LG 32LB561 (32LB561U) very dim backlight

[Khron]

Right, so a client brought in this 32" LG TV a couple days ago, with the symptom of the picture being very dim, and allegedly dimming even further within 5-10mins.

What i've tested and/or discovered so far:

- the voltages produced by the bias generator IC on the TCON all seem to be stable and agreeing with datasheet values (AVDD, VGH, VGL etc)
- LED+ voltage is about 145v, LED- (which goes into the drain of an N-channel MOSFET) around 45v
- PWM-dimming signal seemingly stuck at 35% duty-cycle, regardless of the menu setting (no change, whether at 0 or 100)

After a quick consult with Th3_uN1Qu3, i traced the PWM signal back from the connector on the logic board, through a series resistor, back to the main processor itself. As advised, i pulled the PWM signal up to 3.3v, for testing purposes. Further discoveries:

- no change in brightness at all
- interestingly enough though, the LED- voltage was now 0v (as one might expect, for full backlight brightness)

Attached you'll find a photo of the TV in a dark room, taken with a Canon EOS 550D, f/4.0 aperture and 1/5sec exposure. And it's (still) dim as hell.

The LCD panel itself seems to be made by Heesung, model no. HC320DXN, and interestingly enough, the TCON's "spread" across the bottom edge of the panel and says Sharp on it (with their specific font).

Is this yet another case of crapped-out LEDs? But if one or more had fried, how come at least some of them still work?

A quick googling of the panel produced a few links with allegedly spare LED strips for this model - 19 diffusers visible, and with a 145v supply, that should mean two LEDs under each one (145 / 19 = 7.6; something over 3v per LED is expected).

[budm]

Re: LG 32LB561 (32LB561U) very dim backlight

Are you saying that the Backlights level doe not change at all when when pulling the PWM signal to 3.3V and the LED- is showing 0V? I cannot really see 0V on the return line of the LED since it has to have some Voltage drops on the MOSFET and also on the current sensing resistor.
Since Brightness and Backlights level are not the same thing so I am confused about the symptom.

[Khron]

Re: LG 32LB561 (32LB561U) very dim backlight

Exactly - no reaction when changing the "backlight" setting in the picture menu, and it didn't get any brighter with the PWM pulled to 3.3v.

I re-probed (with PWM still hardwired to 3.3v), and i'm getting 0v on the LED-. I've also tested the regulating MOSFET out of circuit, and it measured fine (both open and closed).

[budm]

Re: LG 32LB561 (32LB561U) very dim backlight

There should be a current sensing resistor connected between the Source pin of the MOSFET and GND, so it has to have Vdrops on that resistor.
Can we see the pictures of the board top and bottom side?

[Khron]

Re: LG 32LB561 (32LB561U) very dim backlight

The current-sensing resistor is actually a group of four 1ohm resistors, one 2ohm and one 8.2ohm, all in parallel.

I've probed the lower of the two wire-jumpers between the backlight connector and the rest of the board (which is coming from the LED- pin), and measured the same 0v as right on the connector's pin itself.

[budm]

Re: LG 32LB561 (32LB561U) very dim backlight

The current flow through the sensing resistors is about 300~400mA at max backlights level, so you can calculate what the Vdrops will be on the resistor, you can also measure the Vdrops between S and D pin of the MOSFET also.

[Khron]

Re: LG 32LB561 (32LB561U) very dim backlight

In that case, math says i should see anything between 1.3-1.8v (those six sense resistors end up being about 0.22ohm in total). Except that that's not really happening.

I soldered back that series resistor on the PWM line, and i scoped it again to make sure the joints were ok (a diddy little 0402 package). I caught the thing while it was still starting up, and it defaults to an 80%-ish duty cycle, then smoothly ramps down to the 35% i was seeing before.

I probed that LED- signal as well - i'm seeing "obviously" the reversed duty-cycle, at about 80v-ish when the PWM signal is low, and very close to 0v (the scope screen can only fit so much) when the PWM is high.

I still find it really odd that, especially if there are indeed bad LEDs in the backlight, it still works as it does. Could a LED (or several) have failed somehow "higher resistance" instead of open-circuit? That still wouldn't / can't explain why the PWM seems stuck at 35% though...

[tw2005]

Re: LG 32LB561 (32LB561U) very dim backlight

In that case, math says i should see anything between 1.3-1.8v (those six sense resistors end up being about 0.22ohm in total). Except that that's not really happening.

I soldered back that series resistor on the PWM line, and i scoped it again to make sure the joints were ok (a diddy little 0402 package). I caught the thing while it was still starting up, and it defaults to an 80%-ish duty cycle, then smoothly ramps down to the 35% i was seeing before.

I probed that LED- signal as well - i'm seeing "obviously" the reversed duty-cycle, at about 80v-ish when the PWM signal is low, and very close to 0v (the scope screen can only fit so much) when the PWM is high.

I still find it really odd that, especially if there are indeed bad LEDs in the backlight, it still works as it does. Could a LED (or several) have failed somehow "higher resistance" instead of open-circuit? That still wouldn't / can't explain why the PWM seems stuck at 35% though...

I've seen very dim LEDs on defective strips where one may be out, others are still normal brightness and others are so dim they're nearly dead.



It's only a 32 inch so should be real easy to work with and dismantle the panel.

[Khron]

Re: LG 32LB561 (32LB561U) very dim backlight

So then, it's definitely the backlight itself, that's at fault?

[csori23]

Re: LG 32LB561 (32LB561U) very dim backlight

Hello.
Please help me. What can cause this error? There is a voltage for 5 seconds, then nothing. Within 5 seconds, the picture is chirping, vibrating and then dark, and only half of the display works. Sorry, I do not speak English, I use only a translation.

Video: https://www.youtube.com/watch?v=xCyY2C1KbOc