Intermittent or missing colors

Note that due to the additive color scheme used in all emissive color displays like CRT or flat panel TV sets and video monitors, a single missing primary color (red, green, or blue) will result in the following appearance (for a white screen):

    Missing Color       Appearance
  ------------------------------------------------
       Red              Cyan (blue-green)
       Green            Magenta (reddish-purple)
       Blue             Yellow

Which color is affected may be even more obvious if the set has a color on-screen display for which you recall the proper colors.

• If gently whacking the set can make the color(s) come and go suddenly, then bad connections are probable. The most likely place for these are solder pads on the little circuit board on the neck of the CRT or even dirty CRT socket pins that are not making solid contact. Try prodding the CRT neck board with an insulated stick to see if you can affect the colors.

  Although not impossible, this is not likely to be a CRT problem.

• If the color fades in and out with a delay of about 10-15 seconds, it is probably intermittent power to the CRT filament for that color and probably means a bad CRT since the three filaments are wired in parallel inside the CRT. One of the internal connections has come loose.

  Look in the neck of the CRT to make sure all three filaments are glowing orange. If one is out or goes on and off, toss the set. Replacing the CRT is probably not worth it. However, if they all go on and off together (all colors would be fading in and out though perhaps not quite in unison), then bad connections for the CRT filaments on the CRT neck board are indicated.

To narrow down the problem:

• Locate the output for the bad color on the video driver board on the neck of the CRT. This will probably read a significantly higher voltage than the corresponding pins for the good colors. A circuit problem is likely - probably on this board.

• Test components on this board for the good and bad color channels. A shorted transistor or open resistor can kill one channel. Swap parts between good and bad colors to confirm.

• Gently pull the CRT neck board off of the CRT and replace it. This will tend to clean the contacts.

• Connect an output of the video/chroma circuit/chip that is working (i.e., a color that appears on the screen) to ALL three color drivers on the CRT neck board.
  • If you now get a more-or-less black and white picture (there may be a moderate color tint as the relative intensities of R,G,B may not be balanced), the problem is likely with the chroma decoder or its support circuitry.

    Note: the picture will be the intensity of only one color channel so it will not be quite NORMAL in any case.

  • If you still have missing or messed up colors, the problem is on the CRT neck board or with the CRT.

Most of the causes of intermittent colors boil down to bad connections of one form or another. For totally dead colors - not intermittent - bad components are also a possibility.

• Printed circuit board on the CRT neck. This is a common location for cold solder joints. Check with a bright light and magnifying glass for hairline cracks around the pins of larger parts. Prod and tap with an insulated tool to see if the problem is effected. Resolder if necessary.

• Cold solder joints elsewhere in TV or monitor usually around the pins of large parts such as transformers, power transistors and resistors, and internal connectors.

• Internal connectors (including CRT socket) that need to be cleaned and reseated. Remove, clean with contact cleaner, burnish, and replace.