The Colossus Rebuild Project, evolving to the Colossus Mk 2

by Tony Sale

Tony Sale's
Codes and Ciphers

Getting started on the Colossus Mk 2, 1996 - 1998

In 1996 we had achieved a demonstration of the basic working functions of a Mk 1 Colossus but only at a 2 bit level out of the five parallel bits coming from the paper tape reader. We had shown that data could still be read from a paper tape at 5,000 characters a second, that valve logic circuits worked and most importantly, 45 to 50 years old thyratron valves could still be coaxed into functioning in a ring circuit to produce Lorenz wheel patterns.

But this and the counters and typewriter logic had only been achieved by lashed up direct wiring from one chassis plate to another, not through the switches or jack sockets on the J rack and K rack. So the first task was to identify and make all the logic plates involved with the J rack switching and the big K2 switch panel on the K rack.

Cliff Horrocks, who was now project manager, did the chassis plate layouts. John Pether drilled chassis plates from templates in his workshop and Cliff made up bags of components for volunteers to wire up chassis plates at home.

Going five wide, 1999.

We next manufactured all the staticiser chassis plates for the R rack. This meant we could route all five tracks from the paper tape reader to the J switch panel and had in place the staticisers for all the K rings and S rings. I tested these with a microcircuit pattern generator.

We now needed more sponsorship and Quantel Ltd. came to our rescue. They gave us the funds to buy enough GT1C Thyratrons to do all the rings. Luckily various dealers had kept these from just after the War.

So now Cliff laid out the ring controller plates and the thyratron plates and various volunteers wired them up.

Next I disconnected the original K4 and K5 controllers and rings. They were in the wrong place on the W rack so I then commisioned the correct controllers and thyratron plates. Meanwhile David Stanley and Bob Alexander had worked out the very large inter-rack looms needed to carry signals over to the S rack to set the wheel start positions on the pointer uniselectors. We decided that all inter-rack wiring should go via tag blocks on the top of each rack, just as it had in wartime.

By this time various volunteers had built all the chassis plates for the counter rack, 5 controller plates and ten counter plates. We then re-arranged the lamp panel to correctly display the counter outputs and the wheel start positions.

Now I turned to the paper tape reader and got that working on all five tracks, but not very reliably. So by 1999 we had repeated the 1996 results but with signals now correctly routed through the switches on the J rack.

2000 and the big K rack switch panel.

This was the major, and very obvious, difference between a Mk 1 and a Mk 2. It was a vast improvement over the complex jack plugging needed to configure an algorithm on the J rack jack socket panel on the Mk 1.

After a lot of detective work we managed to identify the chassis plates needed for the logic behind the switches on the K2 panel. Cliff laid these out and more volunteers assembled them at home.

Then came a setback to the Rebuild Project. In September 2000 I was banned from Bletchley Park by the Bletchley Park Trust. I no longer had access to the Colossus Rebuild so I met regularly with the team in the Enigma tavern in Bletchley. During this enforced absence, which lasted for 18 months, I made a wrong decision as to which logic plates were associated with which sets of switches on the K2 switch panel. David Stanley wired up the whole of the back of the K2 panel, based on this decision, and connected up the logic chassis plates.

David also wired up the counter rack and the set total switches.

Back on the job, March 2002, and a disaster!

In March 2002, under strong external pressure, the Bletchley Park Trust allowed me back in two days a week to continue working on Colossus.

I had, during my enforced absence, built in my garage a replica of Heath Robinson, the precursor of Colossus. This required two paper tape readers and I had built two amplifier chassis for the photo cell outputs. These worked much better than the amplifier chassis on Colossus, so the first thing I did was to install a Heath Robinson chassis on Colossus. This immediately improved the Colossus paper tape signals and Cliff then rewired the original Colossus chassis to the Heath Robinson specs.

So now with good reliable paper tape signals we could start commissioning the K2 switch panel.

Disaster struck.

It became obvious that in my enforced absence we had allocated the wrong logic panels to the groups of switches on the K2 panel.

After a lot of discussion David Stanley said that the only way to get out of this was to re-wire the whole of the back of the K2 switch panel and he said that he would do just that. It took him nearly six months!

But I turned a disaster into an opportunity. The K2 switch panel was out of action but we had the J rack and all its jack sockets. By wiring up the XOR logic and cathode follower chassis plates connected to the J rack jack sockets I was able to plug up the double delta algorithm on tracks 4 and 5 and get this through to the counter rack. This repeated the Mk1 usage before the Mk2 K2 panel was incorporated. So now we could commission the counter rack on all five counters but we only had the top three counters connected to the lamp panel.

We then connected up and commissioned the span counter system. This allowed the Colossus operator to only start feeding sprocket pulses to the counter logic circuits at some point down the tape and to stop feeding them at some point further on. This selected a "span" of the tape over which counts were made. It also provided a very nice way of testing counters. If the data signal to the counters was forced continuously "true" then the count should be the count of the width of the span.

2002, getting the Chi wheels working, K2 switch panel back!

Bob Alexander had divided up the thyratron panels into rings of the size required for the various wheel lengths on the Lorenz machine so I now started to commission the thyratron rings. This was an extremely difficult task. To get a ring working both the ring controller chassis panel and the ring panels had to be coaxed into operation. K4 and K5 were working, now K1, K2 and K3 had to be tackled.

We needed to connect the thyratron rings to the patch panels used to set the ring patterns and make up the complex wiring looms from the patch panels to the pointer uniselectors and up to the top tag blocks on the W rack ready to go over to the ring start jack panel on the S rack.

