Solution: Replace with a 9V regulator diode, and the fault is eliminated. In Changhong TV, after turning on for 20 minutes, the image color gradually disappears, but the contrast of the black-and-white image is normal, and there are no snowy noise spots across the screen.
Analysis and Maintenance: Focus on checking the chrominance processing circuit in Changhong TV. The chrominance processing of this TV is completed by NQ501 (TA8783N). By touching each component in the chrominance signal processing circuit, it was found that the temperature around pins 39, 40, 41, 42, and 43 of NQ501 was significantly higher than other areas. After using the alcohol cooling method for 20 seconds, the color suddenly returned to normal, indicating that the fault was caused by excessive temperature rise of NQ501. Measuring the voltages at pins 61, 63, and pin 6 (+12V) of NQ501 showed normal readings, but the Vcc voltage at pin 4 was significantly higher than the normal value of 2.4V. Upon further investigation, it was determined that the horizontal scanning regulator diode VD401 had poor thermal stability.
Source: Beijing Changhong TV Repair Center http://blog.tianya.cn/blogger/blog_main.asp?BlogID=3764634.