Replaced Flyback Transformer and Horizontal Output Transistor
Had the same problem everyone else describes with this model. Replaced Flyback Transformer and Horizontal Output Transistor. Now the TV powers on and the sound works fine but the picture is so dark it can barely be seen.
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Check and replace damaged component/s at its horizontal scan output section circuit. Check the LOT (Line Output Transformer) too for damage. Replace the horizontal scan output transistor. Check the input voltage to LOT. If the voltage is absent check the feeder resistor; if any. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. http://electronicshelponline.blogspot.com/
The horizontal output transistor that drives the flyback transformer overheats now. There might be other reasons too for this fault. If the flyback transformer has any internal winding short, this transistor will overheat. Just replace the horizontal output transistor. Check its heat. If the newly replaced transistor too overheats, replace the flyback transformer.
Along with the flyback transofrmer, its horizontal output transistor also must be replaced. I don't know whether you have replaced it or not. If not, replace it too. It can be located very near to the flyback transformer, a three terminal component, black in colour, usually screwed to a metal heatsink. Then switch on the TV and measure the video output voltage at the video output card pluged directly to the picture tube neck. The collector of the video aoutput transistors must come around 145VDC. If it also is there, advance the screen control on the flyback transformer to get a raster. If that voltage is not there, it means that the flyback transformer is not working, and horizontal oscillator stage should be checked for oscillation, check the lined drive transformer for its primary winding open, Check the entire horizontal oscillator section. The collector point of the line drive transistor, the transistor that drives the line drive transformer, a small transformer soldered diretly to the printed circuit board; should be checked with frequency meter. The frequency at that point should come around 15525HZ approx: OK.
The F27442 uses the CTC203 series chassis and that symptom is very common. If the horizontal output is shorted you MUST resolder the coil in series with the base of that transistor and the pins of the small horizontal drive transformer. When they have been properly resoldered you can put in a new original horizontal output transistor. If the output transistor shorts out again then your flyback (high voltage) transformer is shorted and needs to be replaced.
The F27669 uses the CTC203 series chassis. The most common cause of that symptom is a shorted horizontal output transistor, caused by a cracked solder joint on a small coil connected to that transistor's base. When this happens, the main B+ supply is shorted to ground so the power supply squeels. Every once in a while I also see the horizontal output transistor shorted due to a shorted flyback (high voltage) transformer. A good tech will replace the horizontal output transistor, resolder the coil and drive transformer, test the flyback (replacing it if necessary) and your set will be back to perfect.
When the horizontal output (2SC4927) is shorted it means one of 2 things is wrong - the horizontal drive transformer has cracked solder joints or the flyback transformer is shorted inside. If the solder joints on the horizontal drive transformer (the small grey transformer connected to the base of the horizontal output transistor) are cracked then the set will play for a little while and then the parts you just replaced will again be destroyed. ALWAYS resolder the driver transformer when replacing the parts you mentioned! When I replace the parts you mention and I do not see any obvious cracks on the driver transformer then I unsolder the B+ pin of the flyback (labeled 135, for 135 volts) and put a 60 to 100 watt lightbulb (drop-light) in series with the flyback. This means that the 135 volts B+ goes through the lightbulb and then into the flyback, so the lightbulb acts as a current limiter to protect the horizontal output. When you turn the set on the bulb should glow no more than about half bright and you should hear the high voltage come up, but you should not see a picture. If you turn the set on and the bulb glows as bright as if it was plugged into the wall then you can bet that flyback is shorted and must be replaced.
Yes, you should check other parts. The horizontal output transistor is more than likely shorted. Also the flyback could have gone bad. Some SONYs have problems with the horizontal driver transformer which is connected to the base circuit of the horizontal output transistor. If this little transformer has a loose solder connection, it will take out the horizontal output transistor.
This may give you some insight as to where to look for your problem.