SONY TRINITRON TV [ONE COLOUR PREDOMINANT SCREEN]
Sony Trinitron sets are of CRT [Cathode Ray Tube] types. The common fault seen in these types of TVs are any one color predominant on screen with sound OK. Either red, blue or green color with some thin horizontal lines at about an inch apart will be seen on the entire screen. This is totally a display card problem, which can be seen plugged onto the picture tube neck. There are three electron guns for every picture tube, which emits red, blue, and green beams, which in turn illuminates the phosphor on the screen to get that color. If no emission from all guns, results in a blank screen, ie black screen without any display at all. The three-color beams together produces proper colored picture what we see on screen. Pins of the picture tube outside to which video signals are given in appropriate proportion to produce a TV picture bring out these three gun’s terminals. Normally, all these three pins are biased with a voltage of about 170 to160VDC+ve. Take one color gun, as an example and let me describe the problem.
Let it be a green gun. These guns are called cathodes of the picture tube. If it gets a voltage with respect to its grid, it will emit electrons according to the voltage applied to it. This is a negative process, that is, as the voltage at this pin drops, its emission increases and vice versa. If the emission increases, the color with respect to that gun also will increase and gets predominant at the screen. So when we see a green predominant screen, there will be less voltage at the green cathode with respect to the other two. If red color is predominant, the voltage at red cathode will be less than the other two. If blue color is predominating, the voltage at the blue cathode will be less than the other two.
We must find out the reason for this fault first. Assume that your TV has a red predominant screen with some thin lines across it, at about one inch apart of the entire screen, it is clear that the red cathode voltage is very lower than the other two. The main reason for this fault, normally seen in most of the Sony Trinitron, are loose soldering points either to the picture tube base socket or the resistor connected between the red drive transistor's collector point. Reordering the most of these points on the base card by applying a little more solder will rectify this problem. Other possibility is shorted video output transistor of that cathode.
A direct short [collector to emitter] of
these transistors will drop voltage to very low at that respective
cathode of the picture tube, and that color will be predominant along
with lines. Replacing the transistor will solve the problem. In some
cases, the resistor connected to that transistor may have blown out. If
so, it should also be replaced. Whatever the case may be, voltage at the
respective cathode that emits color will be very low with respect to
the other two. Find out the reason and rectify it. You can compare the
voltages of the other two cathodes with respect to the faulty color
cathode. Cathode pins will be marked on the base card, and can be
identified. It is best to pull out the base card from the picture tube
neck and place it somewhere a safe place without shorting it with other
parts of the TV. You can measure the voltages of these points very
easily, without fearing the damage to the picture tube. After all are
found Ok, replug the base card and switch on the TV. The set will be all
right. In rare cases, chroma IC also can be a reason for this fault. If
it is faulty, after a short period after switch on the TV, the IC will
overheat and shutdown the entire system
