LG Flatron 24EN33TA No Backlight When Cold

[canadaboy25]

Hi all,

I've got an LG Flatron 24EN33TA with a backlight issue. The problem began with the monitor not powering up after sitting for a long time (overnight or during the day without use). When I would wake the computer up, the backlight would flick on for a second then go out. Sometimes it would take many tries to get it to come on. Once the monitor came on, it worked flawlessly until another long duration of being powered off.

When the backlight fails to stay on, the LCD panel itself is still displaying the image. If the monitor is left "on" but with the backlight still off for several minutes, the backlight will light and stay on the very first try. It's like something is warming up and allowing the backlight to work after it has been powered up for a while.

To test this theory, I used a thermal camera to see which components warm up when the backlight is in the failed state. The only noticable heat sources are the main processor and the 8 pin chip right by the display ribbon cable. I then froze the chip and the processor with some freeze spray to see if I could get the fault to return. I cannot get conclusive results on where the problem is. If I thoroughly freeze the entire board, I can get the backlight to fail, and warming the board with a hot air gun makes it work again. However, freezing just the processor or other components individually is hit or miss.

As a test, I tacked a low value, high wattage resistor over top of the main processor and connected it to the DC input (as shown in the photo). The resistor heats up to ~80-90°C. With the "preheater" resistor in place, the monitor still does not start by itself, but will start up first try 95% of the time if power cycled immediately after waking the computer. So it clearly has made a dramatic improvement.

I read the firmware from the Winbond EEPROM at room temperature.  I then thoroughly froze the EEPROM and read the contents again.  I then heated the EEPROM up with a hot air station to a very high temp and read the contents again.  I compared all 3 firmware dumps and they were all identical.  So it seems the EEPROM is not causing an issue when cold.  I have had other monitors where the EEPROM would start to fail when warmed up but that clearly isn't the case here.

At this point I thought it had to be the main processor failing internally since that is the component that warms up the most when the monitor is on and it is also right under my preheater resistor which seems to help when it is warmed up.  I was able to locate a new Novatek NT68655UFG processor and replaced it.  The monitor worked perfectly after replacement and I thought it was solved.  However, the problem has returned about a month later and it is now even more difficult to get the monitor to start.  So it seems the act of getting the board quite hot during the processor replacement caused the issue to rectify itself for a while.

I have tried replacing both electrolytic caps on the main board with no effect.  I have also tried a known good AC-DC power adapter and tried powering the board straight from a DC lab supply to rule out a power supply issue.

Hopefully somebody has some ideas of what to check next, or a better way of pinpointing which component is being affected by the temperature changes.

[FIXITNOW2003]

its often the 19 volt power brick that goes on these monitors try running it from a bench power supply if you have one of suitable spec

[canadaboy25]

its often the 19 volt power brick that goes on these monitors try running it from a bench power supply if you have one of suitable spec

I have already tried that and the problem persists.

I have tried replacing both electrolytic caps on the main board with no effect.  I have also tried a known good AC-DC power adapter and tried powering the board straight from a DC lab supply to rule out a power supply issue.

[DavidAlfa]

Also the leds can get damaged. Lots of TVs fail this way.
This might discard a LED problem:
Disconnect the backlight connector, turn the monitor on and leave it to warm up for 20 minutes.
Turn off, connect the backlight, check if it turns on at first try.

If  still needs additional warming, it might be the leds.

[canadaboy25]

Also the leds can get damaged. Lots of TVs fail this way.
This might discard a LED problem:
Disconnect the backlight connector, turn the monitor on and leave it to warm up for 20 minutes.
Turn off, connect the backlight, check if it turns on at first try.

If  still needs additional warming, it might be the leds.

That is what I thought as well initially.  By leaving the monitor on in the failed state, the electronics on the board will warm up and then a power cycle usually gets it going.  Although it is getting harder and harder to get it going.

Once I get it to come on, it will stay on and work perfectly until a long cold soak.

Perhaps I should try putting an ammeter in series with each of the (4 I think) LED backlight circuits to compare the current and see if perhaps one string of LEDs is developing a fault.

I have repaired quite a few LED backlights and usually one of the LEDs in the string fails open, taking out the entire string.

