Dell OptiPlex 740 - Manual (Page 196)

If the screen is difficult to read
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Test the electrical outlet — Ensure that the electrical outlet is working by testing it with another device, such as a lamp.
Test the monitor — Connect a properly working monitor to the computer, and try using the monitor. If the new monitor works, the original
monitor is faulty.
Check the diagnostic lights — See Diagnostic Lights .
Check the card setting — Enter system setup (see System Setup) and ensure that Primary Video option is set correctly. Exit system setup
and restart your computer.
Run the monitor self-test — Check the monitor documentation for more information.
Check the monitor settings — See the monitor documentation for instructions on adjusting the contrast and brightness, demagnetizing
(degaussing) the monitor, and running the monitor self-test.
Move the subwoofer away from the monitor — If your speaker system includes a subwoofer, ensure that the subwoofer is at least 60 cm
(2 ft) away from the monitor.
Move the monitor away from external power sources — Fans, fluorescent lights, halogen lamps, and other electrical devices can cause
the screen image to appear "shaky." Turn off nearby devices to check for interference.
Adjust the Windows display settings —
1. Click the Start button, click Control Panel, and then click Appearance and Themes.
2. Click Display and click the Settings tab.
3. Try different settings for Screen resolution and Color quality.