The machine has repeatedly damaged line pipe, down tube Q802 inspection, found that the transistor has been broken down. Thanks to the warning, dare not rashly replaced, first on the line scanning circuit for in-depth and detailed analysis, sorting out the circuit as shown in the figure. Usually, tube breakdown failure reasons: 1. + B output voltage is too high; 2. Line retrace capacitance decreased; 3. For lack of incentives; 4. Load short circuit fault; 5. Horizontal frequency is too high or too low; 6. Pipe from their own bad. Because the machine has been replaced many times, can rule out the cause of bad line pipe, should focus on the first five reasons, the first check + B voltage. After the removal of line pipe, at + B and to access 100W / 220V lighting false load, power detection + B output voltage lt136v, monitoring time for nearly an hour, voltage is always stable lt136v, indicating that the switching power supply is normal. And then down the reverse process of the reverse process, C809, C811, C812, C817 with the capacitance meter to measure its capacity, no abnormal. There are no obvious short circuit in the output of the line. Check the Q802b extreme IC301{27} feet incentive pathway components were normal and weld legs of each component in the pathway were before maintenance repair welding over, there is no weld place, can be ruled out due to weld caused by insufficient incentives, only to determine the is horizontal is normal. Therefore, under the line does not take control of the state, with digital table frequency shift measurement IC301{27} output pin of the line excitation frequency signal, when not in closed antenna measurement to frequency is 15.94kHz, in antenna after falling for 15.62kHz that horizontal normal and TV signal synchronization, oscilloscope observation {27} feet square wave pulse amplitude 1VP-P, was slightly lower than that of normal 1.2VP-P amplitude. Was about to take the oscilloscope probe, found that square wave flashed and to restore the status quo, doubt ic301 or 500KHz crystal is bad. Demolition of the first oscillator X301, with MF47 type million table R x 1K retaining measure resistance on its feet for infinity, when to switch to R * 10K retaining measure but found X301 feet between 50 ~ 100k unstable resistance (normal crystal feet resistance should be infinity), indicating that poor X301. Take a high-quality 500KHz crystal replace X301 were observed IC301{27} feet of the excitation signal increase 1.2VP-P. Will be removed from the false load, change on the new line pipe test machine, the machine is back to normal. Use has been more than half a year, no fault reproduction. 500KHz crystal because of the existence of the unstable leakage resistance, the horizontal oscillation frequency and amplitude of random changes, and serious leakage will make pulse amplitude decreased caused by excessive incentive problems, make the switch state change and loss increase, the tube due to temperature rise of the rapid increase in the pressure drop and be breakdown.