LG 55LM6700 LED/3D- Powers On, No Picture

[Chris24]

Hey everyone- this forum has been a huge help in the past and I have been able to fix three TVs from your advice, my own and some friends. I am current stumped though.

I have an LG 55LM6700 that powers on, the LG logo screen comes up for 2-3 seconds then goes away, and after that I have no picture. The backlight is ON. In a dark room, the front screen is a dark glow, and with the back off, you can see the LEDs come on from inside.

The previous owner said they got 'two boards from LG and it was hard because they are discontinued and out of stock.' I don't know what the symptoms were before that led to them contacting LG.

There is no T-Con board, only a power supply and mainboard. No visual capacitors blown. I tested the large obvious glass fuzes and they are good.

Will post pictures and what I have tested so far. Appreciate your help.

[Chris24]

Re: LG 55LM6700 LED/3D- Powers On, No Picture

Back of TV

Power Supply

Main board

[diif]

Re: LG 55LM6700 LED/3D- Powers On, No Picture

Have you checked under the stand bracket for the tcon ?

[Chris24]

Re: LG 55LM6700 LED/3D- Powers On, No Picture

On this connector from the power supply to mainboard, with the TV off and plugged in I get the standby 3.5V on pins 5 and 6. Here is what I get with the TV powered on, from bottom (power on) to top (p-dim). I also get the same on the mainboard side.

1. 3.4V (why not 24? is this all the TV needs for power?)
2. 24.8V
3. 0
4. 0
5. 3.5
6. 3.5
7. 0
8. 0
9. 11.9
10. 11.9
11. 11.9
12. 2.0

[Chris24]

Re: LG 55LM6700 LED/3D- Powers On, No Picture

Diif-thanks for the quick reply

Here is a pic under the stand of where the two LVDS cables connect to panel. It appears there are two separate small boards in there. Are they TCON boards?

[Chris24]

Re: LG 55LM6700 LED/3D- Powers On, No Picture

Here is the connectors in the LED driver section at top of power supply and the volts i tested with set on:

P210: 81.1V
P211: 0

Connectors Left to right:
Black connector

1. 81.1V
2. 0
3. 0
4. 0
5. 0
6. 19.4V
7. 19.4V
8. 19.4V

White:
1. 19.4
2. 19.4
3. 19.4
4. 19.4
5. 19.4
6. 19.4
7. 0
8. 81.1

[Chris24]

Re: LG 55LM6700 LED/3D- Powers On, No Picture

Okay last picture and the closest I have come to a possible problem. On this small connector (P701) at the top right of power supply to the main board here is what i get with the set on:

top to bottom:
1. 0
2. 0
3. 0
4. 0
5. 0.1V (this seemed odd)
6. 3.2V
7. 3.2V
8. 0

same readings at the main board side

[tvtimmy]

Re: LG 55LM6700 LED/3D- Powers On, No Picture

You have the classic bga fault. Take off the heatsink on the mainboard and hit the chip with a heatgun on low with a reducing nozzle for about 1 min. Let it cool and then try it. It will work but for how long? I had one that made it 3 weeks and 25 that are still going after a year.

[Chris24]

Re: LG 55LM6700 LED/3D- Powers On, No Picture

Thanks- timmy

Dumb question: can I accomplish the same by putting the main board in the oven on 385 for 10 minutes as seen in other places on the web? I do not have a heat gun but could take the board somewhere probably.

[Chris24]

Re: LG 55LM6700 LED/3D- Powers On, No Picture

After reviewimg the forum more on bga fault I went ahead and baked it in the oven, hooked it back up, and got no change to symptoms.

Maybe I didn’t do it hot/long enough? While I search for a heat gun I think I’ll bake again and see what happens.

[Chris24]

Re: LG 55LM6700 LED/3D- Powers On, No Picture

Got a heat gun and took out the main board, removed the heat sink, and heated up the bga chip for about a minute on low. Put it all back together and no changes.

So after baking in oven and attempting to reflow with the heat gun the TV still turns on, flashes a clean LG logo for 2 seconds, then goes blank with black screen that is back lit. All the original voltages I posted are unchanged.

I'm wondering if since the original owner received two replacement boards from LG, if the original problem lies somewhere in the panel or LED strips. Any thoughts or suggestions from the group on anything else to verify are appreciated.

[jasonr2000]

Re: LG 55LM6700 LED/3D- Powers On, No Picture

because the back lights are staying on its not a backlight issue,


if the mainboard is the fault then a replacement is the way forward,


but that don't mean you will get one look on eBay and places for one,

[Nick's Tvs]

Re: LG 55LM6700 LED/3D- Powers On, No Picture

For the love of God, yall need to stop throwing things in the oven every time you suspect a bad main board. Electronics repair should not be mixed with the culinary arts.

This model is known to have issues with the mainboard, but throwing it in the oven won't do anything for you. Most often you will need to replace the BGA chip as it has an internal failure, not something that can be fixed by re-flowing the board. It could also have software failures, in which case you would need to replace the software chip/chips.

This is a scenario where you will need to send your board in for a repair service, or purchase a new one. eBay sellers have them for about $100 to $120.

[Andrew F. Ali]

Re: LG 55LM6700 LED/3D- Powers On, No Picture

For the love of God, yall need to stop throwing things in the oven every time you suspect a bad main board. Electronics repair should not be mixed with the culinary arts.

This model is known to have issues with the mainboard, but throwing it in the oven won't do anything for you. Most often you will need to replace the BGA chip as it has an internal failure, not something that can be fixed by re-flowing the board. It could also have software failures, in which case you would need to replace the software chip/chips.

This is a scenario where you will need to send your board in for a repair service, or purchase a new one. eBay sellers have them for about $100 to $120.

+100
I don't know why members think that baking a Main Bd is a solution. Baking can cause other components to overheat and fail and/or burn other plastic parts, etc.

[Chris24]

Re: LG 55LM6700 LED/3D- Powers On, No Picture

Thanks everyone. I fully agree- now. I think I was rushing it and there is a HUGE following for baking LG boards online that I fell for it - and probably ruined the main board in the process so won't be doing that again. These main boards are not very common and run over $100 online so I'll be passing on this one since its just a curb pick up. too bad

Thanks for the help and once again learned a few new things from the forum.

Chris