Faults to the power supply section
*Over voltage output from switching output section
*Dead set. No response to power or no front panel LED lights up.
*Intermittent self-power ON and OFF.
*Fuse blown up, as soon as the set is switched ON.
If fuse [F600 – mains fuse] blown out as soon as we switch ON the set, never insert a fuse without rectifying the fault. The fault will be surely to some direct short circuit on printed circuit board. There is an MOV [Metal Oxide Veristor] connected across the Ac power line print on the power supply section. This component is indented as a protection device against high voltage surges. If any surge voltage on AC line exceeds the breakdown voltage of this MOV, its internal junction will fuse together to make a direct short circuit to AC power line. The AC fuse is fitted before this device on power section. As soon as MOV detected an voltage, and gets shorted, the fuse in series with this will blow up.
So before do any repair, if fuse [F600] has found blown up, check the MOV [VDR600] for short. You can measure the short of this device by using a multimeter set to Ohms range. If found leaky, replace it. Even though this component is present in the circuit, all other function of the power supply should work on.
If MOV is found Ok, check the resistance across the two terminals of [C613 – main reservoir capacitor]. No short circuit, (low resistance reading) should be there. If low resistance, say less than 40Ohms are measured, suspect the power-switching component [IC-601 STR-F6654]. Desolder out this component from the circuit, and check its pin number (2 & 3) for short circuit. If found shorted, the STR is faulty and should be replaced.
If STR is shorted, many other components also should be checked for open or short circuit. Check the main rectifier bridge [D4SB60L] first. Check all its four-diode points for short. If any one segment has found shorted, replace the bridge rectifier, before replacing the STR. Keep in mind, if this bridge rectifier has some short or any trouble with it, may cause STR too to blow up.
The associated component to be checked if STR is found shorted. [R625 – 1.8E, R628 – 1.8E, R632 – 0.1E, R639 – 0.33E, RR654 – 0.33E, R641 – 0.27E]. All these resistors should be suspected. Check them one by one after disordering the out from the circuit.
When replacing the STR, replace C618 – 680Pf 2000vV also. If this capacitor has, that is if its value deteriated or it have some trouble, it will blow up the STR as soon as you switch ON the set after STR replacement. The is an important fly wheeling capacitor.
