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Steve G.
Steve G., Technician
Category: TV
Satisfied Customers: 17987
Experience:  VP of electronic repair company. Audio nut.
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I have a 42LG50 that the back lights are not turning on. I

Customer Question

Customer: ***** ***** I have a 42LG50 that the back lights are not turning on. I did the flashlight test and I have a picture and I can see menus
JA: Thanks. Can you give me any more details about your issue?
Customer: Ive been reading a post and they all had the same issues, they found that it was due to little black fuses on the inverter that fixed all of their problems
JA: OK got it. Last thing — JustAnswer charges a fee (generally around $18) to post your type of question to TV Experts (you only pay if satisfied). There are a couple customers ahead of you. Are you willing to wait a bit?
Customer: ok
JA: OK. Now I'm going to take you to a page to place a secure deposit with JustAnswer. Don't worry, this chat is saved. After that, we will finish helping you.
Submitted: 1 year ago.
Category: TV
Expert:  Steve G. replied 1 year ago.

Hello there and welcome to Just Answer. My name is ***** ***** I will be assisting you today. I am an engineer with over 30 years of electrical and electronic training, repair and installation experience. I will try and answer your question accurately and precisely so that we can get you on your way.

What sort of help are you looking for? Is it your intention to try and fix this yourself? If so, what have you done so far?

Let me know.

Customer: replied 1 year ago.
I have not made repairs on it just testing with a multimetter
Customer: replied 1 year ago.
Ive been reading a post and they all had the same issues, they found that it was due to little black fuses on the inverter that fixed all of their problems
Expert:  Steve G. replied 1 year ago.

Let me grab the service manual and take a look. Give me a few mins and I'll be right back.

Expert:  Steve G. replied 1 year ago.

Your's is definitely a 42LG50 and not a 5010, 5020, 5030 or 50D?

Customer: replied 1 year ago.
product code is 42LG50-UABUSHLJM
Customer: replied 1 year ago.
42LG50-UA.BUSHLJM
Expert:  Steve G. replied 1 year ago.

Ok. This one has dual inverter boards and I believe that one is fed by the other - like a master/slave arrangement. I would suggest therefore that either the master or the power supply to the master is bad.

These fuses that you mention, have you checked them yet with your meter?

Customer: replied 1 year ago.
no, I am not sure exactly which are fuses and which are resistors. The regular white fuses are easy but the little black ones i'm not sure about. The components all look new, theres no dust or anything. No hot spots, nor swollen caps, nothing to show tell tell signs so i'm not sure which one is bad
Customer: replied 1 year ago.
I have taken many picyures and can take more of anything you'd like.
Expert:  Steve G. replied 1 year ago.

I was going to suggest. Can you post a photo of the entire TV from the back? As close up as you can but I need the whole TV in the photo.

Expert:  Steve G. replied 1 year ago.

I have pics. Just found some.

The fuses are white. Have you checked them with your meter set to to ohms?

Expert:  Steve G. replied 1 year ago.

There are also some black ones. All fuses will be designated with an 'F' on the board. For example F3 or F12.

Check all of these on both inverter boards. Any open fuses will prevent the board functioning. All fuses should measure less than one ohm.

Customer: replied 1 year ago.
I just moved the pics to my computer from my phone. I have close ups of each compomnent will that work? The white fuses are all fine, Its the black ones on the inverters is what I believe is the issue. Ive been searching for answers and that was the only place that had the exact same problems and when they solved it, all of them stopped posting which meant they had what they needed
Expert:  Steve G. replied 1 year ago.

Ok, I need to know if you have you checked the fuses on the two inverter boards, black and white. ALL fuses need to be checked.

Customer: replied 1 year ago.
I said that I dont know how to identify the black ones
Expert:  Steve G. replied 1 year ago.

Ok, All fuses will be designated with an 'F' on the board. For example F3 or F12.

Check all of these on both inverter boards. Any open fuses will prevent the board functioning. All fuses should measure less than one ohm.

Customer: replied 1 year ago.
i checked the big white ones one the power supply and thats all
Customer: replied 1 year ago.
ok will do. which setting is best? 200?
Expert:  Steve G. replied 1 year ago.

Sure. Doesn't really matter when it comes to ohms. I don't see any black fuses in your photo. What are your readings across the two white 3A fuses?

