Electrohelponline: CRT BASED TVS _ FAULT FINDING_ PICTURE COMES ON BUT FADES OFF GRADUALLY _ AFTER SOME TIME, WILL COME BACK, THEN GOES OFF AND THE SEQUENCE CONTINUES

     The main cause to this type of a fault to CRT based TVs, might be of some loose solder terminals. Some components of any color TV, irrespective of its brand and model, will heat up to an extent while it works. repeated heating and cooling may make the solder terminals to these components to get loose contacted, due to difference in the coiafficiant of the leaner expansion of both the solder material and the metal made of the solder terminals of the component. Normally, the fault will be very difficult to detect, as thy might be microscopically small to naked eye. If you examine these terminals by the help of a goog magnifying glass, can see tinty cracks around the solder terminals.

     Suppose that we switch ON the TV for the first time. Actually, these loose solder points are still there, but in cold condition, thy might be in good elelctrical contact, and the set will work properly. But, after some time, the components will heat up, the gap between the solder terminals and solder points may get aparted. The microscopic gap that have been already there befor ewe switch ON the set, will get enough expanded to cuto off the electrical connection, the set will begin to show porblems. When this happens, naturally, we will switch Off the set and call for service,. But, when the service person come to you, and switch On the set, he can't detact any fault with it.

     This is a common porblem caused by loose solder terminlas. As I've explained above, the fault will be very difficult to detect. If the set is either dead, or hav a distorted picture, or with any othr fault, it will be easy to reach the faulty point, becuse, we have visual indications on the screen. But this case id different. The oly thing I did to these faults, is to resolder all the suspected solder terminals by applying a little more freash solder. In most cases I've made it right.

      The most of these type of fault, can be identified by the screen of the TV itself. Switch On the set. Look whether raster is there or not. If raster is there, make sure that the horizontal and vertical secion of the TV isl functioning well, otherwise, there will be no raster. Ask the customer, about the probelem he faces with this set, listen him carefully. By doing this, we can get an idea about the fault to the set.

     Just imagine, the customer complaints that, the set will work fine at switch ON, and after 10 or 15 minutes, its color combination will go out. The customer can say ony this to us, because, he might not know nothing more than this, as he is not a technician. 'Suppose the customer says that, the picture will go out after sometime, and only a red screen with some lines can be seen. But , if switch off the set for some time, and when switch it back On, the picture will be alright, and after some time, the fault said above repeats'. This is the only information that we've got from the customer's side. His complaint might look like this.

See the complaint. Set works fine and after some time the raster will appear like this, with some horizontal retrace lines across it. A clear indication, that the voltage at the 'Red' cathode of the piclture tube is less than normal than the other two; Blue and Green. In this case, if we tune to any station, the audio output will be OK, but will not have any picture except this red screen. The picture and sound was perfect for some time and after 5 or 10 minutes, the picture suddenly goes out, leaving only a red screen like above, still sound is perfect.

From this sympton itself, we can conculde that there is no component failure in the circuit, but some solder defects are.

The first thing to do is. Switch On the set while its back cover opened, and wait till the fault occurs. When the set begin to show this problem, check the voltages at the Blue, Green and Red cathodes of the picture tube base card. Can note that, the voltage at the red cathode will be much less than the other two this time.

This is the fault; because, if the voltage of the red cathode is not almost equal to the two others,the Red Cathode will emit more electrons than the other two cathodes and will cause to a red predominant screen. This is not a picture tube fault. The fault lies on the circuit itself. Check the red output transistor first. Just desolder it out from the circuit. Without this transistor, switch ON the set. There must not be any trace of red color. If so, be sure; the red output section is faulty.

Check the soldered out transistor for collector to emitter leak. Check it well. Replace it with an other one of the same number. Check the resistor that supplies voltage to this transistor. Usually it will be between the the value of 12 to 15 K Ohms 2Watts. Check this resistor for continuity, and be sure that it has the rated resistance value marked on it. If found opened, replace it with an other one of same value. Re-solder all the solder terminals of this board including CRT base socket solder terminals [CRT base board]. Be sure not to make any solder bridge short in between adjacent solder terminals. You've already won in this case. You've repaired your TV. OK? Do you see any other color predominant screen to your TV, Blue, Green? Check for it as I described here for the Red predominant screen. OK?

If you've any more to know, please leave a comment. I'm ready to share my 40 years of service experience in this field to you.