The Colossus Rebuild Project by Tony Sale



Colossus Reborn

The Paper Tape reader

The next problem was the optical paper tape reader system. The details of this are not shown in any of the photographs. However I managed to locate Dr Arnold Lynch who designed the reader system in 1942.
Although well into his eighties Dr Lynch came to my house and using my CAD system we re-engineered the reader system to his original specifications. The part holding the punched paper tape on wheels, known as the "Bedstead", could be worked out from the wartime photographs, but the optics, photocells and amplifiers were not shown on any photographs.
The redrawing of this started with the size of the photocells. These had to be hard vacuum caesium cells to achieve the high frequency response for reading the punched tape at 5,000 characters per second.
Tommy Flowers had located a large stock of suitable cells made for a proximity fuse system for rocket which had not worked. These were mounted into octal valve bases. So this determined how close together they could be mounted. Luckily some of these photocells had been had been kept by engineers when the Colossi were dismantled at the end of the war. They kindly gave these to me so I was able to work out the width of six of them in a row.
Arnold Lynch and I then drew a CAD diagram which confirmed his memory of a 10 times magnification required from the paper tape hole to the photocells and also fixed the size of the amplifier chassis and the size of the angle iron frame holding the amplifier and photo cells.

In the best engineering tradition, I then made a prototype of the optical system and here it is.

On the left of this is a 50 watt car headlamp bulb from my pre-war 1937 TA MG sports car. Next is a wartime condenser lens assembly which I had used in a 35mm slide projector made in 1958. Next is a plastic drum with a piece of punched paper tape wrapped round it followed by a brass plate with a slit cut in it for one row of holes on the paper tape. A camera lens then projects the tape image, first onto the mask and then through the mask onto the photocells on their ramp.
It all worked fine and so I proceeded to build the whole bedstead when, in 1994, I had access to the room in H Block where Colossus number 9 stood in WW II.



This page was originally created by the late Tony Sale, the original curator of the Bletchley Park Museum, and Secretary of the Bletchley Park Heritage Society.