Yellow circle in center of screen & blue shading to side
This will require you opening the back cover of the television.
This could be a heater-cathode (H-K) short in the CRT, a failure of a component in the chroma circuits or video output (driver board), or bad connections there or elsewhere.
Note: before proceeding, it is a good idea to make sure that the screen is degaussed - else you could be attempting to track down problems with the wrong color!
Some simple tests can confirm or rule out other possibilities.
Compare the voltages for the video drive signals to the CRT on the little board on the neck of the CRT with the CRT both connected and unplugged. A schematic will help greatly in locating these signals.
If there is a significant difference especially on the bad color, then the CRT is a likely candidate. Try tapping the neck of the CRT GENTLY (with it plugged in and while viewing a picture) to see if it is an intermittent problem.
If there is no significant difference, you may have a bad driver or a problem in the chroma circuits.
Look for bad connection/cold solder joints, probably on the little board on the neck of the CRT. Use an insulated stick to gently prod the board and its components in an effort to induce/cure the problem. Look carefully for hairline cracks around the component leads.
You can swap components between two colors and/or test with an ohmmeter on that driver board to determine what is bad.
Another simple test: Disconnect the cathode for the full-on color from its drive. If it is still full-on, there is probably an H-K short in the CRT since the only way to get each color on the screen is via the cathode connection to the CRT neck board. If it is removed and there is still that color, the current must be taking another path inside the CRT.
Alternatively, interchange the outputs of the bad color with a good one by jumpering on the video driver board (on the CRT neck). If the bad color changes, then the problem is in the circuitry and not the CRT.
To identify if the fault is in the CRT or a control problem try this (WITH SET OFF):
On the CRT board, lift the output end of the green cathode final resistor. Do the same with the offending red cathode's resistor. Use short insulated jumpers to 'swap' drive signals - drive the red cathode with the green drive and the green cathode with red drive. (Note that if this problem only occurs after a warmup period, color at turn on will be - well - wierd, but it is just a test.)
If instead the symptom becomes 'goes green' then the red drive leg has the fault and the CRT is probably good. (In this case, there may be bad connections or a bad component on the CRT drive board.
Also it involve a technical skilled personnel to handle this..
May 20, 2010 |
Sony Grand WEGA KFWE42S1 TV