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In later color sets with a focus bleeder hanging from the HV, that would discharge the HV and prevent a spot. Sets without a fixed load on the HV were in a race between the sweep collapsing and the video output (CRT cathode) voltage collapsing. Motorola's first solid state B&W chassis had a way of killing the spot when the power was switched off, but they didn't think about the case of someone pulling the plug while the set was on. In that case, the video output voltage had a very slow decay because the output transistor bias voltage died immediately and that transistor was the only load on an independent video b+ supply. About 15 or 20 seconds later, (as the cathode voltage dropped) the beam would come on full blast (because the HV had not been discharged and the cathode was still hot enough) and burn a big black spot into the CRT. Unfortunately (or fortunately for discovering the problem), pulling the plug was the last step in production before dropping the set into a carton.This was only discovered when the first ones were unpacked at the distributor to check a few. There was a rush effort to add a spot killer that did not depend on the power switch.
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