Lg 42lg30 need help with inverter question

[joeyt1111]

Hello,
i would like to thank anyone who responds for their time.I am new to the site but have used some info to fix a few tvs. i am just learning and doing this as a hobby and to add some lcd tv's to my house My problem is i have an lg 42lg30 lcd tv that i bought with a bad backlight problem.The backlight would go on for a second but go off and the screen did not show a usable screen.But if i disconnect the inverters from the bulbs the screen when viewed in the dark with a flashlight shows a good image. Now for the info on what i have found.First when i got the tv i could smell burnt plastic, took apart the tv found the board that holds the ccfl bulbs had a fire on the main inverter side. a bulb was broken as well. Ordered used board and replaced all ccfl bulbs with good ones. still the same problem. 24 volts going into inverters but the thing i have to ask is when checking for a short with my ohm meter i happened to test the plug where the pink and white wire go into the board and when i touched the leads to the pins it showed resistence between them. both inverters do this and i am unsure if that is correct as i would think if voltage was going in and out of the bulbs they shouldnt be showing resistence.Again thank you for any help and as i am new to all this i might not be explaining it well so if you can help and want some info please ask.
thanks again,joe

[1beertap]

Set your meter TO 200 ohm scale. The inverter side will test low resistance at those two pins as they are connected together via printed trace on board and through the windings of transformers. Don't measure the voltage with your meter, as your meter can be damaged and you will run the risk of getting shocked with high voltage. Also, don't measure resistance with the power turned on. On the bulb connector, the pins will have a low resistance. Hope this helps.

Originally Posted by joeyt1111

Ordered used board and replaced all ccfl bulbs with good ones.

One other thing, don't rule out the other board being bad as well.

[joeyt1111]

thanks for your help- i have found other issues that may lead to the repair of my tv.I am adding this info in hopes of getting a better direction of where to check for problems. when plugged in and turned on the power light slowly flashes red twice then goes solid blue. If i plug in my ps3 via hdmi in hdmi one with the inverters unpluged i get image BUT NO SOUND AT ALL then if i switch to hdmi 2 or 3 i get nothing no video or sound. Is it possible its a bad audio video mainboard causing my backlights to shutdown and lose audio? if so is there a place on the board to start testing for a failed part? thanks again for the help.

[1beertap]

In order to get more and better responses, take pictures of all boards in the tv and post them using "Manage Attachments" on the post page.

[joeyt1111]

here is my master inverter that i believe is causing the problem.With the inverter disconnected from the bulbs and turned on i cant get a voltage reading on my meter it just errors out my meter and puts it in hold mode.If i do the slave on ac voltage the meter reads 33volts for a sec then drops to almost nothing.

[PlainBill]

Originally Posted by joeyt1111

here is my master inverter that i believe is causing the problem.With the inverter disconnected from the bulbs and turned on i cant get a voltage reading on my meter it just errors out my meter and puts it in hold mode.If i do the slave on ac voltage the meter reads 33volts for a sec then drops to almost nothing.

Where are you trying to measure the voltage?

PlainBill

[joeyt1111]

hello,
thanks, i was measuring the output at the pink and white wires going to the bulbs.
thanks

[PlainBill]

Originally Posted by joeyt1111

hello,
thanks, i was measuring the output at the pink and white wires going to the bulbs.
thanks

Not the brightest thing to do. The output is close to 2000 volts at 40 Khz and can provide 40 mA or more. More than enough to provide a life ending event for your DMM and for you.

PlainBill

[joeyt1111]

yikes! how is the proper way to test inverter output? other than from very very far away!

[tom66]

Originally Posted by joeyt1111

yikes! how is the proper way to test inverter output? other than from very very far away!

You don't test the output of the inverter, it's very difficult without a high voltage probe or something similar.

[PlainBill]

Tom66 is correct. Although in a sense you have already tested the master output and it is good. Now the slave inverter SHOULD have behaved the same way.

PlainBill

[smason]

Originally Posted by joeyt1111

yikes! how is the proper way to test inverter output? other than from very very far away!

