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Posted on Jan 02, 2017
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Jun 02, 2011 | Oldsmobile Alero Cars & Trucks
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.
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