LG 65lb7100 Popped backlight broken SMPS

[diablo9987]

TLDR; Single backlight LED popped no more power from Powersupply.


Hi I tried to keep the title short but will explained what happened here.

My 65lb7100 had a bad backlight led which was causing the TV to turn the backlight off. I took apart the TV to get to the backlights I powered it with the power supply to see if I can spot the LED. Then when I was powering each strip 1 by 1 I eventually got to which I assume was the bad led but it popped/burned and the lights shut off and i was unable to get anymore power from the supply.

I don't have enough know how on SMPS to trouble shoot fully but probing around i'm getting power on the primary side but on the secondary side there is nothing except 3.5v on the optocouplers and motherboard power cable. There is no visible damage to the board as well.

The Board is A EAY63189001 REV2.0
Here is a nice picture of the Board https://assets.shopjimmy.com/media/c...189001-top.jpg

Hopefully I can get this fixed as I don't know if I damaged the main board as well and don't want to have to buy a new power supply just to have it still not work.

[nomoresonys]

Re: LG 65lb7100 Popped backlight broken SMPS

Check your fuse F101 on power board with a multimeter, see if thats what popped, check with tv unplugged, little how to video here. https://www.youtube.com/watch?v=HuWly36NyEE

[budm]

Re: LG 65lb7100 Popped backlight broken SMPS

You must report the Voltage reading for each pin, except GND PINS, of that black 24-pin connector P201 when TV is plugged in and when power switch is actived.
You must put down pin name an follow by Voltage reading for each pin.

[diablo9987]

Re: LG 65lb7100 Popped backlight broken SMPS

Check your fuse F101 on power board with a multimeter, see if thats what popped, check with tv unplugged, little how to video here. https://www.youtube.com/watch?v=HuWly36NyEE

Ya checked both fuses today they are fine. I have been slowly going through Checking the diodes and resistors and so far they are good.

[diablo9987]

Re: LG 65lb7100 Popped backlight broken SMPS

You must report the Voltage reading for each pin, except GND PINS, of that black 24-pin connector P201 when TV is plugged in and when power switch is actived.
You must put down pin name an follow by Voltage reading for each pin.

I'll have to do that tomorrow as I have some ends of diodes desoldered to check them but from what i remember the TV does not turn on and when checking all the pins on P201 the only reading i got was 3.5V but don't remember on which pins.

I'm guessing the 3.5v is the standby power? maybe the supply is fine but the main board is toast and not sending the startup signal?

[diablo9987]

Re: LG 65lb7100 Popped backlight broken SMPS

OK well I put all the boards back on the TV and now it seems to be working. The display panel is off so all I see is the back lights, but the lights come on and change brightness as I try to move around with the remote. I don't know why it didn't work for the 2 days prior.

Anyways I guess it's back to fixing the original issue. If anyone sees this is there a recommended source to buy the LEDs and maybe a lens as the blown LED melted its lens a bit too.

TV = LG 65LB7100
LED Model = ? 3535 6V Maybe
Back light Strip = ag-d1 94v-0 1415
LENS = No idea

[MichaelCline]

Re: LG 65lb7100 Popped backlight broken SMPS

I was given (and repaired!) an LG 65LB7100 with intermittently failing backlight. Basically the screen would go blank after a few seconds or minutes, and the red LED would blink 4 times. I learned a lot about how the power supply board works while fixing my TV, so wanted to share that knowledge here:

There are taps for the 8 LED- outputs, and LED+ and LED-. If the LED current or voltage is too high on any of the 8 strips, the board will disable that strip and flash error 4. Too low gives error 3. If more than 3 strips get disabled, the backlight board will turn off entirely. It is very hard to notice just one or two disabled strips as these are quite diffused vertically.

The power for the LED's is provided by a central dc-dc inverter to ~63 volts, and then boosted to ~75 volts, measurable at the LED+ and LED-. Less than ~72 volts or more than ~82 volts the backlight board will error out and shut down. If you see ~63 volts the booster is not boosting. In my case the board was producing ~82 volts and would sometimes work, sometimes not, with error 4. The voltage is set by a voltage divider with 6 "05D" smt 105K ohm resistors in series+parallel which is easy to find. The fix in my case was adjusting this divider to bring voltage to 75V by addition of a 330K resistor parallel with the last 4 resistors of the top of the divider. I could not determine the root cause of the voltage being off, and suspect it was internal to the controller IC.

The LED strips are each driven from LED+ and PWM LED- to set the brightness of the given strip dynamically.

In my case I tested all of the electrolytic capacitors and found one small one had failed outright. I replaced all the caps, but this made no difference other than providing a warm fuzzy feeling.

One other thing to know, if you are experimenting with a shutting down board, the fastest way to power cycle it is with the power plug. (Seems obvious, but the power button otherwise takes forever!)

You can test each LED strip easily using a ~500 ohm resistor from LED- to each LED tap and look for the glow while the board is in shutdown. If the strips appear to work, you most likely have an issue with the supply board.

I hope this info is clear and helpful and the community gets some value from my learnings.