We have already checked the MOV and found that it is good. If so, the cause for power protection fuse may be due to some other reasons. Before proceed, just unsolder one end of [RM801: 0E22Ohms] connected to supply the main switching regulator IC [ICM801S: F9222L]. This is the resistor supplies voltage to main power switching IC ICM801. If this IC has internal short circuit, it will cause for blow up main fuse. Now unsolder out the sub power-switching transistor [QP801S:FCPH20L60]. This is a power MOSFET switching transistor that starts work first, as standby supply is derived from switching output of this transistor along with transformer [LP801]. If this transistor has shorted junction, will cause a tremendous current throug it to ground, which will cause fuse to blow up.
After disconnecting [RM801 & QP801S], just put a 5AMPS fuse in place in fuse holder, and switch ON the set by mains power on switch. Look if fuse blows out now. If this 5Amps fuse does not blow up, make sure that either main switching IC ICM801S or power switching MOSFET [QP801S] are faulty. Check the junction resistance of QP801S for short. If found short replace it with same type and number. Be sure to replace this transistor with same number.
If fuse we inserted blows again as we switch on the set, with [RM801&QP801S] are desoldered out from the board, we have to check the components including main rectifier bridge, two line filter transformers [LX801S&LX802S] for winding short. Check both the transformers by desoldering out them from the circuit board. If found shorted, replace them. There is no matter even one among them is not in place. They are line filter, which prevents internal power supply switching oscillations from coming out to the main line back, and disturbing other electronic equipments. There is no special function to these transformers, other than this.
Check the capacitors, connected across the main AC power line, on board, [CX801X:330Kpf 250V & CX802S:330Kpf 250V] for short. Check [NT811S:NTC5D-15], connected in series to AC power line, in between line filter transformers [LX801S&LX802S].
This is an NTC resistor [Negative Temperature Coefficient], connected in series with power line, to limit the tremendous current drawn by charging the main filter capacitor [CP803:150Uf 450VDC], when the set it switched ON each time by power ON/Off switch. A wire wound resistor [4.7Ohms 7W], will also meet the need there.
If all these components said above are found good, again the fuse blows OFF as soon as the set is switched ON by its main power ON/OFF switch, the rectifier bridge is defective. Replace it with same type and number. Here again, insist on same type and number.
To be continued on part - 4
