Before performing any repair work, a close look to all the solder terminals at the video output card is necessary. In most cases, loose solder terminals can cause severe faults that can’t be predictable. In this section, most of the components will get heated while set works. Repeated heating and cooling might make the solder terminals to get loose contacted due to expansion and contraction of component leads. It is advised to re-solder all the suspected solder terminals by applying a little more fresh solder to each terminal, without making any solder bridge short in between adjacent solder terminals. Always give preference to this. In most cases, the cause can be this. Go further only after this check.
This is the most common fault to CRT TVs, irrespective of brand. As far as sound is OK, we can determine that all other circuit sections, Tuner, Video IF, Audio Processing, Audio Output and all the voltage lines to these circuit are OK, unless there might me no audio too. Audio output can be voice or hissing noise. Whatever it may be, is there, and so all the above said circuit sections are functions well. The fault should be with or around the video output section circuit. As you know, from the transmitter, audio, video and sync: signals are transmitted together within the carrier wave. Here, TV set receives all of them, but unable to produce picture on screen. The raster is Ok. So all the voltages, Filament voltage, Screen voltage, Focus grid voltage, EHT voltage are all should be there. So concentrate on the video output circuit, which processes video signals to drive the cathodes of the CRT.
Three are many type of video output circuit designs. Some of them uses discrete transistors, and others uses an IC to process R-G-B signals, or use 3 Ics to processes these three signals. What ever the type may be, functions to them are equivalent to each other.
Let’s take a typical video amplifier circuit for reference. Here I’ve chose a simple video amplifier circuit, uses with 80% of TVs now in production. It uses three discrete transistors to amplify the R-G & B video signals, which in turn fed to red, green and blue cathodes of the picture tube. There is an other transistor too, witch functions as the current limiter to the above said three video output transistors. Check the schematic diagram and follow please. You can enlarge the image here by clicking over it once or two.
Check the circuit. It is the circuit diagram of Matsui 25M CRT TV. The video output section circuit can be located at the hack end of the picture tube, will directly plugged on to it. It can be unplugged out from it, by a slight straight backward pull.
Just see, there are three transistors for color amplification. If you look close, the circuit for each of them is equal to one another. So, for ease of description, let’s take one among them. Take [Q601] B-Out. The transistor number is 2SC2621. It is an NPN transistor used sot amplify Blue signal. See, the collector of that transistor is connected to Blue cathode of CRT via R605 [3Kohms]. The blue cathode of the picture tube gets its drive from the collector point of this transistor. Here, we have to note other main point. Actually picture tube is a vacuum tube, very like one used in old vacuum tube radio sets. It is very similar to construction too. The point to note is, as the voltage to the cathode go down with respect to its screen voltage, the emission of the cathode will increase, and number of emerging electrons will also increase, and thereby that particular cathode will produce its typical color on screen. Thus if the voltage at the Blue cathode go below normal, the screen will have a bluish shade, if the red goes low, will produce a red shade, and if green goes low, will produce a green shade. No I thins that, the reason for one color predominant screen by CRT TVs has cleared.
Now arises the question, why this happens so. Why does the voltage drops below normal. What are the voltages taken to reference? Reasons? Rectifications? I’ll come back to you by next post.