Maintenance of YC separation circuit of Panasonic

2022-08-23
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Panasonic m17w widescreen color TV y/c separation circuit maintenance

1 Only when receiving pal/ntsc3.58 system signal, there is no image. This fault is obviously caused by no bright color separation signal output caused by abnormal operation of digital comb filter circuit. You should focus on checking the y/c separation circuit shown in Figure 2. First, check whether there is video signal input at the ic5501 pin. The electrolyte and diaphragm of lithium-ion battery are becoming more and more important to improve the performance and safety of the battery. If not, check the buffer circuits q5521, q5522, q5516 and their bias circuits and relevant components of the video signal coupling circuit; If yes, check whether the DC working voltage of relevant pins of ic5501 is normal. If the pin voltage is abnormal, check the peripheral circuit of the corresponding pin; If the pin voltage is basically normal, retest that there is a colorless subcarrier synchronous clock signal input at ic5501 pin. If there is, check ic5501 and its peripheral circuit components; If not, check the signal amplification circuit components connected outside the pin

2. Only when receiving pal/ntsc3.58 system signal, there is no color. It can be seen from Figure 1 that there are respective transmission channels after the pal ntsc3.58 system chrominance signal is sent to pins ic5503 and ⑤, and the probability of chrominance signal loss caused by the simultaneous damage of their respective transmission channel circuits is very small. Therefore, the fault element should be connected to the chrominance signal buffer amplification circuit of comb filter ic5501 and its pin. During maintenance, first measure whether there is chrominance signal output at ic5501 pin. If so, check whether q5504, q5503 and their bias circuit and signal coupling circuit components are damaged; If not, check whether the voltages of ⑤, ⑩, and pins of ic5501 are normal (the normal values are 4.8, 4.9, 4.7, and 4.9v respectively when receiving pal signals; the normal values are 0.1, 0.1, 0.1, and 0.1V respectively when receiving ntsc3.58 signals). If the voltage of these pins is abnormal, check the external circuit components and circuits of the corresponding pins; If all are normal, it is judged that the integrated block ic5501 is damaged and needs to be replaced

3 When receiving pal/ntsc3.58 video signal, there is a very dark and distorted color image on the screen, while when receiving black-and-white signal, the black-and-white image is clear and adjustable. It can be seen from Figure 1 that when the received signal is black-and-white TV signal, the image is clear and adjustable, indicating that the brightness signal processing circuit after ic5502 pin works normally, then the fault source of this example must be in the comb filter circuit shown in Figure 1. The fact that distorted color images can be displayed on the screen indicates that the ic5501 pin has chrominance signal output; Therefore, the fault source of this example must be in the internal circuit of ic5501 pin or its external buffer amplification circuit. During maintenance, first measure whether the ic5501 pin has brightness signal waveform output. If yes, check whether q5507, q5506, q5505 and their bias circuit and signal coupling circuit components are damaged; If not, retest whether the ② pin voltage of ic5501 4 stops automatically High level above 5V. If the pin has a high level, check ic5501 and its surrounding circuit components; If not, check whether r5588 is open circuit or its external circuit is broken, or the internal circuit of ic5501 ② pin is short circuited and damaged

4. There is no color when only receiving NTSC color video signal, but the black-and-white image is normal. This fault is obviously caused by the loss of chrominance signal caused by the damage of chrominance bandpass filter circuit in ntsc4.43 y/c separation circuit. During maintenance, first measure whether there is chrominance signal input at ic5503 ⑦ pin. If there is, it indicates that the analog electronic switch circuit of ic5503 ⑦ and ⑧ pin is poor; If not, check whether the band-pass filter lc5501 is damaged by leakage, and whether the r555 notice indicates that 7 and c5529 are damaged by open circuit, etc

5. When receiving color video signals, the images are all normal. When receiving black-and-white TV signals, there is no image on the screen. It is not difficult to see from Figure 1 that the fault source is likely to be in the emitter buffer and filter circuit between q5511 emitter and ic5502 ⑦ pin. During maintenance, first check whether there is video signal input at pin ⑦ of ic5502. If so, check whether the external circuit components of ic5502 and pin ④ are damaged; If not, check whether the base and emitter voltages of q5534 and q5535 are normal (in normal operation, the base and emitter voltages of q5534 are 4.8 and 4.1V respectively; the base and emitter voltages of q5535 are 4.1 and 3.5V respectively). If all are normal, check whether c5538 and its surrounding printed lines are broken or open circuit damaged; If the measured voltage is abnormal, check whether the external circuit components of the relevant pins are damaged, or whether q5534 and q5535 are damaged, etc

pin functions and normal operating voltage parameters of comb filter IC ic5501 (mn5236) are shown in Table 2. Since the y/c separation circuit of the main screen of Panasonic m18w core widescreen color TV is basically the same as that of the core, the content described in this paper is also applicable to Panasonic m18w core series color TV

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