Chanwong tv-china- see circuit below- No Power

Motherboard

Actual Voltages -Circuit

This TV has no power symptoms-first I checked the fuse and it was blown but not blacked inside meaning there was no heavy shot in the supply.

Remember a fuse can tell you a lot about the last moment of any equipment before it died (in medical terms postmortem)

1. Fuse blown, dark inside means the machine died due to heavy shot. Please be sure to use the light serious Lamp to bring up the machine after replacing the fuse.

2. Fuse blown, clear inside means the fuse might have died through natural dead.

After I replaced the fuse it didn’t blown again confirming to me that there was no serious shorted component (I mean component shorted which is taking the supply directly to the ground.

I measured the output voltages and there was none: absolutely zero volts. Then I checked the voltage at the main capacitor and it was 320 v dc which is okay. The voltage was also reaching the s.o.t collector same voltage.

I measured the S.O.T base voltage and it was 0.02 volts. I followed the start up circuit divider and the voltage was ok and no resistor was open.

But from the above voltage measurement you find the Zero volts at the base of the s.o.t is suspect.

Either the S.O.T (v513)is sorted base-emitter because its taking the voltage to the ground. Or V512(c3807) is shorted collector-emitter because it’s also taking the base of the s.o.t to the ground. Or capacitor c517 across the collector emitter of v512(c3807).

I removed the two transistors i.e. s.o.t(v513) and driver v512 and measured them out of circuit together with cap c517.

To my surprise the S.O.T v513 (C4429) was shorted Base-Emitter junction i.e. was 82 ohm both ways on diode test on my Dmm meter. It should have been high and low ( I mean the B-E junction has two reading meaning the transistor is shorted B-E junction which is very rare.

Conclusion

If there is zero voltage at the output then most probably the primary section is the cause of the problem.

Narrow down your faulty circuit (area) by use of voltage testing only. This one is a very fast and valuable method I have come to learn.


Thanks guys

Let meet in the next class