Tip & How-To about Televison & Video

Cathode emission check for colour picture tubes

How to check the colour cathodes of a picture tube for emission


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. 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. Keep the brightness, contrast and colour control to zero position. Take a wire of suitable length, say 50cms or so, tie its one end to TVs chasis [negative]. 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. 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. Just make a touch at each cathode of the picture tube base card solderings. Just one touch will give you a colour of the respecive cathode on the screen. Never hold it for long time. This process can be used for checking cathode emission of any type of Cathode Ray Picture tubes.

Posted by on

Televison & Video Logo

Related Topics:

Related Questions:

1 Answer

friend gave us a toshiba a3642 tv,the screen is green,is this worth fixing?


Hello

Your TV is has no green cathode voltage at the picture tube pin. There are three electron guns for every colur picture tubes, Red, Blue and Green. These three guns together mixes all colours we see on the screen. If any one of them happen to be not working properly, that colour must either be lack, of excess. Voltages at the above said three colour cathodes are somewhat equal to each other. if voltage goes down, the emission of that particular cathode will rise, and as a result that colour will be predominent on the screen.
These cathodes are fed voltages from the video output stage of your Tv. This stage can be located at the neck of the picture tube itself, as a card pluged onto it, called the base card. There will be either three trasistors to drive the each above said three cathodes, in some sets, there will be independent three ICs or only one IC capabale of driving these three catodes will be used.
You just open your TV, after unpluging it from the wall power socket. You can locate the above said base card, at the neck of the picture tube. Before doing anything, check very keenly for any loose solderings are there on the CRT base board. It is very difficult to locate the loose solderings, as they may be microscopic in size. So I advice you to resolder all the soldering points on that board, by applying a little more solder, without making any short circuit bridges in between.
In most cases. this will solve the problem. If not, check the voltages at the above said three cathode, wheere tey were marked on the PCB itsef as R ,G & B. You can see that the green cathode voltage of your set will be much lower than the other tow, in your TVs case. If so, check the resistor connected to the revalent Ic or transistor, whichever it may be, for open circuit. that is check whether it has burned or not. If that resistor is Ok, replace the transistor or IC, which drive the green cathode.
If you wants furthermore assistance, feel free to get me through Fixya. OK.

Dec 15, 2010 | Toshiba 36A42 36" TV

1 Answer

Pink color appearence when swith on tv for 1 minutes & wringles form on the screen


Hello

Pink colour denotes the absence of green colour dispay in your picture tube. I hereby gies an approximate voltages at the three cathodes of your picture tube socket solderings, and you just measure the voltages with reference to this. The voltages are approximate, and can chage up to 2% up or down.
Green Cathode = Pin number 9 [161VDC approx]; Blue Cathode = Pin number 8 [162VDC apprpx]; and Red cathode = Pin number 10 [157VDC approx]

If any of these voltage differes, the clolur related to that cahode will also rise or low, thath is if the cathode voltage goes very low, that clour will be prdominent, and if it goes high, that colour will not be there.
In your TVs case, the green cathode voltage now will be high. t may be due to a soldering loose to either to the picture tube base socket or at the terminals of the related transistor or Ic. It is best to resolder the suspected loose soldering points by applying a little more solder, as we in service field also do. It is very difficult to locate the exact loose soldering point and so I advice you to do so. If needs, replace the colour output transistors, of other devices if used. Just look at the voateg at pin number 9 at the picture tube base card. Resolder all the picture tube base socket pins. If you wants any more calrification, contact me through Fixya. Ok.

Nov 25, 2010 | Sony KV-T29CF1 TV

1 Answer

no green and yellow colour


Hello
If you see the picture in yellow colour, means that there is no blue colour output from the video drive section in your tv, Yello is the mixtrure colour of red and green. Green is actualy there. blue colour is absent.
To detect this fault, you must open your tv. Just look in to the video output section, can be located at the picture tube base card, which is plughed into it diretly. See for any loose soldering points there. Check whether transisotors are used for red, green and blue output stages, or an Ic or three seperate Ics are used for this purpose. Anyway, blue catode of your picture tube pin gets a hig voltage, and so the blue emission is absent. You just check the voltages at red,gree and blue catodes, at the respective picture tube pins, which can be taken at picture tube base card. All of these three pin voltages must be somewhat equal to each other. In your case, no voltage at blue cathode will be higher that the other two. Check for the reason. It may be due to an open video output transistor [blue], or an Ic, of one Ic if used for the three colour outputs. Check and replace the same, will rectify your problem. Ok
If you have andy difficulty, get it to me through Fixya. Ok
Gopakumar, India.

Nov 21, 2010 | JVC AV-27230 27" TV

Not finding what you are looking for?

353 people viewed this tip

Ask a Question

Usually answered in minutes!

Top Televison & Video Experts

The Knight
The Knight

Level 3 Expert

74136 Answers

Cindy Wells

Level 3 Expert

5105 Answers

Electro Med Services...
Electro Med Services...

Level 3 Expert

6694 Answers

Are you a Televison and Video Expert? Answer questions, earn points and help others

Answer questions

Loading...