I just looked at the picture of your Zenith SC300 on Flickr. Nice looking set; I'm sure once you track down and correct the vertical problems it will be an excellent performer as well, as were all Zeniths from that era. I'm wondering, however, without a channel selector knob on the front panel, how on earth the viewer would change channels if the motor drive failed for any reason.
I only looked briefly at the pics and didn't see one of the back of the set; perhaps there is a manual channel selector in the upper right or left rear corner near the tuner--almost all remote TVs, not only Zeniths, with motor driven tuners had this feature.
BTW, after looking at the picture of the inside of your set with the back off, I can see why this particular TV could be difficult to carry up one or more flights of stairs; the power transformer alone looks like it must weigh several pounds, and the CRT, being 19" diagonal (I think) probably weighs ten or 15 pounds as well. Add the weight of the TV chassis itself, plus the tuner and the remote chassis, and the whole thing probably weighs in at something like 30-40 pounds or more.
Again while looking at the pic of the inside of this set, I saw what appears to be an insulator with a large red arrow pointing to a shaft. Could this be the manual channel selector I asked about two paragraphs ago?