LG L246WH No Backlight Inverter Circuit - Page 1
Forum section: electronics / repair

author: canadaboy25
author_rank: Regular Contributor
timestamp: March 31, 2019, 04:20:13 am
content: Hey guys, I have an LG L246WH monitor that I'm working on. For the past couple years the display has had trouble starting up when it was cold and would require a smack on the top to get it going. Once it heated up it was fine. Recently I gave it a smack to get it to start but only one corner of the screen lit dimly then it wouldn't come on again. I figured I must've cracked one of the tubes. This display has 6 curved tubes that run behind the display. I took it apart and tested each tube using the supply of another display that I have. All the tubes lit up just fine.I turned my attention to the power supply which had cracked solder joints everywhere. It was also not putting out 12V or 24V. After fixing all of the cold and cracked joints, I had the 12 and 24V rails back. However, the display still doesn't work.Next I looked at the inverter board. It only has one connection to the power supply and connections to the 6 tubes. The pins for the connector are conveniently labelled on the board. There are some 24V power pins and grounds, an on/off signal, a digital brightness signal, and an analog brightness signal. The analog brightness trace ends up leading to an unpopulated component and has 0V on it so it obviously isn't being used on this model. The 24V rail is present when the display is turned on and there is also a 3.3V on signal present. I can use a flashlight on the display to navigate to the brightness setting and can vary the signal on the digital brightness pin up to 3.3V at 100%.This means that the inverter board should have all of the inputs that it requires to start up. When I turn the power on, I get a few hundred volts on the backlight side of the transformers for about half a second then nothing. There is no blinking of the backlights.I was able to find a datasheet for the inverter controller chip. After doing some testing, I found out that the brightness control was set to "Burst mode with external source" based on the configurations of the ABRT and BRS pins (page 8 of the datasheet). I'm not sure if the control chip is detecting some sort of fault and shutting down or what.Hopefully someone out there with more knowledge of inverter circuits than me can help me along with this.Thanks

author: canadaboy25
author_rank: Regular Contributor
timestamp: March 07, 2023, 07:08:28 am
content: Figured I'd post an update in case someone else stumbles across this thread with the same issue.The reason I had to smack the monitor to get it going was the cracked solder joints on the power supply board. This turned out to be a separate issue from the inverter board. The inverter board likely would've failed on power up even if I did not have to smack the monitor.The issue with the inverter board ended up being an open 0 ohm resistor/jumper. I replaced it with a piece of wire and the monitor was back up and running.About a year later, the monitor quit again and it ended up being another open 0 ohm resistor on the inverter board. Jumped that one with a wire and it fixed the problem again.Seems like this may be a common issue with this board.