LG 55LN5600- Sound but no backlight

[TheEngler]

This is my first post on BadCaps.net. I have an LG 55LN5600 of the dreaded 2013 vintage that the backlight died on. It powers up and the screen will display normally for anywhere between 0.5 to 30 seconds. After the backlight dies, I can see the screen contents by using a flashlight. It was my parent's TV and they decided to replace it so they gave it to me since I'm an electrical engineer. I should also mention the TV was/is covered by a Square Trade warranty which still has 3 months of coverage left.


I read this thread:

https://www.badcaps.net/forum/showth...ght=lg+picture

and immediately suspected the LEDs so I ordered the ones from AliExpress that jbfr_54 had indicated in anticipation of doing the repair since they were cheap.


In the meantime I pursued a repair through SquareTrade and they sent a replacement power supply board to me directly and had a technician come a day or so after we got the power supply. I wasn't around when the tech came (my wife was) and he swapped out the board with the same result. He put the original board back in and left after closing up the TV.


I HATE to throw out something that is still very viable so I disassembled this monster today. Prior to disassembling, Square Trade told me to dispose of the TV in accordance with local laws and is in the process of sending a check. The power supply has no obvious cracks in any of the solder joints and no hint of any bulging caps although there are some flux globs from what was either a bad board wash or bad rework. This led me to believe there were dead LEDs in the backlight.


For the faint of heart, I can confirm IT REALLY SUCKS to tear down to the LED strips on a large TV like this. None of the LEDs showed visual evidence of failure so I hooked up one of these

http://www.ebay.com/itm/Constant-Cur...sAAOSwCEdYT7ID

that I had left from another project to a set of sharp tipped spring loaded probes. I set the voltage to 3.3V and limited the current to 30 mA (using one of the AliExpress LEDs). I proceeded to individually check all 84 LEDs (seven rows of 12, which are split into 2 sets of six in each row). ALL LEDs illuminated. I the raised the voltage slightly and raised the current limit and checked them AGAIN - they ALL are functional.


I have NO reason to suspect the tech's competence but I do wonder if the power supply board Square Trade sent was indeed functional. It wasn't shipped in an ESD bag or anti-static foam. So I am back to suspecting the supply. I can do the voltage test but don't know if I can do so without having the LCD panel connected. The supply is silkscreened with 'PWR-ON' and 'DRV-ON' on the last two pins going to the main board.


Can I disconnect the main board and wrap the supply 3.3 V to both the 'PWR-ON' and 'DRV-ON' pins (in it's current disassembled state) without damaging anything? This also got me to thinking that maybe the 'DRV-ON' signal from the main board is crapping out and not a function of the power supply or LEDs.


Can anyone confirm that I can do the 'PWR-ON' and 'DRV-ON' test without damaging anything? I don't want to put it back together until I am certain the backlight strips are not crapping out somehow. Any thoughts or suggestions?


Thanks!

[budm]

Re: LG 55LN5600- Sound but no backlight

You should leave everything connected and find out what DCV you have on those two pins, more likely the 'PWR-ON' command signal is OK.
<1V = off, > 2V = ON.
DRV-ON is the BL-ON, so monitor that pin when the Power switch is actived and after the backlights go out.
You also need to verify the Voltage at the LED connectors.
We also need good clear pictures of your board so we can see what we are dealing with.

[TheEngler]

Re: LG 55LN5600- Sound but no backlight

So I won't damage anything by not having the panel connected?

[TheEngler]

Re: LG 55LN5600- Sound but no backlight

@budm Attached are pics of the main board and power supply.

Can I power this monster up without the TCON connected to the panel and not cause damage?

[attainteddragon]

Re: LG 55LN5600- Sound but no backlight

you can power it up without the panel, or t-con connected, without any issues.

