After opening the set, I went straight to check on the
safety components:,I found the surge limiter Resistor (dropper) open circuit
but the fuse ok.
I replaced the dropper and powered on but it blows again
instantly, I wish I had used the series bulb but it was too late.
I replaced the dropper again and used the series bulb and I
notice d the bulb was lighting with lots of brightness indicating a heavy
short. I measured the voltage across the man capacitor and found around 65
volts.
Now back to basic electronics I remembered that if the diode
bridge has a problem usually cause the voltage at the main capacitor to fall.
I tested across each of the four (4) with my digital meter
on diode test and I found one diode was shorted (I got reading both ways)
I replaced the four diode (bridge)...Usually whenever you
find one of these diode shorted it is important to change all the diode because
even if they test okay it will be a matter of time before they succumb also and
therefore to avoid call back I usually change all of them.
Now after changing the diodes I again applied power via
series bulb and still the bulb remained bright even after staying for over one
minute, with I concluded there is another shorted component which I have not
yet picked.
My next suspect was the switching transistor (S.O.T), so I
switched of the power and removed the power cable from the outlet and cut the
middle leg (collector of the switching transistor) with a side cutter, this
effectively removed this transistor from the circuit…I usually do this to avoid
wasting time soldering out the whole component. If the transistor is found to
be okay I apply a small solder to the cut leg only.
After cutting the middle leg (collector) of the S.OT I again
applied power via the series bulb and this time the bulb brightness was first
bright as the main capacitor was charging and then went down.
With this I concluded the switching transistor was shorted
and need replacement.
I replaced the transistor and re-tested again, this time I
was lucky and I even saw the dummy load I normally used while doing power
supply troubleshooting lights up indicating that there is power on the
secondary side.
I re-assembled the television and everything was well.
Lesson learnt:
Whenever you find a fuse or dropper gone never replace them
and apply power without using the series bulb-there is always a cause why it
blow i.e. expect a heavy short on the primary area-like the diodes bridge,
Chopper transistor / control chip, degauss Posistor) or component that lie
across the power rail (one leg on the live and the other on the return or
ground) see the figure below.
You can relate to the actual components on the picture below
(diagram used only for identification purposes only, from a different model)
most of the power supply doesn’t have varistor on the power supply.
Thank you guys
Stay safe
