LG 42LG70 blowing fuse

[Th3_uN1Qu3]

Re: LG 42LG70 blowing fuse

Yeah, I have the components for it, I just didn't have the time yet to put it together.
By the way, what kind of bulbs I should use, same as in the description, have different wattage and go from low to a higher one?

60 to 100W will work for any supply you will find in consumer equipment, at least for standby mode. Once you replace all shorted parts and kick it out of standby there's no more need for it anyway.

[tibimakai]

Re: LG 42LG70 blowing fuse

I have removed both FET transistors and checked them out as this Youtube video suggests and both are switching, so they are not shorted(most like the last set it was good too).
http://www.youtube.com/watch?v=b83YUu4Np6A
Actually last time it blew the main 6.3A fuse and not the one next to the FETs.

[tibimakai]

Re: LG 42LG70 blowing fuse

If I cut the trace here, that red continuity line is interrupted and the FET doesn't show continuity either(the one next to the cut). The other FET still shows the continuity. I have started removing SMD components, jumpers, but I still haven't found anything wrong yet. Maybe something is in other areas of the board.

[budm]

Re: LG 42LG70 blowing fuse

So the MOSFET Q111 on the right still showing short between Source (left top pin) and the Drain pin? But this MOSFET does not show short when you test it off the board, if that is the case, lift one leg of C110 which is connected between Source and Drain of that Q111 MOSFET.

[tibimakai]

Re: LG 42LG70 blowing fuse

I have removed it completely and it's the same. I have cut the line from the IC's pin #14, still no difference.
I have removed the little SMD diodes(2x), capacitor(C112) going to pin #16, resistor R117, J29/J123, C114, still no difference.

[budm]

Re: LG 42LG70 blowing fuse

If you remove Q111, is the short still there betweeen Source and Drain pads on the board? Something defying the rules here.

[tibimakai]

Re: LG 42LG70 blowing fuse

I will try it one more time.

[budm]

Re: LG 42LG70 blowing fuse

OK, I tried tracing out the connections from the pictures, the Gate drive may not be exact but close enough. I do not see what can cause the new MOSFET's to be damaged since you had already installed the new SMPS IC already, both outputs of the IC has to be stuck high or turn on at the same time to wipe out the MOSFET's. You may want to Ohm things out around the Gate Zener, resistors, etc.

[tibimakai]

Re: LG 42LG70 blowing fuse

I have tested both FETs, using the Youtube video and both are switching. They are not bad, but to tell you the truth, I don't know how to test these type of transistors.
That short it showed right after replacing the IC and both FETs without powering the board.
It blew the main 6.3A fuse this time.
Today I didn't had time to work on this TV, I have worked on others. I will post these in there proper thread.
Thanks for the drawing, I was about to do it myself, but to me it looks like it's pretty close to the generic drawing.
I will be removing components one by one, until I will find the problem.

[budm]

Re: LG 42LG70 blowing fuse

If the fuse for this section is not installed, and it is blowing the main fuse. The problem will be in the PFC/ main rectifier section.

[tibimakai]

Re: LG 42LG70 blowing fuse

The fuse it was installed when it blew it. This time it was the T fuse and not the fast one as before.
I have a feeling that the short it's somewhere else.

[tibimakai]

Re: LG 42LG70 blowing fuse

Need to come back to this TV, any more things to try?

[tibimakai]

Re: LG 42LG70 blowing fuse

Today I have tried it out without the two FETs installed and none of the fuses were blown.
The bulb lit up and then went off. In this couple of seconds the voltage on the main cap went up to 396V and then dropped. All this without the inverter plugged in.
Not sure what to do next, I would appreciate any input.

[tibimakai]

Re: LG 42LG70 blowing fuse

Any input in what should be the next step?

[tibimakai]

Re: LG 42LG70 blowing fuse

Nobody can help?
Should I try again with new FETs and IC? Just to have a fresh start?

[tibimakai]

Re: LG 42LG70 blowing fuse

I have installed two new FETs and a new IC and turned it on without the bulb tester and I blew up a thermistor. Sparks were flying.
I'm frustrated with this board, I'm ready to get a new(replacement) board.
I measured the short and it was there, but I was frustrated and I wanted to blow it up.
electronic horror picture

[tibimakai]

Re: LG 42LG70 blowing fuse

Last night, I have just purchased another board for $50. Hopefully this will take care of this TV.

[Khron]

Re: LG 42LG70 blowing fuse

For what it's worth, i just revived a 37" LG tv just the other day...

Turned out, two of the four FETs that made up a full-brige for driving the inverters, had shorted. I've got a hunch they're the four FETs in the top-right of the first picture of the PSU board.

Might be worth pulling them out and testing them out-of-circuit

[tibimakai]

Re: LG 42LG70 blowing fuse

Replacement board fixed the problem.