Well its many years later! wow, I finally disassembled the entire spindle
motor from my 1541-II that is direct drive, there is a Mitsubishi 3 phase
motor controller chip M51784P it is a 36pin chip here is the datasheet

http://www.datasheetcafe.com/m51784p-datasheet-motor-control-mitsubishi/

pin 34 says speed control, but it is currently grounded. it shows 1:1 or
1:2 not sure what would happen if it was disconnected from ground.

The 1571 has a hitachi chip HA13432 24pin chip that I cannot translate the
Japanese or Chinese to help my life, but it looks like the speed controller
is changing the R/C circuit to change the pulse modulation that the chip is
referencing to adjust speed.

https://datasheet.octopart.com/HA13432-Hitachi-datasheet-106076.pdf

this post from lord cross references connecting but not where on Lemon64

https://www.lemon64.com/forum/viewtopic.php?t=66390&sid=14068c5de33f2ae59ce65247ffd2951a

Now if only I could figure out what pin 9 does on the 1571 Hitachi chip
then maybe this can be applied to the 1541-II Mitsubishi chip, I think
ultimately this was probably used on 5.25 1.2meg high density floppy drives
and that would be used to change 
the speed from 300 to 360 rpm

have a look and if anybody can figure this one out, then please let us
know. Geez everything else has been figured out, even the PLA and the Vic
Clock chip.

-BBP

--- SoupGate-Win32 v1.05