Tubes are all the tv cathode ray tubes the same and can they be replaced by others as long as they are the same size

NO PICTURE; SOUND OUTPUT OK; PICTURE TUBE BASED COLOR TELEVISIONS.

 

This is a common fault to most of the picture tube based televisions, irrespective of its brand and model number.  There are several reasons for this fault, and 90% of them are related with its video output card, usually located at the neck of the picture tube. Plugged on to its neck.  This card can be pulled out from the neck of the picture tube to repair.  Pull it strait back, no sideways movement should be done, while pulling it out, as it may cause damage to the connection pins of the picture tube.

 

 If you see a color predominant screen that is, if the screen shows excessive blue, red or green color with nice white lines all across the screen, lying parallel to each other and inch apart or so.

 

Check which color is predominant?  Just assume the blue color predominant screen with nice slanting line across the entire screen, about an inch apart to each other.

 

The first thing to do is to measure the voltage at the blue cathode of the picture tube.  You can do this check, while the set is On.  Set a multimeter to measure DC voltage range 500V or above.  Check the voltage at the blue cathode.  You can see markings on the card Bk, Rk & Gk.  All of these are the three cathode connection points.  Among the, Bk is now out discussion point now.

Understand that, the cathodes of a vacuum tube, is negatively biased, that is, if the voltage of the cathode will go down with respect to its screen voltage, the emission of that cathode will increase, and vice versa.  In our case, now, the voltage at the Bk will be less.  Check the voltage of the other two cathode points for reference.  A slight voltage difference, says about 3 volts or so, is Ok, but should never be too below to this.

 

Here one thing to note is, that the voltages of the other two cathodes are high, and the voltage to the blue cathode only is very low.  This is the reason you see a high intensity blue screen.  It were happened to one among other cathodes, say if the voltage at RK is low, you will see a high intensity red screen, and if voltage at Gk is very low, you will see a high intensity green screen.  Of course all this screen will have lines as said earlier [retrace lines] will be there.

 

Let us come to the reasons for this fault.

 

Take the case of blue predominant screen as stated earlier.  It is sure that the voltage at the blue cathode will be low in this case.  That is, the blue cathode pin of the picture tube does not get proper voltage to cut off.

Check the solder terminal of the picture tube base socket pin of blue cathode first.  It is advised to resolder all the crt base connecting socket, soldered on the CRt base board [video output board], by applying a little more solder, because, these solder terminals will heat up while TV works.  Repeated heating and cooling may make the solder terminals to get loose, and no proper connection.

If the solder terminals are OK, there is blue predominant screen, check this resistor connected to the blue video output transistor for value developed or loose soldered.  Check the voltage across the tow terminals of this resistor.  This is the collector current limiting resistor, and will be of a resistance value between 10 to 15Kohms/2W.  If the value of this resistor had developed high, the voltage at the Bk will be low.

If the resistor is found OK, and still the voltage at the Bk is less, than the other two cathodes, check the video output transistor [blue] for Collector to Emitter short.  Replace it is advised.

<p>How to check the colour cathodes of a picture tube for emission<br /> <p> <br> <p>It is very easy to cehck the emission of the cathodes of a colour picture tube. Assuming all other circuit parts are working, including LOT [Line output Transformer] and video output section.<span> </SPAN>There is proper screen voltage; focus voltage and EHT [Extra High Tension]. Locate the three cathode points on the base card, usually marked as Gk, Bk and Rk.<span> </SPAN>Keep the brightness, contrast and colour control to zero position.<span> </SPAN>Take a wire of suitable length, say 50cms or so, tie its one end to TVs chasis [negative].<span> </SPAN>Take one resistor of value 100K 1W, tie its one end to the other end or the wire, thus one end of this resistor is now connected to negative chasis and the other end of it is free now.<span> </SPAN>Hold this resistor with with some electrically noncunductive material [using hand gloves will be better]; switch on the Tv, wait a minute or so to get it warm up.<span> </SPAN>Just make a touch at each cathode of the picture tube base card solderings.<span> </SPAN>Just one touch will give you a colour of the respecive cathode on the screen.<span> </SPAN>Never hold it for long time. This process can be used for checking cathode emission of any type of Cathode Ray Picture tubes. <br> <p>

Sony Trinitron sets are of CRT [Caothde Ray Tube] types. The common fault seen in these type of TVs are any one colour predominant on screen wih sound OK. Either red, blue or green colour 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 pluged onto the picutre 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 colour. If no emission from all guns, results in a blank screen, ie black screen without any disply at all. The three colour beams together produces proper coloured picture what we see on screen. These three gun's terminals are brought out by pins of the picture tube outside to which video signals are given in appropriate proportion to produce a TV picture. Normally, all these three pins are biased with a voltage of about 170 to160VDC+ve. Take one colour gun, as an example and 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 emisson incresses and vice versa. If the emission increase, the colour 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 resepect to the other two. If red colour is predominant, the voltage at red cathode will be less than the other two. If blue colour is predominat, 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 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. Resoldering the most of these points on the base card by applying a little more solder will rectify this probelm. Other possibility is shorted video output transistor of that cathode. A direct short [collector to emitter] of these transistors will drop volatge to very low at that respective cathode of the picture tube, and that colour will be pridominant 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 alo be replaced. Whatever the case may be, voltage at the respective cathode that emits colour 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 colur 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 easly, without fearing the damge to the picture tube. After all are found Ok, replug the base card and switch on the TV. The set will be alright. 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.