Ok so boyfriend did some testing and discovered that the deflection yoke coil is bad - he measured 20+ ohms across the vertical and 300+ ohms between the vertical and horizontal deflection coil.
Unless we are mistaken, there should not be measurable resistance here, correct?

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To the best of my knowledge, the two sets of coils should indeed show no continuity with each other, so it does sound as if there's a short somewhere. Yours would be the first shorted classic Mac deflection coil I can recall, so, um, congratulations.

(I'm assuming that the resistance measurements were made with the yoke disconnected from everything.)