LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power board?

[arman960]

I am getting only sound not video. I checked the screen with flash light and it seems like lcd is in proper working condition. The inverter is not turning on.

The inverter is receiving 24v from power board. But one problem is the power board's output to t-con main board is only 1.9v which should be 3.4 on

My question is is it possible that inverter board not turning on because of low voltage to t-con main board?

or just both inverter board is bad?

[budm]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

Can we see how all the boards are connected? Are you getting BL_ON >3V at the connector of the Inverter board?

[retiredcaps]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

Cross posting in multiple threads is heavily frowned upon by regulars here (especially me). It creates confusion and causes way more work than necessary for us. Pick one thread and stick with it.

I suggest you keep your posts here and not the other thread at

https://www.badcaps.net/forum/showth...341#post266341

[arman960]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

I will take a picture and post it here. That way you will have better idea. Thanks for the reply.

[arman960]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

Ok. I will do that. I am new. Thanks for bearing with me.

[arman960]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa








Here is the pictures. Power going to inverter boards and power going to t-con/mainboard

I am getting 24V for inverter board. But they are not firing up. I am also getting only 1.9v for t-con output where it should be 3.4V on

So I want to know if my inverter boards are bad or my power board faulty.

The tv turns on with sound and faint picture can be seen with flash light.

[cadiman]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

Mostly the inverter boards fail due to ccfls failed two or three will shut down the panel.

2 seconds to black?

The power supplies are not an issue with any of the 42lg 20,30 or 50 models. All of the failures were inverter, panel issues involving au-optronics panels.

The only boards in this tv that are L/G are the power supply and mainboard.

These tvs have a high failure rate with their mainboards too.

I start with the slave inverter looking for shorted mosfets the do the switching for the inverter transformer.

Look for some other posts here as all of the L/G series have been very similar models using the same panel and power supply with the same problems.

[arman960]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

well the tv was working well couple days ago. Had one issue though. Half of the screen was little darker for half an hour at start up. After wamp=up. TV was working very bright and well. So I opened the back to see any faulty connection. I don't know if I shorted anything. Now the tv has no back light. I thing the back light is till good. Has to do something with inverter/power board or main board.

[arman960]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

Yes that is the Inverter board on ebay. I can buy both from ebay for a reasonable price. But I want to make sure it is Inverter board not the power supply board that is faulty.


I checked with one inverter at a time hooked up with the board. No sign of back light. Is it possible that my main board is faulty. I get picture with flash light, two second test.

[arman960]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

Can anybody give me any suggestion how to test my inverter board or how to repair?

[drpj23]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

well the tv was working well couple days ago. Had one issue though. Half of the screen was little darker for half an hour at start up.

That sounds like your ccfl tubes to me

and both inverter boards must be hooked to get a backlight.

[budm]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

You need to post the pictures of the inverter boards. There are fuses, MOSFET's, PWM IC, transformers that you need to check. For the backlights to be on it needs 24V and the BL_ON or ON/OFF signal from Main board which is routed through the power supply board. Same for the Dimming control signal.

[arman960]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

is there any way I can check the inverter board or they are good or bad? Can I check the output from inverter boar which goes to ccfl with a multimeter. I know the voltage is high. Is that ac or dc voltage that is coing from inverter board?

[budm]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

It is ac about 2000~2500VAC which will blow the meter right away. Why don't you at least check the fuse first and inspect real close to see if you can see any blown components.
The inverter circuit will go into shutdown if the lamp dose not fire up ,does not stay on in the first 3 seconds.

[arman960]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

budm I was thinking since my video and sound is ok, mainboard and power supply is also ok.

I am getting 24v into inverter board input terminal.

[drpj23]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

What about the other 4 voltages to the inverter? (ERR, DIM, I_C, BRI)

[budm]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

But are you getting the control signal to turn on the inverter circuits, just think of it this way, you have 120V feeding your desk lamp, but the lamp switch is OFF, then the lamp will not be on, same concept.

[arman960]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

budm, in your opinion the control board is faulty? power board giving low voltage output where it says 3.4v ON, there I get only 1.9v

do you thing that low voltage has a role in this case?

[budm]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

Can't tell since you need to find out if the main board is sending out all the correct signals to turn on the power supply, the backlights inverter. That is hwy you have to find out what signal you are missing, right now the PS_ON from main baord is OK otherwise you will not be getting the 24v, 12V power supply outputs.
What DC voltage are you seeing at the I_C pin. The ERR is the signal generated by the Inverter to tell the main board if it has fault on the Inverter board or not.

[arman960]

Re: LG 42" LCD TV 42LG20-UM No video ONLY sound BAD inverter board or bad power boa

I already checked all the output voltage from power supply. Found only one low voltage at 3.4v ON terminal.

Now how do I test the main board or inverter? Can you please tell me if you know any tricks?

Here is my left side inverter board. Right one is just like this one.

which dc voltage at which IC I look at?

is there any way I can check the dc/ac voltage with multimeter that is coming from inverter output for ccfl?