LG 42LN575V backlight issue.

[Darkstar_]

Hi,

Hoping you guys can come to my rescue here.
Firstly, let me apologise if my knowledge is not as thorough as you may be used to on this forum. I really know nothing about repairing, but I am reaching out in the aim of saving money on a repair and saving my family who are going crazy without a TV (2 young boys).

My LG 42LN575V has developed an issue. About 7 months ago, the backlight seemed to have failed. I could see the image if I shone a torch onto the screen. I sent it to my local repair shop and I think they charged me just over £100 to fix the issue. I say fix because the exact same issue has happened again (they only have a 3 month warranty on repairs).

I've removed the back and did some basic visual inspection to look for anything that looks damaged but I am at a loss.
After some light Googling, it could be the power board(?) which I can get a replacement of for £25. But I don't want to spend that on something which isn't the issue.

I have a very basic voltage tester which I was planning to use on the power to the LED strips to see if they are getting power.

I posted elsewhere about this issue and this response I got confused me further:

These TVs have problems with the LED array, likely the TV repair shop only repaired the strips and not replaced the whole array as recommended by LG, also the firmware must be update when the repair is complete or the TV will fail.

I tried to clarify the firmware point, but got no answer.

So here I am, with a busted TV and no money to put on serious repairs. Can you guys help shine on light on what could be wrong and can some noobie like me repair it?

[jondoe]

Re: LG 42LN575V backlight issue.

Take good pictures of the front and backs of the main boards. Get your multi meter, be careful, take readings of the boards outputs, report back your findings.

[dick_barton]

Re: LG 42LN575V backlight issue.

Do you have a very brief flash of the backlights. You need to look very carefully and in a darken room or looking at the rear of the set with back removed.

[vinceroger69]

Re: LG 42LN575V backlight issue.

as this is a led set quite possibe one or more have failed as above take one picture showing all boards connected and then one of each board also include the board numbers thats printed on the boards

[dick_barton]

Re: LG 42LN575V backlight issue.

Can you also check the following:-

The DC voltage at the capacitor marked with meter probe on each leg.
The voltages at the power supply connector to the main board. 18 pins in total.
The voltage at the led connector.

[ReeceyBurger123]

Re: LG 42LN575V backlight issue.

It will be an Led again did they turn down the Bl and advise you to keep it low? I started terminating warranties if people turn it back up.

[tn245]

Re: LG 42LN575V backlight issue.

It will be an Led again did they turn down the Bl and advise you to keep it low? I started terminating warranties if people turn it back up.

What about modifying the backlight circuit to limit the LED current? I've been playing with doing that with a couple recently. I know tom66 has said he takes out some of the parallel sense resistors on Vestel PSUs. Many LED driver ICs have pins specifically to set the current. Some of the Samsung PSUs even have trimmer pots for this I noticed. Seems better than relying on the customer to keep the backlight down.

[lee0803]

Re: LG 42LN575V backlight issue.

ive had 2 of these and a few other models you will need a backlight strip but this does mean running the risk of damaging your screen as it has to be removed to replace. If you decide to fix it do as the guys are saying and lower the backlight on your set. LED failure was always going to be a issue when these designs came out. good luck

[Darkstar_]

Re: LG 42LN575V backlight issue.

Thanks all for giving me some good advice here. I will work my way through it best I can.
I removed the back cover again and using my terrible meter (which is essentially a series of lights going from 4V to 400V) I tested the contacts (marked on the attached). I assume that while the TV is on it would give out power? Well it doesn't. Every 3/4 seconds my tester flashes for a split second but that is it.
I would assume then that this board is just not providing power to the led strips?

I am leaning towards buying a new board to take a punt, but yeah, it's a punt.

[dick_barton]

Re: LG 42LN575V backlight issue.

That may be because the leds are powering up and then the voltage is falling because you have a led fault.
Have you access to a reasonable multimeter or borrow one preferably with a min/max function.?

There a a number of reasons why this could be happening and is why it is important to give voltage readings so the fault can be found.

I certainly would not be considering a replacement power board at this time.

Can you give the voltage readings on the connector highlighted against what it says it should be on the legend alongside.

[budm]

Re: LG 42LN575V backlight issue.

The LED power supply is putting out, but without correct load the Voltage will go up beyond what are printed chart as shown on the board so the LED power supply over protection kicks in.

[Darkstar_]

Re: LG 42LN575V backlight issue.

Sorry for not replying sooner. Money has been so tight, it has taken me nearly a month to scrape the money together for a meter.

I ended up buying one. I will admit, the settings wheel looks daunting.
To get the results you are after, by measuring certain points, which "mode" do I have the thing in? Or does it not matter?

I am going to try get those readings today, I need to know what is wrong with this damn TV.

[vinceroger69]

Re: LG 42LN575V backlight issue.

That meters fine for testing put black probe in com point and red probe in the right hand side most voltages we will get you to test will be in dc mode setting where switch is in picture as thats dc volts symbol

[dick_barton]

Re: LG 42LN575V backlight issue.

The meter in the photo is currently set to 600V DC. You can leave it set there to measure the voltage between the two capacitor legs. As photo in post#5, black meter probe on one leg of capacitor and red meter probe on the other leg of capacitor. This is high voltage so be careful when using the probes to make the measurement.

To measure the led voltages at connector P801 you can leave the meter set to where it is and use the red meter lead to measure the voltage at the connector pins and black meter lead connected to the panel chassis. I suspect the voltage will rise to a high voltage when you switch on and fall quickly.

To measure the voltages on P201 you can rotate the meter dial anticlockwise to 20V DC. Red meter lead to measure, black lead to chassis.

For all measurement black meter lead is connected to COM and red meter lead is connected to V Ω mA

[vinceroger69]

Re: LG 42LN575V backlight issue.

Thanks dick_barton i was typing on phone your explanation is a lot better than mine.

[dick_barton]

Re: LG 42LN575V backlight issue.

Fortunately I'm not on the phone. And it would take me too long.

[vinceroger69]

Re: LG 42LN575V backlight issue.

yes thats why my reply was a basic answer back on laptop now so alls good.

[Darkstar_]

Re: LG 42LN575V backlight issue.

Thanks guys, really appreciate the detailed responses and thank you for putting up with my total lack of knowledge.

I did what you said (as best as I could) and got the attached readings.
All readings are when the TV is on and not on standby.

[vinceroger69]

Re: LG 42LN575V backlight issue.

there should be 18 voltage readings on that connector as has two rows of pins

[dick_barton]

Re: LG 42LN575V backlight issue.

All the voltages on P201 look fine but you have only read one row of the two rows of pins.
You need to also give DRV_ON & PDIM.

Also the top pin of P801 is given as 230V. Does that remain steady or fall and what DC voltages were on the next two pins.