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Old 9th Feb 2017, 12:29 pm   #1
Panrock
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Default Mirror Screw - quick facts

The mirror screw is a mechanical television scanning device. This one consists of 120 mirror-edged horizontal slats mounted on top of each other in a corkscrew formation.

When a thin vertical line of light is reflected in a horizontal mirror edge, a spot of light is seen at the intersection.

Rotate the screw, and this spot of light travels in an orderly fashion across each slat edge, one at a time, line by line. Do this fast enough and a raster is seen.

Modulate the line of light seen reflected from the whizzing screw with video and a television picture results.

This mirror screw weighs 44lb and rotates 25 times a second. It is located in a stout wooden frame with ball races top and bottom and a thrust bearing at the bottom. It is driven by a mains powered 1/8 horsepower washing machine motor at the rear via belt and pulleys. The motor is fed from a Variac.

Because of the inertia of the screw, the signal source is synchronised to it, rather than the other way round. The source is a WC-01 "World Converter" by Darryl Hock.

The angular positioning tolerances of the mirror edges, when working at 120-lines, are extremely exacting and difficult to achieve in practice. Therefore Karen Orton specially developed and made a Timing Corrector, with the capacity to individually correct up to 128 lines. I am forever in her debt.

The line of light consists of three rows of (red, green and blue) LEDs, side-by-side. These are so bright they are dangerous to look at directly. The necessary vision bandwidth is 240KHz. The modulating driver circuits were designed by Steve Anderson. The reflections of the three colour channels are 'converged' by appropriate small delays introduced by Karen's Timing Corrector.

The aspect ratio of a mirror screw display varies with viewing distance, which doesn't make it very practical! In this case, the display needs to be viewed from right across the room, thus the rather small image in the YouTube video.

Steve
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Old 9th Feb 2017, 1:16 pm   #2
Boater Sam
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Default Re: Mirror Screw - quick facts

Seen it, not in action though. Its a masterly piece of construction.
Good to see you on here Steve, and thank you for all the stuff I collected from you when you were having the clear out some time back.
I have tested some of the old valves and forwarded some to new homes where they are much appreciated.
Sam.
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Old 9th Feb 2017, 1:57 pm   #3
BassoonBloke
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Thumbs up Re: Mirror Screw - quick facts

Hi Steve,

Thanks for the additional info.
Truly a work of art and very impressive.

Cheers,
Alan.
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Old 9th Feb 2017, 2:33 pm   #4
Panrock
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Default Re: Mirror Screw - quick facts

Sam, greetings to the Philippines! I too was once a visitor, to Manila and Cebu. During my stay, there was a 7.8 earthquake, centered in Baguio, so watch out! Since you came to see me here, the doorstop (mirror screw) has been rebuilt (twice!) and is now of a better standard.

Alan, your comments are really most kind and much appreciated.

Steve
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Old 9th Feb 2017, 7:14 pm   #5
McMurdo
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Default Re: Mirror Screw - quick facts

I saw your mirror screw a while ago on your website and thought how nice it looked even when turned off! I've had alot of encouragement from reading your painstaking restoration pages and my last radio cabinet finish (acrylic laquer!) was inspired by your Baird Townsman rebuild. Keep your project pics coming!
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Old 9th Feb 2017, 8:20 pm   #6
Panrock
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Default Re: Mirror Screw - quick facts

Glad the simple cabinet restoration tips were useful Kevin. More restoration picture strips are to follow. You've got a pretty impressive site yourself.

The original YouTube link is repeated HERE for convenience.

Those who will be in the Notts. area in early April can pop in to the NBTVA Convention, where this mirror screw will be demonstrated and there will be other members' NBTV goodies on show too. PM me if you would like details.

Steve
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