LG 42LN5100 no backlights

[provider10]

Hi

I have this LG with no backlights using the backlight tester seems the LEDs are OK
Theproblem seems to be the FET that pulls the LEDS to ground has anybody had tis FET(Q802) fail

Thanks
James

[dick_barton]

Re: LG 42LN5100 no backlights

Are you sure it's the mosfet. There's plenty of Youtube videos showing how to test them. Have you checked if the mosfet is being driven?
Do the backlights flash momentarily as you switch on?
Have you checked if the "DRV_ON" signal is present. Connector P701 pin 2



https://www.electronica-pt.com/esque...s-lb35b-34067/

[chrismurphy]

Re: LG 42LN5100 no backlights

these are notorious for failed backlights
Not worth repairing

[provider10]

Re: LG 42LN5100 no backlights

Hi thanks for the reply. I don't think its the LED strips as I have bought a tester and the panel lights up ok .
The positive voltage is on one end of the strips but there is an FET that i think switches the other end of the strips to earth. I was wondering if anybody has had this fet go faulty or if the circuit driving the IC gives problems

I do not get a momentary flash and yes when switched on I get a voltage on the DRV ON pin

Thanks
James

[dick_barton]

Re: LG 42LN5100 no backlights

The mosfet is driven by the led driver IC so if that is not driving the moseft then the led current will not flow to ground.

Have you any voltage from CS_LED to chassis?

What voltage do you have on pin 16 of U103

[dick_barton]

Re: LG 42LN5100 no backlights

My mistake. Please ignore CS_Led and U103 voltages. I was looking at the wrong diagram.

What is the part number of your power supply?

[provider10]

Re: LG 42LN5100 no backlights

Hi

The number on the power supply is 3PCR00275B

Thanks

James

[dick_barton]

Re: LG 42LN5100 no backlights

The power supply should have a number should be something like "LGP42 13PL1 EAX64905301"

What voltage do you have at the led connector on pin 7 (P801) when you first power up. Does the voltage peak sharply and drop just as quickly.
What voltage do you have on P802 again when you first power up.

Have a read through https://www.badcaps.net/forum/showth...hlight=17ips20

[provider10]

Re: LG 42LN5100 no backlights

The number is LGP421-13PL1 (PLDF-l205B)

There is a very short flash from the LEDs at switch on.

When i measure on the lead to the LEDs on one side I get a varying voltage of around 120 v and on the other side a varying voltage of about 230v. Yet if I measure across the socket I get 0 volts

[provider10]

Re: LG 42LN5100 no backlights

across c214-204 I have a stable 115volts. On switch on this voltage goes unstable varying from 103 to 113 volts

[neilc6]

Re: LG 42LN5100 no backlights

Flash of backlights is a symptom of bad backlights. https://techhometravel.com/lg-ln-ser...ed-backlights/

[dick_barton]

Re: LG 42LN5100 no backlights

The led voltage for that model is 157.5V so it would appear as neilc6 has just said that you have a led(s) or led strip connection problem.

[angelos]

Re: LG 42LN5100 no backlights

Just a quick question. these LN model backlights are those with the concave lenses, right? They are so expensive here in my country that they cost about 35% the price of a new tv. Has anybody come up with a cheaper way to repair or replace them? This would really help me out.
And... yeah, i agree with what was said. LG's are notorious for having their backlight fail anything from 1.5 years from purchase and onwards.

[angelos]

Re: LG 42LN5100 no backlights

Another note! Don't think if your backlight tester drives your LED's that they are 100% ok.
One symptom is a drive voltage on the tester that is bouncing around. In other words it goes 57.4V then 63V then 52V. That indicates a faulty LED strip. The other thing is that even if the LED tester drives the backlights steady you don't know for sure if that is correct because the tester drives it at a reduced current and not at 100% of full current like the TV set does.
You will have to open up the panel and observe what happens. Are there dead led's etc

[neilc6]

Re: LG 42LN5100 no backlights

LB/LF series uses the large concave lens. Also 55" LN series as well. Lots of vendors of strips on Aliexpress that ship worldwide.

[angelos]

Re: LG 42LN5100 no backlights

Yes, aliexpress has but what they don't have is a reliable shipping method to this dump of a country of ours so there is no cheap shipping method.
For example. A 55" LG TV's backlights cost $22, lovely. Then comes our favorite rip offs DHL charging $123 and on top of it once the package has arrived at their customs then thy want an importer license etc. So now they want to ship it to my area, a remote outlying area so they want more money. At this rate let me do the conversion, R2175 at an exchange rate of 15:1. Hey guess what, i can buy them from the LG agents at the same price. Aliexpress has completely failed us here in South Africa because our local postal service has collapsed.
Anyone heard of www.bangggod.com, they are a online sales platform somewhere in the orient and they have circumvented this shipping nighmare.
They have their own customs (China) divizion called Buffalo logistics and they send your parcel via a chain of clothing suppliers (PEP Stores) right to my town i a matter of 18 days. There the shipping is $6, bloody excellent. Problem is banggood do not do much electronic spares. pity though.
Just thought i would vent my frustration here, sorry.

[angelos]

Re: LG 42LN5100 no backlights

Oh yes and if you find a suppler that is willing to send it via free shipping method you wait 4 months to maybe receive it. Again, collapsed postal service.

[nomoresonys]

Re: LG 42LN5100 no backlights

Yes, if you mean Banggood they are ok, probably ships even faster than aliexpress, maybe recheck your spelling.

[angelos]

Re: LG 42LN5100 no backlights

yes, like i said 18 days from payment to delivery. Currently wit alibullcrap i wait 4 months on average for a parcel to arrive. A huge difference hear. I am trying to convince aliexpress to change their shipping method but all i get to speak to is a bot.

[provider10]

Re: LG 42LN5100 no backlights

Thanks to everybody that replied to my post.
75 and still learning.
I don't get much work as I am semi retired but I bought an LED tester from Banggood.
And as the lights seemed to light up OK. I looked for other faults anyway opened it up and found 3 LEDs SC
So once again for the help and patience

I will know in future

Regards

James