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The most frequent causes of loss of luma are 1. Dirty Service Switch (but I don't believe CTC-5 has a Service Switch) and 2. Heater-cathode short in the CRT.
Try tapping gently on the CRT neck with something like a screwdriver handle. That'll usually show up a H-K short.
Other things can cause loss of luma too, including open delay line.
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