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Schreibmax
Enigma Printer - wanted item

Schreibmax was a small printer that could be attached to an Enigma machine instead of the light bulbs. It printed the text on a small paper strip. Although the Schreibmax could theoretically be attached to any lamp-based Enigma, it was intended for the Naval Enigma machine (M3 and M4).
 
The Schreibmax consists of two units: MZSE the actual printer that is mounted on top of the Enigma machine, and MZSS, the external power supply unit (PSU). Schreibmax printers are extremely rare and are only occasionally on display in museums. The power supply unit is even rarer and is often missing or broken.

The printer is a small black qubic box, on a metal base plate, that can be mounted on top of a Naval Enigma machine. Behind the top lid of the printer is a 9mm paper reel that exits at the top right of the front panel. The paper is running past a print-wheel and is driven by a capstan in combination with a pressure roller.

Two keys are present at the bottom: one for inserting a plus (+) and one for an empty space.
  
Enigma M4 with Schreibmax

Mounting the Schreibmax on top of the machine is not a straightforward task. First of all, the light bulbs have to be removed as the printer attachment has 26 pin-shaped contacts that take over the function of the bulbs. More importantly: the printer is attached to the machine instead of the wooden top lid. For this reason, the printer can only be mounted on top of a Naval Enigma machine, as it is the only model that allows the wooden top lid to be removed.

In order to use the Schreibmax with the M4 Enigma the 4.5V battery has to be removed and the two pins of the external power socket must be shorted. We have currently no idea how this was done in practice, but we've done it here with a short test wire with two crocodile clips.
 
Marc Simons removing the lamps from the M4 Enigma M4 with Schreibmax Schreibmax printer unit Connector at the right of the printer Shortening the power connector of the M4

 
The printer
The Schreibmax consists of a metal frame with the actual printer mounted on top. The frame physically replaces the top lid of the wooden Enigma case. For this reason, the wooden lid of the Naval machines was removable. Before mounting the printer however, the 26 light bulbs of the Enigma must be removed and the lamp panel must be made permanently available to the printer.
 
This is why the top cover of the Naval Enigma machines consisted of two halves and the parts that normally covers the lamp panel can be removed. After the lamp panel is freed, the printer-frame is attached to the machine instead of the wooden lid by sliding it onto the hinges.

The underside of the metal frame contains 26 pin-shaped contacts that mate with the 26 spring-loaded lamp sockets of the Enigma's lamp panel, when lowering the printer. Once in position, the frame is held in place by two bolts at the front corners of the base plate.
  
Schreibmax printer unit

A rectangular cut-out in the base plate of the Screibmax leaves the cipher wheels accessible, so that the basic setting of the machine can be altered at any time. The printer-frame is slightly wider than the machine itself, sticking out at the right. This is done to accomodate the large 30-pin socket that connects the printer to the external power supply (PSU).
 
Schreibmax printer front view Schreibmax printer unit Schreibmax close-up Schreibmax bottom view Contact pins The wheels are accessible through a cut-out in the base plate of the Schreibmax Removing the lamp panel M4 without the light bulbs
Close-up of the print head Warning on the base plate: Do not rotate the cipher wheels whilst holding down a key. Schreibmax, rear view Schreibmax with open top lid, showing the paper reel Inserting the plus-character (+) Feeding paper Inserting a space after each 5-letter group Checking paper transport

 
Power Supply
The Power Supply Unit (PSU) is a rather complex device which has puzzled people for a long time. In 2010, our good friend Arthur Bauer [1] finally managed to crack the mystery when he was restoring his old Schreibmax and his newly acquired PSU. The PSU supplies power to the printing device but also protects the switches of the Enigma's keyboard against the sparks caused when operating the solenoids inside the printer. He reconstructed the circuit diagram as follows:

Schreibmax circuit diagram

In this circuit diagram, the Enigma has been simplified somewhat. As we are connecting the Schreibmax to the lamp panel, the actual path between the keys and the lamp contacts goes through the ETW, the wheels, the UKW and back. In order to keep the currents as low as possible, a relatively high voltage (115V DC) is used for driving the solenoids. Nevertheless, the solenoids inside the printer (yellow in the diagram) cause enough sparks to cause permanent damage to the switch contacts of the Enigma. An this is where the extra circuits inside the PSU come in.

Each solenoid has a 1K5 resistor (R) connected in parallel and a 1µF capacitor (C) connected in a network. This so-called Snubber network [3] suppresses the voltage transients when operating the solenoids and effectively protects the contacts of the switches by 'killing' the sparks.

As the high-voltage 1µF capacitors used for the Snubber network are relatively large, they could not be placed inside the printer itself and were mounted inside the PSU instead. As a result, each individual key contact has to be wired to the PSU separately. 27 of such snubber circuits are present: 26 for the letters of the alphabet and and extra one for the plus-character (+) that is available on the printer itself. All wiring between the printer (MZSE) and the PSU (MZSS) goes via the large 30-pin connector on the side of the Schreibmax. All wires of this connector are used:

Schreibmax connector when looking into the socket

The drawing above shows the connector at the right of the printer, when looking into the socket from the bottom. Two index holes are present, in order to prevent the connector from being inserted the wrong way around. Please note that this connector carries 220V AC which can be potentially dangerous. The pin-out of this connector is shown in blue in the drawing above.

Schreibmax PSU (MZSS) circuit diagram

As there is not sufficient room inside the printer itself, the capacitors (C) and resistors (R) are placed inside the PSU. This is the only reason why we need so many wires between the printer and the PSU. The image above shows the wiring diagram of the PSU. All resistors (R) are wire-wound 1K5 types and all capacitors (C) are 1µF/1000V. Capacitor (C1) is 375µF/200V.
 
PUS socket at the underside of the Schreibmax M4, Schreibmax and PSU PSU front view Connector at the side of the PSU

 
Investigation
Without the dedicated MZSS power supply, the printer can not be used. In 2011 we had the unique opportunity to examine the only working Schreibmax that we know of, at the Foundation for German Communication in Diemen (Netherlands) [1]. The museum curator, Arthur Bauer, spend many hours getting his unit to work and his efforts have really been paying off.

The image at the top of this page shows his Schreibmax on top of our Enigma M4. The yellow plug sticking out at the right is the connection with the external PSU.
 
References
  1. Arthur Bauer, Foundation for German Communication and Related Technologies
    Schreibmax printer featured on this page courtesy Arthur Bauer.
    December 2011.

  2. Arthur Bauer, MZSS, The power supply of the Schreibmax
    Foundation for German Communication and Related Technologies.
    2 December 2010. Retrieved May 2012.

  3. Wikipedia, Snubber
    Retrieved May 2012.

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