Old 21-inch Changhong "color cast" repair case - Appliances

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KNA-21-400-W3

An old user should be repaired, the TV is Changhong 21 inch, [previously repaired is three no faults, the chip is LA7688, spare parts, and took an old circuit diagram], the user describes the fault is color cast. Figure:

Hey, this fault is the best, nothing more than a green gun on the tailgate or LA7688. First turn off the color saturation, look at the image, stuffy... is a pure black and white image, which shows that the green gun circuit is normal, including the entire tailgate matrix and the picture tube are normal. The 34 feet of the LA7688 [green gun] output pin voltage is higher than the red, the blue gun is high, the color saturation is turned off, and the red, green and blue three gun voltages are measured. Actually, the balance is turned off, and the three gun resistances in the road test are also balanced. Is it LA7688 or a line of baseband delay circuits? Next, the baseband delay integrated LC89950, and the peripheral components are still the sound of the sound, the only thing related to the color cast is LA7688. Fortunately, there are components, and the 7688 is removed with confidence, and the new 7688 is replaced with joy. . I thought: Oh, this ticket is imported into the bag! Check undoubtedly immediately after the boot, hate! ! ! Like a glimpse, it is green! green! green! green! Take out the drawings and scan them carefully from 1 foot to 52 feet. Only 33, 34, 35, 36, 37, 38, 39 are related to the color cast. They are all checked, normal! At this time, I couldn't sit still. The user was also supervising the whole process. The user was a reasonable person. He said, "Master, slow check, this is hard to beat you." After listening to this, I feel a lot better. At this time, I calm down and think about it. It seems that I have missed it. It is a standard! ! ! This movement has three standards: PAI/NTSC/SECAM. Try to press the three systems with the remote control, the images are all green, and the user inserts a sentence next to them. "It is like this, it is all green." The speaker is not interested, the listener has the heart, there is! ! NTSC should be black and white in any case of color cast. What does it mean? There is only one explanation - that is, the three systems have not changed, and in the SECAM system, because the SECAM system is greenish, it is too smart. The standard electronic conversion IC is a well-known 4052, taking into account the conversion of images and sound systems. This time, I learned. The voltage of pin 1 is in normal TV state, SECAM is 0-1.4V, PAL/NTSC is 1.7-2.6V. After understanding this point, immediately measure the voltage of 1 foot [AV/SECAM/SIFIN] of LA7688 and convert according to the standard. The voltage is only about 1.3V, which means that it belongs to the SECAM state, no wonder it is green. Haha, LA7688 has also been replaced, the only remaining 1 foot external components, check the 1 chip outside the 1K resistor connected to the ceramic capacitor C173 [103PF], the result of leakage, and 103PF ceramic capacitor. Needless to say, the above picture first:
For everyone to observe, the capacitor has been soldered to the back, remember C173, is 103PF! ! !

The repaired picture, there are pictures to prove:
This fault was discovered for the first time and I hope to help everyone in the future.