Aspire 5742z black screen - Acer Aspire 1640Z Notebook

Had the same problem, but product still covered by warranty. Came back working with screen replaced. Could also be backlight fault and/or invertor board.See you tube for how to replace and test to eliminate various parts. Good luck.

The screen Inverter, and the Backlight would NOT affect using an external display.
You are right, isimi.

Primer:
An LCD screen cannot produce light by itself,

http://solutions.3m.com/wps/portal/3M/en_US/NA_Optical/Systems/BehindTheScenes/Optics101/

(Click on the monitor icon)

It needs an additional light source. A Backlight is the additional light source.
A Backlight can be a CCFL, (Or two CCFL's), or a series of LED's.
(Light Emitting Diode)

A CCFL is a Cold Cathode Fluorescent Lamp.
Similar to Fluorescent lighting used in homes , and businesses, but on a MUCH smaller scale.

[Average thickness of a CCFL is 2mm. That is a little larger than 1/16th of an Inch. Average length is a little shorter, than the LCD screen is in height,

http://www.laptoprepair101.com/laptop/2007/12/09/replace-laptop-backlight-ccfl-lamp/

(Observe the last 5 photos)

5742z is a Model Series. The Model Number comes after this.
The 5742z model series can use a CCFL for a Backlight, or a series of LED's. Just depends on what the model number is,

http://www.screentekinc.com/acer__Aspire__5742Z_Series-series-laptops-lcd-screens.shtml

When LED backlighting first came out, there were some models that still used a screen Inverter. Generally, (To my knowledge), the LED backlit models now do not use a screen Inverter.

A screen Inverter is used to convert the power from the laptop, for the LCD screen, and Backlight.
Again, for a CCFL backlight.

The Video Cable is plugged into the motherboard, leads across, and under the left Hinge, then routes up into the internal Display Assembly, and connects to the back of the LCD screen.

Connecting an external display bypasses the Video Cable.
This is how you check if the problem is the graphics chipset, or the Video Cable, or inside the Display Assembly.
(Screen Inverter if used, and LCD screen)

If you do not have a display on the external monitor, isimi, there may be one step you have omitted;

There are at least three display options;
1) Internal monitor of laptop ONLY
2) Internal monitor of laptop AND external display
3) External display ONLY

Press the Fn key, and hold it down. At the same time tap once on the F5 key. The F5 key is the Display Toggle-Over key.
Still no?

Hold the Fn key down again, and tap once more on the F5 key.
Still no?
Suggest your graphics chipset is bad.

It may be the mounting of the graphics chipset to the motherboard.
(BGA surface mount. Ball Grid Array)
Or it may mean the graphics chipset is kaput, and this means motherboard replacement.

If you have a display on the external display, (TV), post back in a Comment.

Regards,
joecoolvette