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This is a very long shot, but I ran in to it on a DC-coupled scope many years ago.
Measure your CRT voltages both with and without a signal applied to the vertical input.
If anything except for the deflection plate voltages change, suspect something is amiss in the DC coupled sweep amplifier that causes it to pull too much current and create a power supply voltage drop that is causing the CRT to go out of focus.
Jas.
Last edited by earlyfilm; 10-24-2013 at 11:56 AM.
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