Rebuilding the Tunny Machine

Back in May, I wrote about a project to reconstruct a Tunny machine at Bletchley Park in the UK, home of the National Museum of Computing.  Bletchley Park was also the home, during World War II, of the Government Code and Cipher School, also called Station X, which was the center of British code-breaking efforts against the Axis powers.  The Tunny machine was used in breaking the German Lorenz cipher, used for communications between the Nazi high command in Berlin and its field commanders.

The UK magazine PC Pro has an interesting article giving a closer look at the reconstruction project.  The volunteers who undertook the task did not, at least initially, have a lot to go on.

A single photograph, scraps of circuit diagrams drawn from memory and a pile of disused components – it isn’t much to go on, but from such meagre beginnings, engineers rebuilt one of the precursors to the modern computer.

The team did have one photograph of a Tunny machine, but it was a general image of the room which housed the machine.  They did have one or two lucky breaks.

Another lucky break was the discovery of the lead engineer’s notes stashed away in an envelope in a toilet after the war. Pether’s [John Pether, one of the reconstruction team] fellow restorer, John Whetter, jokes that the team would have had more to work from if only British forces had stocked more toilet paper.

Perhaps the team’s most valuable asset was its collective experience working in telecommunications.  The Tunny machines, along with the Colossus computer, were built be technical staff from the Post Office’s telephone operations.  Naturally enough, the original engineers designed and built the equipment using the telephone system parts and circuit elements they were familiar with; those parts were also reasonably available during the war.

Pether worked for the GPO and BT for 36 years, and the Colossus and Tunny machines were built using standard telecommunications equipment – the very same bits that made up the GPO’s, and subsequently BT’s, network.

The use of telephone exchange components turned out to be advantageous to the restorers in another way.

The team started work on the code-breaking machines in the mid-1990s, about the same time as another massive project was coming to an end: BT’s move to digital exchanges. The decommissioning of the old equipment gave the engineers their pick of parts for the restoration.

“BT was kind enough to donate a lot of the components from these old exchanges that we needed for the Colossus rebuild and the Tunny,” Pether said.

Had it not been for the supply of old components, some of the electro-mechanical parts, such as relays, might have required custom manufacture.

Looking at photographs of the reconstructed machine, one is impressed by the enormous amount of skilled labor that  was required to build either the original or the replica.  Many hundreds of wires had to be carefully routed, according to a detailed plan, through the frame of the device, which contains ~5,000 solder joints.  Modern test equipment did make calibration of the machine’s temperamental timing circuits a bit easier.

The Museum now has a complete exhibit replicating the Lorenz code-breaking operation.

The National Museum of Computing now has a full, working WWII code-breaking system set up at Bletchley Park: visitors can hear the radio signal, see the Colossus being programmed, and watch the message being processed through the Tunny.

It’s instructive and fun to see this early technology brought back to life.  It is also good that the really extraordinary work that was done at Bletchley Park is being recognized, after having been kept secret for so long.


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