LG 42lg30-UD... no picture

[stratos976]

I was given a LG 42g30-UD, because the picture doesn't work. With the help of this forum I was able to repair my Samsung Plasma so I thought I'd give this a shot also.
I've searched on here but need some direction to troubleshoot further, as other sets close to this model seem to be "-UA" and have more boards in the back.
when you power on the set there is no flicker or flash of backlight, just dark, but there is sound. With a flashlight, I am able to "see" snow on the screen and also the input options if I scroll though them. I've added a couple pics of the opened back for reference.
thanks in advance!

[KevinCro]

Re: LG 42lg30-UD... no picture

There is a fault somewhere with the backlight system. Either a bulb is bad or the power to the backlight is not being supplied. Can you post a close up of the power board front and back? The inverter board might be bad because usually the screen will light up for at least a few seconds.

[CapLeaker]

Re: LG 42lg30-UD... no picture

Need hi res pics of the whole power supply and areas with the cables unplugged so we can read the pin outs.
Basically you have to see if the inverter board is getting the supply voltage and the BL ON voltage. If either is missing, the backlight is off.

[stratos976]

Re: LG 42lg30-UD... no picture

Ok I'll work on the pics. I'm assuming the power supply board is the tan one, would the board on the far left under the metal cover be the inverter board?

[CapLeaker]

Re: LG 42lg30-UD... no picture

yes

[selldoor]

Re: LG 42lg30-UD... no picture

Please post the pictures using manage attachments so that they appear as thumbs so we dont have to have them open unless needed.

[stratos976]

Re: LG 42lg30-UD... no picture

Here are pics of Inverter board front and back. I took a close up of the caps as the connections to me looked like they got hot. This board is getting
24.8v on pins 1-5,
pin 11 (VBR-A) was 1.6v,
pin 12 (on/off) 1.4v,
pin 13 (VBR-B) 2.6v,
pin 14 (errout) 0v

Can the output to the bulbs be checked?

[stratos976]

Re: LG 42lg30-UD... no picture

These are power supply pics. I checked voltage out on all the pins, I wrote down what I saw but I really don't know if they were proper or not. Nothing on the board to me looked visually bad or that it had become overheated. both fuses were good. pics are clockwise around board starting at bottom right corner where power enters.

[CapLeaker]

Re: LG 42lg30-UD... no picture

Yep, but need those pics from the power supply so you can force the power supply and the backlight on.

Can you make another one of the 2 connectors so we can see the writing?

[stratos976]

Re: LG 42lg30-UD... no picture

Here are connectors on power board.

[CapLeaker]

Re: LG 42lg30-UD... no picture

Looking at what you have posted so far is that the on / off voltage for the inverter is too low to turn on the back light. I'd expect more than 2.5V there.

All in a nut shell, you have to disconnect the main board so that only the inverter is hooked up to the power supply. Then you have to jumper the PWR ON and the ON/OFF pin through a 1k ohm resistor to the 5V Stanby.

With that the power supply is forced on and the backlight should light up.

[stratos976]

Re: LG 42lg30-UD... no picture

Thanks for the manual!
A couple questions.
on the double pin connector of the power board the 16.5v pin is only putting out 15.6v, could this be a problem?
the on/off pin on the double pin connector has 2.8v, the pwm-dim has 1.6v.

the PWM-dim pin of the inverter has 2.6v, while the on/off only has 1.4v

Are the ON/OFF and PWM-DIM pins the "same" on the double pin connector and the inverter cable connector? seems weird to me to have 2.8v at ON/OFF and 1.6v at PWM at the double pin connector, then at the inverter connector have 2.6v at PWM and 1.4v at ON/OFF, like they swapped??

All in a nut shell, you have to disconnect the main board so that only the inverter is hooked up to the power supply. Then you have to jumper the PWR ON and the ON/OFF pin through a 1k ohm resistor to the 5V Stanby.

With that the power supply is forced on and the backlight should light up.

just to clarify this, what I'm trying to do is take the 5v standby and with a 1k resistor put more than 2.5v onto the on/off and PWM-dim inputs of the inverter board to get it to turn on.

[stratos976]

Re: LG 42lg30-UD... no picture

I tested as posted, I forced on the inverter by jumping the 5v standby voltage trough a 1k resistor to the on/off signal of the inverter board. This powered up the inverter board and backlights ARE working.

Where would the problem be that would keep the inverter board from getting the on signal, in the power board or the main board? or something else

[budm]

Re: LG 42lg30-UD... no picture

The main board generates the BL_ON and dimming signasl to turn on and control the inverter circuit, the main board also generates the PS_ON signal to turn on the main power supply.

[stratos976]

Re: LG 42lg30-UD... no picture

Thanks, so next step would be to replace main board I assume.

[CapLeaker]

Re: LG 42lg30-UD... no picture

I am glad you narrowed down the problem!
Either it is main board replacement time, or you try to figure out why the back light on signal isn't generated properly.

[stratos976]

Re: LG 42lg30-UD... no picture

What things or where would I need to check to see if the signal isn't being generated properly. That sounds like an internal board thing, but it may be something I could fix?

[selldoor]

Re: LG 42lg30-UD... no picture

You need to find the BLon pin on the main board and try and trace it back. See if you can find it in the schematic and see where it goes. ( usually main processor eventually)
Post good clear picture of the main board and the area 3-4 inches around the blon pin

[stratos976]

Re: LG 42lg30-UD... no picture

To update thread, I went ahead and purchased a mainboard from a seller on E-bay. It arrived today and I installed it. Set is working all looks good so far.
Thanks for the help!

[CapLeaker]

Re: LG 42lg30-UD... no picture

Thanks for the follow up!