The machine using small-signal processing LSI LA76810A complete the whole. Above symptom occurs most likely to occur in high frequency circuit. Check the LA76810A related to the intermediate frequency processing circuit (3~10, 48 and 49 feet voltage, most pin voltage anomalies, suspect LA76810A damaged, replacement failure does not change. Examines 3-10 pin peripheral components, no problems found. Then replace the 48, 49 feet add-38MHz week T101, high frequency surface acoustic filter in first class, failure cannot be ruled out.

Considering that LA76810A is I2C bus control, if the I2C bus is faulty, can also cause non-silent. Then measure the 11 foot (I2C bus communication), 12 feet (I2C bus clock end) voltage is 4. 6V, preliminary judge I2C bus correctly.

Check information, LA76810A video detection using PLL PLL synchronous detection mode. See photos, image after image medium frequency amplifier amplifier if signals synchronous detection circuit, this circuit for intermediate frequency signal and the reference signal in the image (generated by the image-if voltage-controlled oscillator) for detection, and SIF II signal demodulated video signal. Synchronous detection needed to reference the carrier signal, from the image-if voltage-controlled oscillator (38MHzVCO). The voltage-controlled oscillator actually is a phase-locked loop (PLL), the course of their work are: LA76810A48, 49-foot circuit and LC components of external voltage-controlled oscillator, the resulting oscillation signal send synchronous detection circuit, the other sent upon frequency discriminator. 4. 43MHz color Deputy carrier by crossover Hou also sent mirror frequency device, in mirror frequency device in the two signal for frequency and phase compared, dang both of frequency and phase inconsistent Shi, seized out errors signal, by 50 feet add-ins resistance capacity components composition of double time constants filtering circuit smooth filtering, get DC voltage, sent 38MHz pressure controlled oscillation device, control oscillation device of frequency and phase. Oscillation frequency of the oscillator and phase images always with the tuner output frequency (38MHz) the frequency and phase of the signal line, the synchronous detection circuit registered the largest image of the demodulated signal level. Adjust the image in the week T101, you can fine-tune the image if VCO frequency. Above of the PLL voltage-controlled oscillator reference frequency, phase and amplitude of the carrier signal is stable and not under the influence of changes of image content. Improves image detection characteristics of the synchronous detection circuit and has good immunity, making the image and sound quality was greatly improved.

Based on the foregoing analysis, 4. 43MHz color subcarrier recovery circuit is faulty, not 4. 43MHz color subcarrier signal or frequency offset too large can cause image VCO does not have a normal 38MHz reference carrier, causing the video sync detector circuit is not working properly. Cannot be demodulated video signal if signal from the image and the second sound signal, there will be no video and no audio. Then repair shift to LA76810A color subcarrier signal circuitry. Measuring voltage of 36, 38, 39 feet, 36 feet 3. 1V below normal 3. 8V,38 foot is 3. Normal 9V. 39 foot 4. 2V higher than normal 3. 5V。 Check the color subcarrier of 36 foot external VCO low pass filter capacitor C210; 39-foot external PLL circuit low pass filter for color subcarrier-VCO circuit components, C207, C208, R205, R206; Finally, authentic 4. 43MHz Crystal replacement G201, image and sound are back to normal.

Summary: Sanyo A12 as with LA76810A complete the whole small-signal processing, LA76810A just a 4. 43MHz crystal. PAL chroma demodulator circuit required 4. 43MHz color subcarrier signal. Can also be frequency NTSC chroma demodulator circuit required 3. 58MHz color subcarrier signals in addition to the machine's 4. 43MHz colour subcarrier after pulse-frequency, if voltage-controlled oscillator (38MHz VCO) compares the frequency, if voltage-controlled oscillator frequency correction. Therefore, the A12-4. Bad 43MHz crystals, light, resulting in images without color, severe cases can lead to frequency drift (running), as well as automatic search does not exist and even result in no image, no sound.