Hey guys, I've got a bit of a troublesome one here. This might be a bit of a long post because I'm going to try my best to give as much detail as possible.
I have an LG Flatron L246WH that I've been using as my main monitor for about 7 years now. I originally got it for free because it wasn't working and someone tried to repair it. When I got it the power supply board was in rough shape. It looked like someone took a screwdriver and jabbed it to death. After patching up all the broken traces, I found it had a bad mosfet, zener diode and blown fuse. Replaced all and it worked fine. I've been using it daily for about 7 years without touching it again. However, about 2 years ago, when waking the computer up, the monitor would come on but the backlight would either flicker or not come on. Giving it a good smack on the top would get it going and once it warmed up it would be fine. I've been using it that way since. Today it didn't come on so I gave it the signature smack on top but this time I only got a dim light from the lower right hand side of the screen and then nothing. I could still see the image on the screen, just no backlight.
I tore the thing down and took a look at the power supply. All these years of use have not been kind to it as there were several cracked solder joints all over the board. I used some flux and resoldered just about every through hole component on the board expecting to solve the problem but no dice. Same symptoms.
This display uses 6 curved CCFL tubes that run behind the screen, making it a very thick, but also very bright display. I thought that I might have cracked a tube when I smacked it so I took apart another monitor I had laying around to test the tubes in this one. I hooked each tube up to the working monitor to test them and they all light just fine. So I know the tubes themselves are ok.
This display also has the inverter circuit for the backlight on its own separate board that connects straight to the power supply board. The only connections on the power supply board are the inverter board, and the main logic board. LG has conveniently labelled all of the pins on these connectors with the voltages and signals making it very easy to test.
The first two pictures are of the front and back of the power supply board.
The third picture is of the inverter board.
The fourth picture is of the connector on the main logic board that goes to the power supply board.
The fifth picture is of the 3 connectors on the power supply board from the bottom side. The 2 pin connector on the left goes to the logic board. The 12 pin connector in the middle connects to the inverter board and the 14 pin connector on the right also connects to the logic board. There are 3 data lines that come from the logic board and just go straight to the inverter board.
The sixth picture is the same 3 connectors but from the top.
The seventh picture is the connector on the inverter board that connects to the power supply.
When the power supply is plugged into power, the 5V and 12V rails are present and once you turn the display on, the 24 rail for the inverter is present as well. Since these rails and the on/off signal pin are the only actual connections to the power supply, this makes me think it is working fine.
When testing voltages at the inverter board with the display turned on, I get 24V on all 4 VCC pins. I get 3.3V on the on/off signal pin. I get a steady 3.3V on the PWM pin. This was confirmed with a scope, there was no PWM signal, just steady 3.3V. There is nothing on the V-DIM pin. I thought this might be a brightness setting that wasn't present so I tried applying 3.3V to the pin but it made no difference.
If you look at the picture of the connector on the logic board, you can see that the pin which eventually connects to the PWM pin on the inverter board is labelled PWM DIM while the pin which connects to the V-DIM pin is labelled ANAL DIM... which I assume means analog dim? I'm not sure what this ultimately means though.
So basically I have all my voltages present, the inverter board is getting a power on signal but the two brightness signals are worrying me.
If anyone has any ideas that'd be great because I'd really like to get this display back.
Thanks
I have an LG Flatron L246WH that I've been using as my main monitor for about 7 years now. I originally got it for free because it wasn't working and someone tried to repair it. When I got it the power supply board was in rough shape. It looked like someone took a screwdriver and jabbed it to death. After patching up all the broken traces, I found it had a bad mosfet, zener diode and blown fuse. Replaced all and it worked fine. I've been using it daily for about 7 years without touching it again. However, about 2 years ago, when waking the computer up, the monitor would come on but the backlight would either flicker or not come on. Giving it a good smack on the top would get it going and once it warmed up it would be fine. I've been using it that way since. Today it didn't come on so I gave it the signature smack on top but this time I only got a dim light from the lower right hand side of the screen and then nothing. I could still see the image on the screen, just no backlight.
I tore the thing down and took a look at the power supply. All these years of use have not been kind to it as there were several cracked solder joints all over the board. I used some flux and resoldered just about every through hole component on the board expecting to solve the problem but no dice. Same symptoms.
This display uses 6 curved CCFL tubes that run behind the screen, making it a very thick, but also very bright display. I thought that I might have cracked a tube when I smacked it so I took apart another monitor I had laying around to test the tubes in this one. I hooked each tube up to the working monitor to test them and they all light just fine. So I know the tubes themselves are ok.
This display also has the inverter circuit for the backlight on its own separate board that connects straight to the power supply board. The only connections on the power supply board are the inverter board, and the main logic board. LG has conveniently labelled all of the pins on these connectors with the voltages and signals making it very easy to test.
The first two pictures are of the front and back of the power supply board.
The third picture is of the inverter board.
The fourth picture is of the connector on the main logic board that goes to the power supply board.
The fifth picture is of the 3 connectors on the power supply board from the bottom side. The 2 pin connector on the left goes to the logic board. The 12 pin connector in the middle connects to the inverter board and the 14 pin connector on the right also connects to the logic board. There are 3 data lines that come from the logic board and just go straight to the inverter board.
The sixth picture is the same 3 connectors but from the top.
The seventh picture is the connector on the inverter board that connects to the power supply.
When the power supply is plugged into power, the 5V and 12V rails are present and once you turn the display on, the 24 rail for the inverter is present as well. Since these rails and the on/off signal pin are the only actual connections to the power supply, this makes me think it is working fine.
When testing voltages at the inverter board with the display turned on, I get 24V on all 4 VCC pins. I get 3.3V on the on/off signal pin. I get a steady 3.3V on the PWM pin. This was confirmed with a scope, there was no PWM signal, just steady 3.3V. There is nothing on the V-DIM pin. I thought this might be a brightness setting that wasn't present so I tried applying 3.3V to the pin but it made no difference.
If you look at the picture of the connector on the logic board, you can see that the pin which eventually connects to the PWM pin on the inverter board is labelled PWM DIM while the pin which connects to the V-DIM pin is labelled ANAL DIM... which I assume means analog dim? I'm not sure what this ultimately means though.
So basically I have all my voltages present, the inverter board is getting a power on signal but the two brightness signals are worrying me.
If anyone has any ideas that'd be great because I'd really like to get this display back.
Thanks
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