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Looks to me as though something is clamping the vertical sweep wave... i.e. it's rounded off on the bottom instead of a nice sawtooth.I'd be tempted to say it's a passive component (resistor, capacitor) that's the culprit, but if that's the case, you'd see it at the top of the sweep too.
Does anything happen if you adjust the vertical centering control, and if so, can you bring the top edge of the raster in to view?
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