View Full Version : Backlight flickers on any brightness level except maximum

30-04-2010, 05:41 PM
Hi guys,

I have just had a laptop come in with a faulty backlight. I diagnosed this as the backlight tube, not the inverter, as a spare inverter produced the same symptoms (which were the backlight lighting for only one second and then going out).

I replaced the panel with a brand new one, fired it up, and now I notice that it runs perfectly on full brightness. However, if I turn it down more than one notch, the screen begins to flicker. This is not an on-off flicker, it's a 'soft' flicker where it pulses from dim to bright at about 10Hz at my estimation. It does not make any noise.

The replacement panel did have a different backlight plug, as older Toshiba laptops have a larger plug, but I just soldered it to the plug of the old panel. I cannot see this causing problems as it only resulted in a 4cm or so extension and the soldering is fine.

This behavior occurs with both the original backlight inverter and my spare (from a different model laptop, a Tecra M3 with a 14" screen). The laptop is a Toshiba Tecra P10 with a 15.4" WXGA screen.

Any ideas on what may be at fault here? I have had it running on full brightness for an hour now with no issues.

30-04-2010, 06:17 PM
Maybe it has some sort of ambient light correction as this is a top model laptop I think, perhaps there is an adjustment for sensitivity. Clue is it does not do it at full brightness because it can't. Put your finger over any likely looking light sensor and see if the screen brightness changes, maybe block it off completely.

30-04-2010, 06:26 PM
Had a look just now and cannot find any hole that could be such a thing. Tried covering the webcam in case there was a sensor in that board, but it made no difference.

I notice though that it has 8 brightness levels, and only on level 4 or below does it flicker. However, the strange thing is that level 5-8 are all the same brightness - in other words, turning the brightness down from full to half does NOT change the brightness.

I would put it down to a faulty inverter (presumably damaged by the faulty CCFL tube), but it does to to my spare too, so I'm absolutely baffled, hence the post...

30-04-2010, 07:31 PM
On my Toshiba screen brightness is continuously variable using Fn and F4 and F5 keys I think, when I got it they didn't work, I downloaded drivers for this from the Toshiba site, suggest you try reinstalling the relevant drivers/software.

It is possible that it is a hardware fault, you will have to work through by a process of elimination, pity you don't have another similar laptop, maybe put it aside until you do.

You could also try this:

There is an add-on from Nirsoft that can control brightness on any laptop, I use it but not on laptop at moment, I will find it and post back.

30-04-2010, 07:33 PM
Here it is:


Look in the options you will find an option (the last one at the bottom of the drop-down list as I remember) to change brightness using Mouse wheel and Alt key, worth a try.

30-04-2010, 07:46 PM
The brightness control does work (software-wise), as it pops up an OSD to tell you the level when you adjust it using the function keys.

Can't put it aside really as my customer will need it back. However in normal use it is an unnoticable fault as the normal brightness levels don't cause it to happen. So I may give it back to her and see how it goes.