COLOR TROUBLES TO COLOR TELEVISIONS
Absence of colour on screen can be occur to many causes. Some sets will show only one primary color. Some others will show a plain white raster. Others may show a white plain raster. In all the above states, assuming that, the audio output is OK and let's see which might be the fault.
In a set, audio output is OK, means, tuner, video IF, audio output, power supply and all other parts are working fine, but there is some fault with the video section. Before going to troubleshoot, let's see what is happening in the circuit of the TV.
The TV transmission signals will contain these signal combinations. A carrier wave, which carries all these signal. The carrier wave will have a frequency. This frequency is usually called the transmitting frequency, as we say, that a particular TV station is tranmitting at 106.8Mhzs, 308Mhz, 609.3Mhz and so on. These frequencies only indicates the carrier frequency of the transmission. The carrier frequency is one, which carries, as it stats, more signals included in it, For sample, these signals will have synchronisation signals; both vertical and horizontal, which will be retrieved at the receiver circuits, seperates them, and makes the picture to a stable frame. The horizontal synchronisation signals helps to synchronise the picture horizontally and vertical signal stabilizes it vertically. We can't feel these signals are comming to TV, and are processing, becuse, among these two signals, one on the upper sider of the carrier wave, and the other will be at its lower part. Actually these signals are visible both at the left and right, top and bottom of the secreen, but will be scanned away from our visinity, when we see picture.
The other signals moldulated with carrier frequency is Luminance signal [actually a main part of video], Blue color signal, Red color signal. It should be noted that there is no Green signals are transmitted from the transmitter, and there is no such signal in the carrier wave too. Then how this green signals are produced?
Just see, there are actually three signals comes modulated with carrier wave R,B &Y. Here 'Y" stands for luminance signal otherwise, back and white part information to a particular screen. As you know, there is chroma processing Integrated Circuit, {IC}, in every color TVS. This IC have a matrix section inside and it compares the incoming R,G &Y signals. The comparison is done with resepect to Y signal. Thus the matrix section seperates [Y-R],[Y-B] . The resultant signals [voltage levels] are further processed to get the Green signal. Acually Green signal produced inside the TV itself and is the difference between [Y-R] & [Y-B], and it not supplied to it.
Let's check a set which have no Green color on screen. We know that the green colour is processed inside the TV circuit. If there is no green color, the picture will come appear ofcourse, but the color combinations will be strange, You will see a mixture of Red and Blue colors, as you know, all the colours we see are the combination of the three primary colors, red, green and blue. If any one is absent, abnormal color will be the result. Here we see some combination of colors on picture that both Red and Blue together can make. Zyan color for example. Actually, they toghther can make for color combinations
In fact there is no need ot say, that audio informations also is being tranmitted along with these color signals withing the same carrier wave. It is also ther. But, the techneque used to transmit audio signal is different to that of color.
The video information is transmitted in AM [Amplitued Modulation} and audio with FM [Frequency Modulation]. Therefore, the video signal receved at the TV end may have some sort of noise, where audio will not have. More over, television signals cannot reach a vast area like radio signals. The reception will be best when we are in near visinity of the transmitter, and there is no obstruction to signals. In other words, the signals must be strong enough to overcome the noise, to get a clear and crisp reception.
Frequency Modulated [FM] sound signals are transmitted to different standards to different countirs. The standard used will said to 4.43; 5.5; 4.32 and so on. These are nothing, but indicates the position of audio signal, inside the transmitting envelope. For example, 5.5, means, the audio signal is transmitted 5.5MHz aprt from the video signal. going very deep into transmission technique is more confusing.
At the TV end, there must be sufficant signal strength at the antenna, to get a clear picture. The transmitted signals, comes in contact with antenna, comes to the tuner section of the TV. The tuner converts the very high frequency carriar wave along with video and audio signal, to an intermediate frequency, shortly [IF]. This intermedeate frequency is given to video IF stage of the TV and then detects out the video and sudio signals sperateley, gives to respective amplifiers to process. The audio signal will guided to audio amplifier to produce sound, and video signal is passed to video amplifier to process picture. Let we go very simple.
Come to our case. There is no Green signal to the TV under concern. As we know, there is no green signal transmission from the transmitter, and the matrix section inside the chorma ID derive this green signal and gie it to video ampllifier to process. This no means that the chroma section inside the IC is faulty. There might me some other reasons. We have to find out, whether such a reason is there, before confirming the IC has been damaged. This is the diificult part of service, not to this only, but for all electronic devices. Fault finding, point out the exact faulty section, locate the exact faulty component, the reaon for the failure of that component, etc- are the key factors, and a technician must know what he does with set, and must aware of its circuit diagram, its working and all otheres. He must be very patient, and well expereinced.
In fact, troubleshooting faults to an electronic equipement is an art.
If you come across the fault described above, lack or absence of Green color, the first test to do is to measure the voltage at the green cathode of the piclture tube. It is very easy to measure, as it is assembled and pluged onto the neck of the picture tube. You can see a square or rectangular printed circuit board pluged to the back side of the necd of the picutre tube.
Locate the Green cathode there. In normal cases, for all TVs, it will be marked on the base card, very near to respective pin as Gk,Bk&Rk. Gk-means Green cathode, Bk-means Blue Cathode and Rk-means red cathode.The easisest way to measure and compare the voltage is, measure the voltage of a good cathode which works fine. Here take the case of Red cathod[Rk]. There is no fault with it. It works fine, that is whey we see combination of red colors on screen.
Check the voltage at red cathode, It will be in the order of 270VDC to 189VDC. The voltage stated may vary a little. Mark this voltage on a piece of paper. Now measure the voltage at Green cathode [Gk]. It will have much difference between the voltage to other two cathodes; no doubt.
Here starts the play. To proceed , you must have sound knoledge in electronics, or anybody at your home must have. I'll give more details about troubleshooting, somewhere after in this blog.