According to the aircraft maintenance experience, when the machine appear "point" described in the irregular automatic shutdown occurs, is one of the more common reasons for damage of photoelectric coupler D811 and D836 open. When D811 open, 0827 trigger conduction, and D836 when open, although no voltage D823 is end, according to the protection circuit principle analysis, no trigger current Q827, does not result in a protection shutdown, but after D836 open makes C824 voltage rise. Switching circuit work normally when the voltage on the C824 sources have two road, one is 300 v voltage by R805 and R806 through R858 supply after partial pressure. Although R805 and R806 partial pressure point of high voltage (100 v or more), but the R858 Ω (180 k), R859 (10 k Ω) partial pressure again after the voltage value is very low. Therefore, the actual voltage on the C824 for T801B1, B2 winding output ac voltage by D822 rectifier, its value is about 8.1 V. When D836 open, R859 lost partial pressure action, R805 and R806 pressure point voltage will through all add to the C824 R858, when the voltage more than Q826 c and e very pressure, breakdown, Q826 Q827, Q804 enter a state of saturated conductivity, switching power supply to stop working. As a result of the 0826 c and e breakdown voltage measured at about 100 v, therefore, when the mains voltage is low, R805 and R806 partial pressure point voltage is relatively low, Q826 in critical breakdown state, when the mains voltage fluctuated slightly, will more than Q826 breakdown voltage value and cause failure. As a result, the failure of irregular and without any abnormal phenomena appear in front of the failure. qfUservice manual download site
 qfUservice manual download site
Had come across the fault, for example, are all D836, D811 damage. Therefore, if encounter the machine fault, irregular automatic shutdown can check first C824 terminal voltage, if too much doubt D836, D811 damage, are sometimes can get twice the result with half the effort effect.

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