LG 47LN5400 TV Backlight problem

[abduct]

Re: LG 47LN5400 TV Backlight problem

could be also a bad LED inside the panel. Measure the voltages on the LED power connectors on the PSU. See what you get. Does it ramp up and fall off or what?

These are the readings for the original PSU (Note I don't officially know which set of outputs are which I just gave them channel names to help analyses the data):

-LED + CH1 to ground goes to 76v then when the backlight attempts to turn on goes to 103v and ramps down to 76v
-LED + CH2 to ground goes to 76v and then 171v when the backlight attempts to turn on, then ramps down to 76v.
-LED - CH1 to ground goes to 20v, then 46volts when the backlight attempts to turn on, then ramps down
-LED - CH2 to ground stays at around 28 volts, and then ramps down after backlights attempt to turn on.

The backlights turn on/off for less then a second.

These are the readings from another PSU from a 55" LG which makes the 47" TV work when used with it:

-LED + CH1 to ground goes to 93v and then the backlight STAYS on and slowly lowers until stabilizing at 56.9V
-LED + CH2 to ground 0v at all times
-LED - CH1 to ground 0v at all times
-LED - CH2 to ground -1v at all times (might of had probes backwards, red was on LED -, and black was on grounded part of the PCB)

The backlights turned on and stayed on and the TV functioned as normal, no noticeable dark spots or other damage.

Did you measure the PFC Caps using VDC and the negative leg as ground?

If so the PFC circuit is dead

*EDIT* Then again you said you can still see the image.... *EDIT*

Which power filter caps are you talking about. I tried measuring the power filter caps closest to the LED output (as marked by the link in the original post) and they showed similar VDC results as measuring from the LED output pins. (When I measured the caps, I put the red probe on the positive side, and black probe on a grounded part of the PCB.

When I test positive side to negitive side of the cap, I get DC voltage with similar results as in the table above for the 47" LG PSU.

With the original PSU, I can still see image with via the flashlight test and go through the menus, it's just the backlights fail to stay on for more than a split second.

I find it weird that what I think is channel 2 on the LED power output is putting out 0 volts on the 55" LG PSU when placed on my 47" TV. Might this be a feature that when it notices damage, it tries to keep on working, but closes off voltage to the damaged strip? Then again I don't see any fluctuation in voltage at all when measuring on that channel or noticeable dark spots on the screen where no LEDs might be shining.

Thanks for the help guys. If you want high res images of the power supplies front/back let me know.

[newtothis]

Re: LG 47LN5400 TV Backlight problem

Did you measure the PFC Caps using VDC and the negative leg as ground?

If so the PFC circuit is dead

*EDIT* Then again you said you can still see the image.... *EDIT*

[CapLeaker]

Re: LG 47LN5400 TV Backlight problem

could be also a bad LED inside the panel. Measure the voltages on the LED power connectors on the PSU. See what you get. Does it ramp up and fall off or what?

[abduct]

I have recently come into a bunch of TV's for cheap and they all need repair. I've had success with getting one working, in an odd ball way so I am here for advice or recommendations.

First the conditions of the TV's:

I have 3 LG 47LN5400, one LG 55LB5900 (broken panel) that I am working with (I have a few others 50" and 42" but they have broken panels that I am not going to bother with besides for parts.

The 3 47" LG's I am trying to fix all have the same problem. When powering on, the backlight comes on for a split second and turns back off. When the backlight is off, I've done the flashlight test and the image is still there. This tells me the main board is likely fine. All the caps look in mint condition and there isn't any obvious damage such as arcing or black burn marks. I've been reading some threads and have done some measurements and I will post those later.

In order to test the 3 47" LG's I took the powersupply from the 55" LG with the broken screen and tried each one, and the TV's work flawlessly so that limits the problem to the power supplies of the 47" TV's.

In my searching I cam across this image, where someone kindly layed out the parts of my PSU

https://www.badcaps.net/forum/attach...1&d=1430269466

I did some measurements like the thread suggested at Filter Caps 1, and Filter CAPs 2, and here are the results.

Filter Caps 1: 78v (peaks at 103v when backlight turns on then off)
Filter Caps 2: 75v (peaks at 182v when backlight turns on then off)

In another thread a similar issue arrised and they said the problem is the LED drive controller IC, which makes sense since when I swapped the other larger power supply in from the 55" TV, everything worked fine.

Here is some other info:

47" TV model number: 47LN5400
47" power supply model/serial number: EAX64905501 (2.0) Rev 1.0

55" TV model number: 55LB5900
55" power supply model/serial number: EAX 65423901 (2.1) Rev 2.0

-------------------------------------------------------------

So I guess the real question is, is it possible to purchase these IC's to solder them on, or is there a cheap place to buy new power supplies?

Also is it safe to use the 55" LG TV power supply in the 47" TV? The PCB physically doesn't fit so I will have to make my own mounts and cut a hole in the back plastic casing, but other than that is there any risks?

Like wise I have a 42" TV with a smashed panel but the backlights stay on so I assume the PSU is good, but it only supports power to 1 LED string. Can I remove the LED controller from this and use it on a 47" LG PSU?

Thanks!

Edit: I am not 100% positive that it is the LED controller, but from what I read, it sounds like it could possibly be it. Any testing/tips will be appreciated.