LG 42LV5400 powers on, no video, no sound

[Coder45]

Good evening,

I have LG 42LV5400-UB that suddenly stopped working.
It successfully powers on with the touch panel or remote (power LED is white) but there is no video nor sound.

When it powers on I hear a click of relay but no backlight seen from the rear panel in the dark.

Measured voltage on the connector P501/502 between the main board and PSU, all seems good.

PWR_ON is 3.3V

All other voltages 3.5V, 12V, 24V are OK as indicated on the attached picture.

INV_ON (Pin 18) is 0.6V

If I understand properly the latter corresponds to High for this model. (see the attached picture)
Right? So it is Ok.

Voltage on two big capacitors in the top part of PSU is 79V.

I followed the service manual troubleshooting process (except frequencies measurement) until "Change module".

Does it mean the LED panel is dead or still something else?

Mainboard: EAX63333405
Power board: EAX62865401

[budm]

Re: LG 42LV5400 powers on, no video, no sound

'INV_ON (Pin 18) is 0.6V' It should be >2~2.5V for on, <1V = OFF.
How about the ERROR signal?

[Coder45]

Re: LG 42LV5400 powers on, no video, no sound

'INV_ON (Pin 18) is 0.6V' It should be >2~2.5V for on, <1V = OFF.
How about the ERROR signal?

Thank you for the prompt response.

Err OUT (Pin 24) is 0 V.
Also INV_CTL on the base of Q504 is 3.3V

[budm]

Re: LG 42LV5400 powers on, no video, no sound

Thank you for the prompt response.

Err OUT (Pin 24) is 0 V.
Also INV_CTL on the base of Q504 is 3.3V

That is why the Invr is in low state, that transistor should be turned off by the processor for the Collector Voltage to go high to turn on the backligts circuit.
Right now that pin 18 invr is not changing states when the power swithc is toggled on and off, right? As shown on the flow chart, pin 18 must go high when TV is turned on.

[Coder45]

Re: LG 42LV5400 powers on, no video, no sound

That is why the Invr is in low state, that transistor should be turned off by the processor for the Collector Voltage to go high to turn on the backligts circuit.
Right now that pin 18 invr is not changing states when the power swithc is toggled on and off, right? As shown on the flow chart, pin 18 must go high when TV is turned on.

When TV is powered off, the pin 18 shows 0.3V, when powered on it goes lower to 0.06V (sorry, I missed a zero in the first message).

If I send +3V via resistor on the pin 18, I see backlight working.

The signal INV_CTL is managed by the pin 10 of the processor IC602. No idea why it is keeping it high.
I attached the TV manual if it may help.

[budm]

Re: LG 42LV5400 powers on, no video, no sound

When TV is powered off, the pin 18 shows 0.3V, when powered on it goes lower to 0.06V (sorry, I missed a zero in the first message).

If I send +3V via resistor on the pin 18, I see backlight working.

That is why I indicated that the main board is not sending the HIGH to turn on the backlights, you have the problem on the main board, and if it is the only problem with board, bad invr signal, but everything else is fine then you should see the pictures on the screen, if not then you have multiple problems on the main board beside bad invr signal.

[Coder45]

Re: LG 42LV5400 powers on, no video, no sound

That is why I indicated that the main board is not sending the HIGH to turn on the backlights, you have the problem on the main board, and if it is the only problem with board, bad invr signal, but everything else is fine then you should see the pictures on the screen, if not then you have multiple problems on the main board beside bad invr signal.

I tried a test with a flashlight and see no pictures.

Strangely, the service manual says that if INV_CTL is not On, then there may be an issue with the power board or "module" (LED panel?).

Can a defective LED panel cause this?

[budm]

Re: LG 42LV5400 powers on, no video, no sound

I tried a test with a flashlight and see no pictures.

Strangely, the service manual says that if INV_CTL is not On, then there may be an issue with the power board or "module" (LED panel?).

Can a defective LED panel cause this?

You are able to force on the backlights so the backlights are working.
BTW, did you force on the backlights with all the connectors in place?

[Coder45]

Re: LG 42LV5400 powers on, no video, no sound

You are able to force on the backlights so the backlights are working.
BTW, did you force on the backlights with all the connectors in place?

No, I disconnected the mainboard and connected pins with a resistor.

[budm]

Re: LG 42LV5400 powers on, no video, no sound

You can remove the wire from pin 18 on the main board side so you can attach the resistor to that end of the open wire to force on the backlights on while having all the connector in place so you can find out if you do have the pictures on the screen or not when the backlights are on with all the connectors in place.

