Whenever you find the main fuse is blown and darkened inside please don’t replace it and power on the equipment because chances are very high that it will blow again.
In this case substitute the main fuse (across the two pins of the main fuse holder with a series bulb.
You can make one easily as shown here below.
Replace the main fuse with the series bulb (100Watts) and apply the power.
If the bulb remains bright, power off the TV and remove the power cable from the socket.
Go to the switching transistor and cut the middle leg (collector) with a side cutter.
Then apply the power again to the TV and observe the bulb again.
If the bulb now goes dim, this means the switching transistor is shorted and must be replaced.
If the bulb remains bright means the short is before the switching transistor.
Let me point out here that the only shorted component that can make the bulb to remain bright are the only component with one leg on the live(voltage) and the other leg on the ground.
Kindly refer to the follow diagram for more information.
On the diagram I have circled two components with red colour (V512 and C517). Which although they are not directly on the main Vcc line they also have one leg with voltage (+) and the other leg on the ground.
I have personally an experience where the entire component with blue circle tested okay and still the bulb was bright, testing transistor V512 (2SCC3807) was found shorted collector-emitter junction and after replacing it the problem was solved.
Below you can the actual component which you should check out when you suspect the short is before the switching transistor.
Sometimes a component may be failing under load which means getting the culprit is not easy with a meter.
Tip: in this case solder out one leg of the suspect component one by one and applying power, the last component you remove and the bulb brightness disappear is the actual culprit.
If any of the component below short the bulb will remain bright.
For the supply using control I.C, then you will have to find out which pin is for the main Vcc (300vdc or 155vdc) and cut it out and repeat the procedure in the previous example.
Thanks you guys
See you in the next class.