Dell Latitude D610 screen suddenly goes black

06 Jan 2009
Posted by Laptop Junction

2009 has started well, but not for the screen of the D610. After coming back from Windows standby the screen was black, although the harddisk made all the familiar noise of startup. At first it seemed key code Fn+F8 (CRT/LCD) was pressed - this redirects graphics output to the external monitor VGA connector. But that wasn't the case.
 

DELL Latitude D610 External monitor test

Connecting a flatpanel TFT screen and pressing Fn+F8 showed the familiar desktop again. The graphics card was still doing it's job apparently. The D610 now starts with a black screen. After typing the BIOS boot password blindly, then Fn+F8 the desktop shows at the external monitor. BIOS modifications and such have now become very difficult through a faint blacked out screen.

In the story below you find a simple CCFL inverter test.
 

Screen inverter or CCFL backlight

After a little research, it seems either the screen inverter LTN141XB or the CCFL screen backlight tube had died.The screen itself is not black but very faint. Looking at the screen from another angle shows it's actually working.

The problem now is to find out if it's the backlight tube or the inverter. A backlight tube repair seems pretty difficult with a high risk of failure. The CCFL inverter swap seems easier. The inverter sits in the screen, just underneath the DELL logo between the hinges. 
 

Temporary screen replacement

Before going into finding replacements, the screen was installed on a different D610 to check if it's not software bug / feature that can turn-of the backlight. Too bad it wasn't - it's either the backlight tube light or the inverter electronics driving the tube light.

In the meanwhile a temporary TFT screen is installed by cannibalizing a dead D610. I got hold of it in Pakistan and took it as a parts spare. The screen has a lower resolution unfortunately - but at least it works.
 

Screen inverter and CCFL backlight specifications

The screen with the broken backlight and/or inverter is a TFT Active Matrix 14.1 Inch SXGA+ 1400 x 1050 from Samsung with type number LTN141P4-L04. The backlight is a ccfl tube (Cold Cathode Fluorescent Lamp) driven by an inverter that produces the high voltage that makes the tube glow.
 

** Read about the symptoms of CCFL tube failure here. **

DELL Latitude D610 TFT Screen inverter caused the black screen

 
DELL D610 screen inverter diagnosis and repair

Because of the sudden black out, first suspicion is that the ccfl fluorescent tube is broken. This is actually the most sensitive and difficult repair job. Before the black-out there was no indication of faulty screen behavior of any kind. The brightness of the screen was ok and there was no history of flickering or strange noises from the inverter.

First thing to test is the laptop screen inverter - preferably without buying one first. In Dubai the place to go for laptop repairs is the 2nd floor of the Al-Khaleej Center, opposite Computer Plaza shopping mall. Several small laptop workshops can help with the repair. Phoenix Computer Trading, hidden in a side-alley, had the most 2nd hand spare parts laying around. Including a screen cable with a screen inverter for a DELL Latitude D610.

The test was quick, the result also - the screen inverter had died and the ccfl fluorescent tube in the screen was still OK. A fifteen minute repair.
 

DELL D610 screen inverter test in hindsight

Another non-destructive and non-intrusive test is a simple signal frequency measurement. A screen inverter outputs a high voltage, between 500 V to 700 Volts AC at a frequency between 30 KHz and 60 KHz. A signal like that can be easily measured by an average digital voltmeter with Herz (Hz) frequency measurement function. This without cutting or touching the CCFL power leads. Holding the measurement leads close to the CFFL wires shows a signal frequency reading created by the electromagnetic field. A dead screen inverter will most likely output no electromagnetic field at all.
 

  Dell D610 CCFL Inverter frequency test setup

Simple CCFL inverter test by measuring the CCFL wave frequency in the proximity of the CCFL power leads

So lets give that one a try with a working CCFL screen inverter. Without even opening the plastic screen frame - a Fluke 123 Scope-meter picked up a 58 Khz frequency reading while holding the measurement leads close to the right side of the screen frame. Switching the screen off by pressing Fn+F8 made the frequency reading drop to 0. A simple and effective test without the risk of burning your fingers or destroy a possible working inverter.

Dell D610 CCFL inverter wave form test results
CCFL inverter waveforms at low and high screen intensity.

This test can be repeated with an ordinary digital multi-meter with frequency measurement function by:
 

  1. Keep screen intensity at the maximum so the waveform is continuous like on the right.
  2. Run the laptop on battery so the 50-60Hz AC mains waveform isn't interfering with this test.

Even the AC voltage indication - 254 mV - can be a sign the inverter is working. (don't forget to switch to AC measurement.) Switching the screen off (Fn+F8) drops the voltage to a lower level.

See CCFL inverter testing / replacement tools what can do the job.

WARNING: Unless you know exactly what you're doing :: Do not connect a voltmeter directly to the output of the inverter. The resistive load of a simple AC voltmeter can easily destroy the inverter output.

 
 

D610 replacement screens and laptops

A new TFT Active Matrix 14.1 Inch SXGA+ 1400 x 1050 from Samsung (type number LTN141P4-L04) goes for the asking price of AED 600 (€ 127 / US$ 165) at the start of 2009. These high resolution screens with 3:4 aspect ratio are a bit more expensive than the standard 1024x768 version.

Most surprising was the huge amount of DELL D610 Latitude laptops available, all imports from the US. The asking price was about AED 950 (€ 200 / U$ 260) without any haggling.


 

CCFL inverter testing / replacement tools

A simple digital multi-meter with frequency measurement will do the test above. The replacement of the CCFL inverter board requires a micro "philips" screw driver. It's attached with 2 tiny screws - the philips cross is too small for the standard screw driver sets.