[Coder45]

Re: LG 42LV5400 powers on, no video, no sound

You can remove the wire from pin 18 on the main board side so you can attach the resistor to that end of the open wire to force on the backlights on while having all the connector in place so you can find out if you do have the pictures on the screen or not when the backlights are on with all the connectors in place.

I will see if I can do that without too much damage.

The thing that I still do not understand is why sound is also absent.
This makes me think that the problem is more serious than just the backlight circuit.

With all that tests is it possible to say that the issue is located somewhere in the mainboard?

Searched for replacement mainboards, they are not easy to find

[budm]

Re: LG 42LV5400 powers on, no video, no sound

I will see if I can do that without too much damage.

The thing that I still do not understand is why sound is also absent.
This makes me think that the problem is more serious than just the backlight circuit.

With all that tests is it possible to say that the issue is located somewhere in the mainboard?

Searched for replacement mainboards, they are not easy to find

We already know that the main board is not sending the signal to turn on the backlights, and as I indicated, if you can get the backlights to come on and you do not see any pictures on the screen then you have more than one problem with the main board. Sound problem may or may not be unless you know for sure that the TV is set to the correct source input but since you cannot see the pictures on the screen to verify.
You can try heating and cooling the board, disconnect the T-CON board, check the regulator ICs Voltages, good visual inspection, etc.

[Diah]

Re: LG 42LV5400 powers on, no video, no sound

video audio, Dimming , backlight all those responsible on them one IC 101 i think.. the main one. since it turn PWB on, but no other parts, i would suggest trick this IC to inter ( INSTART ) service menu, as this step saved on first bits, so easy to let it switch on backlight and show the menu on the screen. of course only if the set response to RC. if this not help then reflow it by heat gun.

between forcing BL to turn on while set ON not good idea, as this BL had dimming control of 8 steps easy will damage.

also using flash light enough to know if there menu or not

[Coder45]

Re: LG 42LV5400 powers on, no video, no sound

You can remove the wire from pin 18 on the main board side so you can attach the resistor to that end of the open wire to force on the backlights on while having all the connector in place so you can find out if you do have the pictures on the screen or not when the backlights are on with all the connectors in place.

Update.
Tried to send INV_ON on the pin 18 while the connector P501 is attached to main board. The backlight is there but still no picture.

What else could I try to identify the faulty component?
I am wondering in which conditions IC602 does not enable INV_ON and keeps INV_CTL high.
May it be a defective LCD that is detected by the processor?

According to the attached picture and my measurements, the startup sequence is followed pretty well until the step 8 (INV_CTL).

It looks like the mainboard replacement is quite hard to find.
Called LG and local repair centers, no one is selling mainboards.

[mrjiffy6]

Re: LG 42LV5400 powers on, no video, no sound

Salut Coder45,

Have you resolved your problem? I've got the exact same model than yours and the exact same problem, it seems. Thanks for the service manual, i'll investigate the problem on my own and let you know of any updates. Meanwhile, i'm willing to know the outcome of the problem

[drkthundr]

Re: LG 42LV5400 powers on, no video, no sound

I have the same TV and same issue as listed above. I will be posting soon the measurements I'm getting and what I've done. I know the issue is on the main board, I've just got to narrow down the component or components that are causing this.

[drkthundr]

Re: LG 42LV5400 powers on, no video, no sound

Here are the measurements from P201 on the power supply. It looks allot like the original posters readings. I don't think the power is getting to the main board to turn the power supply on. I think it could possibly be ic602 or q504 on the main board or somewhere along the way. The hunt is on.

[microvibe]

Re: LG 42LV5400 powers on, no video, no sound

I know this is silly but most time with lg tvs have no video or sound I just bake the board in oven and they work. Solder on the processor cracks I believe. Thus definitely sounds like you have main board issue. Try freeze spray first on cpu see if cooling it enough makes contact

[drkthundr]

Re: LG 42LV5400 powers on, no video, no sound

Using the startup document posted on the post above, I have been using it to check voltages. I am not getting any voltage on the INV_ON / DRV_ON connection. I'm sure voltage on this line is needed to tell the power supply to turn the set on. I am posting an image of the document with the voltage readings i am getting. Its looking to me like it might be R546, R532, R547, and or Q504. Let me know what you think.

[drkthundr]

Re: LG 42LV5400 powers on, no video, no sound

Here are more measurements of the area. Measurements are in the pictures. What do you think?