NotTaR of Television Sets : Dark picture                            
 Copyright © 1994-2007, Samuel M. Goldwasser. All Rights Reserved. Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied: 1. This notice is included in its entirety at the beginning. 2. There is no charge except to cover the costs of copying. I may be contacted via the Sci.Electronics.Repair FAQ (www.repairfaq.org) Email Links Page.

  << High voltage to focus sho.. |  Index  | Brightening an old CRT >>

Dark picture

A TV or monitor with a picture that is too dark may have a fault or the CRT may just be near the end of its useful life.

First, confirm that your video source - computer, camera, etc. - is producing a proper signal.

Is the brightness at all erratic? Does whacking the monitor have any effect? If so, then you may have bad connections on the CRT driver card or elsewhere. If the brightness tends to fade in and out over a 10 to 20 second period, a bad filament connection is likely. Check for the normal orange glow of the filaments in the neck of the CRT. There should be 3 orange glows. If they are excessively reddish, very dim, or fade in and out, you have located a problem. See the section: Picture fades in and out.

Common causes of brightness problems:

  1. Dirty CRT faceplate or safety glass. Don't laugh. It sounds obvious, but have you tried cleaning the screen with suitable screen cleaner? It is amazing how dirty screens can get after a few years - especially around smokers!

    Wipe gently with a slightly dampened cloth - not soaking or you may end up with real problems when the water drips down inside and hits the electronics! On TVs with a separate protective faceplate, clean both the front and rear surfaces of this plate as well as the CRT itself.

  2. Old CRT. The brightness of the CRT deteriorates with on-time. It does not matter much how bright your run your TV. An indication of a weak CRT would be that turning up the SCREEN (G2) or master brightness control only results in a not terribly bright gray raster before the retrace lines show up. There may be indications of poor focus and silvery highlights as well. A CRT brightener may help. See the section: Brightening an old CRT.

  3. Bad component in filament circuit or bad connection reducing filament voltage. This should be easy to check - there are only a few parts involved. If it is erratic, bad connections are likely.

  4. Brightness control faulty - bad pot, bad connections, or problem with its power supply. Depending on specific problem, control may or may not have any effect. If digitally adjusted, there could be a problem with the logic or control chip. If the button or menu item has no effect at all, then a logic or control problem is likely.

  5. Improperly set SCREEN (G2) voltage (usually on flyback) or faulty divider network. See the section: Adjustment of the internal SCREEN and color controls.

  6. Improperly set video bias (background) levels or fault in video drive circuitry. See the sections starting with: "Optimal procedure for setting brightness/background and screen adjustments".

  7. Fault in video amplifiers. With all three color affected equally, this would most likely be a power supply problem. A video amplifier problem is likely if turning up the SCREEN (G2) or master brightenss control results in a very bright raster before the retrace lines appear. Cheack signals out of the video/chroma(IC.

  8. Fault in beam or brightness limiter. Many TVs and monitors measure the beam current (possibly indirectly) and limit the maximum to a safe value. The purpose of this may be to protect the CRT phosphors, and/or to assure that the power supply does not go out of regulation, and/or to limit X-ray emission. If this circuit screws up, a dark picture may result. Checking the signals and voltages at the CRT socket should determine if this is the problem.

  9. High voltage is low. However, this would likely result in other symptoms as well with focus, size, and geometry.

 <<High voltage to focus sho.. | ToC | Brightening an old CRT>>