I'm seeing a symptom I can't explain:

I replaced a faulty LCD (a Quanta QD15AL01 rev.5) in my notebook (a Dell
Inspiron 6400) with a "compatible" display (a Chi Mei N154Z1-L01). The
new display's backlight does not respond to changes in brightness correctly:

The brightness can be set from 1 to 8, which should be dimmest to
brightest. With the new display, the dependence is not monotonic: 4
appears brightest, followed by 8, then 1. 2, 3, 5, 6, and 7 are fairly
dim, but show some variation. Even 4 does not appear to be full
brightness. The pattern is perfectly repeatable and stable as I change
the brightness setting.

I've tried two different inverters, including the original inverter.
Both inverters work fine with two other displays I've tried, but both
exhibit the same symptom with this display, so I attribute it to the
backlight responding incorrectly to the output of the inverter, rather
than it being a fault with the inverter. This seems strange, though,
since the backlight is a relatively simple CCFL, I think.

Is it possible that this particular backlight is reflecting power into
the inverter in a funny way that causes the inverter to malfunction for
certain dimming settings?

Is it possible that the inverter is doing PWM and giving the backlight
frequencies it doesn't like for some dimming settings?

Is there any communication from the inverter back to the graphics
card/motherboard, such that it could be telling the software something
confusing about the display? I thought the connection from the
motherboard to the inverter was just an input, with power and dimming
signals.

I am convinced that this is not a software problem, as the symptom
persists while in the BIOS screen, and is unique to this display, which
does appear to be otherwise compatible with the computer.

Thanks much for any advice.

On Wed, 14 Jan 2009 22:03:53 +0000, Rafael Delgado
wrote:

I'm seeing a symptom I can't explain:

I replaced a faulty LCD (a Quanta QD15AL01 rev.5) in my notebook (a Dell
Inspiron 6400) with a "compatible" display (a Chi Mei N154Z1-L01). The
new display's backlight does not respond to changes in brightness correctly:

The brightness can be set from 1 to 8, which should be dimmest to
brightest. With the new display, the dependence is not monotonic: 4
appears brightest, followed by 8, then 1. 2, 3, 5, 6, and 7 are fairly
dim, but show some variation. Even 4 does not appear to be full
brightness. The pattern is perfectly repeatable and stable as I change
the brightness setting.

I've tried two different inverters, including the original inverter.
Both inverters work fine with two other displays I've tried, but both
exhibit the same symptom with this display, so I attribute it to the
backlight responding incorrectly to the output of the inverter, rather
than it being a fault with the inverter. This seems strange, though,
since the backlight is a relatively simple CCFL, I think.

Is it possible that this particular backlight is reflecting power into
the inverter in a funny way that causes the inverter to malfunction for
certain dimming settings?

Is it possible that the inverter is doing PWM and giving the backlight
frequencies it doesn't like for some dimming settings?

Is there any communication from the inverter back to the graphics
card/motherboard, such that it could be telling the software something
confusing about the display? I thought the connection from the
motherboard to the inverter was just an input, with power and dimming
signals.

I am convinced that this is not a software problem, as the symptom
persists while in the BIOS screen, and is unique to this display, which
does appear to be otherwise compatible with the computer.

Thanks much for any advice.

Since you tried the original inverter with the new display, it almost
has to be a problem with the CFL tube. You could rule out the display
by disconnecting the LCD video cable.
Andy Cuffe

You could rule out the display
by disconnecting the LCD video cable.

True. Nope, it's not the LCD misbehaving, it's the backlight.
Thanks for your reply!

One more piece of information: The backlight responds correctly to a
different model of inverter on a different laptop (Lenovo T61). So it
appears to be an incompatibility between this display (Chi Mei
N154Z1-L01) and this inverter (6632L-0266B), even though each component
works with other setups.

Have other people seen that kind of incompatibility with other
supposedly compatible inverter LCD combinations?