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Loss of luma with chroma still intact can be caused by a heater-cathode short in the CRT. Tapping the CRT neck usually reveals it.
In a solid state set with the heaters powered from a winding on the flyback, you can sometimes 'float' the whole circuit, thereby isolating the short from signal ground. Sometimes this is as simple as cutting a single trace on a pc board.
Some Sanyos were particularly prone to this. Never encountered in a Zenith before.
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