Model: Sony KV-2565MT TV
Fault Analysis and Resolution: From the phenomenon that both the image and sound are normal, this fault may be caused by the misoperation of the protection circuit. During normal operation, the measured power supply voltage is 136V, and the collector voltage of the horizontal push tube Q801 is 56V. The power supply voltage is normal. After the automatic shutdown, the measured power supply voltage and the electrode voltage of the horizontal push tube are both 144V, and the base voltage of the horizontal push tube Q801 is 0V. After the automatic shutdown, the output voltage of the switch power supply is higher than the normal operating voltage, which is estimated to be related to the fact that the horizontal output circuit is not working. The base of the horizontal push tube is 0V, indicating that the horizontal excitation pulse forming circuit is not working. There are three reasons for the failure of IC301 horizontal oscillation: first, the abnormal supply voltage of the integrated block; second, the X-ray protection circuit is not caused by the abnormal supply voltage of the integrated fast and the integrated block itself or the poor thermal stability of the external crystal oscillator, but by the X-ray protection circuit and overcurrent protection circuit. Measuring the power supply voltage as 136V proves it's not overvoltage protection but overcurrent protection. The overcurrent protection circuit is composed of R340, D311, Q312, Q313, and the bias circuits of Q312 and Q313. When the circuit works normally, the voltage across R340 is small, at 0.3V, the base voltage of Q312 is high, at 135V; the emitter voltage of Q312 is 134.7V, and Q312 is in a reverse-biased cut-off state. The 22nd foot of IC301 is at a low level, and the circuit works normally. When the current in the circuit is too large, the voltage across R340 increases, and the positive pole voltage of D311 decreases. This voltage goes through D311, R338 to the base of Q312, making Q312 conductive, the base potential of Q313 rises, the collector potential of Q313 drops, the base potential of Q312 further decreases, accelerating the conduction of Q312, making the collector voltage of Q312 rise further, forming a positive feedback loop. When the potential of the 22nd foot of IC301 rises to 5V, the horizontal circuit stops oscillating, serving a protective function. Upon measuring the voltage at the 22nd foot of IC301, it was found that the voltage had already reached 4.8V, which is 1V higher than the normal operating voltage. The increase in the voltage at the 22nd foot indicates that the externally connected overcurrent protection detection circuit has started and entered the working state, so the key part of the circuit should be inspected. Upon inspection, it was found that the nominal value of R340 is 1Ω, but the actual measured resistance is 3Ω, indicating an increased resistance. After powering on, the voltage drop across R340 gradually increases, causing the circuit to mistakenly believe that the current is too large, forcing the horizontal stop oscillation and automatic shutdown. After replacing R340, the fault was resolved.