LG 42LN5400 backlight issue, double-check my troubleshooting

[gustagen]

Hi everyone! I am 3 for 3 in repairing a monitor and two TVs with this forum's help. I've also been able to repair a few without the forum's help.

I'm back today with an LG 42LN5400 that has a backlight issue. It was rescued from the curb.

During power up, the backlight flashes briefly. I can see the on-screen display using a flashlight.

During power down, the backlight flashes quicker and dimmer.

I understand that failing LEDs are common with this model, but I want to verify my testing before proceeding.

I've followed along with the troubleshooting steps in this thread and have found the following:

at connector P201:

Pin - Standby, Power Up
GND - 0V, 0V
12V - 0, 12
12V - 0, 12
GND - 0, 0
24V - 0, 24
GND - 0, 0
3.5V - 3.5, 3.5
3.5V - 3.5, 3.5
PWR ON - 0, 3.3

GND - 0V, 0V
NC - 0, 0
12V - 0, 12
GND - 0, 0
24V - 0, 24
GND - 0, 0
PDIM2 - 0, 1.1
PDIM1 - 0, 1.1
DRV ON - 0, 3.3

at P801, cable connected, black lead to chassis ground:

Pin - Peak At Power Up, After 10 Seconds
Pin 1 - 114V, 27V
Pin 3 - 75, 66
Pin 5 - 0.5, 0.6
Pin 7 - 172, 90

When Pin 1 of P801 is disconnected, the backlight does NOT flash at power up.

Attached are photos of the power supply PCB and the chassis label.

Do my measurements indicate a working power supply and faulty LEDs?

Thanks for any help you can provide!

[budm]

Re: LG 42LN5400 backlight issue, double-check my troubleshooting

It indicate you have open circuit in one of the two LED strings. That is why the Voltage went up to 172V.
Pin 5 - 0.5, 0.6
Pin 7 - 172, 90

[gustagen]

Re: LG 42LN5400 backlight issue, double-check my troubleshooting

Thanks, budm. I suppose the next step is removing the panel.

I have another question. In the other thread dhoff024 reported these readings at P201 on power up, which you indicated were on the low side:

PDIM2 - 0.8V
PDIM1 - 0.8V
DRV ON - 2.8V

My measurements are:

PDIM2 - 1.1V
PDIM1 - 1.1V
DRV ON - 3.3V

Are my measurements in range? What should those pins read?

I will proceed with removing the panel and report back. I plan to record a slow-motion video during power up to see which LEDs aren't lighting.

[gustagen]

Re: LG 42LN5400 backlight issue, double-check my troubleshooting

I successfully removed the panel and was able to use my phone to make a video of the emitters flashing during power up. I've attached a screenshot.

Only the top and bottom LED strips are flashing during power up. The middle three rows remain off.

I connected three 9-volt batteries in series and tested each row individually at the contacts nearest the PCB.

Rows 1, 3, and 5 lit up fully with my test rig.

Row 2 partially lit up, but its 4th emitter did not. I removed the optic and found it was shorted.

Row 4 did not light at all. I applied power to just the left half of the row, and those five emitters did light.

I tested row 4's five right-side emitters with my DMM in continuity mode and found one that was faulty.

I swapped the left sides of rows 2 and 4, which are both labeled L2. Now I have four fully working strips, with row 4 needing just two emitters.

Is it worth the effort to simply reflow two new emitters, or is it inevitable that more LEDs on these strips will fail?

Am I better off replacing all of the strips with a new 10-piece kit?

[budm]

Re: LG 42LN5400 backlight issue, double-check my troubleshooting

You may be better of replacing the whole set instead of taking a chance of taking the delicate panel apart more than once. When done you need to turn down the backlights level (not Brightness) to may be 50% to help extend the LED's life.

[gustagen]

Re: LG 42LN5400 backlight issue, double-check my troubleshooting

That's good advice, budm. Saving a few dollars now might result in a cracked panel later!

In addition to lowering the backlight level, I've read that a firmware update is also advised when replacing these backlight strips. Apparently the new firmware doesn't drive the emitters quite so hard.

I'll order the parts today and report back when they arrive. Thanks again!

[budm]

Re: LG 42LN5400 backlight issue, double-check my troubleshooting

Update to latest FW will probably help but I still think the LED's are still driven hard with the FW update. Just make sure to lower the backlights on all input any way.

[neilc6]

Re: LG 42LN5400 backlight issue, double-check my troubleshooting

If you are going to be doing more tv repairs, you should invest in a backlight tester. Bonus is that you get a pair of sharp probes you can use with your multimeter.

https://www.aliexpress.com/store/pro...909018049.html

[gustagen]

Re: LG 42LN5400 backlight issue, double-check my troubleshooting

Success!

The replacement LED strips arrived and the TV is working great.

I turned the backlight level down to 60% to help these emitters last a bit longer. I'll do the firmware update, too.

Thanks, everyone!

[budm]

Re: LG 42LN5400 backlight issue, double-check my troubleshooting

Thanks for the follow-up on the repair.
Can you please provide the link to the seller you got the replacement strips from so it will help other people as well? Thanks

[gustagen]

Re: LG 42LN5400 backlight issue, double-check my troubleshooting

I found the lowest price for replacement LED strips on eBay by searching for "LC420DUE" which was on the chassis label indicating the panel model.

I was sure to order NEW strips, not used. My set was about $33, including shipping from/to the USA.

[budm]

Re: LG 42LN5400 backlight issue, double-check my troubleshooting

Do you still have that seller name and link? Most of EBAY sellers sell used strips.

[neilc6]

Re: LG 42LN5400 backlight issue, double-check my troubleshooting

I've purchased both new and used strips from Chinese ebay seller meihua361000 with no issues. Quick epacket shipping (1-2 weeks) and packaged securely.

https://www.ebay.com/str/meihua361000

[budm]

Re: LG 42LN5400 backlight issue, double-check my troubleshooting

Thanks, I like to keep the good sellers in my list.

[gustagen]

Re: LG 42LN5400 backlight issue, double-check my troubleshooting

I purchased mine from eBay seller 'luckyelectronic'.

https://www.ebay.com/itm/Replacement...E/222996774308

[budm]

Re: LG 42LN5400 backlight issue, double-check my troubleshooting

Thanks!