LG 60LB7100 screen goes black seconds after start up

[tstew]

TV was fine for two years and one day it just goes black and still has sound. I kept trying to turn it off and on most the day with no luck. The next day I turn it on and it worked fine for another month. Then it did it again. I did the flashlight test and could see the source playing in the screen. When I first turn it on it has full back lights come on with the LG life is good coming on like normal then it just goes black. I purchased a power supply with LED inverter built in and put it in. It's still doing the same thing. On start up it does fine for a few seconds and then goes black while still having sound and with a flashlight you can see the source playing. With the back panel off you can see the back lights come on during start up and then go off. It also has a code light blink rapidly 9 times on the power supply board. I'm uncertain if I purchased A bad part or if this is A bad main board or T con. I appreciate any help or advice.

[dskall]

Re: LG 60LB7100 screen goes black seconds after start up

Life will be good again after you replace the bad leds. For a while at least.

[tstew]

Re: LG 60LB7100 screen goes black seconds after start up

LED strips ahg. Not easy to replace or find online.

[dskall]

Re: LG 60LB7100 screen goes black seconds after start up

Well you can replace them individually or use the stacking method. 60" does require some finesse. May not be bad leds but to me LG=bad leds. You can wait and see what others say.

[attainteddragon]

Re: LG 60LB7100 screen goes black seconds after start up

i would just pull the screen out and look at the led's, but i'm very eager to pull things apart.

+1 for led failure!

[tstew]

Re: LG 60LB7100 screen goes black seconds after start up

My question is why will the back lights come on and then go off if any are bad? Wouldn't a bad back light just not come on at all?

[budm]

Re: LG 60LB7100 screen goes black seconds after start up

Can we at least see good clear pictures of the whole back side of the TV showing how all the boards are connected together? We also need straight shot pictures of the whole board, closed up pictures around the connectors so we can see the pin labels for each pin.

[budm]

Re: LG 60LB7100 screen goes black seconds after start up

My question is why will the back lights come on and then go off if any are bad? Wouldn't a bad back light just not come on at all?

It will still light up if:
1) If the TV has all the LED strings connected together in series like Christmas lights, if one or more LED/s shorted out, the whole string will still light up but it may/will trip protection circuit.
2) If there are more than one string of LEDs driven by more than one LED driver circuit, the one driver circuit with bad (open or shorted) LED/s will still light up the good LEDs for seconds and then it goes into protection shutdown.

The protection circuit will shutdown the circuits if:
1) Open circuit (one of the LED in the string is open) is detected, in this case the output Voltage feeding the LED string will go to high so it shuts down the circuit.
2) Over current if some of the LEDs shorted out, or shorted wiring, etc.

[tstew]

Re: LG 60LB7100 screen goes black seconds after start up

Here's a few pics.

[budm]

Re: LG 60LB7100 screen goes black seconds after start up

http://www.shopjimmy.com/lg-eay63072...led-board.htm#
LG EAY63072701 Power Supply / LED Board for 60LB7100-UT
I cannot read the P/N of your board, I believe that is the one you have.
Looking at the bottom side of the board, this TV has SIX LED driver circuits to drive SIX LED strings,
So what you have to do is this based on your picture: https://www.badcaps.net/forum/attach...5&d=1495392900
Turn the TV OFF, leave the Black probe of the DCV meter on the metal chassis, out the red probe on the far right pin of the connector, LED+, turn the TV on and watch how high the Voltage goes up as you see the backlights flash on for seconds, report that reading in this format LED+ = xxxV, after backlights go off = xxV
Turn the TV OFF and then move the red probe the VC_1 (Cathode return of the LED string #1), turn the TV on and see what the Voltage goes up too and the Voltage afetr the backlights go out, report the reading as VC_1 = xxV, after backlights go off = xxV.
Turn the TV OFF and repeat the same test for VC_2 ~VC_6.
This tst will give us the clue about the LED power supply and the LED strings. The reading should come out to be the same if all LED strings are OK.
LED power supply Voltage is shown as 69.3V 1.2A as printed on the board.
Pictures provided by SHOPJIMMY.

[tstew]

Re: LG 60LB7100 screen goes black seconds after start up

I'm using a klein meter. not a fluke by any measure.
LED+76.7v
after lights off 55.8v
VC_1=78.6v
=55.9
VC_27=78.6
=55.9
All of the other four had exact same readings. I was testing the red positive.

[budm]

Re: LG 60LB7100 screen goes black seconds after start up

I'm using a klein meter. not a fluke by any measure.
LED+76.7v
after lights off 55.8v
VC_1=78.6v
=55.9
VC_27=78.6
=55.9
All of the other four had exact same readings. I was testing the red positive.

Those readings on the VC_X do not make any sense since the LED+ 76.7V is the feeding Voltage to the Anode of all the LED strings and the VC_X are the Cathode of the LEDs which return to GND through MOSFETs and current sensing resistors. So I cannot see how the Voltage on the return lines of all six LED strings can be higher than the feeding Voltage of 76.7V.

[tstew]

Re: LG 60LB7100 screen goes black seconds after start up

I'm testing on the plug pins from the board that go to the ribbons. Also I'm testing the positive of the VC. Maybe I'm testing the wrong spot or wrong setting with my meter? I'm not skilled at this.

[budm]

Re: LG 60LB7100 screen goes black seconds after start up

You leave the black probe of the meter on the chassis at all time and then the red probe is on each pin of the connector starting with pin 1 which is the LED+, and then pin 2 (VC_1), and then pin 4 (VC_2)........on and on...., right?

[tstew]

Re: LG 60LB7100 screen goes black seconds after start up

On first test I tried putting meter to negative of VC1 thinking that's what you meant by return side. All back lights stayed on until I unplugged and tried again.

[budm]

Re: LG 60LB7100 screen goes black seconds after start up

Well, you did not follow my instruction on post #10:

"Turn the TV OFF, leave the Black probe of the DCV meter on the metal chassis, put the red probe on the far right pin of the connector, LED+, turn the TV on and watch how high the Voltage goes up as you see the backlights flash on for seconds, report that reading in this format LED+ = xxxV, after backlights go off = xxV
Turn the TV OFF and then move the red probe the VC_1 (Cathode return of the LED string #1), turn the TV on and see what the Voltage goes up too and the Voltage afetr the backlights go out, report the reading as VC_1 = xxV, after backlights go off = xxV.
Turn the TV OFF and repeat the same test for VC_2 ~VC_6."

[budm]

Re: LG 60LB7100 screen goes black seconds after start up

"All back lights stayed on until I unplugged and tried again." What do mean by STAYED on? The backlights do not black out after a few seconds?

[tstew]

Re: LG 60LB7100 screen goes black seconds after start up

It stayed on until I pulled power from it. That was during testing vc1 with probe to negative/black wire side of pin. When I gave it power again the back lights went off after a few seconds. I two questions before I test again. Of the two pins marked LED+ and LED- the one on the far right is negative, the left pin is positive. Which pin do I test? Also when you say return on the VC testing do you mean test the negative/black wire pin side? I'm grateful for your help. I'm sorry if this is frustrating teaching me. I won't be able to test again until this weekend. Single dad with two little girls and a busy work week.

[tstew]

Re: LG 60LB7100 screen goes black seconds after start up

These two pins for LED+ and LED-?

[budm]

Re: LG 60LB7100 screen goes black seconds after start up

VC_1 through VC_6 with ref to the chassis GND, just leave the black probe on the chassis and measure VC_1 ~ VC_6 with red probe as instructed in post 16. We want to know what the Voltages are from VC_1 ~ VC6 each time you turn on the TV, you have to do that 6 times.