Expert:  Steve G. replied 1 year ago.

Also, all surface mount fuses will have the amp rating written on it and etched close by on the board along with the voltage rating. The master inverter - the one on the right, definitley has some.

Customer: replied 1 year ago.
looking at theback of the tv, I found one that that didnt read anything, the rest were 1.6. that happens to be the same one the guy on the forum jimmy rigged to fix his. I'll send his pic now
Customer: replied 1 year ago.
those are mine that i thought already uploaded sorry here is his
Expert:  Steve G. replied 1 year ago.

When you say one fuse didn't read anything, do you mean that it was open? Or that the resistance was very close to zero.

Customer: replied 1 year ago.
the other fuses on the other one are fine. I didnt know they were marked with an F. One of the ones on the left master inverter didnt read as anything.
Customer: replied 1 year ago.
i need to change that out. I will be billed $18 for this right?
Expert:  Steve G. replied 1 year ago.

Ok, so the fuse is open? In other words, when you measure resistance across it, the resistance is very high?

If that is the case, then yes, the fuse is bad. What you can try temporarily is bridging the fuse with a pair of small needle nosed pliers and see if the backlights come up.

Want to give that a try?

Customer: replied 1 year ago.
I will assemble the tv with the 2 components that i removed, test with needle nose pliers and come back here
Expert:  Steve G. replied 1 year ago.

Ok, if the screen lights up, you will need that fuse. I would get a few just in case something else is popping the fuse. That is also why you shouldn't keep the needlenoses on there for too long.

I'll be here. Which fuse is it, I'll try and find you one. What is the rating and voltage?

Customer: replied 1 year ago.
ok i soldered a jumper on there temporarily. The screen now comes on but shuts off after a sec. any ideas why?
Expert:  Steve G. replied 1 year ago.

If you turn the TV off and then back on, does the screen come on again?

Customer: replied 1 year ago.
just for a sec
Expert:  Steve G. replied 1 year ago.

Ok, so you still could have a fault with one of the boards. It would be interesting to see if a replacement fuse blows immediately. Hold on for a second please.

Customer: replied 1 year ago.
i circled the bad fuse
Expert:  Steve G. replied 1 year ago.

Ok, unplug the TCON board and see if the backlights stay on.

Customer: replied 1 year ago.
no, same thing. on for 1 sec and shuts itself off. thank you for helping me btw
Expert:  Steve G. replied 1 year ago.

You are most welcome - if you would, please rate the service when you have a second.

This could be a few things. I really would like to know if the fuse blows again. Have you looked for the fuse? You should be able to buy then at www.digikey.com or www.mouser.com or www.alliedelec.com

If you put the fuse back in and it pops, then we know the problem is on the inverted boards.

If it doesn't we can start looking elsewhere.

Customer: replied 1 year ago.
My 32" monitor just stopped working on my main pc. How ironic is that? Is there any other way to figure this 42" LG out, looks like im going to need it. I'll order the fuses now
Expert:  Steve G. replied 1 year ago.

Ok. One other thing to check is that all the zero ohm jumpers (these have a 'J' designation usually) are good and none are open. There are many of these on each inverter board.

Customer: replied 1 year ago.
ok those fuses are .80 cents each 3.10L x 1.56W. I'll check those jumpers now.
Expert:  Steve G. replied 1 year ago.

Sounds good.

Let me know how you get on.

Customer: replied 1 year ago.
all of them did exactly the same thing, started above 1 and settled at .8. the larger caps read 115 each
Expert:  Steve G. replied 1 year ago.

You can't measure caps with a meter very well, just make sure they are not shorted.

Ok, let's try something. I had forgot that the TV is not actually shutting down.

Let's set the meter to DC volts. And let's deal with the master inverter which is the larger one - the one with that bad fuse. FInd the 12 pin connector that powers it up from the PS board. Usually on these things, the first 5 pins are tied together, and 6 through 10 also. MEasure the voltage, in VDC from say pin 1 to pin 6. This would be done with the connector still connected - poke the probes down through the top of the connector

Customer: replied 1 year ago.
I did that before and they all came up right but I will double check them. This is done with the power on and tv on, it would have to be.
Customer: replied 1 year ago.
I went through both ribbons that attach from the power supply to the inverters and they all read 24 to 25 volts each. I also checked the ribbon going to the main board on both sides and they all checked correctly to the specified voltage marked on the ps board. the reason it took me so long was, i started getting inconsistent readings all over the place and found that one of my leads from my multi-meter was bad so I have a new one.
Expert:  Steve G. replied 1 year ago.

