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home>>Tech Articles>>Tech Article #1
"How A CRT Projector Works"
CRT is short for "cathode ray tube"; the technology is similar for direct view tubes as well as projection types. Most CRTs employ a funnel-shaped glass envelope (see PIC 1) containing an electron-emitting mechanism or "gun" (see PIC 2), and a glass plate coated with light-emitting phosphor compounds (see PIC 3).

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After assembly, the tube's air is pumped out until a vacuum results. The cathode of the gun is heated by its' filament, or heater (see PIC 4), to enable electrons to easily boil off. The tube face is charged from a power supply with 30-40,000 volts, and some other bias voltages cause a stream of electrons (the beam) to fire from the gun end and strike the phosphor see PIC 5).

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The incoming video signal is amplified from 0.7 volts at the displays' input to 140 volts or more by a circuit board mounted right on the pins of the electron gun on the rear of the tube neck, hence the boards are called "neck boards"; the neck boards employ high frequncy amplifier stages to drive the cathode and modulate the electron beam. We see three neck boards in this view of the open M8500 chassis (see PIC 6)

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Less cathode voltage allows for more beam current, a higher voltage shuts it off, so the cathode drive signal waveform is inverse relative to the input source signal waveform. A tube with a blown drive circuit but other voltages intact runs wide open with retrace lines visible and would soon shatter from overheating the tube face if allowed to. The modulated cathode signal represents pixels of the source image; this beam can be steered about the tube face by magnetism; the pixels are distributed across the tube face by signals fed to magnetic windings of the sweep yoke (see PICs 7 & 8); a vertical sweep signal of 50 or 60hz for typical PAL or NTSC video, up to 180hz for some computer signals.

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The output waveform for the vertical yoke drives is usually in the 35 volt range. The horizontal yoke windings are driven much faster; a scan line may be drawn 15,750 times per second (15.7khz) for normal NTSC video, 31.5khz for a line-doubled or de-interlaced NTSC feed, 33khz for HDTV at 1080i, 48khz for line-tripled, 63khz for line-quadrupled, and even higher horizontal rates for computer graphics and some HTPC outputs. Horizontal yoke pulses can vary in the 1000 to 1500 volt range; one wants to exercise caution when reaching around the sweep yoke solder points (see PIC 9) and the exposed copper windings of the yoke! Other magnets or magnetic windings mounted on the tube neck see PIC 10) assist in controlling focus, also known as spot size; stigmation correction, or dot shape; dot flare; and convergence correction

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In a projector, the three images from each tube are fired at the screen from angles but need to be square at the screen and matched to each other; the convergence circuitry distorts the image (raster) shape on the tube face so as to be square and linear and matching the other colors at the screen. Some geometry functions such as linearity, bow and pincushion are executed through the convergence circuits. The image drawn on each tube face is delivered to the screen surface through a lens assembly (see PICs 11 and 12), and there you have your typical three-tube video projector.

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In subsequent articles we will explore some of the projector subsystems in more detail.
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