The Colossus Mk2 Rebuild

Description and maintenance instructions.

Colossus is made up of two large racks, a paper tape reader called the “Bedstead” and an electric typewriter on a stand. In the centre of the front rack is a sloping plug panel used to set wheel patterns.

The intercepted cipher text, punched onto paper tape in the Baudot teleprinter code, is loaded onto the Bedstead and read optically at 5,000 characters per second. This is the data going into Colossus, known as Z.

The Z data is then operated on by the electronic valve (tube) circuits on the racks, ending up as counts in the electronic counter circuits on the C rack on the left hand end of the front rack.

When the end of cipher text is reached in the Bedstead, a special signal causes the contents of the electronic counters to be dumped onto relays in the relay rack. These counts can then be sent to the electric typewriter during the next reading of the cipher text on the Bedstead.

In order to emulate the cipher action of the German Lorenz SZ42 cipher machine, Colossus also generates bit streams

corresponding to the SZ42 wheel patterns set by the German operator and used to encipher the intercepted text now

loaded onto the Bedstead.

These patterns are plugged up and generated in the W rack in the centre of the rear rack of Colossus. The stepping round

of these patterns is caused by the sprocket holes being read optically from the paper tape containing the cipher text and

loaded onto the Bedstead.

Two sets of patterns are generated, one set of five are the X wheel patterns. The other set of five are the S wheel patterns.

There are also two M (Motor wheel) patterns and the action of these will be described later.