(At least) one answer to How were video tape recorders adapted to work in orbit in 1962? argues that the recorder should not be called a "video" tape recorder because the record/playback head was static and not scanning the tape, and links to
NASA Technical Note TN D-1542 A Precision Endless-Loop Magnetic Tape Recorder for Space Applications which covers the AC system that maintains the motor's constant speed, but not the recording and playback electronics.
update: it now links to Contract No. NAS5-3173; Final Engineering Report, TIROS-X Meteorological Satellite System, Volume I which may in fact contain the answer to this question.
However if this were a conventional tape recorder and you just took the amplified output of the vidicon and drove an audio record head with it, it would not work when playing back. For example a solid white area or solid back area would result in constant magnetization of the tape and therefore zero output independent of brightness level; the signal during playback would be proportional to dB/dt in the head.
Simply put, conventional tape recorders have a low frequency cut-off; they do not record long stretches of DC levels that a video signal would contain if looking at a blank gray level, which a video tape recorder absolutely must do!
So they must have used some modulation or encoding scheme to convert brightness levels to a modulated carrier tone of some kind (e.g. AM, FM, something else) so that a signal would be present during playback even for constant brightness levels.
Question: How did they do this? How did they make an otherwise audio tape recorder function as a video signal recorder and playback device?