LG 47LD452B no power to backlights, blown fuse, shorted FETS, what did I miss?

[DXseekerMO]

I picked up this LG 47LD452B today that tries but won't turn on. The stand-by light turns from red to blue then flashes red.

I checked voltages and appropriate voltage is found at the power to main board connector including INV on/off, 3.2v/0v. The main caps switch from 165V to 395V when the power button is pressed. There is zero voltage at the backlight plug and no voltage on either side of the backlight transformers.

So I pulled the LIPS and found the 4A 250V backlight protection fuse open. I checked the MOSFETS and three of them are reading shorted in-circuit.

So my question is: it looks like I need to replace all 4 FETS, but I'm really wondering what else got taken out or what I may have missed, even if it's the mark entirely?

Thanks all

[DXseekerMO]

Re: LG 47LD452B no power to backlights, blown fuse, shorted FETS, what did I miss?

So how about it guys, can I get an opinion?

[budm]

Re: LG 47LD452B no power to backlights, blown fuse, shorted FETS, what did I miss?

Check all the parts connected to the GATE of the MOSFET.
Those MOSFETs are driven by the isolation GATE drive transformers so I do expect the IC to be damaged when the MOSFETs shorted out.
Verify that rectangular DC Blocker/coupling cap right next to the inverter transformer is OK, you need to remove it to check the capacitance and leakage resistance.
The MOSFETs may have gone bad by itself or may be bad inverter transformers are bad.

[DXseekerMO]

Re: LG 47LD452B no power to backlights, blown fuse, shorted FETS, what did I miss?

Check all the parts connected to the GATE of the MOSFET.
Those MOSFETs are driven by the isolation GATE drive transformers so I do expect the IC to be damaged when the MOSFETs shorted out.
Verify that rectangular DC Blocker/coupling cap right next to the inverter transformer is OK, you need to remove it to check the capacitance and leakage resistance.
The MOSFETs may have gone bad by itself or may be bad inverter transformers are bad.

About how often do you see those inverter transformers bite the dust?

[budm]

Re: LG 47LD452B no power to backlights, blown fuse, shorted FETS, what did I miss?

Not very often on the failure on the primary winding, but the secondary winding does fail a lot so you may want to check the secondary winding resistance of both transformers to see how close they are with each other.

[DXseekerMO]

Re: LG 47LD452B no power to backlights, blown fuse, shorted FETS, what did I miss?

Ok I will do that this weekend.

[DXseekerMO]

Re: LG 47LD452B no power to backlights, blown fuse, shorted FETS, what did I miss?

Not very often on the failure on the primary winding, but the secondary winding does fail a lot so you may want to check the secondary winding resistance of both transformers to see how close they are with each other.

I went ahead and de-soldered the two MOSFETS, circled in red. They did test short across all pins.

What I find interesting is the other two (outer, circled in green) do not appear to test short in-circuit, although it looks like they trace to different legs of the same transformers. Questions:

Are these the isolation gate drive transformers you were referring to?

Is electricity flowing from these to the MOSFETS?

The item circled in yellow, is this the IC that is likely damaged?

The backlight transformers and the two small transformers all test very close to short, not even an ohm primary or secondary. Funny thing is the readings are the same on all four transformers, leading me to believe something else in that circuit has to be shorted, or this is how they usually ohms-test?

The other thing I find interesting is there are no signs of any transformer being overheated. The board is nice and clean under all of them.

[budm]

Re: LG 47LD452B no power to backlights, blown fuse, shorted FETS, what did I miss?

The two arrows are pointed to the Isolated Gate drive transformer, the IC is more likely to be fine when the MOSFETs shorted out since the circuit is isolated by the transformers, but the resistors and diodes connected to the GATE may be damaged which you need to check and verify.
I would replace ALL 4 MOSFETs at the same time.
The DCR (DC resistance) of the primary winding will be very low which is normal.

[DXseekerMO]

Re: LG 47LD452B no power to backlights, blown fuse, shorted FETS, what did I miss?

The two arrows are pointed to the Isolated Gate drive transformer, the IC is more likely to be fine when the MOSFETs shorted out since the circuit is isolated by the transformers, but the resistors and diodes connected to the GATE may be damaged which you need to check and verify.
I would replace ALL 4 MOSFETs at the same time.
The DCR (DC resistance) of the primary winding will be very low which is normal.

