NEC PA241W 1/8th normal brightness

[przemhb]

Hi,

I have fixed "2 seconds to black" in my NEC PA241W monitor, which uses LG LM240WU4-SLB1 panel.
Conductive rubbers, which connect inverter outputs and CCFLs' electrodes were the ones to blame. (I have used EMC flat springs to do so; Harwin S7061-42R).

After reassembly of the panel and my whole monitor another problem arise - the screen is really dark.
When I set the monitor to it's maximum 400cd/m2 I only get 55cd/m2.

I have disassembled the panel again and measured it's brightness when inverter was set to full brightness (PWM=100%). It is >1700cd/m2. CCFL current is about 21mAp-p at the time.

So it looks LC module transmission ratio is very poor. I am trying to figure out what might have gone wrong, but I'm out of ideas at the moment.

Image looks fine, with smooth gradients, but much too dark.

What do you think might have happen? How to diagnose this and fix it?
I will be thankful for your help.

[przemhb]

Re: NEC PA241W 1/8th normal brightness

Previously given backlight brightness (1700cd/m2) was wrong. Out of range of my i1 Display Pro colorimeter's main sensor, so it saturated. Using ambient light sensor gives 11800cd/m2. It seems to be reasonable as, according to information from Japan Display, IPS transmitance is in a range of 3% to 7%. It seems backlight is not the problem.

[petehall347]

Re: NEC PA241W 1/8th normal brightness

does turning up brightness help ?

[przemhb]

Re: NEC PA241W 1/8th normal brightness

Monitor's brightness control is working, but at full brightness it gives only about 50cd/m2 (whereas it should about 400cd/m2, which is specified for the LCD).

It's definitely not a problem of LCD panel's dimming control. It is dimmed by PWM. Currently I am using power supply to supply inverter and USB-UART converter as PWM output for panel dimming. As I have written, when PWM is set to 100% the panel brightness is merely 50cd/m2.

New finding - LVDS channel TE, which transmits RGB 9&8 bits of a 10-bit input to T-CON board are all set high when LCD displays white. It proves video signal coming from main board is not to be blamed. It seems there's something wrong with T-CON.

[przemhb]

Re: NEC PA241W 1/8th normal brightness

Using ambient light sensor was wrong. It is not meant for luminance measurement.
So 1700cd/m2 I given at the beginning was correct.

I have measured the LCM has transmittance of 3%, which is normal for IPS panels. It means backlight simply does not give enough light.

I cannot find reasonable explanation to the fact the monitor's brightness was fine even at the time of failure and since it was repaired the brightness of all CCFLs is reduced. Especially taking CCFLs current (7.5mArms) and voltage (1.3-1.6kV).