Observe the machine first boot about ten minutes after the image gradually appear "nap", about half an hour after the image details can not be resolved, its like a video with repeated after ripping replay of the situation. At the same time, the whole image has a dark shadow interference. After the failure of the brightness has declined. After a while restart the image normal, but the failure is soon re emergence. From the starting to the failure of the time and the length of time with the machine temperature, the higher the temperature the faster the failure occurs.
Analysis and inference: from the failure phenomenon analysis, this is a temperature related failure. After the failure of the image details can not be resolved, the high-frequency components of the video signal is lost. Since the image synchronization state is always good, so the inspection focus should be placed on the video part of the circuit.
General TV video portion of the circuit is relatively simple, from put the video signal output circuit enters the decoding IC for color decoding and brightness signal amplification, to output depending on the final directly promote the imaging tube cathode. But the machine uses Panasonic m18m movement, the multifunctional machine, and the use of the various video processing technology. Therefore, the video signal process twists and turns, is very complex. According to the circuit diagram of sorting out the PAL signal process is shown in figure.
Put the video signal circuit IC101 (M52760SP) output by IC201 M52317SP TV mode selection of, rushed to the AV / TV conversion circuit IC3001 AN5858K, IC3001 select the desired video signal and by {21} pin output, rushed to the Y / C separation circuit separation excellent signal (c) and the luminance signal (y), back again IC3001{20}, {19} feet. By IC3001{17} feet in the output signal Y, {15} foot output C signal. The Y signal in turn into the image of artificial intelligence (A1) circuit, horizontal sharpness enhancement circuit for processing, and then sent to the color processing circuit IC601 (ta1215an).
By C signal IC3001{15} pin outputs, PAL4.43MHz chrominance signal by the foot of the IC601{49} into, the demodulation of IC601 a R - y, B-Y signal, respectively by {40}, {41} feet in the output and sent to 1H delay line IC603 TA8772AN {27}, {26} feet. IC603 and after 1H delay r - y, B - Y signal respectively from {30}, {29} output pin, return to the foot of IC601{37}, {38}, then within the IC601 by color difference matrix circuit to produce a G Y signal.
Image artificial intelligent circuit and horizontal sharpness enhancement circuit for processing a luminance signal input by the IC601{60} feet, in the interior of the IC601 for further processing of evacuation to the IC's internal color matrix circuit, and R - y, g y, B-Y color difference signal together to demodulate R, G and B color signals, and then by the {16}, {17}, {18} pin output to the IC3401 TA8889AP {1}, {3}, {6} feet. IC3401 is the I2C bus control of white balance adjustment circuit, the adjusted R, G, B signal by the {16}, {14}, {12} output, send to the last stage depending on the circuit IC351 (tda6103q) {1}, {2}, {3} feet. After amplification by {9}, {8}, {7} feet in the output to the imaging tube cathode.
From this, the video signal of the machine is more than the path, it is not easy to determine the loss of the high frequency component in the video signal.. However, it should be noted that this fault with image quality becomes poor and brightness decline phenomenon, and brightness changes, will some point appeared in the DC potential changes, from the lookup caused tube brightness decline reason of, may be able to find out fault reason.
Maintenance process: according to the above analysis, first check overall offer small signal processing circuit of 9V, 12V voltage, in to appear before and after the fault had no change, excluded by the bad power supply caused by reasons.
Then measured IC351 R, G, B output end {7}, {8}, {9} pin voltage, found after a failure in the voltage from the normal value about 160V were up to 202V, 197V, 197V. Because the IC351{5} pin is the reference voltage sample, the change of the end voltage will affect the DC voltage of the output of G, B and R simultaneously.. Therefore, the monitoring of the IC351{5} pin voltage is not changed before and after the failure. The next step is to detect the DC voltage of the IC351 input.. Because of its {1}, {3}, {2} foot and IC3401{16}, {14}, {12} foot as DC coupling, the voltage of IC3401{12}, {14} and {16} was measured directly for the short overhaul time.. Just when the phone is switched on, the IC3401 the tripod DC voltage respectively 2.67v, 2.85V, 3.02V. With the boot time decreased to 2.10V, 2.17V, 2.31v, its input end {1}, {3}, {6} feet DC voltage is respectively from the 2.10V, 2.23V, 2.39V fell 1.70 V (SCE). And 1.69V, 1.69V. Because of IC3401{1}, {3}, {6} feet and IC601{16}, {17}, {18} feet through and DC voltage variation IC601 input end of the may also affect the output DC voltage changes, therefore, measuring IC601 RY, B-Y input end {37}, {38} pin voltage, stable without change