I would suspect a bad lamp (CCFL) or bad transformer on the inverter board. DO you have 24V at each transformer primary? Check them all.

Customer: replied 1 year ago.
whats a transformer primary look like?
Customer: replied 1 year ago.
are they the black blocks that give the back lights amplified power or something like that?
Expert:  Steve G. replied 1 year ago.

HI again. Yes, the larger black components are transformers. You should have 24V going into each of them.

Customer: replied 1 year ago.
i'm not crediting anyone anything until my tv works.
Customer: replied 1 year ago.
we can continue today but i've been busy. ok?
Expert:  Steve G. replied 1 year ago.

I'm here.

Customer: replied 1 year ago.
they billed me $53 already and without confirming anything and without fixing my tv, Why is everything as close to a scam as possible?
Expert:  Steve G. replied 1 year ago.

I am not privy to billing I'm afraid. I am a contractor that is paid by the site. I have no involvement with payments, credit card etc.

I think you can go to www.justanswer.com/help and challenge any billing issues.

I'm sorry that you are experiencing this.

Customer: replied 1 year ago.
its just that whatever is wrong with this tv is probably something small. I just need to find it. you're having me test what ive tested but you have to know per troubleshooting protocol and you did help me find the bad fuse. so iwill duoble check the transistors, i do remember them being all the same so i believe they are good
Expert:  Steve G. replied 1 year ago.

We seldom repair circuit boards to component level. The accepted method as you will find on various blogs and message boards is to swap out the boards.

Again, I would like to see if that fuse blows again when replaced. When do you think you will have the fuses?

Customer: replied 1 year ago.
that i found out and is why i am here.
Expert:  Steve G. replied 1 year ago.

Ok. Any idea on the ETA of your fuses?

Also, have you checked the transformers? What you should do is turn off TV and set meter to ohms and measure across the outside two terminals of each transformer.

Customer: replied 1 year ago.
I havent ordered them because it has another unknown issue. I am back home now so I can check the transistors now.
Customer: replied 1 year ago.
they are all 25volts
Customer: replied 1 year ago.
this is a pic of the 2 spots on each transistor that I metered at 25 volts each.
Customer: replied 1 year ago.
what is next?
Expert:  Steve G. replied 1 year ago.

Measure the voltage across the outside two pins of the transformer and then the two outside pins on the other side.

Customer: replied 1 year ago.
they were all 0. If they had power, the lights would come back on
Expert:  Steve G. replied 1 year ago.

This is true, I just wanted to check. Keep in mind though that if one lamp fails, the all turn off so that is another possibility.

So all the transformers have voltage, they initially get a switch on signal and then all turn off. That could be a lamp.

Make the room dark and observe the screen as you repeatedly turn the TV on and off. During that brief light up of the screen, do you see an dark sections around the edge where one lamp may not be lighting?

Customer: replied 1 year ago.
I have been trying to open this up but it kept saying 'problem loadiing page' seems fine now.
ok I am going to go check the tv for dark strips for a bad lamp now brb
Customer: replied 1 year ago.
its hard to tell because the only part of the screen thats really lighting up is the channel input. I also noticed that the power light is sometimes blue now. thats a first. its was not easy to get but heres a photo of it on
Customer: replied 1 year ago.
here is one a slpit second before the channel input lights up
Expert:  Steve G. replied 1 year ago.

Hi again. I'm not going to be able to tell from a photo unfortunately.

Do you thing any part of the screen edge is not lit?

Customer: replied 1 year ago.
those photos are exactly what i am seeing;. I'll send a video
Expert:  Steve G. replied 1 year ago.

I know but It's not the same as being there. Are there any darker spaces around the edges that would indicate a bad lamp?

Customer: replied 1 year ago.
No
Expert:  Steve G. replied 1 year ago.

Ok. Then something else is turning off the transformers.

At this point, I think I'd rather turn this over to other LG experts. I'm not confident I know the absolute cause.

Someone should pick this up soon. Hold on.

Steve