X20 backlight died

[GnatGoSplat]

I picked up my X20 while it was running and set it on the desk. The A/C adapter came unplugged from the back and as soon as I plugged it back in, I saw a bright flash and suddenly the screen went blank. I can see the LCD itself works fine, but there's no backlight.

Does anyone know what voltages go into the inverter so I can test it?

I'm not getting any voltage out (or at least not that my meter can measure), but I don't want to replace the inverter if it's not the problem.

[teetee]

It sounds like you've already opened the LCD assembly. Maybe you can check the back light tube(CCFL) and see if the two ends are black. I suspect the bright flash was from the light tube. However if you smelled something burning right after the bright flash then it might be the inverter got burnt. I really doubt that though since any "bright flash" can only mean the circuit board is severely damaged.

[jamiphar]

Unfortunately, you probably can't test it safely with a normal multimeter. As Ray has documented here, you can use a neon bulb to see if the inverter is turning on.

Now, you might be able to use your oscilloscope to measure the current. I couldn't tell you how, I don't have a 'scope (yet ). But in that same thread, SMA said how he was able to do it.

[GnatGoSplat]

Thanks, I'll have to try the scope. It won't cost me anything and I don't have any spare neon bulbs laying around. Hopefully the fact that the X20 controls the scope and will share a common ground with it won't affect the readings. Good thing it still works with an external monitor!

I've opened the LCD assembly, but I didn't try removing the tube. I didn't smell anything burning, just saw a flash from the CCFL before it went out.

I'd still really like to know the voltages going into the inverter though. It could be the inverter is fine but there's something wrong with the power supply section. I did measure about 10V across the tiny capacitor near the ribbon.

[GnatGoSplat]

I finally got around to picking up an inverter from eBay. It still doesn't work, I guess that's $15 down the drain.

When I did the test with the scope, it didn't pick up any frequencies.

I discovered what I thought was a capacitor on my original inverter is a fuse, and it's blown. When I short across the fuse, the backlight turns on for about a second then goes off. I guess some kind of protection circuit kicking in. Strangely, the fuse never blew on the eBay inverter.

Blown fuse and inverter shutting down made me think perhaps something is shorted in the motherboard. I found 16V going to the inverter. Using a variable power supply, I fed my original inverter 10V... and it works. Any higher and the shutdown circuit kicks in. I noticed though, that a chip-looking thing on the inverter gets quite hot. I think it might be a transistor or MOSFET in an IC package because several of its pins are connected together. Strangely again, the eBay inverter doesn't work on 10V. It only flickers. The original inverter is an ALPS and the eBay one is an LG, although they're both shaped the same with the same connections in the same places.

I don't know what to think right now, my only guess is maybe the motherboard has a short in it, although I can't find it.

Anyone have a pinout or know the voltages should be at the inverter input? That would help a lot.

[Brad]

From what I have read on this forum it sounds like the CCFL is drawing too much current and the inverters built in current limiting design is kicking in.

Usually this happens when the tube fails.

The difference between the two inverters may just be components with ever so slightly better specs.

I may be totally wrong.

Brad

[GnatGoSplat]

That's possible, although I'm doubting that because I connected a CCFL tube from an AU Optronics 8.4" LCD to it, and it does the same thing.

I could be wrong though... maybe the AUO CCFL needs more power to begin with. I'm leery of buying more parts to find out that's not it considering this laptop is almost obsolete.