|
|
NotTaR of Television Sets : Red, green, or blue full on - fog over p..
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.
|
<< Bad CRT causing retrace l.. |
Index
| Shorts in a CRT >>
|
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.
Don't panic - heater-cathode shorts in CRTs can often be worked around.
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. The nice thing about
color monitors and TVs is that there three copies of each of these
components. Swapping and/or comparisons between these is an excellent
diagnostic technique.
- 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.
Here is the procedure in more detail (example for red full on):
(From: J. K. Emerine (jkemerine@aol.com).)
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 the symptom returns = 'goes red' the CRT is shorting. (See the section:
Providing isolation for a CRT H-K short. --- sam)
- 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 or further back
in the chroma circuitry. --- sam)
<<Bad CRT causing retrace l.. |
ToC
| Shorts in a CRT>>