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I don't know what was done in the old porthole sets, but in the ZOOM feature in some later Zenith color sets, they increased the retrace time. So, the scan started just beyond the edge of the faceplate, but it started late and ended early. I imagine the porthole sets may do the same thing. If you can put a scope on the deflection coils, you could measure this easily. Watch out, thouhg, the retrace pulse voltage may be quite high (several times as big as B+)
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