David finished his massive task of rewiring the K2 switch panel and with due ceremony we re-installed it on Colossus. After correctly connecting up the relevant logic panels the K2 switch panel began to work, much relief all round. So now we had signals coming onto the "Q" bus on K2, 5 wide from the paper tape reader plus rings K4 and K5.

At this stage Charles Coultas had joined us and we commissioned the delay or "remembering" circuits. This part of Colossus provided the 5 fold increase in processing speed which was such an impressive part of the Mk 1 to Mk 2 upgrade. This gave us the "R" signals on the K2 switch panel.

Bob Alexander and Peter Merriman designed the system for setting and incrementing the ring pointer uniselectors on the W rack and the chaser uniselectors on the S rack. Rob Dickson designed the typewriter drive logic but was taken ill before he could complete it. Phil Hayes took up Rob's design and implemented it on the S rack.

All Chis working, 2003.

By early 2003 I had beaten all the K rings into submission and we could now begin some real testing of the Colossus Mk 2.

We re-installed the original tape amplifier chassis which Cliff Horrocks had rewired and at last got good reliable signals from all 5 data tracks and good start/stop signals at the beginning and end of the cipher text on the tape.

I had, in the meantime, upgraded my Virtual Colossus on this web site to a Mk 2 complete with all 12 rings and patch panels. This, together with the Walter Fried weekly reports that I found in the American National Archives, enabled me to work out what algorithms we needed to break the Chi wheel settings on a real cipher text. So now we could try setting up these algorithms on the Colossus Rebuild. At first results were not very good. We just could not get reliable and consistent counts. But gradually by changing valves and getting rid of dry joints and bad connections it began to settle down.

Meanwhile David was installing the massive inter-rack wiring between the W and S racks and making up and installing the looms for the chaser uniselectors. It was these uniselectors which "followed" the ring pointer uniselectors on the W rack and drove the ring position lamps on the lamp panel.

John Whetter joined us, helping to wire up the pattern patch panels.

By December 2003 we were getting reliable counts on the (1+2)= . algorithm for setting K1 and K2, the 4=5 given 1=2 algorithm for setting K4 and K5 and the slash (/) count for setting K3. This was on a cipher text of a re-encipher German decrypt using the K2 one back limitation and BREAM patterns from the Walter Fried reports.

The wheel start positions were set by using a croc clip lead feeding the strike pulse onto the ring pattern patch panel. The pointer uniselectors were not yet working.

The home run to 1st June 2004!

At a team meeting in January 2004 we decided to go all out for a working Colossus Mk 2 on 1st June 2004, the 60th anniversary of the first running of a Colossus Mk 2 in Bletchley Park in 1944.

The critical path was the inter-rack and chaser wiring looms to implement wheel starts from the wheel start position jack field on the front of the S rack. We decided to just do the Chi wheels and we thought that this was achievable.

Phil Hayes thought that he could get some if not all, of the typewriter driver system working. At least giving wheel positions and counts. So we all set to.

The result was that on Thursday 20th May I filmed Colossus Mk 2 setting all five Chi wheels on the BREAM cipher text and after editing, this video was shown to 120 people at our commemoration event at the Science Museum in London on 1st June 2004. This was a marvelous occassion with over 30 of the original wartime people there, WRNS, maintenance men and Knockholt interceptors.

A DVD of this video is now available from this web site.

What next?

We now have to get the automatic stepping of the pointer uniselectors working and get the set totals and the typewriter output correct.

Then we need to demonstrate the speed up obtained by using the "remembering" circuits. We think that about 4 months is required in which to evaluate, video and document the Colossus Mk 2 breaking wartime Lorenz cipher texts.

The Colossus Mk 2 rebuild

The rebuild can be seen, in H Block in Bletchley Park, in the original room where Colossus No. 9 stood in World War II.

It is a marvelous tribute to Tommy Flowers, Allen Coombs and all the engineers at Dollis Hill and a great tribute to Bill Tutte, Max Newman, Ralph Tester and all the code breakers involved at Bletchley Park...

...not forgetting all the WRNS who operated and supported Colossus and the interceptors at Knockholt without whom there would have been no messages to break.

Lastly I would like to thank my wife Margaret for agreeing to the use of our own money to start up the project and for her continuing support and encouragement.

The financial sponsors:

A E & M D Sale

Mr Frank Morrell

The Mrs L D Rope Third Charitable Settlement

Mr Keith Thrower OBE

Quantel Ltd

The Computer Conservation Society

Contributions by special low prices:

Charles Head (Blacksmiths)

Billington Exports Ltd

Claude Lyons Ltd


Thanks also to:

The Bletchley Park Trust which has allowed free use of the room in H Block where Colossus has been rebuilt.

The many hundreds of individuals who have searched their garages and lofts and sent valves for Colossus.

      The Colossus Team

      Manager, Cliff Horrocks,
      Bob Alexander
      Charles Coultas
      Ken Day
      Phil Hayes
      Peter Merriman
      John Pether
      David Stanley
      John Whetter

Lorenz cipher index page

You can now see more of the Colossus Reconstruction. Continue to Walk Around Colossus.

You can also use an interactive Virtual Colossus.

These pages may not be suitable for use with a Macintosh and there may be other browser-dependent features which are not yet optimised. Tony Sale will welcome your comments.

You should also see the original 1945 document, The Newmanry History, which was released from secrecy in 2000. It details the methods by which the Lorenz cipher was broken.

Tony Sale has now made available HTML and PDF editions of this crucial document.

Tony Sale's Codes and Ciphers index page

For information about when Bletchley Park and its Museums are open to visitors you must go to the Bletchley Park Trust which is responsible for it. This website has no connection with the Bletchley Park Trust.

This page was originally created by the late Tony Sale
the original curator of the Bletchley Park Museum