20MB MFM HDD spindle motor speed control

[fzabkar]

This is not strictly a repair question, but it follows on from this thread:

https://www.eevblog.com/forum/repair/ambient-temperature-dependant-fault-(old-mfm-hard-drive)/

In that other thread it appears that the 10MB MFM HDD has a trimmer to adjust the speed of the spindle motor to a tolerance of +/-1%. ISTM that this kind of design is asking for trouble, so I then looked at the design of a 20MD HDD that was supplied with the IBM PC-AT.

Here are the circuits from IBM's Tech Ref Manual:

http://www.users.on.net/~fzabkar/PC-AT/AT_20MB_HDD/AT_20MB_4_50.jpg  (spindle motor + stepper positioner)
http://www.users.on.net/~fzabkar/PC-AT/AT_20MB_HDD/AT_20MB_3_50.jpg  (read/write analogue circuits)
http://www.users.on.net/~fzabkar/PC-AT/AT_20MB_HDD/AT_20MB_2_50.jpg  (digital logic for motors)
http://www.users.on.net/~fzabkar/PC-AT/AT_20MB_HDD/AT_20MB_1_50.jpg  (uC + MFM interface)

AFAICT, HE0, HE1, HE2 are the Hall Effect outputs from the spindle motor and are used for commutation. HE2 provides the Tach for the uC. The motor also produces an "R Index" signal for the uC. There are 3 phases -- 0A, 0B and 0C.

AISI, there is only one way to control the speed of the motor, and that is by PWM-ing the -MOTOREN and/or MOTOR signals. Would that be how it is done?

[shakalnokturn]

Although I'm no expert on the matter I suppose the same.
-MOTEN seems more convenient to PWM considering C4 on MOTOR line.