Screen not coming on - LG 55LD520-UA

[kingkiller]

I had some burnt jumpers between the master and slave boards. The screen was dim until I replaced the two jumpers along with one of the master boards, because the jumper connector was destroyed. After replacing the jumpers and master board the screen looked fantastic...for about 10 minutes. After 10 minutes or so the screen went off and has not come back on. I'm looking for any suggestions on how to begin troubleshooting the issue. If anyone has any ideas I'm all ears.


Thank you,

Michael

[kingkiller]

Re: Screen not coming on - LG 55LD520-UA

Any help would be much appreciated.

[kingkiller]

Re: Screen not coming on - LG 55LD520-UA

I tested end-to-end connectivity between all 40 points between the two sets of inverter boards and they all tested good. I noticed some of the inverter board screws were missing and thought maybe that could be causing a ground issue but replacing the screws didn't make a difference.

When I hit the power button I noticed there is a short delay before the indicator light starts to move on the TV, then it goes blue shortly after like it normally would. The light flickers when I hit buttons on the remote, but the screen stays completely black. Does anyone have any suggestions? What is this indicative of?

Thank you,

Mike

[kingkiller]

Re: Screen not coming on - LG 55LD520-UA

The power supply board outputs 24V to the inverter boards when the inverter boards are not connected to the power supply. When I connect them and wait for the light on the front of the TV to turn blue then test voltage, there is no voltage going to the boards. I'm guessing there is some kind of POST test the TV is doing then it is cutting power to the inverter boards? How can I test which is bad? The bottom master board is suspect.

[budm]

Re: Screen not coming on - LG 55LD520-UA

Can we see some good clear pictures?

[kingkiller]

Re: Screen not coming on - LG 55LD520-UA

Can we see some good clear pictures?

The only place I see any signs of damage are on the Slave 2 board, but I'm not sure how to test it.

Pictures attached

[kingkiller]

Re: Screen not coming on - LG 55LD520-UA

If I power on the TV with the master boards disconnected from the power supply board, then it never cuts power to the slave boards. Not sure if that indicates anything.

[kingkiller]

Re: Screen not coming on - LG 55LD520-UA

What are these things I've circled in black? It seems the fourth one down on both the slave2 and master2 boards is non-existent. I'm going to look for pictures of others to see if that is "normal"? It does look like one was there at one point though.

Edit: Red Herring, they are missing in all of the pictures I've seen of the boards elsewhere on the interwebs.

[selldoor]

Re: Screen not coming on - LG 55LD520-UA

Hi - I think they are mosfets - what is the part number.

Also on another thread this youtube was posted - it shows a test on the feedback
on the inverter boards. They are different to yours but the principal may be the same.
http://www.youtube.com/watch?v=oE6K4xBVweA

[kingkiller]

Re: Screen not coming on - LG 55LD520-UA

Nikos P4004ED is the number.

I had watched that video but I wasn't sure where he was putting the common, he only shows the red probe. Is the common just going to a good ground?

[selldoor]

Re: Screen not coming on - LG 55LD520-UA

Here is the datasheet

You can see a test for mosfets in retired caps 2 seconds to black guide
https://www.badcaps.net/forum/showthread.php?t=10419 start at post 19.

I would just use a groundscrew where the board you test attached to the chassis.

[kingkiller]

Re: Screen not coming on - LG 55LD520-UA

More info: If I fire up the set with the cable from the master inverter to the power supply board disconnected, I get sound. When I fire it up with the cable connected, I get nothing. I also confirmed that the panel is displaying something, there's just no light, so I'm hoping, at least, that that indicates my panel is not bad.

I can't seem to get any AC reading on the inverter boards when I turn the TV on. It also doesn't turn on the same. When I hit the power button with the inverter board connected to the power supply board the TV hesitates for a few seconds before the indicator light starts to go back and forth, finally going blue indicating that it's on. Normally the red indicator light starts going back and forth immediately and then goes to blue. Also, once it is blue, if I try to power it off it hesitates before the light turns red again.

[Ltank]

Re: Screen not coming on - LG 55LD520-UA

Disconnect the cable supplying power to the inverter boards. Does the P.S. fire up and stay on? If so then the ther inverter boards are the problem. Connect the master with the slave disconnected. Does it power up and say on lighting half the screen? If so then the master is good and the slave is bad

[selldoor]

Re: Screen not coming on - LG 55LD520-UA

Really need budm back on the case or tom66. You do have the lamps connected when testing?
Is there no flash at all? try in a darkened room?

[kingkiller]

Re: Screen not coming on - LG 55LD520-UA

There's no flash at all with everything connected, or when the master is disconnected from the power supply board. I just tried a couple more tests. The bulbs do flash on when I disconnect the jumper between the master1 and master2 boards. The bulbs do not flash on when I have both the master and slave jumpers disconnected, or when I have only the slave jumper disconnected. So, when the slave jumper is disconnected, the bulbs do not flash. When only the master is disconnected the bulbs flash. I'm not really sure if that helps isolate things or not :-/

Also, I hear some white-noise when the bulbs flash on.

[kingkiller]

Re: Screen not coming on - LG 55LD520-UA

Well, either I'm doing something horribly wrong, or the transistors on the master 2 replacement board I bought are all shorted. The resistance measurements are around 2 ohms or less. Am I missing anything here?

[kingkiller]

Re: Screen not coming on - LG 55LD520-UA

Well, I went ahead and reached out to the eBay seller I bought the board from. Before I request an exchange or refund is there anything I should investigate as a possible cause for the shorted transistors? It seems suspect that all the transistors on the master2 board would have gone bad on their own all at the same time.

[selldoor]

Re: Screen not coming on - LG 55LD520-UA

How do the readings compare with the old board/slave board.

If I recall correctly the mosfet test says Remove from circuit and repeat the tests to verify.

Did you do that? there may be other component on the board affecting the result.

[kingkiller]

Re: Screen not coming on - LG 55LD520-UA

How do the readings compare with the old board/slave board.

If I recall correctly the mosfet test says Remove from circuit and repeat the tests to verify.

Did you do that? there may be other component on the board affecting the result.

Hey, I actually trashed the old board after replacing it, guess I should have kept it. The numbers on the slave board are very very different. Since the numbers are so drastically different, and I'm not confident I'll be able to put them back on the board if I remove them, I didn't take them off the circuit, especially since I might have to return it.

The seller suggested pulling the ribbon from the screen to see I if the inverter is bad. Any idea what he meant by that? He also stated that 90% of the time it's a slave problem, not with the master. Any last suggestions? :-/ I'm at a loss.

[kingkiller]

Re: Screen not coming on - LG 55LD520-UA

Ok I bit the bullet and removed it from the circuit and luckily (so I didn't feel like I had to put it back on) it tested the same. I'm kinda concerned as to why they would have all shorted out like that though. Is there anything I should check before sending this board back for a refund? I don't want to ask the seller for a refund if it's something in my set that caused the board to fail.

Thank you.