About LED backlight…

[Behemot]

So I got this korean 27" wonder with LG display. No backlight. There are two backlight aluminum strips with 36 SMD diodes on each, in three channels (so 12 LEDs per channel). It is powered by 24V brick, I got 24 V on each channel or about 2 V on each diode. I disassembledthe panel to get the acklight off. Tested each diode, each one of them produced ligt being powered by multimeter. I measure those about 2 V on each single diode when it is plugged in.

So why the hell I got no light even when there is voltage on the diodes?!

[stj]

Re: About LED backlight…

2v is hardly enough for a white led, they are usually 3.5-4v

[ReeceyBurger123]

Re: About LED backlight…

2v isnt enough the booster is not running to drive the series circuit.

[Behemot]

Re: About LED backlight…

It produces light with the multimeter, what voltage does it have?

Is it because in series it does not reach the treshold voltage so the diodes do not "turn on"?

[ReeceyBurger123]

Re: About LED backlight…

Yes normally on edge lit they drive them in groups of 6 or so. Each Led needs 3-4v as Stj says. If you have 24v and nothing else and all the Leds light the booster is not woking. What Psu/Led driver does it have ?

[Behemot]

Re: About LED backlight…

The power supply is some korean brick, I started with that but it was OK. Recapped it right away.

There are two PWM drivers, one for each strip, BIT3251. Seems like they are not getting the enabling pin pulled high.

The SOT23 transistor which is supposed to do that for both of them is MMBT3904, a PNP one. Its base is just about 0.65 V, collector and emitor are at zero. It is driven by the Weltrend WT61P8 MCU, pin 43 (GPIO E3 share with PWM3 output).

The PWMs enable at 1.2 V so I guess I can jumper it to one of the 1.8V regulators to check the operation?

ADD// yes, putting 1.8 V on the enable pin makes it shine like hell

ADD2// I've checked the resistor in front of the MMBT3904 transistor, it is 4k7 as should be. The MCU outputs about 3.3 V so that is OK, on the other side of the resistor I only have 0.69 V. I think the transistor is shot…?

[budm]

Re: About LED backlight…

So are you getting the bl-on signal from the logic board?

[Behemot]

Re: About LED backlight…

There is 3.3 V on the resistor in front of the PNP base, which is also the supply voltage of the MCU, so I would guess yes. I won't get any more of the MCU if it's fed with 3.3 V.

If the PNP is busted it draws more current on the base which means there is higher voltage drop on the 4k7 resistor, right?

[Behemot]

Re: About LED backlight…

Correction - the transistor is NPN of course. The PNP is the complementary part (MMBT3906).

Replaced with MMBT2222A did not help, obviously. Being NPN, the enabling pin is actually pulled high normally, when disabling, the MCU puts voltage on base which opens the transistor to ground and pulls the EA pin down. I tested this by removing the voltage from the base (grounding the other side of the 4k7 resistor closer to the base) which also turns the boost controllers on.

So everything behaves normally, just that the MCU is not supposed to put the voltage on the base when the backlight is to work.

So, I have a bad MCU (or the program in it)?

[Behemot]

Re: About LED backlight…

BTW the display in question is the this wonder.

This thing seems to be quite problematic even though how simple it is. The ITE IT6251FN signal converter overheats quite much as it is discolored beneath it. I am not sure if it is the reason but I've been told this board has already been baked. I fail to see the reason other than people who call themselves electronics repairman are stupid pricks. They even made a video so first thing every fool does now is to bake it https://www.youtube.com/watch?v=By7hrUXJCPk

I think I don't have to state it explicitely, but there is NO BGA chip on this board. So if you think there is a problem with cold joint, just take micro soldering pen and go ahead!

[Behemot]

Re: About LED backlight…

I've jumpwired it to stay ON for testing, besides brief flash of some color stripes upon plugging in the power, it stays black than. I don't have dual-link nothing and as it has no scaler (cheaped out) I can not run it at any other resolution. But I think the MCU or the converter may be shot, especially when it has already been baked once.

Than I am sure if it already was (as these were sold with cheap defective panels), or I damaged it, but the last difusor foil beneath the LCD itself has defect and I think the panel is also scratched. Fortunatelly 27" displays with this resolution went down in price and they have scaler, multiple inputs and all.