**Fault Cause Analysis:** The fault was caused by poor contact of the line synchronization adjustment potentiometer R626, which led to line asynchronization. This triggered the activation of the X-ray protection circuit, causing the line scanning circuit to stop working. The reason why line asynchronization can activate the X-ray protection circuit can be explained as follows: When the winding ②~⑧ of the line output transformer T602 does not produce a line retrace pulse or has an insufficient amplitude due to abnormal operation of the line scanning circuit (such as line asynchronization), the voltage across capacitor C631 cannot change instantaneously. As a result, there is no voltage or a reduced voltage across C631, causing Q606 to conduct. The current from its collector passes through resistor R643 to pin 30 of IC801, increasing the voltage at pin 30 and triggering the X-ray protection circuit.
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**Edited and organized by Beijing Philips TV After-sales Service Center**
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**Fault Symptoms:** During normal viewing, there were irregular occurrences of line frequency asynchronization and raster disappearance faults, but the audio function remained normal.
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**Circuit Characteristics Analysis:**
1. The switch power supply uses a thick-film circuit IX0689CE, with the +12V voltage generated by the switch power supply circuit.
2. The remote control power on/off circuit is composed of microprocessor M50436-560SP pin ⑨ (power on/off control terminal), interface circuit (V1201), and relay K1201, using a relay control method (controlling the AC input voltage 220V of the switch power supply to turn on/off).
3. TA7698AP pin 30 contains an X-ray protection circuit. When the anode high voltage is too high, the beam current is too large, the field output capacitor shorts, or the line frequency asynchronizes, it serves a protective function (stopping the line circuit from working).
**Maintenance Techniques:** Monitor the voltage at TA7698AP pin 30 (X-ray protection). If the voltage rises to about 1.5V during a fault, it indicates that the X-ray protection circuit has been activated. Analyze and identify whether the fault is caused by overvoltage, overcurrent, short-circuited field capacitors, line asynchronization, or a fault in the protection circuit itself. The method for judgment is to momentarily short pin 30 to ground and observe the phenomenon. In this case, line asynchronization occurred, indicating the fault lies in the line synchronization circuit.
There are two causes for line asynchronization faults:
1. Abnormal operation of the AFC circuit;
2. Issues with the line oscillation frequency control circuit.
The method for judgment is to adjust the line frequency synchronization potentiometer R626. If the image can momentarily stabilize in the horizontal direction, the fault lies in the AFC circuit; if the image cannot stabilize horizontally, the fault lies in the line oscillation frequency control circuit or the TA7698AP line oscillation circuit. Upon inspection, it was found that the line synchronization potentiometer R626 had poor contact. After replacement, the fault was resolved.
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**Philips TV:** Irregular occurrences of line frequency asynchronization and raster disappearance faults.