In article , Dave Plowman
writes

They're pulling your plonker. I can't think of any tube fault that would
cause a zig-zag pattern - or to cause the SMPS to shut down the set.

It's quite common. A cathode short in one of the RGB guns will cause
the entire screen to turn that colour, and the zig-zag pattern is the
retrace lines. As the cathodes are fed from a ~200v supply, the load
caused by a short on this can cause the PSU to go into shutdown to
protect itself.