Check the insurance tube normal F1, and electrical capacitance C2, C4 at both ends of the voltage 300V normal, measurement of IC3, the pin voltage 300V normal, measuring the pin voltage is approximately 4.6V, check the D7, R16, C15 were normal.
The short-circuit of the secondary load capacitance C7 was found by the inspection. The high frequency pulse voltage of feedback winding output is rectified by D7 and R16. In the C15 on both ends of the capacitor voltage decreased to 4.6V, under voltage protection circuit IC3. Internal foot make switching tube stops working, to prevent because of insufficient power supply, leading to the switch pipe is damaged due to lack of the excitation voltage. Replacement capacitor C7, troubleshooting.