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Sounds like retrace lines to me.
They usually are not all that horizontal.
During vertical retrace the beam rapidly moves from the bottom of the screen
to the top while the horizontal sweep keeps running.
Early sets used the fact that the beam is moving rapidly to the top and therefore dimmed (because the brightness is spread out along the path) to not need a blanking circuit. That can only work to a certain degree.
It depends on how the video connection is made verses the original circuit but you might be upsetting the DC bias at the injection point, this could upset the video amplifier.
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