February 11, 2015

TVInfo

They Don’t Build Them Like They Used To

The best of the best were the old style CRT, big tube sets. Sadly, they are almost gone, since there are no mainstream manufacturers out there making them. Those designs lasted for 20 to 30 years, but today the new flat panels are lasting for about 3 to 5 years before they need repairs. Many are worth repairing, but some are not.

Our opinion, based on thousands of repairs and years of experience, is that the old style big-tube CRT’s were the best, including CRT projection TVs.  Next come the LCD projectors followed by plasma for best overall viewing, with LED/LCDs flat panels next, and the standard LCDs bringing up the rear.

What’s the Difference?

Starting with the CRT sets, they were built very heavily and designed to last for many years.  They have a picture tube that is most of the weight. The picture tube works like this: it has a phosphor coating on the face of the screen that gives off light when an electron hits it. The gun loaded with electrons sits at the back of the tube and is heated, and when heated the tube gives off electrons that just float around until the high positive voltage on the front of the screen attracts them.

Left to their own devices, they would all hit the middle of the screen and make a very bright dot. But there are other forces that act on the tube to divert or bend the beam! It is the “yokes” job to do this; it is comprised of a vertical and horizontal deflection coil. The horizontal coil can pull or steer the beam to write a single line across the screen. The vertical coil pulls it from the top to the bottom, and when the set is working properly it starts off at the top left and is pulled across the screen to the right. It then snaps back to the left, the vertical coil moves the line down one line and the horizontal does its job once again.

This continues until it reaches the bottom of the screen and the whole process repeats.  A device inside the tube called a “grid” this device allows for changes in the brightness in the picture by reducing the amount of electrons. Color TVs use three guns to hit a series of three phosphor dots on the screen: one red, one blue and one green.  Using a variance of these three colors they can reproduce almost any color. The phosphor dots on these sets can burn in if the same image is left on the screen for a long time in the same location.

But the sets were designed to give years and years of worry-free operation.

Plasma

These sets started out unreachable by most of us; their initial cost was incredible. They work by firing an electrode that light hundreds of thousands of red, green or blue dot pixels on the screen. A processor determines which one lights and how bright it should be. Each pixel’s electrode sparks which makes the material glow the color of its pixel.

They are very complex in their operation and produce a decent amount of heat. Until the advent of modern computer processors they were not technically feasible. However, once that technology and the technology to mass produce the panels (screens) came to be, they flew off the shelf and nobody wanted the big old heavy CRT TV’s any more.

LCD and LED/LCD i.e. “Liquid Crystal Display”

There are three types of LCD TV’s: LCD projection, regular LCD flat panel, and LED/LCD flat panel. In each case they use LCD technology to operate. LCDs require an external light source, because unlike the CRT or Plasma they give off no light of their own. Instead, they allow light to pass or not pass.  More on that later.

There is an array of hundreds of thousands of crystals that when turned on or off allow light to pass or not to pass. The processor will determine which ones will let light pass and runs the signal into a timing control board or “T-Con”, which sends the information to the display panel. In order to see an image on the display panel, it requires a light source. The light source is generally a series of thin tubes that run horizontally behind the screen and are powered by ballasts or inverters. If one goes out, they all go out.

Then there is the LED/LCD. Most everything is the same except the backlight. Instead of thin fluorescent tubes, LED/LCDs have rows and rows of many of small LEDs. The good thing about them is that they do not run at high voltage but they will all turn off if one goes bad.

If you have a projection LCD you will have an “optical block” or light engine that houses three panels about 2 inches by 1.5 inches, one for each color. The light source is a very high tech lamp that is incredibly bright. They use mirrors that allow one color of light to pass through each of the three LCD screens, which is then bounced off of a mirror and through a lens to hit another mirror on the back of the set which reflects the image to the front screen.

DLP “Digital Light Process”

This technology was developed in the late 70’s by Texas Instruments. It uses mirrors, usually around 900,000 of them, that are controlled by a processor that tells each one when to turn and reflect light. If a row of them turn, it will give a straight line across the screen. The light source can be in the form of a lamp or red, green and blue VERY bright LED’s. The LED sets are few and far between today and do not use a lamp or color wheel!

DLP is the most common type that require a lamp and do have a color wheel. This device spins at aproximately 9000 RPM and contains 6 or 7 segments. 2 red, 2 blue, 2 green, and sometimes a clear or white. As the wheel spins the processor knows exactly which one of the colors is in front of the lamp, it then tells the DMD (Digital Mirror Device),  which mirrors to turn to display each color in that scene and how bright depends on how far the mirror is turned. Just before the light goes onto the DMD it passes through a “light tunnel” which removes any shadows that may be present. When it goes bad, it can leave a shadow on the right or left side of the TV screen.