This site by  Sami Rautiainen:  

http://www.devili.iki.fi/Computers/

..has a HTML version of the Commodore 8050/8250 Service Manual at

http://www.devili.iki.fi/Computers/Commodore/8050/Service_Manual/

Relevant technical information on floppy disk drives is as follows:


MICROPOLIS AND MICROPOLIS SAFARI DRIVES

RESISTANCE CHECKS

Stepper motor has two sets of three wires, each set to a coil with a center tap. 
Both sets are white/yellow/black,
with yellow at center. 28 ohms either side of center or 56 ohms total.

PARTS LIST

805003101 Micropolis Drive 8050
825001601 Micropolis Drive 8250

31416701 MIC 8050 R/W HEAD Assy.
31416801 MIC 8250 R/W HEAD Assy.

TANDON DRIVES

RESISTANCE CHECKS

Stepper motor is shows as two pairs of two coils in parallel. Pin 1/2 is 37 ohms, 
pin 3/4 is 37 ohms. Notibly, the wiring is "RED/YEL pin 1, BRN/ORG pin 2; WHT/GRN 
pin 3, BLK/BLU pin 4". This suggests the coils could be connected in other ways. 
Tandon docs elsewhere refer to this 4-wire stepper as the "bipolar" stepper
configuration.

PARTS LIST
 
805004701 Tandon Drive 8050
825001501 Tandon Drive 8250

31419401 TAN 8050 R/W HEAD ASSY.
31419501 TAN 8250 R/W HEAD ASSY.
31419601 TAN STEPPER MOTOR
31419801 TAN HOUSING/SPINDLE ASSY.
   1. Housing Base
   2. Spindle/Tach Assy.
   3. L/R Guide Shafts 

NOte: no mention of a servo board. 

P12 is wired: 1/RED, 2/BROWN, 3/GREEN, 4/BLACK for the stepper motor.

MPI DRIVES

RESISTANCE CHECKS

Stepper motor has two sets of three wires, each set to a coil with a center tap. 
One set is brown/black/green, the other red/black/white. 75 ohms either side of 
center or 151 ohms total.

PARTS LIST

805005901 Drv0 MP1 Drive 8050*
805005902 Drv1 MPI Drive 8050*
825002901 Drv0 MPI Drive 8250*
825002902 Drv1 MPI Drive 8250*
	*DRV0 and DRV1 are interchangable.

31421901 MPI 8050 R/W HEAD ASSY.
31422001 MPI 8250 R/W HEAD ASSY.
31422701 MPI W/P Servo PCB Drv1
31422801 MPI Analog PCB Bracket Drv0 

31422301 MPI HOUSING/SPINDLE ASSY.
   1. Housing Base
   2. Spindle/Tach Assy.
   3. L/R Guide Shafts 


PS: While looking at this document I followed some links to THIS Web site:

http://www.ktverkko.fi/~msmakela/8bit/c2n232/index.en.html

which has a simple dongle to connect a C64 cassette port, to an RS-232 serial connection
on a Windows PC. Windows software loads tape files. The device, an Atmel microprocessor,
converts the ASCII codes to the analog signals needed to emulate the cassette. The
site includes software and hardware descriptions. Building the device is very simple if
you can assemble chips onto your own boards, otherwise you have to obtain boards.