|The Colossus Rebuild Project by Tony Sale|
The Photocells and the Mask
Here is an older bit slightly clearer picture of the photocells on their ramp at the back of the amplifier chassis.
The light through the sprocket holes and through the smaller aperture in the mask, is detected by the fourth PEC from the left. Here is the sprocket amplifier circuit.
This circuit forms the electrical signal from the PEC into a narrow pulse of about 40 microseconds width.
The picture also shows clearly the offset photo electric cells(PEC) which read the start and stop holes punched into the paper tape.
The PEC standing up at the back reads the stop hole between tracks 4 and 5 on the tape. The lower PEC on its little platform reads the start hole between tracks 3 and 4.
On the mask on the left of the picture you can see the vertical slots through which the light from the start and stop holes gets to the PECs. These are slits to stop light from three data tracks 3, 4 and 5 getting to the start and stop PECs.
Here is the circuit of a start stop amplifier.
The sprocket pulses are fed to the suppresser of the centre valve.
The effect of this is that one sprocket pulse is selected by the wider start or stop wave form amplified from the PEC.
The mask shows the contoured shape of the apertures in front of each PEC. Arnold Lynch was asked by Tommy Flowers to produce a square light pulse to the PECs as the circular image of the tape hole swept across the mask. He achieved this by a "double eyebrow" design. Each eyebrow was the differential of the circle thus giving the required fast rise and fall in the transmitted light energy. However this design produced problems.
Light leaked through from adjacent holes causing the base line electrical signal to heave causing loss of some data. The Newmanry history records having to doctor data tapes to overcome this problem.
I started by using the original design of mask. I also found this adjacent hole problem and decided to experiment with a different mask design.
I abandoned the need for a rectangular light pulse, (the electrical circuits were quite capable of squaring up the signals), and concentrated on a design which gave a good light cut out between adjacent holes.
This double concave design worked very well and is what is shown on the mask in the picture above.
The irony of this is that when I mentioned this to some of the original WW II Colossus engineers they said that they also had changed the mask design and ended up with one like mine!
This is not recorded in any of the histories.
This page was originally created by the late Tony Sale, the original curator of the Bletchley Park Museum, and Secretary of the Bletchley Park