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Originally Posted by loomis
You turn the tuner knob, and a vertical line appears on the tube, which you line up with vhf/uhf numbers to tune in a channel. The line is blue for uhf, and green for vhf. After a station is tuned in, the line fades away.
At first I thought something was wrong with the tube, because it had a line running through it. Sometimes the vhf line turns into a white/black/green line, although I don't quite understand how you use these lines to tune in a station, and why only vhf has the multiple lines.
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The line is like a pointer used in radio sets with a "slide rule" dial. The dial is the part just below the picture tube with the channel numbers written across it. You twiddle the tuning knob to make this line point to a desired TV channel. Instead of a set of pullys and a string that moves a mechanical pointer, the line on the picture tube is slaved to the variable tuning voltage that feeds the TV tuner. The line maker circuit is some form of voltage controlled variable delay generator that creates a pulse some controlled period of time after the horizontal deflection flybacks to the left of the screen. Another circuit shuts this pulse off after you last play with the tuning. The pulse is added into one or more of the red/green/blue video amplifier circuits that drive the CRT guns.