

References
  1. Wikipedia, Universal Motor
    Detailed explanation of the series-wound AC/DC motor.

  2. SMID, C-446A en M-209 Beschrijving en Analyse
    Descryption and analysis of the Hagelin C-446A and M-209 (Dutch).
    Dutch Department of Defence, Military Intelligence School.
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