Boot will be the weakest position, with the use of the meter to monitor the IC30050 pin DC voltage, power, 2.2V, the brightness is gradually enhanced, IC30050 pin voltage is also increased, to the brightness of the signal character is raised to the 2/3 of the screen, the foot voltage rose to 3V, this time to enter the state of protection. Then determine is the 50 foot peripheral circuit anomaly and the automatic shutdown. Respectively, the protection of the input port voltage to be analyzed, the field scan of the protection input port in C Q1513. The voltage of the Q1513c is 3.5V, and the pin voltage is not changed during the enhancement and weakening of the brightness. If the field scanning circuit is working abnormally, the Q1513 will be cut off when the D1505 is off, and the 9V voltage is applied to the IC30050 pin, which is protected by the D1504 voltage. Therefore, it can eliminate the field output circuit fault. Check the beam current, X ray protection port, that is, the C898 of the non ground, measured at the point of the voltage is about 12V, when the voltage of the pin voltage only tiny changes, is normal state. When the pressure is too high, C898 non voltage high, when it increases to a certain value (18V~~20V), the voltage R319 r320 pressure, through R341 added to IC30050 foot, 50 foot internal circuit in action, for protection. There is also a line of protection for the line, when the boot time to measure the flow of the protection transistor C Q561 pole voltage of 24V, when the enhanced brightness, the C voltage will also be followed up, when the voltage is C 35V. While the C Q561 is a normal operation with the voltage of 0V, obviously this fault for the line over current protection. After shutdown were used to detect the related components, and finally find out R858 (for protection of sampling resistance flow) damage. The resistance by 1.2 Omega into 2.7 ohms. From the circuit principle analysis, when the R858 resistance becomes large, with brightness enhancement, the electron gun emission electron beam ability enhancement, anode voltage drop and amplitude of the screen also enhanced. The current increases with the flow through R858, the pressure drop increases, when more than Q561, the 1V is biased, the C pole voltage by R564, R565 partial pressure, and then through the D591 plus to the IC30050 pin, internal circuit action to achieve the purpose of protection. After the replacement of R858, the IC30050 pin voltage is 1.7V, the change of the brightness of the pin voltage is not changed, and the fault is eliminated.