Yep here it is, Technical Introduction to the Macintosh Family p. 227:
”Note: On the older 400K disk drives, a buffer in RAM (actually the low-order bytes of words in the sound buffer) is read by the Analog Signal Generator (ASG) to generate a pulse-width modulated signal, like the sound signal, that controls the speed of the disk motor. This speed variation is responsible for the characteristic humming of the disk drive. The Macintosh Operating System uses this speed control to store more sectors of information in the tracks closer to the edge of the disk by running the disk motor at slower speeds. On the BOOK drives, the variable disk speed is automatically controlled by the disk drive hardware.”
You need to recap it before you do ANYTHING with it period.
These logic boards are known for failing catastrophically with bad caps. Sometimes to the point where you end up the waiting queue for a replacement logic board as a few others are in.
Excellent, there’s the answer. If indeed all it needs is a PWM on one of the pins, a fun and entirely useless project would be to build a little Arduino hack to enable use of a 400k drive with an SE or Mac II ...