LG 65UH5500-UA Backlight flash on first power up only

[budm]

Re: LG 65UH5500-UA Backlight flash on first power up only

PWR-ON is the PS-ON for the power supply boar, the LED driver board is used for routing that signal to the power supply board, it is not used by the LED driver circuit.
For the backlights LED driver board to works it needs DRV-ON (BL-ON) and the P-DIM signal.
So you are able to force on the Backlights by using the PS-ON signal, at this point the image, sound, and all other function of the TV are OK?
What do you get at the P-DIM pin?
What is the Voltage reading between the two leg of cap C409?

[jason123]

Re: LG 65UH5500-UA Backlight flash on first power up only

Ok, further measrement on the LED Driver board yielded something intersting:
The PWR-ON (pin 1 on CN402) has 3.45 V when I turn TV ON. DRV-ON (Pin 11) however has only 1.5 V. Momentary contact between these 2 pins (Pin 1 and Pin 11) turns the Backlight on and keeps it on.

[diif]

Re: LG 65UH5500-UA Backlight flash on first power up only

Very odd.. Brand new set of LEDs. With a tester, they all light up..
I'm going to try replacing the LED driver board - Can't think of anything else.

Swap LED1+ and LED1- with LED5+ and LED5- and see if the slightly different voltage reading moves.

[jason123]

Re: LG 65UH5500-UA Backlight flash on first power up only

So this TV has 24 strips, each with 5, 3-V LEDs. There are 6 supplies on the LED Driver board, so each supply should be providing at least 60 V Differential for the Strips to turn on (I'm assuming each supply is driving 4 strips = 20 LEDs)

The fact that I'm getting only ~40V differential tells me the LED driver is not doing its job?? Am I on the right track?
Is there another control signal that tells the LED Driver to turn on other than the STB that's going to the power supply? Or is the Power supply turning the LED driver board on by simply turning on the 80V rails (From 60 to 88?) I see PWR_ON and DRV_ON on the Driver LED coming from Main board.

Just trying to determine if this is a LED Driver board issue or still a power supply issue and I simply got a bad power supply this time too (2 bad supply boards)

[jason123]

Re: LG 65UH5500-UA Backlight flash on first power up only

Not sure, maybe one weak diode in backlights??

Very odd.. Brand new set of LEDs. With a tester, they all light up..
I'm going to try replacing the LED driver board - Can't think of anything else.

[nomoresonys]

Re: LG 65UH5500-UA Backlight flash on first power up only

Not sure, maybe one weak diode in backlights??

[jason123]

Re: LG 65UH5500-UA Backlight flash on first power up only

Ok, so
When TV is on (STB = 3.4V)
LED1+ to LED6+ are reading 88.25V
LED1-, LED2-, LED3-, LED4- & LED6- are reading 42.26 V
LED5- is reading 44.6 V

When TV is off (STB = 0 V)
LED1+ to LED6+ are reading 66.2V
LED1-, LED2-, LED3-, LED4- & LED6- are reading 20.24 V
LED5- is reading 22.3 V



Is that meaning string LED5 has defect? I'm measuring on the back of the connector (Side that is directly connected to LED strips so no connector "connectivity issue" it seems

[budm]

Re: LG 65UH5500-UA Backlight flash on first power up only

I got derailed by the large caps having 385v all the time... never seen that before

If i didn't misunderstand you #40, if STB is 0 v, then PFC Should be off. So why would large caps have 385v?

That is why you need to check pin 22 of the SMPS/PFC IC, but as I indicated it may be stuck on, it the least of the problem right now, you need to find out why you do not have backlights.

[jason123]

Re: LG 65UH5500-UA Backlight flash on first power up only

I got derailed by the large caps having 385v all the time... never seen that before

If i didn't misunderstand you #40, if STB is 0 v, then PFC Should be off. So why would large caps have 385v?

[budm]

Re: LG 65UH5500-UA Backlight flash on first power up only

So the power supply is putting out the Voltages like it should, so the LED drivers board should be working as long as the LED driver board has all the control signals for the backlights to be on.

[jason123]

Re: LG 65UH5500-UA Backlight flash on first power up only

Yes the 13.2 V is 10.5 v when TV Is off. When the STB is ON (3.4 V) the voltage jumps to 12.62 v (when the back lights happen not to turn on) and 13.34 v when it powers up properly.

80 v is 66.4v when TV is off and ~82V when STB is 3.4 v

[budm]

Re: LG 65UH5500-UA Backlight flash on first power up only

Edit: Looking at the spec sheet of the SMPS/PFC IC combo, the PFC has enable pin 22 (see page 34).
OPTO IC 1 looks to be for the PS-ON controller OPTO.
BYW, are the 80V and 13V present when running the power supply board by itself?
The key point right now is that if you do have the 80V output of the power supply board to feed the LED driver board with Error pin is LOW, and the LED driver board has all the control signals then the back lights should be on.

[jason123]

Re: LG 65UH5500-UA Backlight flash on first power up only

So I replaced the IC.. same issue - with only AC connected to the power supply, I see 385 V across large caps, and 0 V on STB pin

Bought another power supply board - it's acting exactly the same 382 V across large caps with only AC connected. Could it be that both boards are bad? or are they supposed to work this way??

[jason123]

Re: LG 65UH5500-UA Backlight flash on first power up only

Is it the NXP Pfc IC, or...

[jason123]

Re: LG 65UH5500-UA Backlight flash on first power up only

So to be clear, when the TV is OFF, the Voltage on the main cap stay at 385V?

Correct...

[jason123]

Re: LG 65UH5500-UA Backlight flash on first power up only

Not anymore. Recall i replaced the full LED Set and fail signal is 0 v

[budm]

Re: LG 65UH5500-UA Backlight flash on first power up only

At this point , the FAIL pin is the issue since you have to GND it to get the TV to come on, need to trace out the FAIL pin to see where it leads to as RJ suggested.
https://www.shopjimmy.com/lg-cov3369...er-supply.htm#
https://assets.shopjimmy.com/media/c...401-bottom.jpg

[budm]

Re: LG 65UH5500-UA Backlight flash on first power up only

So to be clear, when the TV is OFF, the Voltage on the main cap stay at 385V?

[jason123]

Re: LG 65UH5500-UA Backlight flash on first power up only

With everything connected.. StB is 3.4 v when tv is on and 0.057 when tv turns off

[budm]

Re: LG 65UH5500-UA Backlight flash on first power up only

So does the STB change state when TV is turned on and off with everything connected?
Yes the main board control the STB signal.
The STB control is being routed through the LED driver board.