We've noticed that on our 9420's and the Precision 3551 that when you lock the screen it will go black within 30 seconds. It happens when using the WD19tbs, external HDMI port to a monitor or just the laptop screen by itself. We've checked the normal power and advance settings for screen savers, sleep, monitor turn off, etc and all seems to be set correctly. We've even checked the registry, local policy's and bios. I even moved the computer into an OU that had no screen saver set. It seems that it's not related to a screensaver though and more of a power setting for the screen. I've checked device manager and unchecked any options that are set to "allow windows to turn off this device to save power", but it still turns off within 30 second and you need to tap the external keyboard, mouse or laptop keys to turn the screen on. When this happens no laptop keyboard or power lights are flashing and power button remains on, not flashing. 

I'm just not sure what else to check unless it's a setting in the bios or special to the Dell laptops that's set to conserve power. Any addition areas to check?