Monday, November 21, 2011

Blank picture, power light on

Does 'blank picture' means a totally black screen with the brightness and
contrast controls having no effect whatsoever? Or, is there is no picture
but there is a raster - light on the screen? The direction in which
troubleshooting should proceed differ significantly depending the answer.

Verify that you computer has not simply entered power saving mode and
blanked the screen or shut off the monitor video and power circuits

Confirm that the video source is not defective or blank - try another one.

Here are some questions:

1. Is there any light on the screen at any settings of the brightness
and contrast controls, and/or when switching channels. Can you see any
raster scanning lines?

2. Can you obtain a raster of any kind by adjusting the screen (G2) control
(probably on the flyback) or master background or brightness?

3. Looking in the back of the monitor, can you see the glow of the CRT

4. Do you get that static on the front of the tube that would indicate that
there is high voltage? Any cracking or other normal or abnormal sounds
or smells?

If the answer to all of these is 'no', then you have a power supply and/or
deflection problem. Refer the the section: "No picture but indications of power".

Possible causes of no raster:

* No or low high voltage (low voltage, deflection, or high voltage power
supply failure).

* Fault with other voltages like G1 or screen (G2) to CRT.

* Filament to CRT not getting powered.

* Drive to CRT bad/shut off as a result of fault elsewhere. For example,
failure of the vertical deflection may disable HV or blank the screem to
protect the CRT from burn-in due to the very bright horizontal line that
would result. With some monitors, it is possible that the X-ray protection
circuitry will blank the screen.

Possible causes of no video: problem in video input, video amplifiers, video
output, cutoff due to other fault.

It could be as simple as a bad connection - try gently prodding the boards
with an insulated stick while watching the screen. Check for loose connectors
and reseat all internal connectors.

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