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This really is a job for a first class scope.
Look at the video output to the picture tube and the input to the horizontal
output tube. Also examine the output of the horizontal output tube
BY HOLDING THE PROBE CLOSE TO THE PLATE LEAD but NOT touching it.
Look for the ringing. Then simply remove the horizontal output tube and see what
happens to the waveforms. Make sketches of the waveforms and report back. Use
the grid of the horizontal output tube for sync of the scope.
If its an AC-DC set you can't pull the horizontal output tube ... in that case,
disconnect the plate cap lead.
That's the first step.
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