According to the phenomenon of check by 300 v dc voltage rectifier, filter. The multimeter monitoring power supply output voltage, instant startup, multimeter after a pointer to the right, back to 0 v, shows the power supply has a lift, a voltage output, due to the protection circuit starts to make the output voltage of 0 v. Disconnect + B line to the output circuit of power supply (disconnect L903), add a dummy load (220 v, 100 w bulb). Boot, bulb light, bulb on both ends of the voltage is 110 v, normal, that may be a thyristor Q904 conduction that launched the protection circuit. Check Q904 control voltage of 0.7 V, determine whether Q904 conduction into the protection state. pCUservice manual download site
 pCUservice manual download site
 pCUservice manual download site
Adopt the method of one way to remove protection detection circuit, boot failure phenomenon. When the disconnect ZD909 boot is no longer protected. Check ZD909, R939, R918 found R939, R918 normal resistance value, use "replace", will be 22 v zener diode to replace the original ZD909, restore the original circuit, commissioning, everything is ok.pCUservice manual download site