LG 55LF6090 Blinks - No life

[jason123]

Hi folks,
Another problem I would love your help with.

So TV front LED is solid red. Pressing Power on remote results in 3 blinks then LED goes off (normal startup I believe)

No picture, no sound, and the Optical audio that typically turns red (LED) on the Main board doesn't do so.

Suspected backlight, there are 2 LED+ and 2 LED- (LED+ are 104.3 V, first LED - is 0.04V and the other LED- is 9.8 V)
I suspected the LEDs as always, disassembled the panel but all LED strips tested ok - No dead LEDs.

Next I looked at the TCON, and it looks like it's not getting power. Traced this back to main board, Q2407. Little SOT23. Marking is MP1, and I found this to be DMP2130 P-Channel FET. The D doesn't seem to get the 12V, though the G & Source seem to be getting 12V when the power is turned on.
Impedance of the power to GND on the TCON is high. Therefore I suspected the FET to be dead.

This might not be smart, but I shorted across the FET (S and D), and now the 12 (11.89 V) is getting to TCON, but still no Backlight or life in this thing.

Thoughts??

[Andrew F. Ali]

Re: LG 55LF6090 Blinks - No life

From your voltage test readings, you do have an open LED on the backlight strip/s. The test current may not be high enough to simulate the bad LED/s as the working drive current.

[jason123]

Re: LG 55LF6090 Blinks - No life

that's what I thought.. but - every stip tested fully with 10 mA through it. I'm using power supply with current limiting resistor not an LED tester. Would that explain why the FET is not allowing current to the TCON?

[jason123]

Re: LG 55LF6090 Blinks - No life

So, I connected external strips to the LED+ and LED- that was reading 104 and 0.04. Now it's reading 102V and 5.8V, the other string is reading 102 and 8.9V. Still, no luck. No backlight, or power to tcon
. Ideas?

[budm]

Re: LG 55LF6090 Blinks - No life

You have two LED driver circuits, one produces 145V @230mA, another one is 218V @230mA as printed on the board.
So when you test the Voltages on each one of the LED+ pins, did you leave the black probe on the chassis and Red probe on the LED+ pin then turn on the TV while watching the meter to see how high the Voltage jumps up to before going back down to 102V, that is always the way to test the output Voltage of the LED connector for the LED + and the LED - pin with and without LED connector in place. So repeat the test on all 4 pins of the connector which means you will have to turn the TV on 4 times and off 4 times.
BTW, you need to test the LED assembly as the whole string not just each strip, and by applying the Voltage right at the female pins of the LED connector. Testing just the strips does not tell you if you do have the complete circuit all the way to the connector or not.

[jason123]

Re: LG 55LF6090 Blinks - No life

Hi budm,
Yes, all measurements are referenced to Chassis (GND)
What's odd is even with the PWR ON line is 0V, there is roughly 87 V on the LED+ (both of them.) This is true with connector (to LEDs) plugged in or not.

When I turn on LED+ simply rises to the 102-103 V range. LED- one only goes from 0.02 to 0.04 and the other goes from 0.9 to 8.xx V (fluctuates) no significant overshoot. None of the LED+ (with or without connector) reach even close to 145 or 218 V

[budm]

Re: LG 55LF6090 Blinks - No life

87V is the feeding Voltage to the two boost converter, without LED connected the Voltage for the 145V will be over 180V, the 218V will be way over 218V.
"What's odd is even with the PWR ON line is 0V" Are you sure it does not change state when power switch is toggle? Are you 100% sure? That will be impossible to have 0V but the LED power supply Voltages can still change state. 0V also means the PFC Voltage booster will not be on either to produce about 400VDC on the main filter cap.
You need to also report all the Voltages of all the pins of the power supply that goes to the main board.

[jason123]

Re: LG 55LF6090 Blinks - No life

No PWR-ON toggles to 3.45v when the power is switched on. I just didnt expect LED+ voltages with pwr-on being off
All the voltages seem to be good
3.5 v is 3.4v
12 v is 11.87 v
24 v is 23.82v
PWR_ON (3.45 V when on, 0.012 when off)
P_DIM1 and P_DIM2 is 0.072, 0.041
DRV_ON is 0.013V

[budm]

Re: LG 55LF6090 Blinks - No life

OK, so we get the wrong Data then, so it does go high.
We still need the rest of the Voltage reading of the power supply connector pins that goes to the main board.
So do you have about 400VDC on the filter cap on the hot side when TV power switch is on?