However, if it was the LEDs failing, I wouldn't think freezing the control board should cause the fault to show up.

[elecdonia]

A question: How old is this monitor? If manufactured prior to (roughly) 2012 it may contain CCFL or EEFL fluorescent backlight tubes rather than LED backlights. Fluorescent tubes gradually require higher start-up voltages with age. They also require higher voltages to start up when cold. Failure of backlight to turn on from cold is more common among monitors with fluorescent backlights vs. newer monitors with LED backlights. One way to quickly ID the type of backlighting in a given TV or monitor is by its weight. Units with fluorescent backlights are always much heavier. They weigh at least 3 or 4 times more than the same size unit with LED backlights.

With that out of the way, if this is a newer unit with LED backlights then try to find a schematic and study the backlight driver circuit. Any small electrolytics in this circuit are highly suspect. Electrolytic capacitor ESR drops as the temperature increases. My experience is that small and medium size electrolytics develop high ESR sooner than really large electrolytics. Many “won’t turn on when cold” faults are caused by small electrolytics with high ESR.

Check both DC input voltage and current from the power brick. Ideally put a small series resistor in the negative input wire from the power supply: 0.1 to 1 ohm. Then use an oscilloscope to monitor input voltage on channel 1 and input current on channel 2. The scope ground goes to the main ground point inside the monitor. Note that the current flowing through the series resistor in negative power supply wire will create a negative voltage on channel 2 of the scope. Look for a momentary overcurrent (or absence of current) during a failed startup. There should be an observable difference in voltage/current between a successful startup vs. a failed startup.

“Multichip” LEDs are notorious for open circuits at startup. Nearly every mains-powered LED light bulb which fails or flickers intermittently is caused by open multichip LEDs. I know this because I’ve autopsied about 25 of them. But I don’t yet have much experience with LED failures in monitors and TV sets. Perhaps they tend to drive the LED strips with lower voltages? In this case the LEDs themselves might be single-chip. Note that a single-chip white LED has ~4V across it. Multichip LEDs have multiples of 4V, such as  8V, 12V, 16V, and up.

[canadaboy25]

Thanks for the detailed response.  I have some updates.

I had taken the monitor off my desk since I could not even get it to start up anymore.  I finally got around to looking at it today.
I plugged the monitor in to start troubleshooting and it started right up 

I tore the LCD panel assembly apart to see what the LED configuration was.  See the attached picture.  There are 2 strings of 16 LEDs along one side.  I applied voltage and measured the current of both strings.  At 50V, one string draws ~45mA, the other draws ~60mA.  I would expect them to be equal.  So it seems one string is starting to die.

I also found a schematic of a similar monitor, but it seems to match my board as well.  (Attached).

On the backlight connector pins 1 and 2 are shorted together as well as 5 and 6.  Seems they have a generic driver circuit for up to 4 strings of LEDs and they just tie 2 pairs together for the 2 string monitors.

Probing FB1 gives the following:

Probing FB4 gives the following:

There is a clear difference between the two.

An even bigger difference is seen between VFB1 and VFB4 which are inputs to the main processor:

VFB1:

VFB4:

It looks to me like VFB4 is right on the edge of not having enough amplitude to switch logic states on the input pin of the controller.

Since the LEDs seem to be dying, I figured I would try to trick it into giving a bit more service.  I tied FB1 and FB4 together at the LED connector on the main board.  I figured this should make it operate as a 1 string LED panel and maybe the strong string would compensate for the weak one.

These are the waveforms now:

FB:

VFB:

This seems to have boosted the amplitude on all the VFB pins to the same level as VFB1 before the link was made.

The monitor seems to be working fine, but I will post an update tomorrow after being off overnight.  Perhaps the temperature dependency I was seeing was just due to component tolerances changing with temperature and just pushing the already marginal signal over the threshold?

Let me know your thoughts.

EDIT: If anyone knows why my inline attachments aren't working, please let me know and I'll fix them.

[canadaboy25]

It has been two days since the "fix" and both mornings and evenings that I started the computer, the monitor started up fine as well.

Hopefully this hack will last for a while.  I'll post an update if it fails.