This damage is usually over-current damage. In order to check the power supply filter capacitor rectifier elements, startup resistor and Q3, Q3, if damaged, use the same type of high-speed NMOS switch instead. Another source resistor chain reaction susceptible to damage, it must be measured R5 is normal; should measure the regulator DZ1, DZ2, and sometimes burned with breakdown by Q3. Moreover, you should check the excitation circuit, output circuit and damage if caught, otherwise the Q3 replacement will not burn again.bnSservice manual download site