According to the MPEC decoding has the characteristics of image and sound interlocked, due to broadcast the sound normal on the basis to determine decompression circuit and the circuit is normal, the fault should be in and peripheral circuit with video coding circuit U18 (AV3169), as shown in Figure 6. The video coding circuit of the machine uses the special digital video coding IC AV3169 (U18). The work process is: single chip MT1369EE (U3) decoding output video data (YOVO~YUV7), from the U18 11 ~ 18, 16 ~ 19 foot input AV3169. In the U18. First of all, the processing of digital video signal into y, u, V signal, then press the PAL / NTSC encoded request, to compile a luminance and chrominance data, then by the DAC converter reduction into analog composite video signal Y, red color difference component CR signal (R-Y), blue color difference signal components CB (B-Y), respectively from U18 level 31, 33, 35 foot output, after low pass filter output port. , the required line, the field synchronous signal from 22, 21 foot input. Video encoder U18 is required for the 27MHz clock from 2 foot input.4haservice manual download site
First pilot device measuring U18 (y video output), pin 33, 35 feet were no output signal, and measuring the U18 11 ~ 16, 18, 19 feet input video data signal YOVO~YUV7 normal. Then check the U18 20 feet field synchronous signal and 21 feet of row synchronization signal is also normal, and then measured normal 27MHz clock signal input pin 2, 6 feet reset signal is normal, used for control of 23 feet (SDA serial data), 24 feet SCL (serial clock) are also no problem. Use multimeter measurement U18 pin of the direct current voltage, a power supply end 5, 10, 22, 29, 32, the normal working voltage 36 feet 4.82V, but a stomp the voltage is not normal, which the voltage is not normal pin 31, 33, 35 feet voltage are 0V (normally the 0.6V ~ 1.1V), 37 foot 0.6V (normal 1.3V), 38 feet 4.4V (normal 2.6V), 39 foot 0.1V (normal 1.3V), 40 feet of 0.1V (normal 1.3V). Therefore, the suspect AV3169 internal circuit has been damaged. However, the same type of IC after the failure of the same replacement. After testing each foot in resistance and found that 40 feet of positive and reverse resistance than normal resistance is much smaller, is about 1K, judge the foot connecting chip capacitors C136 leakage. When the C136 was welded and then measured the 40 pin to the ground and the reverse resistance recovered to normal. The leakage resistance of C136 is about 1K.. Because the capacitance is not clear C136, decided to use ceramic capacitors with different capacities, through experiments to determine the value to. Find the machine this capacitance requirement is not strict, large capacitance in small dots machine can normal work, play all kinds of disk images and sounds perfectly normal, even not to install C136 nor to machine the performance have adverse effects.

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