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Old 03-30-2019, 01:02 AM   #1
canadaboy25
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Default LG L246WH No Backlight

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
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Old 03-30-2019, 04:48 AM   #2
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Default Re: LG L246WH No Backlight

Why not start with the visibly bulging 3300F cap next to C205?
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Old 03-30-2019, 09:51 AM   #3
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Default Re: LG L246WH No Backlight

Quote:
Originally Posted by Per Hansson View Post
Why not start with the visibly bulging 3300F cap next to C205?
You're right. I was going to but forgot. Replaced both of them but same issue.
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Old 03-30-2019, 11:46 AM   #4
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Default Re: LG L246WH No Backlight

when you turn the monitor on, does the backlight flash for a second then go out? Did you check the inverter fuse?

Last edited by R_J; 03-30-2019 at 11:55 AM..
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Old 03-30-2019, 12:02 PM   #5
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Default Re: LG L246WH No Backlight

Quote:
Originally Posted by R_J View Post
when you turn the monitor on, does the backlight flash for a second then go out? Did you check the inverter fuse?
Nope, no flash. I get absolutely nothing from the backlights.

The fuse is fine and i get 24V on both sides of it when the monitor is on.

Attached are pictures of the control chip on the inverter board and a picture of the rest of the circuitry on that board.
Attached Images
File Type: jpg P1030230.jpg (557.0 KB, 13 views)
File Type: jpg P1030233.jpg (595.4 KB, 12 views)

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Old 03-30-2019, 05:03 PM   #6
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Default Re: LG L246WH No Backlight

It seems like I'm definitely dealing with an inverter issue. The analog brightness pin leads nowhere on the inverter board so it obviously isn't being used. That leaves the supply voltage, brightness signal, and on/off signal. I get a steady 24V whenever the display is on and a steady 3.3V on signal when it is on. So they are working. I was able to use a flashlight and navigate the display settings to turn the brightness up and down. The voltage on the brightness pin went up and down with the setting. So it seems the inverter board is getting all the required inputs but just not working. The tubes themselves were also tested so the fault must be with the board.

I found a datasheet for the inverter controller chip. It has multiple modes for brightness control. It is currently configured as "Burst mode with external source" with the ABRT pin and BRS pin tied to 6V while the DBRT pin coming from the main board is supposed to be a PWM signal. The chip is receiving the brightness control and on/off signal so I'm not sure what the issue with it is.
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Old 04-05-2019, 09:09 PM   #7
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Default Re: LG L246WH No Backlight

Can I see the pictures of the whole Inverter board and how all the boards are connected together?
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Old 05-26-2019, 11:02 PM   #8
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Default Re: LG L246WH No Backlight

Quote:
Originally Posted by budm View Post
Can I see the pictures of the whole Inverter board and how all the boards are connected together?
Sorry for the very slow reply. I've had a string of quite bad luck with the display. As I was picking up my multimeter, one of the leads swung around and shorted between the gate and drain of one of the MOSFET's on the inverter board. I heard the small spark of the 24v rail capacitors being drained through the controller chip, killing it. I bought a replacement chip which just recently arrived and it turns out I ordered the wrong package size and this one has a much larger footprint. Figuring I had nothing to lose by trying, I flipped the chip upside down and glued it to the board and ran wires from all pins to their respective locations on the board. Same problem as before I blew the chip up.

Just to recap the current situation: The inverter board is only connected to the rest of the circuitry by one cable. The cable has an on/off signal, brightness signal, power, and ground. The on off signal turns on and off with the monitor. The brightness signal varies up and down with the monitor settings. The main power rail and ground are present. All of the tubes have been tested with the inverter from another monitor and all tubes light and stay lit. This leaves the inverter board as the only possible fault. It is receiving all of the required inputs and all the tubes are good.

I've attached the pictures you requested and also the unsightly job I did wiring up the too large controller chip.
Attached Images
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File Type: jpg P1030263.jpg (842.8 KB, 5 views)
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Old 05-28-2019, 04:38 AM   #9
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Default Re: LG L246WH No Backlight

have you checked the transformers on inverter board........
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Old 05-28-2019, 09:15 AM   #10
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Default Re: LG L246WH No Backlight

Quote:
Originally Posted by digby1951 View Post
have you checked the transformers on inverter board........
They all have the same resistance across their windings if that's what you mean.
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Old 05-29-2019, 06:52 AM   #11
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Default Re: LG L246WH No Backlight

hi....that's what i meant........i had one with low ohm's reading.......no picture until replaced the faulty transformer...........good luck
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Old 07-08-2019, 07:43 AM   #12
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Default Re: LG L246WH No Backlight

ok thanks
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Old 07-17-2019, 06:55 PM   #13
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Default Re: LG L246WH No Backlight

ic problem
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Old 07-17-2019, 08:01 PM   #14
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Default Re: LG L246WH No Backlight

sounds to me like a poor connection between the invertor and tubes - the slap was the givaway.
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