Backlight or Inverter? How can you tell?
May 23, 2008 8:54 AM   Subscribe

Backlight or Inverter? How can you tell?

Generally, when a laptop's LCD screen is functioning but extremely dim to the point of not really being able to see, this means that either the Backlight or the Inverter for the Backlight needs to be replaced.

Is there any way to determine whether the problem is due to the Backlight or due to the Inverter, without testing by replacing one or the other?

I am trying to avoid having to buy one part only to find out the problem is the other one.
posted by doomtop to Computers & Internet (6 answers total)
Did it just go all of a sudden? Like someone flipped the switch?
Or was it a slow process with the colours going out of whack and then it went dark?

If it was the first one (remove battery), just pop the bezel off and make sure the all little connectors are in properly and all that kinda stuff. (Mine did this the other day and that's all it was. One connector just wasn't quite in properly. Too easy.)

If that doesn't fix it. Um.. do you have a multimeter? That'll tell you if power is getting past the inverter (meaning it works and it's probably your backlight) or if it stops dead (it's toast).
Do you know someone that would let you borrow their inverter for 5 minutes?
(Don't know what the odds of that might be though? A compatible part AND not being too anal to let you try it?? Meh?)

Another free way to check what's wrong with a screen is to connect another monitor. If it looks great then you can rule out everything else. It's definitely the screen.
posted by mu~ha~ha~ha~har at 10:05 AM on May 23, 2008

If, on powering on, the screen is visible for a second or split second and then goes dark the rest of the time, then it's definitely the backlight.

If it's just dark all the time, then you need to diagnose with a multimeter.
posted by randomstriker at 10:12 AM on May 23, 2008

The inverter puts out about 500 volts at 30 KHz so you aren't going to be able to test it with equipment available to most consumers.

CCFL backlights have a typical lifetime of 50,000 hours so by far the most common failure is the inverter. You can get inverters for less than $50 so I would just replace that first. Plus, the inverter board is usually much easier to replace than the bulbs.

You should first check that the input to the inverter board has 5V power. If not, then you may have a hinge switch or cable problem.

If you are into pain you could very lightly touch the output of the inverter with a dry finger, but that will only tell you if the inverter is working, but not if it is working to full specifications. I don't recommend this test.
posted by JackFlash at 10:24 AM on May 23, 2008

Generally, if the screen went out quickly, it was the inverter. If it went out over a slow period of time (flickering etc..) I would bet it is the lamp.

Now, backup just for a second. You say you would want to know which part went out so you can replace the correct one the first time. I hope you are incredibly good with your fingers and soldering iron. Assuming you are, then replacing the lamp should be pretty challenging. as that requires complete breakdown of your LCD, extraction of your dead lamp, and soldering in your new lamp.

The inverter on the other hand is an easy swap-out a high school freshman could do. I would start with the inverter board. Buy it off ebay, swap it in, and see if that works. If it did not, no big deal, sell it back to ebay.

Here is a good guide which explains how to replace the ccfl lamp.
posted by yoyoceramic at 12:26 PM on May 23, 2008

Thanks for the responses so far.

I am experienced with replacing both backlights and inverters, as well as the entire LCD. I am not concerned about which fix is easier, nor am I looking for help with a specific LCD.

This is something that I will be doing regularly and since I don't carry any of these parts on hand, whenever I fix a laptop LCD I will need to order the parts for it.

I am trying to figure out how to determine whether it is the backlight or inverter ahead of time, so that I can order the right part the first time. This will save me time in getting the repair done quicker, as I don't have the risk of getting the wrong part and having to send it back and wait again for the correct part. I also wouldn't have to order both parts when I need to make sure I get the right part the first time.

Seems like testing the inverter is the only way to really do this. I'll have to start a new Question asking what the right equipment is for doing so.
posted by doomtop at 2:27 PM on May 23, 2008

Really, the best way to test the inverter is to replace it with a known good one. Inverters are fairly complex circuits and use many different designs and controller chips. Some controllers will attempt to strike an arc on the lamp multiple times and then shut down if the bulb is bad, so the fact that they are not putting out a voltage does not necessarily indicate that the inverter is bad. It could just mean the bulb failed to ignite and the inverter shut down. Different controllers behave differently so you won't have one single symptom that applies to all inverter boards.

Another option is just to have a couple of spare bulbs around. These are cheaper than inverters and you should be able to just connect a generic bulb to see if that fixes it. If so, order the proper replacement bulb. Otherwise it is probably the inverter.

If you are getting into the repair business, one thing you might consider is repairing inverter boards. The most common failure is the FET switches that drive the high voltage transformer. These cost about 10 cents and a new board costs $50 or more. The second failure point is the controller chip which may cost a couple of dollars. This may require a bit more knowledge of electronics.
posted by JackFlash at 4:44 PM on May 23, 2008

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