[TheEngler]

Re: LG 55LN5600- Sound but no backlight

Thanks for the responses @budm and @attainteddragon. I finally got back to this monster this afternoon. The 'PWR-ON' and 'DRV-ON' signals appear to be solid (LOW when off and constant 3.2-3.4 VDC when ON) so it's not the main board.

It 'appeared' all LEDs were briefly coming on (flash) but just to make sure I used my cell phone in the video mode while I did so. After stepping through frame by frame, it appears the bottom rows weren't coming on. There are seven (7) rows of LEDs with each row split into halves. The top four (4) rows are wired in series (I will call them the upper LED group) and the bottom three (3) rows are wired in series (which I will call the lower LED group). Upon closer inspection, the LEDs in the lower LED group are NOT coming on. ALL LEDs in the upper LED group come on briefly.

The connector driving the LEDs has four (4) pins - two (2) feed the upper LED group and two (2) feed the lower LED group. I disconnected the connectors feeding the upper group and lower groups. The upper cable was not long enough to reach the lower LED group but the lower cable could reach the upper LED group. I connected the lower cable to the upper LED group and the upper LED group came on briefly. So it is something with the lower LED group. I checked the continuity of the cables in the lower LED group as well as the connection between the two halves of the rows. All seemed good.

To reiterate from my first post, I checked ALL LEDs individually with a constant current source (~100 mA, ~3.5 V) and each and every LED (upper LED group and lower LED group) illuminated when I did so one by one. As to checking the LED+ to LED- voltage, my DMM (a 20+ year old Fluke 79, no max hold) can’t react quick enough. It shows ‘OL’ for the brief instant when the LEDs blink and then settles to under 0.5 V.

So WTF? Went and ate dinner and came back afterwards. Swapped the left strip-to-strip cable. Powered it on – no change. Swapped the right strip-to-strip cable. Powered it on – no change. Swapped the end strip loops. Powered it on – no change. Leaned on each of the LED strip end connectors while powering on – no change. Leaned on the bottom strip-to-strip connection and powered on – SUCCESS! Tapped this connection – went out. I must press this connection for the backlight to work – consistently.

FYI - if backlight does not come on at power on, it won't come on at any point after. You must cycle power EVERY TIME.

Since I want to sit down and relax, I will simply solder jumper wires over this connection point (+ test point to + test point and – test point to – test point) in the morning. Hopefully I can put the screen back in without breaking.

I will follow up with the final outcome.

[DjKrish]

Re: LG 55LN5600- Sound but no backlight

I maybe wrong but it sounds more like cold solder to me

[TheEngler]

Re: LG 55LN5600- Sound but no backlight

@DjKrish That is quite likely the case but the connections are mostly under the connector. To quote Arlo Guthrie's Alice's Restaurant:

"at the bottom of the cliff was another pile of garbage. And we decided that one big pile was better than two little piles, and rather than bring that one up, we decided to throw ours down."

Since it was a B!TCH to handle a wobbly 4 ft x 2.5 ft LCD panel, I felt it was better to solder around the connection and know it was going to work rather than try to fix. I did NOT want to open this sucker again. I had a 31" RCA TV (circa early 1990's vintage) that had the notorious TOB (Tuner on Board) issue. I had to re-sweat the tuner joints every 3-4 years to keep it going. That sucker was almost 100 lbs...

Seems to work like a champ!

[budm]

Re: LG 55LN5600- Sound but no backlight

Glad to see it is working.
Same problem with these SMD connector, poor design without good mechanical support.
https://www.badcaps.net/forum/showth...578#post761578

[TheEngler]

Re: LG 55LN5600- Sound but no backlight

@budm, read your post. Same style connector and you're absolutely right. I always prefer through hole vs. SMT on connectors (in most cases) but in this case would have to worry about shorting to the the 'tub'. Amazon says that TV was first available April 9, 2013 so roughly same vintage too. Hopefully they change the strip design.

So if no back light, voltages check out, no obvious LEDs failures --> check strip connectors.