LG 47LN5400 Will not power up after re Cap

[budm]

Re: LG 47LN5400 Will not power up after re Cap

At 78V means the Boost converter circuits are not running, 78 V is the feeding Voltage to the Boost converters which boost the Voltage up to 100V as printed on the board. You need to put Black probe on the chassis, red probe on LED + the turn on the TV while watching the meter to see if how high the Voltage jumps up to before settling down to 78V. The Boost converter will not be running if the BL-ON is not present.
BTW, LED tester that you have probably only putting out 30mA at the most, the TV LED driver will drive them in hundreds of mA.

[nomoresonys]

Re: LG 47LN5400 Will not power up after re Cap

The proper way to test P201 is number 3 in my signature below, should do the whole test, pay particular attention to DRV-ON.

[R_J]

Re: LG 47LN5400 Will not power up after re Cap

It looks like you have a problem with the 1&2 led string. Most likely bad leds on one or more of the strips.

[Nevillet]

Re: LG 47LN5400 Will not power up after re Cap

Please see my updated post above.

[budm]

Re: LG 47LN5400 Will not power up after re Cap

The current measurements were with a DC clamp meter (UNI-T UT210E)

I can put a Fluke 85 series in series with the circuits to get more accurate measurements if required.

Using the automatic LED tester mentioned above the voltages for 1&2 = 94V (dimly lit) and for 3&4 80V (normal brightness)

n.b. Brightness being assessed via the screens rear holes.

Using the Fluke in record mode and in series with 1&2 the current max measures 28mA

Using the Fluke in record mode and in series with 3&4 the current max measures 256mA

This was measured with the TV off - then turned on using the hard wired buttons.

...............
I measured the following screen voltages (Fluke 85, manual range, record mode) using the same switch on method above.

On Pin 2, LED+, with the LED's connected the max was 175.2V and reduced to a steady 78.4V after 20 or so seconds
On Pin 2, LED+, with the LED's DISconnected the max was 173.0V and reduced to a steady 78.0V after 20 or so seconds

On Pin 4, LED+, with the LED's connected the max was 102.2V and reduced to a steady 37.2V after 20 or so seconds
On Pin 4, LED+, with the LED's DISconnected the max was 174.8V and reduced to a steady 78.0V after 20 or so seconds

You have one bad LED string.
I hope it does not look like this inside: https://www.badcaps.net/forum/showth...ht=LG+47LN5790

[R_J]

Re: LG 47LN5400 Will not power up after re Cap

I measured the following screen voltages (Fluke 85, manual range, record mode) using the same switch on method above.

On Pin 2, LED+, with the LED's connected the max was 175.2V and reduced to a steady 78.4V after 20 or so seconds
On Pin 2, LED+, with the LED's DISconnected the max was 173.0V and reduced to a steady 78.0V after 20 or so secondsOn Pin 4, LED+, with the LED's connected the max was 102.2V and reduced to a steady 37.2V after 20 or so seconds
On Pin 4, LED+, with the LED's DISconnected the max was 174.8V and reduced to a steady 78.0V after 20 or so seconds

You confirmed that there is an OPEN led string on connection 1&2 by disconnecting the led's and getting the same 175v
If the string WAS working the voltage when connected would have been 102v just like the other string.

[Nevillet]

Re: LG 47LN5400 Will not power up after re Cap

Many thanks for your continued help on this.

My friend has made the decision to replace the set with a new one and says I can salvage any bits I want. Which in a way is good because I can strip the screen down to inspect the possibly repair the LED's at no risk to the owner.

Looking at videos on uTube the process looks fairly daunting (the LCD ribbon cable and dust between the layers in particular).

Some questions:

Are there any videos/instructions recommended by this forum that would help?

Would the LED's be specific to the LG screen or are they fairly generic?

Was the original failure of the PSU precipitated by faulty LED's?

Thanks

[nomoresonys]

Re: LG 47LN5400 Will not power up after re Cap

Probably match up your strips on ali-express, make sure you do as instructed in post #16, as the backlights are overdriven and made to work too hard which means early failure if settings are left at max.

[neilc6]

Re: LG 47LN5400 Will not power up after re Cap

https://youtu.be/B7o-P7yXa3I

https://ebay.us/5pn5V5

I am seeing repeat failures from just replacing bad strips with used ones even after turning down the backlight level. I now recommend full replacement. Never had any issues with strips from ebay seller meihua361000 and I have purchased many from him. Speedpak shipping arrives in 6-10 days. Avoid rzconne branded strips on Aliexpress.

[nomoresonys]

Re: LG 47LN5400 Will not power up after re Cap

Lots of bad sellers on ebay too, seems that they all come from China, it's good that you found one you can trust.

[Nevillet]

Re: LG 47LN5400 Will not power up after re Cap

I've just ordered a replacement set from https://ebay.us/5pn5V5.

What's your view on replacing the individual surface mount LED's by hand?
And if so what would be the search term for eBay?

[diif]

Re: LG 47LN5400 Will not power up after re Cap

I've just ordered a replacement set from https://ebay.us/5pn5V5.

What's your view on replacing the individual surface mount LED's by hand?
And if so what would be the search term for eBay?

Do a search for LG 47LN5400 led on ebay, there are a couple of sellers.
I replace individual LEDs using a mini hotplate to ensure no damage to the PCB or LEDs. I bench test repaired strips and then I use 5 second fix to secure the lenses. They need careful alignment otherwise you get bright spots on the screen.

The parts of the LCD panel you need to be careful of are the flex cables/PCBs. I usually free them and tape the PCB to the front with painter's tape and also the corners.

Double check the corners when sitting the panel back in the frame and ensure it's flat in it's channel, and check again when putting the metal frame back on the top, the LCD can pop out and sit high on it's mount and get crushed when you snap the metal frame back.

[neilc6]

Re: LG 47LN5400 Will not power up after re Cap

"I am seeing repeat failures from just replacing bad strips with used ones even after turning down the backlight level. I now recommend full replacement."

Replacing all strips with new ones is the ideal solution since the other leds have wear and may fail in the future. If just replacing the LED, misalignment of the lens will cause bright spots as noted by diff. Sometimes, you cant find new strips or they are too expensive. Search 2835 3V LG LED on Aliexpress or Ebay.

[Nevillet]

Re: LG 47LN5400 Will not power up after re Cap

I'm going to wait until the replacement strips arrived before I open up the screen and I totally see the logic of replacing all of the LED's - not just the faulty ones.

When they arrive in a few weeks i'll report back on the state of the faulty ones together with my efforts on the repair.

Many thanks.