### Maintenance Techniques:
Based on maintenance experience, it is known that the self-boosting voltage switch tube TL54 (BU807) of this machine is prone to damage. Therefore, before powering on, check if the TL54 is damaged; if not damaged, then power on and test whether the key pin voltages of the IL03 integrated block (such as 12V at pin 8, pulse width output voltage at pin 7, overcurrent protection voltage at pin 28, etc.) are normal to determine the fault location. After inspection, it was found that the TL54 was not broken or damaged, there was a 12V operating voltage at pin 8 of IL03, but the voltage at pin 7 was around 4.0V, which was too high (normal value is 1.5V), and the voltage at pin 28 was around 1.0V (normal value is 0.6V), indicating that the internal overcurrent protection circuit had been triggered. The current issue is to find out the reason for the overcurrent, so voltage tests were conducted on all pins of IL03, and it was found that most pin voltages were abnormal. No abnormal damage was found in the peripheral components. Possible reasons: 1) IL03 block is damaged; 2) Key peripheral components are damaged. According to the analysis of the working principle, the self-boosting voltage switching power supply circuit works under the control of the line frequency pulse voltage, and the line frequency pulse is generated by the crystal QL11 (CSB503B) and the internal 500kHz voltage-controlled oscillator and 32-divider at pins 17~19 of IL03. Therefore, when the performance of the QL11 crystal is poor or leaks and changes, it may likely cause the line frequency oscillator to work abnormally, leading to operational chaos. A multimeter was used to detect the QL11 crystal, no leakage was found, and the problem was solved using the replacement method.
### Fault Cause Analysis:
The fault was caused by poor performance of the QL11 crystal, making the internal line frequency oscillator of the IL03 block work abnormally, causing the line scanning circuit to malfunction. Specifically, the output voltage at pin 7 of IL03 increased, meaning that the line frequency pulse output by the integrated circuit became wider, making the conduction time of TL54 longer and increasing the conduction current. The voltage drop across the emitter resistor RL58 of TL54 (i.e., the overcurrent sampling resistor) increased. This voltage drop was sent to pin 28 of IL03 through RL59 and RL61, and compared with the internal reference voltage of the circuit, resulting in an increased error voltage that triggered the trigger, forcing the line to stop oscillating. In addition, when the thermal performance of the switch tube TL54 (BU807) is poor or the potentiometer PLA9 has poor contact, it will also cause an automatic shutdown fault.
### Circuit Characteristics Analysis:
(1) The switch power supply circuit consists of a primary power conversion circuit (self-excited parallel type, outputs unstable voltages of 90V, 22V, and 13V) and a secondary self-boosting voltage circuit (boosts the unstable 90V voltage to a stable 108V voltage);
(2) The self-boosting voltage switch circuit consists of a dedicated power supply integrated circuit TEA2026T (with functions such as field-line oscillation, pre-excitation output of field-line, pulse width modulator, and overcurrent protection circuit, etc.), switch tube TL54 (BV807), and energy storage inductor LL54;
(3) The remote power on/off function is composed of microprocessor HD44840A65 and drive interface circuit TR74, with electronic switch (IR61) control method, controlling the on/off of the line oscillation block IL03 (IEA2026T) pin 8 line start-up power.
### Fault Phenomenon:
During viewing, the TV suddenly automatically shuts down with character display. After turning off and restarting, sometimes it can still be watched, but soon it automatically shuts down again, and sometimes the characters flicker.
Sharp TV, automatic shutdown during viewing.
### Data Source:
Beijing Sharp TV Repair http://blog.tianya.cn/blogger/blog_main.asp?BlogID=3682347.