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The image usually fades or darkens after the monitor has been used for many years. Again, it depends on the quality of the picture tube or cathode ray tube (CRT). Some grade A CRTs can last many years, but lower grade ones can only last a couple of years. Don’t be surprised if I come across a picture tube that had gone dark in less than a year! If you turn your monitor on for 24 hours without turning it off, expect the tube to quickly darken. In addition to attenuation (weak emission), CRT can also develop many problems, such as open filament, short circuit between cathode and heater, short G1 and G2, poor gamma performance, stripped cathode and focus, and short G2. In this article, I will only cover the weak emission problem and show you one possible way to illuminate the picture tube.

The cathode ray determines the brightness and sharpness of a picture tube. Its electrons are emitted from the cathode surface into the vacuum of the cathode ray tube. The electrons then strike the screen in the form of lightning, causing it to glow. Weak or depleted cathodes produce few electrons and emit weak electron beams. This causes the image to become dark and blurry. Sometimes the cathode emission ceased because a contamination coating layer covers the cathode surface. This contamination prevents electrons from leaving the cathode surface, resulting in a dim image.

To identify if it is the tube that is causing the attenuation or if it has faulty components, a CRT tester/meter can always be used to verify this. Some call it CRT Rejuvenator, CRT Restorer and others call it CRT Regenerator. These testers are designed to test and verify CRT and fix it if the picture tubes have problems. Personally, I had two CRT testers: the Muter BMR 2005 regenerator and one with less features that I bought in Thailand. With the help of CRT tester, I can know whether a picture tube is working or not. If you don’t have one, don’t worry because by checking the voltage on the CRT socket pin outputs you’ll have some ideas if the CRT is good or bad.

From my experience, I noticed that older tube can be lighted with the help of CRT tester, but not for tubes made in year 1994 and later. I suspect that the getter (made of barium) inside the tube has been used up. The functions of the getter inside the tube are to absorb the gas that was released when the cathodes were heated. If the getter fails to absorb the gas, then there is no point in restoring a picture tube. I have illuminated many picture tubes with the help of the CRT tester and some look like a new tube. But somehow after a day or two the picture tube goes dark again because the getter can’t absorb the gas and the gas will travel back to the cathode surface and convert it to carbonate. The emission of electrons will cease again. I even tried a homemade CRT restorer downloaded from the internet and the result is still the same – the picture tube will dim after a couple of days.

After some research and experimentation, I found that the best way to light the tube is to increase the voltage of the heater. The normal voltage of the heater or filament is about 6.3 volts and if you increase the voltage to about 8-9 volts this means the cathode will produce more electrons and therefore the screen will become brighter. Please note that do not apply any voltage higher than 9 volts to the heater, otherwise the internal filament will burn and break. Once it is open, there will be no way to rescue it.

If the picture tube is headed for the dump, then there’s nothing wrong with trying to restore it. About how long it will last, I can’t guarantee. Some will last even more than two years and some just a couple of weeks. Recently there was a flagship 15” Compaq and Hp monitor which has the symptom of blurry screen with bright image. No matter how hard you try to adjust the focus adjustment on the back of the flyback transformer, the image still looks slightly blurry. Using the above methods, I had saved a lot of picture tubes (I don’t need to replace a second-hand one for the customer).

It is a simple modification and this only applies to the 14 and 15” monitor picture tube. First you need to find which secondary outlet has a 12 to 15 volt output. Once you’ve located it, solder a wire to the cathode pin of the diode and attach it to the input pin of the 7808 voltage regulator (8 volt output). Do not use the B+ voltage which is usually 45 volts or more and this can kill the 7808 IC because this IC can only handle voltage up to 38 volts max. The current drawn from the 14 and 15″ picture tube is less than the 17″ tube and if you use this way to light the 17″ tube, the power supply may go into shutdown mode and sometimes even will cause a flicker of the power.

I know some technicians like to use the method of twisting a few turns of wire on the flyback ferrite core to restore brightness. This method may work on Television but not on Monitor. Computer monitors work at many resolutions and the higher the resolution, the higher the B+ voltage will be and therefore the output voltage on the cable will increase. This will cause a sudden brightness and may turn off the monitor. Even if it was not turned off, the client may be annoyed with the monitor (raster) brightness every time the monitor resolution changes.

Now solder pin 2 of the 7808 IC to cold ground and the output of pin 3 of the 7808 IC to the CRT heater point as shown in the picture. Remember to cut the 6.3 volt supply line from the power supply because you no longer need it. What you need is the 8 volt supply to the filament in the picture tube. You can also connect a low ohm protection resistor in series to the heater point. In case of a short circuit, the resistor will open the circuit. Be sure to connect the 7808 IC to the heat sink, preferably the heat sink on the flyback transformer.

This method only works for tubes that are slightly blurry and not too dark. If an image is very dim, even if you increase the heater voltage to 12 volts, you still won’t see a bright image. As mentioned above, there is nothing wrong with trying to modify it and who knows, the tube may come to life and serve the customer for many years. I hope you enjoy the article.

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