Judging by the phenomenon of failure, the case of the fault is certainly a comprehensive protection circuit control caused by. In order to judge the fault source is in the switching power supply circuit, or row output or a protective circuit before pulling the plug in x1, off line output DC power supply circuit, in third, D11 feet and cold end of a 100W bulb is connected with a false load. Short boot, found the external lighting glows red. At this time the voltage between the two ends of the bulb detection for normal values of 140V pursuant to rule out and comprehensive protection of switching power supply circuit bad this factor, and concluded that the fault element row output and the load circuit. After the shutdown with 500 multimeter R * 10 Omega in the way for detection of Q552 output tube collector is reverse resistance is 0 ohm, remove the Q552 detection, the emitter has short circuit breakdown, caused by the factors of Q552 breakdown are: (1) the bad performance of Q552 itself; (2) line output transformer internal short circuit or T501 local field output circuit short circuit fault or circuit work guide often causes the picture tube beam current is too large or tube damage resulting in very touch line output Q552 collector current is too large for the breakdown; the excitation signal caused by abnormal or excessive excitation resulting in less incentive and damage due to breakdown power surge line; 4 + B output over-current protection monitoring circuit protection function failure caused by bad elements. Therefore before replacing the Q552, first on the road were detected after current protection monitoring element Q553, D551 R557. Results found open D551 anode pin of weld and other elements were normal. Boot then fill welding D551, remove the dummy load, and connect the plug-in X1 (D11), and substitution Q552 with a 2SC4927 line and edge detection Q552 C pole voltage, edge observation screen grating quality, found that just machine Q552 the very C voltage is normal, and normal sound, but no image only had light grating and full screen flyback bright line, after about 20 seconds + B output terminal over current protection circuit control and automatic shutdown. The reason is that the current of the tube bundle is too large. Should focus on the examination of the board (L board) circuit and its related circuit components. After power failure unplug the CRT tube socket, the beam current of 0, boot test depending on the plug-in board L3 foot DC voltage of 90V, obviously lower than the normal value of 220V, accordingly judgment depending on the power supply circuit defective. The machine's place as the 220V power supply is formed by the line output transformer T501 4 foot line retrace pulse obtained by D559, C504 rectifier and filter. After power off, the suspicious components were removed and the quantitative tests were carried out. The results showed that the leakage of the filter capacitor was C504. After replacing the capacitor, in good seat tube, and then boot detected l plate put the drive pipe Q360 ~ q362 emitter voltage in order to 158V, 159V, 162V, were within the normal range. At this time, the observation of the screen, the light, the sound is normal, fault exclusion.