A B C D E F G H I J K L M N O P Q R S T U V W X-Z

Repairing a dim monitor

Picture become weak or dim usually happen after the monitor have been used for many years. Again it depends on the quality of the picture tube or cathode ray tube (CRT). Some grade A CRT can last for many years, but for the lower grade one it can only last for couples of year. Don't be surprise that I do come across picture tube that had gone dim in less than a year! If you switch on your monitor for 24 hours without turning it off, then expect the tube to become dim fast. Besides dim (weak emission) , CRT may also develop many problems such as open filament, short between cathode and heater, G1 and G2 short, poor gamma performance, stripped cathode and focus and G2 short.  In this article, I'm going to touch on the weak emission problem only and show you a possible way to brighten up 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 cathode ray tube's vacuum. The electrons then strike the screen in the form of a ray and cause it to shine. Weak or used up cathodes produce few electrons and emit weak electron rays. This causes the picture to become dark and fuzzy. Sometimes the cathode emission ceased due to a layer of contamination coating covers the cathode surface. This contamination prevents the electrons from leaving the cathode's surface thus you get a dim picture.

In order to identify whether is the tube that cause the dim or from bad components, one can always use a CRT tester/meter to check it. Some call it CRT rejuvenator, CRT restorer and others named it as CRT regenerator. These testers are designed to test and check CRT and fix it if the picture tubes have problems. I personally owned two CRT testers- the BMR 2005 Muter regenerator and a less feature one I bought from Thailand. With the help of CRT tester, I can know if a picture tube is working or not. If you do not have one, don't worry because by checking the voltage at the CRT socket pin outs, you will have some ideas if the CRT is good or bad.

From my experience, I noticed that the older tubes can be brightened up with the help of CRT tester but not for tubes that was made in the year 1994 and above. I suspect the getter (made of barium) inside the tube have been used up. The functions of getter inside the tube are to absorb gas that was release when the cathodes are heat up. If the getter fails to absorb the gas, then there is no point to restore a picture tube. I have brightened up many picture tubes with the help of CRT tester and some looks like a new tube. But somehow after one or two days the picture tube get dim again because the getter cannot absorb the gas and the gas will travels back to the cathode surface and turn it into carbonate. Electron emission will cease again. I even have tried some home made CRT restorer downloaded from the internet and the result is still the same-the picture tube will dim back after couples of days.

After done some research and experiment, I found that the best way to brighten up the tube is to increase the heater voltage. Normal heater or filament voltage is about 6.3 volt and if you increase the voltage to about 8 to 9 volts, this means the cathode will produce more electrons and thus the screen will become brighter. Bare in mind that do not apply any voltage that is higher than 9 volts to the heater otherwise the internal filament will burn out and broken. Once it is open then there will be no way to rescue it. If the picture tube is heading to rubbish dump then there is no harm to try to restore it. About how long it will last, I cannot guarantee it. Some will last even more than two years and some only couples of weeks. Recently there was a badge of Compaq and Hp 15" monitor that have the symptom of blur display with bright picture. No matter how hard you try to adjust the focus adjustment at the back of flyback transformer, the picture still look slightly blurs. Using the methods above I had saved lots of picture tube (I don't need to replace a second hand one to customer).

It is a simple modification and this only applies to the 14 and 15" monitor picture tube only. First you need to find which secondary output that have output from 12 to 15 volts. Once you have locate it, then solder a wire at the cathode pin of the diode and join it to the input pin of 7808 (output 8Volts) voltage regulator. Please don't tap from the B+ voltage which is usually 45 volts and above and this may kill the 7808 IC because this IC can only withstand voltage up to 38 volts maximum. The current drawn from the 14 and 15" picture tube is less than the 17" tube and if you use this way to brighten up the 17" tube, the power supply may go into shutdown mode and sometimes will even lead to power blink.

I do know some technicians like to use the method of turning few rounds of wire at the flyback ferrite core to restore the brightness. This method can work in Television but not in Monitor. Computer monitors runs in many resolution and the higher the resolution the higher the B+ voltage and thus making the output voltage at the wire increased. This will lead to a sudden brightness and may shutdown the monitor. Even if it didn't shutdown, the customer may get annoyed with the Monitor brightness (raster) whenever the customer change the monitor resolution.

Now solder the pin 2 of 7808 IC to cold ground and the output of 7808 IC pin 3 to the CRT heater point as shown in the picture. Remember to cut off the 6.3 volt supply line from the power supply because you don't need it anymore. 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. Just in case if any short circuit happens the resistor will open circuit. Make sure you attach the 7808 IC to the heatsink, preferable the heatsink at the flyback transformer.

This method only work for tube that is slightly blur and not too dim picture. If a picture is very dim even if you raise the heater voltage to 12 volt, you still would not see a bright picture. As mentioned above there is no harm trying to modify it and who knows the tube can be brought to life and serve the customer for many years. Hope you enjoy the article.


About the author: Jestine Yong is a electronic repairer and a writer, for more electronic and monitor repair information, please visit his website at http://www.electronicrepairguide.com/computer-monitor.html
 

 

 

Copyright 2006 Wilson's Electronics