42LG50 no backlight

[rb0746]

I have an LG4250. Powers up and I can see the menu etc when I have enough light directed at the screen (no backlights are working) this unit was covered under warranty the set was replaced by LG . I have purchased this set with hopes of fixing it . the inverter boards are getting the required 25 volts . what really puzzles me is I don't see even a flash of backlight at all during startup .I have checked all the cabling and all seems to be okay .
Any Ideas here to help me along would be greatly appreciated

[PlainBill]

Re: 42LG50 no backlight

I have an LG4250. Powers up and I can see the menu etc when I have enough light directed at the screen (no backlights are working) this unit was covered under warranty the set was replaced by LG . I have purchased this set with hopes of fixing it . the inverter boards are getting the required 25 volts . what really puzzles me is I don't see even a flash of backlight at all during startup .I have checked all the cabling and all seems to be okay .
Any Ideas here to help me along would be greatly appreciated

Blown fuse, missing control signal, or any of a number of other possible flaws that also prevent the inverter from turning on. Part number of the inverter controller and pictures of the inverter would be helpful.

PlainBill

[rb0746]

Re: 42LG50 no backlight

Some photos of the 42lg50 innards

[PlainBill]

Re: 42LG50 no backlight

Some photos of the 42lg50 innards

Looks like dual inverters. Scratch most of the ideas. Check the power supply to the inverters, as well as the control signal.

PlainBill

[rb0746]

Re: 42LG50 no backlight

Looks like dual inverters. Scratch most of the ideas. Check the power supply to the inverters, as well as the control signal.

PlainBill

Thanks for the reply PlainBill
I could use a little help in a procedure for checking the control signal

[PlainBill]

Re: 42LG50 no backlight

Thanks for the reply PlainBill
I could use a little help in a procedure for checking the control signal

Well, if you look at the 12 pin connector on each inverter you will see that 5 of those pins are tied together; and another 5 are also tied together. One set is ground, the other set is the inverter power. That leaves two pins - one is the On / Off control, the other is brightness. Check the voltage across the two sets of power pins, and check the voltage from the ground set to each of the other two. Report the three voltages.

PlainBill

[rb0746]

Re: 42LG50 no backlight

Well, if you look at the 12 pin connector on each inverter you will see that 5 of those pins are tied together; and another 5 are also tied together. One set is ground, the other set is the inverter power. That leaves two pins - one is the On / Off control, the other is brightness. Check the voltage across the two sets of power pins, and check the voltage from the ground set to each of the other two. Report the three voltages.

PlainBill

Hope this helps PlainBill

Master Inverter : 14 pins
1-5= 25v
6-10=Ground
11(BR1)=1v
12(on/off)=3v
13(pwm-Dim)=0v
14(error)-5v

Slave Inverter : 12 pins
1-5=25v
6-10=Ground
11(BR1)=1v
12(on/off)=3v

[PlainBill]

Re: 42LG50 no backlight

Hope this helps PlainBill

Master Inverter : 14 pins
1-5= 25v
6-10=Ground
11(BR1)=1v
12(on/off)=3v
13(pwm-Dim)=0v
14(error)-5v

Slave Inverter : 12 pins
1-5=25v
6-10=Ground
11(BR1)=1v
12(on/off)=3v

Unfortunately, it doesn't. I've never seen a schematic for master / slave inverters, so I don't have any good suggestions except to check any fuses to make sure they measure good (less than 1 ohm).

PlainBill

[kc8adu]

Re: 42LG50 no backlight

if the fuse goes on the master the slave will not start either.had a set where that happened.no problem found so i replaced the fuse.its still running after 6 months.

[PlainBill]

Re: 42LG50 no backlight

if the fuse goes on the master the slave will not start either.had a set where that happened.no problem found so i replaced the fuse.its still running after 6 months.

Duh!!! Of course!!! Thanks for injecting a little intelligence into the problem.

PlainBill

[rb0746]

Re: 42LG50 no backlight

Well PlainBill upon checking fuses I found a little a bugger dead (the color of this fuse is black and the others are white is this a special fuse ?) it was on the master inverter now I need to order and replace said fuse the fuse has the number 15 on it on the board i believe it is printed beside it .75A 63v.
how would I go about ordering this fuse ?

[PlainBill]

Re: 42LG50 no backlight

Well PlainBill upon checking fuses I found a little a bugger dead (the color of this fuse is black and the others are white is this a special fuse ?) it was on the master inverter now I need to order and replace said fuse the fuse has the number 15 on it on the board i believe it is printed beside it .75A 63v.
how would I go about ordering this fuse ?

