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#1
01-04-2011, 09:12 PM
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Can anyone explain this strange CRT fault?
On an '88 Sears/Sanyo 20" TV, that uses a Zenith CRT, as well as other Zenith CRT's from the late '80's-early '90's; I've noticed a problem as these tubes age.
The emission will test OK, there will no leakage or shorts when checked on a CRT tester and the picture will be bright & sharp; but, the color temperature is constantly changing with changes in scene brightness, etc. I've also seen some Zenith tubes display a blurry horizontal bar across bright areas of the picture. It's not as bad if the contrast is turned way down; but, it's still there. A normal picture will be on the screen; then, a blurry horizontal band will appear. I know these two conditions are tube related because I've changed the CRT's in sets that had these problems and the trouble went away. Does anyone know what CRT defect causes these problems? 
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#2
01-04-2011, 09:56 PM
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As CRTs age the cathode may start to exhibit several undesirable traits.... some that I know of offhand:
1. Double peaking... as grid drive is increased, cathode current will increase normally to a point then start to drop as drive is further increased, eventually current will bottom out and increase again with higher drive. Imagine a color tube with 3 guns each "dipping" a different drive points. 2. Cathode "interface"... the emissive surface of the cathode behaves as if it does not have a direct connection to the cathode lead, but appears to have a R-C time constant. 3. De-activation, Re-activation.... certain drive conditions may cause the cathode emission to "slump" while other conditions may promote recovery. Basically, a tired CRT can become very dynamic, non linear and unstable. jr
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#3
01-05-2011, 01:21 AM
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Quote:
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#4
01-05-2011, 09:00 AM
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Quote:
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#5
01-05-2011, 02:35 PM
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Lol x2!!
I have had a Zenith set do this exact same thing. But what I found is when I replaced the CRT the issue was still there, just not as bad oddly. I found a resistor in the grid circuit out of spec. Replaced it and issue was gone. Put the old CRT in and it came back.
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