Budm, what are the items on the board marked "BD", and what is their purpose? Looks similar to a diode?

[DXseekerMO]

Re: LG 47LD452B no power to backlights, blown fuse, shorted FETS, what did I miss?

Ferrite Bead? (BD) For reducing noise?

[Dannyx]

Re: LG 47LD452B no power to backlights, blown fuse, shorted FETS, what did I miss?

"BDs" are ferrite beads indeed. That BD IC however is the backlight controller. Replace all 4 FETs, but just after you finish removing all of them, another test I'd do is (with no FETs in place, so nothing blows), check the datasheet for that IC and identify the VCC pin - measure it and make sure it's within spec. Assuming the TV functions properly but lacks backlight, the IC should have VCC and you should be able to get away with just replacing the transistors. If it's got a blown CCFL lamp however, you won't know until you try it. Fire it up without the lamps connected first to see if if blows the fuse again - it shouldn't. Let us know how it goes.

[DXseekerMO]

Re: LG 47LD452B no power to backlights, blown fuse, shorted FETS, what did I miss?

"BDs" are ferrite beads indeed. That BD IC however is the backlight controller. Replace all 4 FETs, but just after you finish removing all of them, another test I'd do is (with no FETs in place, so nothing blows), check the datasheet for that IC and identify the VCC pin - measure it and make sure it's within spec. Assuming the TV functions properly but lacks backlight, the IC should have VCC and you should be able to get away with just replacing the transistors. If it's got a blown CCFL lamp however, you won't know until you try it. Fire it up without the lamps connected first to see if if blows the fuse again - it shouldn't. Let us know how it goes.

Is that the IC I have circled in yellow in the previous picture?

Thanks for the reply!

[Dannyx]

Re: LG 47LD452B no power to backlights, blown fuse, shorted FETS, what did I miss?

Yes, that's the one.

[DXseekerMO]

Re: LG 47LD452B no power to backlights, blown fuse, shorted FETS, what did I miss?

I looks up the datasheet and found the pin, I don't have a voltage spec unless it's somewhere I haven't seen just yet. Datasheet shows 4V nearby.

[Dannyx]

Re: LG 47LD452B no power to backlights, blown fuse, shorted FETS, what did I miss?

It should most definitely have the maximum/minimum VCC stated somewhere. Even the most obscure, most disgusting datasheets have them

[DXseekerMO]

Re: LG 47LD452B no power to backlights, blown fuse, shorted FETS, what did I miss?

Pulled the other two FETS, they will not go in to "on" state. I will have to order fuses and try it with no FETS installed. I may have a fuse in another board I can steal.

[Dannyx]

Re: LG 47LD452B no power to backlights, blown fuse, shorted FETS, what did I miss?

Pulled the other two FETS, they will not go in to "on" state.

What do you mean by this ? Do you mean you're not reading shorts across them ? That's fine, but still, whenever dealing with multi-FET arrangements like these it's a good idea to replace all of them nonetheless. Just like with a bridge rectifier made up of 4 individual diodes: if two pop, you don't replace just those two - you replace the entire set.

[DXseekerMO]

Re: LG 47LD452B no power to backlights, blown fuse, shorted FETS, what did I miss?

Completely agree, they were gonna be replaced no matter what. When I touch the probes of the voltmeter on diode mode, + on gate, - on drain FET is open. Touch + to source momentarily then touch it to gate and it's still open. What I meant is it looks like it won't switch on.

[Dannyx]

Re: LG 47LD452B no power to backlights, blown fuse, shorted FETS, what did I miss?

Much easier to test diodes/FETs using the beeping continuity thingy on meters: it instantly tells you if a semiconductor is faulty if it beeps. FETs never/rarely(?) fail open - they always fail shorted, unlike resistors which is the opposite, so if you don't get a reading that means it's ok AFAIK, because it's a very high impedance device.

[petehall347]

Re: LG 47LD452B no power to backlights, blown fuse, shorted FETS, what did I miss?

shorted is one thing .open is another that can happen pretty rarely . been there seen that . only once though.
just a guess but are all those the fets same specs ?