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
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