I test it with a lamp.

[joeyt1111]

hello,
i know i havent given any photos that would be of help to get an idea of what is going on and i also know my information makes more sense in my head as i type it than it does when you read it but i thank everyone for thier help so far and for putting up with me.
I was doing some poking around tonight on the tv and happen to come across something and wanted to know if anyone can give me an idea of if the way this is reading is wrong or right.
on the connector from the power supply to the mainboard there is a pin marked acd (ac detect i am guessing?) now the connector has another pin across from the acd that is marked error. When i test to ground on the power supply connector i dont show a ground on either of these two pins but when i check on the mainboard where this connector goes to the error pin is showing ground.Is that normal? or is this causing me an issue with my backlights? I also dont understand that when i plug the tv in without touching anybuttons the status light flashes red twice then goes solid blue and thats when the backlights come on for a sec buzzing and shut off and light stays blue.If i hit the power button again the tv repeats the flashing and on for sec with buzzing then off.
any help would be great but again i know i get confusing and its hard to understand a problem across the net.just looking for ideas to look at. i will add a pic of the connector now.
thanks

[cadiman]

All of the problems I have had with these AU-OPTRONICS panels were bad ccfl bulbs and slave inverters.

You will get symptoms of a bad inverter (2 seconds to black) even if you un-plug the slave inverter. The backlight will stay on longer (3 seconds).

I was getting 1200 volts on my industrial fluke.

If you don`t make a solid firm connection it will ark and smoke the test meter. (I have done this in the past)

When you take apart the panel you will see the bad ccfls. (darkened or burnt spots)

The slave inverter will have shorted mosfets.

[joeyt1111]

hello,
thanks for the response.I did take apart the panel and there was broken bulbs but from the buld holder board had started on fire on the master inverters side. i replaced the board with a used on and replaced the bulbs from another 42 inch lg but the board started arching at the exact same spot! i shut it off but can not determine why this is occuring. can you tell me how you check the mosfets? i had read you read the ohms and they should read 8.5 but the ones on the slave are reading 10. i took a backlight panel i had from a 37 lcd that i had taken apart and i knew the backlight was good.it only has a pink wire on each side so i connected it to my inverters where the pink wires connected and the backlight came on and stayed on. so am i to take from that the inverters are good? bad bulbs and boards?
thanks

[PlainBill]

Originally Posted by joeyt1111

hello,
thanks for the response.I did take apart the panel and there was broken bulbs but from the buld holder board had started on fire on the master inverters side. i replaced the board with a used on and replaced the bulbs from another 42 inch lg but the board started arching at the exact same spot! i shut it off but can not determine why this is occuring. can you tell me how you check the mosfets? i had read you read the ohms and they should read 8.5 but the ones on the slave are reading 10. i took a backlight panel i had from a 37 lcd that i had taken apart and i knew the backlight was good.it only has a pink wire on each side so i connected it to my inverters where the pink wires connected and the backlight came on and stayed on. so am i to take from that the inverters are good? bad bulbs and boards?
thanks

I'm not real familiar with the design of this type of backlight system, but it appears the voltage is getting too high. This is based on the fact that an arc forms. This might be due to a flaw in the inverter, or in whatever circuitry balances current through the CCFLs.

PlainBill

[cadiman]

If you leave a gap it will jump it and the v will skyrocket out of control.

Some of these bulbs are plug in and the spring tabs are junk at best.

Some of these are soldered (much better) You can solder these.

Most mosfets will have a base/ collector short (fail on) and easy to detect with a simple meter

I will need to dig up an old board and let you know

The better set up is the l/g inverter single board It will also work on this panel you will need a longer lead wire (special) if you go this route.

There are tons of parts for these and they are cheap.

All of the model 30s have very much in common.

you can use a 32lg30 mainboard in a 26lg30 and I have also used the 32lg30 mainboard in a 42lg30-ua for testing the panel

What is the full model?

42lg30-ua or ud ? these are very different

The ua have the au-optronics panel and have given me the most problems. I have not seen any ud series 42lg30s.