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There's a fault condition endemic to those sets that you're probably aware of already. If not, it's the two ground stakes on the far right end of the chroma board which go open and kill the heaters on the 6GU7s. They need to be re-flowed, and this should be routine on every CTC-12, '15, '16, '17 or '25 you ever run into.
What happens is - (since the 6GU7 plates are DC-coupled to the CRT grids), the plates go too positive, driving the CRT grids positive, which runs the beam current full-on, overloading the snot outa the 3A3, flyback etc. Symptoms are - raster blooms, loses focus, then fades out completely.
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