LG 42LG70 blowing fuse

[tibimakai]

I have picked up this TV and found a 3.15A fuse blown in the middle of the PS.
Found all cheap Chinese capacitors, two shorted FET transistors.
Replaced almost all capacitors(made a mistake with the ordering, but the rest is coming), even the main one and the two FET transistors(they are next to each other on a smaller heatsink).
After installing all these and a fuse course, it's still blowing that fuse.
Bridge rectifier checks out good.
Second time I FETs don't seem shorted. That's why I say they seem, because sometimes I measure 1.1OHm and sometimes much higher value. It's kind of confusing. I'm switching the transistors on with the DMM?
Any help would be appreciated, not really lucky lately, to many opened up TVs and monitors and it seems that I can't get to the bottom of any of these.
I also have a Philips 42PFL7422D totally dead and I can't find the problem.

[tom66]

Re: LG 42LG70 blowing fuse

Trace the fault. Do you have low resistance between + and - of caps? What about AC incoming?

If + and -: Pull the FETs and bridge, check if fuse still blows.
If ~ and ~: Pull the bridge and check it.

Start another thread regarding the Philips.

[tibimakai]

Re: LG 42LG70 blowing fuse

One thing that I have just realized by reading another Philips problem, that I have replaced the T 3.15A fuse with a F 3.15A fuse. That is bad?
Bridge it's OK.
I will try removing the FETs and see what will happen.

[tom66]

Re: LG 42LG70 blowing fuse

It will be fine for now. T3.15A is time-delay. F3.15A is fast. F3.15A fuse will blow quicker, but it will take some time to do so.

[tom66]

Re: LG 42LG70 blowing fuse

Actually, since the fuse that blew is in the middle, I think you can rule out the bridges and PFC FETs. Your problem must be with the main switching FETs. (Q110, Q111.)

[tibimakai]

Re: LG 42LG70 blowing fuse

Those are the ones that were shorted and replaced. It still blows the fuse though.
Everything that I measure it seems OK around there.
I'm not 100% sure about the state of the FETs at this point. Even when I compare them with a new one, sometimes I'm getting(out of circuit) 1.1Ohm between two pins(don't remember right now which one), but then if I recheck it again, it shows a high value.
Could you please walk me through how to check these FETs and what kind of results should I get?
Do you think that the fuse matters, since it's not a T fuse?

[tibimakai]

Re: LG 42LG70 blowing fuse

If I test them this way, they do the same thing which means that they are OK, am I right?
http://www.youtube.com/watch?v=7piJzaBzZ5o
If I resolder them and check continuity it looks like both FETS are open between D & S and shorting to the ground(black tester on the negative of the main cap).
Pin #14 on the NCP1396AG seems that it's shorting to the ground, but not sure if that is the way it should be or not.
http://pdf1.alldatasheet.com/datashe.../NCP1396A.html
In this diagram it looks like between the pin #10 and #14 should be a L1, a T1 transformer's winding and a C1(this could be shorted?).

[tibimakai]

Re: LG 42LG70 blowing fuse

This is the way is conducting(open):

[tibimakai]

Re: LG 42LG70 blowing fuse

Any help would be appreciated.
I have even replaced the NCP1396A, since it was only $2.00. But still blows the fuse as fast as I plug in the cord.
Can't find any schematics for this TV.

[budm]

Re: LG 42LG70 blowing fuse

So the Source and Drain pins of the Q110/111 are they showing shorts again, check that DC blocking cap (brown cap C114) to see if it has any resistance reading.
If the fuse that feeds the Q110/111 blown right away even the TV is not commanded to turn on yet, those two MOSFET's should be sitting there without being turn on at all. so some how they both turn on at the same time, they only to be on one at a time not both at the same time. If Q110/111 are removed, it does not blow the fuse?

[tibimakai]

Re: LG 42LG70 blowing fuse

Your short tester would save fuses?

[budm]

Re: LG 42LG70 blowing fuse

Yes, you should build one to save the pain.

[tibimakai]

Re: LG 42LG70 blowing fuse

I had time only to check the capacitor and the value in circuit is 0.2 ohm and out is 27uf.
I have purchased all the components for the tester, I just didn't have the time to put it together, but eventualy I will do it. Friday I'm leaving in Costa Rica for a week and after that I will put it together for sure.

[budm]

Re: LG 42LG70 blowing fuse

0.2 Ohm in circuit means the bottom MOSFET Q111 is shorted! You also need to check the PFC MOSFET and PFC diode also (those 4 devices on the heatsink by the two PFC chokes).

[tibimakai]

Re: LG 42LG70 blowing fuse

It's shorted the replacement transistor. How can I avoid shorting another one? Actually both show the 0.2 as marked in my picture above. The red line means that all those traces show 0.2 ohm.

[tibimakai]

Re: LG 42LG70 blowing fuse

How should I check a FET? If I Google it, it comes up with batteries and such.

[budm]

Re: LG 42LG70 blowing fuse

They are already have Source to Drain shorts, no need to check further. Some how the output drive from the SMPS IC is sending out the Gate drive at the same time so both MOSFET's switched on. So it is like putting a dead short across the power supply output, that is why I am asking you to see if any devices in the PFC got damaged or not.

[crashp3]

Re: LG 42LG70 blowing fuse

Can't find any schematics for this TV.

[tibimakai]

Re: LG 42LG70 blowing fuse

Thanks, but it doesn't show the power supply. I guess that LG doesn't want us to see those schematics, since I have tried other similar model TV's schematics and in all of them it was missing the power supply.

[shepp]

Re: LG 42LG70 blowing fuse

The time delay fuse is normally used for in rush current, so the fuse can handle well above the rated current for a short time, 3.15a quick blow will blow on the in rush current, I would fit the correct fuse then test again