No standby light. Set dead. AC fuse blows out as soon as set is switched ON.
The fault is with its main power regulator section. There is some direct short circuit to either its rectifier section or switching section.
SMPS switching is done with [STR 50115B]
Start troubleshooting with its main rectifier section. AC is rectified by a bridge rectifier ( D601_ KBU4JL-6088). Check this rectifier for any internal diode short. If there is any shorted diode junction inside the rectifier diode. Protection fuse (f601_ 3.5Amps) will blow out, as soon as you switch on the set.
Before starting trouble shooting to power supply section, it is recommended to connect a filament type 100W bulb, in series with power in. It can be done very easily. Take out the glass cartridge type fuse from its holder. Connect two wires on both end of the fuse holder and connect the other end of these wires to an ordinary bulb holder, with bulb.
By doing this, we are inserting a series resistor [resistance of the filament of the bulb] with power supply input. If there is any short in the circuit, it can be detected by the intensity of light comes from the bulb. Normally, if the switch mode power supply works well, the bulb will light up wit high intensity at first, when we switch ON the set by its main power switch, and will come lesser intensity, after the switch mode section has been attained stability. If the bulb glows with full intensity, make sure that there is short-circuited component/s at the power rectifier or power regulator section.
The first thing to do, is to desolder out the switching device, STR50115B from circuit. Without it, switch ON the set. If bulb starts a blink, and goes OFF, the STR 50115B is faulty. It is shorted internally. This is a common type of fault with these sets.
This STR has 5 solder terminals. The normal voltages to these terminals are:
1 = Ground (0V)
2 = Start base drive voltage (117V)
3 = Collector supply voltage to internal switching transistor (Depends on AC input voltage; as this DC voltage can vary with AC input voltage)
4 = Voltage out to main board (116.5VDC0 [Steady stabilized]
5 = Reference voltage (6V3)
Here, pin number 3 & 4 of the STR should be checked with a multimeter set to measure Ohms range. If the meter shows approximately (0 or very low resistance up to 20Ohms); make sure that the collector to emitter junction of the internal switching out transistor is leaky, and the STR is defective.
Before replacing this STR, check and replace [9C608 _ 560pf 2Kv]. Even though this capacitor is a low value one, function of it when STR starts it switching process is very great. Check and replace the protection zener diode [8608 _ R2m _ 120V-zener] too.
If STR is found defective, check the main rectifier stalk too. It will be best to replace it too.
Caution; Before starts troubleshoot, desolder out the degaussing posistor [THP 601 PTC]. Resoled it only after all works on power board has been completed.
If the power supply switching section is OK; when you switch ON the set by its power switch, the bulb connected in series, will light up at high intensity at first, and will go down to low intensity, and will remain steady. But the set wont work properly as there will be insufficient AC voltage in due to the introduction of the filament of the bulb in series. Now you can replace the bulb with a fuse in place.
If (R604 – 470K) is open, the power supply start up. It supplies the start up voltage at first when we switch On the set. So if you feel any start up problem, replace this resistor.
