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

[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

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

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

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

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

[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

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.

[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)

[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

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.

[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

P-DIM is pin 12 and is not accessible - I'll get a pair of sharper tip leads and report shortly.
C409 reads 88.x V

Traced DRV-ON back to the motherboard - seems something has changed, either resistors changed value from nominal (not labeled - 603 package) or leakage in the path is higher, but the main board generates 3.3 V, but after resistor networks by the time it gets to the LED Driver Pin 2 of OZ9906 (Chip Enable) it's down to 1.5 V - not sufficient "ON" logic.. given that I don't have 603 resistors and I can't see the darn things anyway, I replaced one of the series resistors (there are several between the main board and the LED Driver) with a blob of solder and that did the trick.. so far so good.. LED Driver is seeing 2.9 V on the EN pin of the controller - Backlight comes on..

Now.. back to the weird problem that appeared on 2 power supply boards - why are the large caps seeing 385V always... PFC not operational on 2 boards ??? very odd

[budm]

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

P-DIM must be present since the backlights are on and you can dim the backlights at this point.
The PFC circuit, need to look at pin 22 of the SMPS/PFC IC as suggested.

[jason123]

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

Pin 22 of the PFC IC reads 5.53 V when TV is off.. jumps to 8.6 when it's turned on.

[budm]

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

Pin 22 of the PFC IC reads 5.53 V when TV is off.. jumps to 8.6 when it's turned on.

That must be measurement error. Did you use the chassis as the GND for the meter? You need to use the negative leg of the main filter cap as the ground for the meter because you are making measurement in the hot side.
See spec sheet of the IC on the page that I pointed out to for what the Voltage should be on pin 22.

[jason123]

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

So not the cleanest solder job...but pin 22 connects to cap then to hot gnd, so i soldered across the cap. Just measured again from pin 22 referenced to large cap gnd and same results.

I see what you're referring to on page 34.. table 5 on page 32 lists range for that pin as -1.4 to 12 v..(abs max maybe)...used in a different mode? This pin doesn't seem to be driven by a voltage.. just has a cap to hot gnd?

[budm]

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

It's page 34 (see my post #40).
Enable input (pin SSHBC/EN).
There may be trace going under the IC.
I cannot see how you can have the same Voltage readings using the chassis GND and using the Hot side ground.

[jason123]

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

Both measurements are referenced to hot gnd. First reading was pin 22 references to the pad the capacitor is connected to, which is shorted to negative lead of large cap. Second measurement to confirm for you was pin22 referenced to negative lead of large cap.
I tried continuity check between pin 22 and the traces exiting top and bottom of chip and there doesn't seem to be a connection

[budm]

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

Pin 22 is the ENABLE pin so it has to be controlled some how, and it does change state, but I cannot see how the control Voltage, 5.53V/8.6V can be that high either..
At this point you will have to find the schematic to really find out what is going on.

[jason123]

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

Section 7.8.8.1 (page 26) explains the function of this pin when used as soft start. I think the voltages i am reading are as intended. Don't you think?