Maybe I connected to tape out instead of tape in? I used pin 1, with pin 2 for ground.

This computer came with a schematic and a couple tapes. The schematic looks identical to the ones on the vector06c.fdd5-25.net site except the page numbers are different at the bottom.

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In fact this could be a regeneration cycle (pin 15 is CAS, inverted)

Can you monitor pin 4 (RAS, inverted) along with pin 15?

Pin 4 should be low and pin 15 should be high to trigger regeneration.

But anyways the low frequency "envelope" on your screenshot from the oscilloscope does not look good at all.

Channel 6 is pin 4, channel 7 is pin 15, so it seems like it's regenerating.