For the Sony KV-S29MH1 TV, during normal viewing, the entire unit would suddenly shut down automatically with the standby indicator light remaining on. After power disconnection and re-powering, the TV could work normally again for a short while before the fault reoccurred.
Analysis and Maintenance: After opening the casing, the first check showed that the main power supply voltage of +135V was normal. When inserting an ammeter into the horizontal deflection circuit to monitor the automatic protection circuit, it was found that the current in the horizontal deflection circuit was significantly higher than normal, indicating that the fault was caused by excessive horizontal deflection current. Testing by increasing the brightness and contrast levels resulted in quick activation of the automatic protection mechanism after turning on the TV, suggesting potential issues with the ABL (Automatic Brightness Limiter) circuit.
In this model, the ABL control signal is extracted from pin 11 of the horizontal output transformer and sent to pin 26 of the integrated block IC3501 (CXA1464AC) on board A. It then goes to board B where Q373~Q377 form the automatic brightness signal control circuit. If the ABL control signal is abnormal, it can lead to loss of control of the brightness signal, causing a sudden surge in horizontal deflection current. Upon checking pin 11 of the horizontal output transformer, it was found that resistor R564 had changed from its normal value of 1kΩ to infinity. This was due to poor thermal stability of resistor R564, which prevented the BL (Brightness Limit) signal from being delivered to both boards A and B.
Repair Method: The issue was resolved by replacing resistor R564.
Source of information: http://blog.sina.com.cn/sonydianshi.