Question about Televison & Video
I have a china t.v that has no display on screen. flyback cap oscilate and there is no horizontal or vertical issu. 180 voltage on base and heater 5 voltage is ok... picture tube is ok what should I do plz help me fixya team..
OK I will try a little bit, If we are talking about a glass tube TV then No picture can be a number of things, first is there a tube heater glowing? have we got EHT and Focus (static when tube front touched or crackle on turn on) next thing is are its colour guns Off by some means (blanking circuit).
This will occur in response to a fault mostly, in the main Frame or Field collapse which causes a line mid screen, this will damage a tube so a sets reaction is to clamp off the guns (no picture).
If I determined EHT and Focus volts existed (if not suspect flyback transfomer) my next move would be to naughty and flash the tube with a resistor, A resistor of about 56K or little higher and a wire at one end long enough to reach from its earth to the tube base, with the set on JUST DAB QUICKLY one of that tubes cathodes while looking at its screen via a mirror. Please note this is non technical way but all you need to know is, is there just a line or a single colour screen (yes I know you tech guys but it is a quick way).
If it is a line then you have frame collapse and thats where to look, if its blank then there is a lack of video drive, possibly a IC or in some old sets I worked on their output emitters sat raised via a simple zener of about 7.5v which goes o/c and its tube guns lift to a high voltage then cut off if I remember rightly cathodes would be around 140 to 165v depending on tube size etc but do not quote me.
Now a thought occurs to me has this flyback transformer got control pots on it, one for focus (top one) and a second for G2 (a high voltage for the tube) note its position before you move it, if after flashing that tube you see a picture then this voltage is low (been got at or faulty transformer) if this transformer has three controls then I would get worried as this idea was employed on monitors to enhance their focus and presented an extra high voltage those I did see with tubes blown (internal flashover) because these transformers had failed.
This is enough to start with hope this helps.........
Posted on Apr 03, 2015
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Posted on Jan 02, 2017
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