|Hawes Mechanical Television Archive||
How to Build a Color TV Delay Line
Why a delay line? A delay line keeps the faster, luminance image in step with the slower, chrominance image. The narrowband chrominance signal requires more processing than the wideband luminance signal. This processing takes a brief, but significant time. The delay line assures that the two images start scanning across your screen at the same time. Without the delay line, you might notice that the monochrome picture starts before the color overlay does.
Wind Your Delay Line
Not hand-wound. A delay line isn't a hand-wound coil. This job requires a lathe, or some suitable coil-winding jig. For information on coil-winding, see Lindsay Technical Books:
The coil winders in these books only provide you with reference information. To make a delay line, you need to adapt these winders in some way.
Time vs. turns. Our final coil delays the luminance by about 1.3 μS. Your results might vary. For an exact delay, you must trim the coil. Delay time is proportional to the number of turns. Yet the coil isn't a pure inductance. The number of turns changes both the coil inductance and distributed capacitance. The trimming procedure appears later in this article.
Original art by James T. Hawes
Trim your Delay Line
Circuit design by James T. Hawes
Modular Construction. A 23-inch piece of coax makes a long, floppy and unwieldy coil form. This form won't fit any conventional hand winding jig. An easier way to complete the winding might be to break it into modules. Cascading delay lines is perfectly all right. A builder could then wind a 22-inch delay line in say, four sections.
Each modular winding would be 5.5 inches long. The winding would have 704 turns of wire. Each coil form (length of coax) would be some 6.5 inches long. The extra inch in length prevents the winding from extending to the end of the form.
Connect the ends of the form modules together. Terminate the first and last form module with a 1,000-ohm resistor.
Making a winder. A homemade coil winder might adapt fairly well to making modular delay lines. I have some ideas on how to proceed. In a typical winding machine, a crank turns a central rod. Hardware stores carry suitable rod stock. Posts with bearings support the rod. Two cone-shaped mandrels clamp to the rod. The rod them becomes an axle to turn the two mandrels. These mandrels hold the coil form. A mechanism applies friction to the axle. This friction prevents unwinding and backlash.
Look, ma! No mandrels! To wind a coil on a small coax form, slide the form over the rod. If the coax is fairly tight, you need no mandrels. If the coax is loose, you might secure it with a couple of wire ties. When the winding is done, you cut the ties and remove the coil.
About the wire gauge. Number 36 wire is mighty small. Some type of feed system seems like a good idea. Otherwise, maybe someone will try to wind a delay line with # 30 gauge wire instead. The coil would be longer, and the distributed capacitance would be greater. I don't know the difference in inductance per turn of wire. I do know that # 30 is easier to work with than smaller gauges are. Maybe one of you experimenters will provide the missing data on # 30 wire. Please!
Copyright © 2006 by James T. Hawes. All rights reserved.
•Reference: Radio TV News for December 1954 & January 1955