[budm]

Re: LG 55LF6090 Blinks - No life

No PWR-ON toggles to 3.45v when the power is switched on. I just didnt expect LED+ voltages with pwr-on being off
All the voltages seem to be good
3.5 v is 3.4v
12 v is 11.87 v
24 v is 23.82v

We need the rest of the Voltages, your report does not show any control Voltages.

[jason123]

Re: LG 55LF6090 Blinks - No life

Across the large cap, I get 165V when off, and 386.5V when the TV is turned on

[budm]

Re: LG 55LF6090 Blinks - No life

We still need to see the control Voltages on the connector.

[jason123]

Re: LG 55LF6090 Blinks - No life

See updated please.. None of the other control voltages are on, they're in the logic low state

[budm]

Re: LG 55LF6090 Blinks - No life

Well, the PWR-ON is the control Voltage and it does toggle, so the BL-on (DRV-ON) has to toggle for the LED Voltage to kick up from 87V to 102V.

[budm]

Re: LG 55LF6090 Blinks - No life

At this point based on your report on post #1, you have bad main board.
BTW:'The D doesn't seem to get the 12V, though the G & Source seem to be getting 12V when the power is turned on." The GATE has to be 3 ~ 4VDC lower than the Source pin for this P-CH MOSFET to be on, the processor is not sending the control signal to turn on that MOSFET.

You can test the power supply board by itself by using pull up resistors to pull up the PWR-ON, DRV-ON, and may be the P-DIM pins to test the power supply circuit and the LED driver circuit.

[jason123]

Re: LG 55LF6090 Blinks - No life

Thanks budm. The DRV_ON is definitely not toggling. It's close to zero just like the P_DIM1 and 2

Thanks.. I'll order a replacement main board.
Thanks again for the help

[jason123]

Re: LG 55LF6090 Blinks - No life

Replacement main board arrived and it TV is working great. Thank you.

One thing to note. The original board of the TV is EBT63746906. The one I received from shopjimmy is EBT63728202 with a white (home-made sticker) on the part number, showing EBT63746906. The board works so I'm not too concerned, but I find it odd that they would do that at ShopJimmy. Maybe the same board and they flash it with the correct TV's firmware? I don't know

[Andrew F. Ali]

Re: LG 55LF6090 Blinks - No life

Well at least now you have a working TV

[howardc64]

Re: LG 55LF6090 Blinks - No life

Got one with exact same failure symptom (plugin AC, red standby light, power on, 3 blinks, no standby light after) Looks like another eMMC failure. Checking here to see if any one knows failures that isn't eMMC with these symptoms.

Exact same voltage readings as OP. All PSU power rails present. (3.5v standby, PWR_ON changes to 3.4V on power button press. No DRV_ON. 12/24v rail present)

LED tester shows 1 string = 120V (11 LED/bar x 2 bar x 6v) and the other 180V (11 LED/bar x 3 bar x 6v)

Main has 1.2v 1.5v 3.3v 5v power rails. Audio section power 0v, USB plugs no power, SPDIF no light.

Has 4pin UART port but service manual suggest this is for loading security keys.

I believe this LG has a separate small boot NVRAM besides eMMC so it is able to blink the standby light even with dead eMMC. But without getting boot code from eMMC, most of the main board will not boot up.

Here is the service manual

https://cdn.badcaps-static.com/pdfs/...5337af891b.pdf

[howardc64]

Re: LG 55LF6090 Blinks - No life

Thermal imaging and checking with schematic likely confirm MTX SOC <-> eMMC path is dead. Don't know which one. Maybe eMMC.

This image shows the 3 hot spots when pressing power button

• SOC heatsink
• SO-8 chip producing 1.2v MTX CORE
• SO-8 chip producing 3.3v NORMAL

This image shows bottom edge of eMMC heat up briefly (1-2s) before returning to ambient temperature after pressing power button



There are no heat signatures on eMMC after this initial brief heat. So clearly MTK SOC is not talking to the eMMC.

Briefly heating the heat sink with hair drying didn't change anything.

I guess time to order replacement main board. Main is fairly expensive and OOS ($70 on ebay, $60 incl ship on tvpartstoday, ShopJimmy OOS) so likely high demand due to high failure.