Sony TV, the standby indicator light is on, but there's no response inside the unit. Source: Beijing Sony TV after-sales service hotline http://blog.tianya.cn/blogger/blog_main.asp?BlogID=3682079. Fault symptom: The standby indicator light is on, but there's no response from inside the machine.
Analysis and Repair: The standby indicator light being on indicates that the switch power supply is working, and the 5V voltage regulator IC002 supplying power to the CPU is also normal. Measuring the pin ② of the 9V voltage regulator IC521, there is no 9V voltage; measuring its control pin ④, when pressing the on/off button on the remote control, there is a change between 5V and 0V, which indicates that the CPU system is normal. After disconnecting the power, the resistance between pin ② of IC521 and ground was found to be less than 50Ω, suggesting some faulty components in the load causing overcurrent protection for IC521. Therefore, the short circuit lines from the output pins of IC521 to each circuit were disconnected, and it was discovered that the resistance between the power supply pin of IC354 and ground is less than 50Ω. Since IC354 (TDA83957) is a surface-mount component, not only is it hard to purchase, but it is also difficult to solder, so accurate judgment is necessary.
To prevent the internal resistance of IC521 from increasing and thus reducing the load capacity leading to misjudgment, the pin ② of IC521 was disconnected from the circuit board, and a genuine AN7809 was used as a replacement for testing. As a result, after powering on, the machine returned to normal. Since AN7809 cannot be controlled by the CPU, it generates 9V DC voltage after turning on. During standby, there is neither light nor sound because the horizontal start switch at pin 27 of the CPU (IC001) outputs a high level, which, after amplification by Q030, prevents the horizontal excitation tube from working. At the same time, pin 03 of the CPU has no high-level output in the standby state, making Q208 and Q207 sequentially cut off, so the audio amplifier does not work. However, most of the circuits in the entire machine are still in a working state under the effect of 9V voltage. This not only increases power consumption during standby but also reduces the lifespan of this circuit. Therefore, replacing IC521 with AN7809 is just an emergency repair. When PQ09RE11 becomes available later, it should be replaced back to ensure the original performance of the machine. However, if the user does not use the standby function, replacing it with AN7809 is also a good emergency repair method.