As with all electronics, a TV has a shelf-life or operating life-span. Heat from interior components of all electronics can–over time–cause the decay of wires and circuitry within the device. Other external factors such as humidity, electricity and magnetism can also cause the deterioration of these devices. A picture tube or CRT (Cathode Ray Tube) is the device that produces the colors for a TV using a vacuum tube and an electron gun, connected to a fluorescent screen. Over time these tubes can decay through heat, magnetism, and other factors. Troubleshooting your TV to tell if the CRT is bad can be accomplished when you know what to look for.
Look for any blurred areas on the television screen when it's on. These may appear as circular bluish-green spots, or one large spot. This is an indication that a magnetic disturbance has affected the picture tube. Nearby electrical wiring, a large metal object near the TV, or moving the TV after a long period of time can cause this problem.
Turn the power to the TV on after a lightning strike. Turn it on well after the storm threat has passed. If your power was knocked out or a lighting strike occurred near your home and the TV now appears blurry or has large spots, this could indicate that the picture tube has been magnetized. This is similar to the effect of an EMP (electro-magnetic pulse.)
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Turn the TV on and wait for the picture to show. If you hear sound and have a black screen, then your CRT (picture tube) has shorted out completely. Heat from the interior of the TV can cause this, typically on TVs that have been in use for many years.
Look for dimming effects on the screen when watching the TV. If the TV all of a sudden appears dim, or darkened, this is an indication that the picture tube is starting to decay. If this effect is combined with a phenomena known as "afterglow" (a glow or subtle brightness after the TV has been turned off) then the picture tube has been in a state of decay for a long time and will soon need to be replaced.