T40 backlight won't stay on with both battery and AC

[Jas]

Hello. Sorry if this has been addressed somewhere else, but I haven't come up with an exact match of the problem I'm having. I think I may know what's causing it, though. Please affirm or correct my conclusion.

The backlight on my wife's T40 has suddenly started turning off within about 2 seconds of plugging in BOTH the battery and AC. If I remove either the battery or the AC (not both, obviously), the backlight comes back on. The backlight goes off at boot after two seconds if both are hooked up, or if I go into BIOS settings or Access IBM, so this isn't a software issue. It's also definitely the backlight, since you can just make out the image on the LCD. Finally, I tried plugging the laptop into a TV, and the problem doesn't exist there. I can even use Thinkvantage controls to have the screen display on both the LCD and TV, but as soon as I reinsert the battery or plug the AC back in - so both are plugged in - the LCD backlight goes off (again, about two seconds), while the TV image stays on. Oh, also, I've tested with two batteries, and there's the same problem, plus the computer seems to be charging and discharging the battery correctly.

So, given what I've seen around these forums, I figure I must have a bad video inverter. I hope that's it, because it should be an easy fix. If anyone can confirm or refute this, I'd appreciate it.

TIA

[Jas]

FYI, I told my wife to let the battery charge up (it's a fairly new 9-cell) and once it got pretty well charged, this problem disappeared. If we swap in the old battery, which has very little charge, again the backlight goes out until we either unplug the AC or remove the battery.

So, a bad inverter?

[Jas]

Anyone?

[GomJabbar]

It could possibly be a weak backlight that is sensitive to low voltage. Another possibility is the voltage regulator circuit on the motherboard is not working properly. Another possibility is the AC adapter (charging brick) is weak. Have you tried a different AC adapter that has the same voltage and polarity? If you don't have a second one laying around, maybe you can borrow one from a friend for a test, or else bring it in to a store where you can test it with one of their AC adapters.

[Jas]

Thanks for the response, and I'd begun to figure that it could be more than the inverter. Given that the computer works fine when the battery is out, don't you think that the AC adapter is the least likely culprit? My wife does have another adapter, but she doesn't have it with her at the moment, so we'll try to eliminate that as the cause first.

If the Mobo is the culprit, would this mean replacing the entire board? We actually just did that a while ago, because it became clear that the original one (we bought it used) had spill damage that began to cause problems over a year after we bought it. The new board (purchased used, but guaranteed in working order) has been functioning fine for several months. My wife did just spill a "speck" of coffee on the far right side of her keyboard a couple of weeks ago. This isn't close to the AC port or inverter, so I figure that's just a coincidence, but it could be an issue. I haven't opened the laptop up to see what damage there might be. From the way she describes it, there may be none.

Thanks again for the advice.

[GomJabbar]

Actually I forgot to mention the inverter in my previous post. The inverter is another possibility.

I'm sorry I can't be more definitive in my reply. I just have to make educated guesses. I have not seen your particular problem posted before.

[Jas]

Thanks. After looking into the AC adapter as the culprit, I might just have to go in order of expense to eliminate other possibilities.

I forgot to say that IBM actually replaced the screen with a brand new one about a half-year ago. I think I'll call them to see if they also replaced the inverter and if they have any more ideas about the problem.