Lg 37lg10 no backlight

[turtle9832]

I have a two year old LG 37lg10. I turned it on yesterday. The led in the front of the tv went from red to green. The sound came on, but the picture stayed black. I made the room dark, used a flash light, and saw snow on the screen. I have done some research here. I did not see and signs of damage to any of the capacitors. I read the voltage coming into the inverter (VIT71060.50) at 25.3 volts. It is also 25.3 volts after the fuse.

The only other place I tried to read the voltage was at the seven connectors to the screen. There the voltage is 0.0.

I am looking for hints on what to look for.

Attached are three photos of the board.

Thanks in advance.

This board is great.

[PlainBill]

Re: Lg 37lg10 no backlight

I have a two year old LG 37lg10. I turned it on yesterday. The led in the front of the tv went from red to green. The sound came on, but the picture stayed black. I made the room dark, used a flash light, and saw snow on the screen. I have done some research here. I did not see and signs of damage to any of the capacitors. I read the voltage coming into the inverter (VIT71060.50) at 25.3 volts. It is also 25.3 volts after the fuse.

Snow on the screen implies a problem with the video processing circuitry, not the inverter. Did you have the TV connected to a signal source? Some inverters have multiple fuses.

The only other place I tried to read the voltage was at the seven connectors to the screen. There the voltage is 0.0.

Don't do that. It can be over 1000 volts at 40 Khz.

I am looking for hints on what to look for.

Do the backlights come on even briefly (usually visible from the back of the TV)?

Have you checked the backlight control signals at the input connector of the inverter?

Attached are three photos of the board.

I see only two pictures.

Thanks in advance.

This board is great.

Pardon me while I go on a minor rant.

Many of the regulars here have a lot more experience working on electronics than you do. However, there are thousands of models of TVs in use, and it is unlikely that anyone will be intimately familiar with a particular model of TV. So we have to depend on experience, logic, and the information you provide. You did better than many (most?), but it is really difficult to glean meaningful information from pictures less than 900 pixels wide, and not sharply focused. One simple rule of thumb. If you can't read a component identifier in a picture, neither can we.

PlainBill

[turtle9832]

Re: Lg 37lg10 no backlight

Thanks Bill.

Excellent use of smilies.

The reason I mentioned the snow was I read that if you observed snow when adding light, you should expect an inverter issue. The tv was in antenna mode.

Thank you for the guidence.

I will try again with the pictures.

[turtle9832]

Re: Lg 37lg10 no backlight

I payed close attention to the taking pics tutorial this time.

The wide shot is the power supply board (PLOTECH 6 94v-0)

Can someone tell me what the voltage values should be at LCD - on and
LCD- BR? This is the connection to the backlight inverter.

Also, when using the volt meter to test the LCD on and LCD BR, do i just ground to the chassis?

thanks in advance

[turtle9832]

Re: Lg 37lg10 no backlight

each 24v connection is 27.0 volts.

LCD - on 3.5 v

LCD - br 3.5 v

[PlainBill]

Re: Lg 37lg10 no backlight

each 24v connection is 27.0 volts.

LCD - on 3.5 v

LCD - br 3.5 v

LCD-ON is probably correct. LCD-BR seems high, but it depends on the design of the inverter.

PlainBill

[turtle9832]

Re: Lg 37lg10 no backlight

When you say depends on the design of the inverter, what do you mean?

Thanks in advance

[PlainBill]

Re: Lg 37lg10 no backlight

When you say depends on the design of the inverter, what do you mean?

Thanks in advance

Hoo, boy. Are you sure you want a half hour long lecture? Better get a cup of coffee, because you are going to get one. Nah, forget it, I'll give you the Cliff's Notes version.

The inverter used in a backlight system is a constant current AC source. Because of the way a CCFL responds, the easiest way to adjust the brightness is by changing the pulse width of the drive voltage. This is done by applying a variable DC voltage to the brightness input of the inverter controller. The relationship between the voltage and the brightness is up the the designer of the inverter controller IC. He can decide if full brightness is at 0 volts, or at 3 volts, 5 volts, or whatever. He can decide if a higher control voltage results in increased brightness or lower brightness.

Quite often when we are troubleshooting backlight problems I ask for the voltage on the inverter control pins. On monitors it's almost certain that 0 volts is off, and 3.3 volts is on. The brightness pin is usually in the 1 - 2 volt range, but it doesn't have to be. Further complicating things, most monitor inverter designs use an analog voltage, which required the designer of the signal card to include a digital to analog converter. Now many inverter controllers include provisions for both digital and analog brightness control. As I was writing this, I looked up one inverter controller - the MAX753-MAX754. It uses only digital brightness control.

PlainBill

[Village Electronics]

Re: Lg 37lg10 no backlight

Quite often when we are troubleshooting backlight problems I ask for the voltage on the inverter control pins. On monitors it's almost certain that 0 volts is off, and 3.3 volts is on. The brightness pin is usually in the 1 - 2 volt range, but it doesn't have to be.
PlainBill

Would it be helpful to monitor the LD brightness pin while adjusting the brightness up and down?

[PlainBill]

Re: Lg 37lg10 no backlight

Would it be helpful to monitor the LD brightness pin while adjusting the brightness up and down?

Yes. If nothing else, that will indicate the range of the voltage (assuming it's an analog control signal).

PlainBill

[turtle9832]

Re: Lg 37lg10 no backlight

checked the voltage on LCD BR pin while adjusting the brightness with the remote. Voltage varied from 2.5 v up to 3.5.

i decided to replace the inverter board VIT71060.50.

plugged the new inverter board in and the set is repaired.

thanks for your help.

ANy ideas on trouble shooting the bad board besides the fuse?

thanks again

[PlainBill]

Re: Lg 37lg10 no backlight

checked the voltage on LCD BR pin while adjusting the brightness with the remote. Voltage varied from 2.5 v up to 3.5.

i decided to replace the inverter board VIT71060.50.

plugged the new inverter board in and the set is repaired.

thanks for your help.

ANy ideas on trouble shooting the bad board besides the fuse?

thanks again

Many of these inverter designs have multiple fuses. Make sure all fuses are good. Trace the backlight ON/OFF line from the connector to the inverter controller.

PlainBill

[sabre504]

Re: Lg 37lg10 no backlight

Had a similar board in a tv i worked on and resoldering the inverter transformers cured that one ,no promises that this will cure yours but worth a shot .