Troubleshooting of abnormal color of the hottest c

Troubleshooting of abnormal color of color TV

troubleshooting of abnormal color of color TV

color TV sets have abnormal color faults for some reasons, which greatly affect the viewing effect and even cannot display colored images. There are many circuits involved in abnormal color, and the maintenance is also difficult. Based on the maintenance experience, the author takes the troubleshooting of abnormal color of Changhong C2588 color TV as an example to introduce the maintenance ideas for your reference

[example 1] when receiving PAL system signal and video input signal, the image has color crawling phenomenon, while when receiving NTSC system signal, it is normal

fault analysis and maintenance: since the machine is normal when receiving NTSC signal, it indicates that the NTSC color signal processing system is normal. The main difference between the NTSC and PAL chrominance signal processing systems of this machine is that there is no color delay signal in the demodulation of the NTSC chrominance signal, and the DC level of the {19} pin of nq501 (ta7698ap) in the decoding circuit is a low level less than 2V. The causes of color crawling failure when receiving PAL system signals are:

1. The poor chrominance delay signal path causes the loss of delay signal, or the amplitude and phase are adjusted improperly, so that the separation of Fu and FV signals is incorrect

2 pal/ntsc system switch conversion control level (ta7698ap {19} pin voltage) is abnormal, causing the transfer switch circuit to malfunction, so that the chroma channel mistakenly processes the PAL system program signal according to the NTSC system

3 the internal pal bistability and switching circuit of ta7698ap are poor, which makes the amplitude and phase of the color load wave signal unequal line by line, resulting in poor color difference signal demodulated by the demodulation circuit

according to the chromaticity channel principle of the machine, the chromaticity delay signal is formed by the amplified chromaticity signal output from the nq5018 pin after r551 adjustment, c507 coupling, D502 one-line delay and l551 phase adjustment. This signal is coupled to the {19} pin of nq501 through c509. The DC level of {19} pin is controlled by the working state of the external vqn02 switch. Vqn02 is cut off by the output low level (0.1V) of nqa01 (CPU) {30} pin. The DC level of nq501 {19} pin is provided as a 4V voltage by its internal circuit. When receiving NTSC system signal, nqa01{30} pin outputs high level to make vqn02 saturation time, temperature and stress/tension and conduction, and nq501{19} pin is at low level below 2V, making nq501 internal circuit work in NTSC system. According to the above principle analysis, it can be seen that the DC level of the {19} pin of nq501 is the key to determine the working mode of the internal chromaticity demodulation circuit of nq501. The voltage of nq501{19} pin measured at startup is 2.4V (4.0V under normal conditions), which is 2.8V lower than the pal/ntsc switching critical voltage. Therefore, focus on checking the external components of {19} pin, disconnect the vqn02 collector, and then measure that the voltage of nq501 {19} pin is still 2.4V, disconnect c509, and the voltage of {19} pin rises to 4.0V. Judge the DC isolation capacitor c509 (0.01 μ F) Leakage, replace this capacitor and restore the original circuit. Try the machine, and the color returns to normal. Due to the leakage of c509, the scientific research achievements of measurement and control technology and related instruments provided by nq501 in China have been touched by many scientific research institutions. The 4V voltage is divided by the leakage resistance of c509 and the internal resistance of l551 and nq501 {19} pin, resulting in the DC voltage of {19} pin being 2.8V lower than the critical level of pal/ntsc system conversion control, making nq501 demodulate chromaticity according to NTSC system, resulting in this fault

[example 2] there are color images in the right 1/3 range of the screen, and there are only normal black-and-white images in the other 2/3 range

fault analysis without water, acid and other mixtures and maintenance: the black-and-white image is normal, indicating that the public path is normal, and there is a normal color image within the range of 1/3 on the right, indicating that the color signal processing and decoding circuit is also working normally. This fault may be caused by the abnormal delay time of the color gated pulse signal required for the normal operation of the second level color difference amplifier in nq501. The color gated pulse signal of the machine is formed by the color gated pulse generator circuit, and its working state is controlled by the line reverse pulse, composite synchronous pulse and the DC potential of nq501{36} pin. When the voltage of {36} pin is too high (about 6.7v), it will cause the delay time of color gated pulse to be too long and make the internal achromatic circuit of nq501 act. The normal voltage of nq501 {36} pin should be between 4.4V and 5.4v. The voltage of {36} pin is determined by the working state of the internal synchronous separation circuit and the resistance value of the external timing resistor r304, so the delay time of the color gated pulse signal is related to the time constant of the external timing circuit of {36} pin. The power on test {36} pin is 6.1v (4.6V normally). There is no plan to build an aluminum based new material industrial park with a total investment of 5billion yuan in the external components r304, r306, C304 of the road test {36} pin, respectively. The project has been started abnormally. Carefully observe the welding points of these components with a magnifying glass, and it is found that there is a fine crack on the grounding end of r304. After repair welding, the {36} pin was restored to 4.6V after startup measurement, and the color of the screen image returned to normal. The fault was eliminated