Electronics > Repair

LG Flatron 24EN33TA No Backlight When Cold

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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:

--- Quote from: FIXITNOW2003 on September 11, 2023, 06:15:47 am ---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

--- End quote ---

I have already tried that and the problem persists.


--- Quote from: canadaboy25 on September 11, 2023, 04:19:18 am ---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.

--- End quote ---

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:

--- Quote from: DavidAlfa on September 11, 2023, 04:09:28 pm ---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.

--- End quote ---

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.

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