Question about Televison & Video
This means you have lost the luminance input to the chroma decoder or final video chip. A failure of the brightness limiter may result in similar symptoms.
A few common causes are:
Check the service switch (if any). Its contacts may be dirty and moving it back and forth a few times or using contact cleaner may be all that is needed.
Check for open high value resistors around the chroma decoder IC.
Check for open high value resistors in the brightness limiter circuit.
With a scope and schematic (or even just a pinout for the chip), you should be able to trace the luminance signal to see where it is getting lost.
I have had several TVs and monitors where the delay line in the luminance circuitry has failed. Usually it's made out of glass, and inherently is fragile. Sometimes whacking the monitor would make it come back, leading to the thought of connectoritis or a cold solder joint -- where in fact it was the delay line (long rectangular unit with two to four leads). Replacing the delay line was the solution, but to check it first it'd be a good idea to look for 'in' and 'out' on the line and short the pins. The picture may be shifted, colours may not line up, but it'll tell you that it's the delay line if the picture comes back at all. It's better than looking at a saturated picture with no luminance! :)
Posted on Jun 01, 2010
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Posted on Jan 02, 2017
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