Using the internet or by telephone usually works.

Sorry, it's been a hellofaday. I just spent more to have the AC fixed on my wife's car than I spent for my first three cars combined.

There are two approaches. The easy way requires adequate clearance above the inverter. If there is enough space, you can use a Picofuse or even a pigtail fuse in heat shrink tubing. It's not going to look good, but it will work.

The PROFESSIONAL way is to measure the fuse as accurately as possible - length, width, and height using a metric ruler or calipers. The go to www.digikey.com, type in fuse in the Part Search box. Select Fuses, then select 63V, 750 mA, surface mount. That will result in 37 hits. Select the proper size under package / case and you should have only a few selections.

Since you live in Canada, perhaps one of the members who resides in that fair land will have a suggestion for a source.

Suggestion: Before ordering a single fuse, check to make sure you have repaired whatever caused the fuse to blow.

PlainBill

PlainBill

[retiredcaps]

Re: 42LG50 no backlight

Since you live in Canada, perhaps one of the members who resides in that fair land will have a suggestion for a source.

If the fuse is going to be picofuse or pigtail fuse, your local electronics store will probably have it. See if you have a MRO electronics in your city.

Otherwise, digikey shipping and handling is $8USD + GST + HST for orders under $200 and I'm told it takes about 1 week or so to arrive.

[torpex505]

Re: 42LG50 no backlight

I just aquired the same monitor for the same reason. The tv turns on, but does not have any backlight. Flashlight test revieled snow(no video source connected at the time of test,but also saw menus). So I opened the back and dove in with my multimeter. I found two fuses on the white board, but both are fine. I tested all caps on the inverters and found that all but one(which is on the primary inverter) tested fine at 24v, the one single cap tested @ 14v... I did drop the probe initally which arched the leads on this cap... is that what fried it, or is there a whole other beast to tame here... I can't figure out where to go next...

Thanks,
Eric

[torpex505]

Re: 42LG50 no backlight

"the one single cap tested @ 14v..."

I found where the voltave drops is at a 3amp fuse on the primary inverter. It tests at 24v on one side of the fuse and 14v on the other side. Is this just a bad fuse or could something else be causing this? Am I better off forking out the $100 for a new primary?

I also noticed that in the wire harnes that goes from the Main (White Board) to the Primary, there is a wire that is labeled as "error" on the main board that is showing voltage. Is this an indication of another issue?

Any advise is appreciated.

Thanks,
Eric

[torpex505]

Re: 42LG50 no backlight

Here are the pics of the Primary Inverter around the area of the problem. One is a close-up of the fuse there the voltage drop is occurring.

[Dgtech]

Re: 42LG50 no backlight

"the one single cap tested @ 14v..."

I found where the voltave drops is at a 3amp fuse on the primary inverter. It tests at 24v on one side of the fuse and 14v on the other side. Is this just a bad fuse or could something else be causing this? Am I better off forking out the $100 for a new primary?

I also noticed that in the wire harnes that goes from the Main (White Board) to the Primary, there is a wire that is labeled as "error" on the main board that is showing voltage. Is this an indication of another issue?

Any advise is appreciated.

Thanks,
Eric

Remember this: there should be no voltage (0V) measured across a good fuse. What you did was perfect. If there is 24V on one side of the fuse and 14V on the other, this means that if you read directly across the fuse ( one lead on one side and the other side has the other lead) you will read 10V. Your fuse is bad. Now you have to ask yourself what took it out. It may have just blown due to a spike or you could have something shorted in the circuit.

[Dgtech]

Re: 42LG50 no backlight

By the way, I'd check the FET's on that board too. Make sure they are not shorted. That could possibly cause that fuse to blow. If one is shorted and you only replace the fuse, the fuse will blow as soon as you power it up.

Attatched is the data sheet for that FET. Only really needed if you need to find an alternative to the original part. (if the original type cannot be obtained)

[torpex505]

Re: 42LG50 no backlight

Thanks for the reply. I'm new at this and didn't even think to check those. I will check this afternoon and report back my findings.

[torpex505]

Re: 42LG50 no backlight

I checked the FETs and they seem to be OK. The FETs that are not affected by the fuse measure at 1.9V on G2, 24.7 on S2, 0.0 on both G1 & S1, and the drain pad measured in at 24.7. On the two FETs that are affected by the fuse, G2=.7V, S2=2.4, S1 & G1=0.0, and the drain was at 2.4V. Is it worth replacing the fuse just to see? Like I mentioned earlier, I arched the leads on the bottom cap which I think may have caused this. Is this something that I can pick up at Radio Shack? Thanks for the direction Dgtech.