Preliminary judgment is to protect the circuit starts. Power block with a multimeter to measure the output switching power supply and line pipe, no short circuit leakage phenomenon. Further determine the voltage measurement method: boot, measuring + B output voltage, output voltage at the moment and then turn down to 0V. Measuring Q814 and Q554 base has 0.7V voltage, diode D560 are extremely high potential 1.4V, proved access to protected status.4maservice manual download site

     Unprotect method using judgment. Disconnect the line output circuit, connected to dummy load, short circuit STAND-BY, then boot into the protected status still shows not due to the load circuit protection caused. Each protection detects disconnection Q554 base-by-trigger circuit, when power is disconnected R833, switching power supply is no longer protected, but the output voltage as high as 180V, and issued a "popping" sound, SCR Q821 breakdown cracking. Quickly shut down by resistance measurement method to check the voltage regulator circuit, no exceptions. Substitution optocoupler output voltage returns to normal. Replace burned SCR Q821, recovery circuit fault completely ruled out.4maservice manual download site