The SDS9300 I used to service had a drum memory, unfortunatley can't remember the type but I do recall that the drum was slightly tapered and had a rising bearing assembly. As the drum finally ran up to speed it rose an amount to bring the heads to the correct position and gap. Basically a "parking" system, when stopped the drum had dropped, so the heads were not at their working position. Heads were adjusted for position with the drum stopped using gauges, so as it was stopped and down you did not risk damaging the track. Spin it up to speed and the heads automatically got to the tracks and flying height. I recall it being about 2ft in diameter and about 4ft tall took about 30 seconds to come to speed and had its own motor generator set as it ran on a dedicated 60 cycle supply not the usual 50 we have here in the UK. I think it had 4 banks of 24 heads in fixed position. It was the one bit of kit we did not touch, a specialist used to fly in from the US once a year to check it and rewrite the timing tracks. Mike