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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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