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The high voltage is generated by a flyback transformer in the plate circuit of the horizontal output tube. Check the tube, make sure it has proper voltages, and a goodly amount of drive.
When the tube cuts off with a negative grid pulse, the plate current stops but the inductance of the flyback transformer primary causes a violent polarity reversal. This creates a large voltage in the secondary, as well as a bit of juice to light the high voltage rectifier (maybe a 1B3GT?). Put the oscilloscope on the grid and see what you have. Refer to the waveforms in the manual.
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