Commodore 1541 User manual

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COVER PAGE
The enclosed pages are revisions to tne 1541 Single Drive Floppy
Disk MAINTENANCE MANUAL. They include additional information for
the 1541 Disk Drive and information on how to maintain the
VIC-1541 (1540). At the time of the first printing, this
information was not available. The author wanted all purchasers
of this manual to have all available inform2tion so this
supplement is being furnished to all ?ersons who purchased the
first printing. Simply insert the enclosed pages in the
appropriate places in the manual. If any questions arise, the
author may still be contacted at his address listed on the
Acknowledgements page. Please note the alignment system discussed
in the new Paragraph 5-5. Thank you for your business.
@ COPYRiGHT 1984 MICHAEL G. PEL flf;ff

PREFACE
This manual contains information for the maintenance and repair of
the VIC-1541 and 1541 Disk Drives. This manual can help you save
on repair bills, reduce down time and improve the reliability of
your VIC-1541/1541. A brief description of the information
contained in this manual is given as follows:
Introduction-A narrative description of the use of this manual, as
well as a brief description of the assemblies contained in the
VIC-1541/1541.
Theory of Operation-A narrative description of electrical and
mechanical theory of operation, including block diagrams and
schematic excerpts.
Initial Configuration-Step by step procedures for verifying that
the VIC-1541/1541 is operating properly.
Calibration-Step by step procedures for calibrating disk rpm's,
head alignment and adjusting Track #1 stop.
Disassembly/Reassembly-Step by step procedures for the
disassembly/reassembly of the VIC-1541/1541.
Preventive Maintenance-Step by step procedures for preventive
maintenance, including cleaning and head care instructions.
Troubleshooting-Written in two parts: Part 1 gives step by step,
fully illustrated, procedures for isolating a problem down to the
PC Board or assembly level and is written with the novice in mind;
Part 2 is written for the experienced technician and contains
narrative instructions, backed up by 33 waveforms, to isolate a
problem down to an integrated circuit or to a discrete component.
Schematics and Parts Layout-Complete schematics, interconnect
diagram, sub-assembly identification and parts layout.
Appendices-Contain test equipment specifications, assembly
instructions for test accessories, CMOS handling precautions,
parts list and, in Appendix F, a narrative discussion of the
differences between the VIC-1541 and the 1541, along with
cross-reference tables.
All references in Sections 1 thru 9 of this manual refer to the
VIC-1541 disk drive. Appendix F must be consulted for information
concerning the 1541 disk drive.
Commodore Business Machines, Inc., and the author of this manual
advise that any attempt to repair the VIC-1541 or the 1541 disk
drives during the warranty period will void the factory warranty.

I
4-1
4-2
4-3
4-4
5-1
5-2
5-3
5-4
5-5
5-5-1
5-5-2
5-5-3
5-5-4
5-6
5-6-1
5-6-2
5-6-3
6-1
6-2
7-1
7-2
7-3
7-4
7-5
8-1
8-2
8-3
8-3-1
8-3-2
8-3-3
8-3-4
8-3-5
8-3-6
8-3-7
Section 4-PERFORMANCE TEST
General
Equipment Required
Preparation for Performance Test
Performance Test
Section 5-CALIBRATION
General
Equipment Required
Preparation for Calibration
Calibration
Head Alignment
General
Equipment Required
Preparation for Mechanical Alignment
Head Alignment
Mechanical Alignment of TLack #1 Stop
General
Preparation for Mechanical Alignment
Mechanical Alignment
Section 6-DISASSEMBLY/REASSEMBLY
Disassembly
Reassembly
Section 7-PREVENTIVE MAINTENANCE
General
Equipment Required
Preparation for Preventive Maintenanca
Preventive Maintenance
Cleaning and Inspection
Section a-TROUBLESHOOTING
General
TROUBLESHOOTING-PART 1
TROUBLESHOOTING-PART 2
Frame Assembly
Power Supply
Timing Circuits
Computer
Read Circuit
Encoder/Decoder
Write Circuit
Page ii
4-1
4-1
4-1
4-1
5-1
5-1
5-1
5-2
5-4
5-4
5-5
5-5
5-5
5-8
5-8
5-8
5-8
6-1
6-3
7-1
7-5
7-5
7-5
7-6
8-1
8-2
8-10
8-10
8-10
8-11
8-11
8-12
8-14
8-15

8-3-8
8-3-9
8-3-10
8-4
Track Select Circuit
Optics Circuit
Drive Motor/Servo Circuit
TROUBLESHOOTING-WAVEFORMS
Section 9-SCHEMATICS AND PARTS LAYOUT
9-1 General
Appendix A
Appendix B
Appendix C
Appendix D
Appendix E
Appendix F
APPENDICES
Test Equipment
Timing Strobe Fabrication
MOS Handling Precautions
Parts List
Video Detector Fabrication
VIC-1541(1540) vs. 1541 Disk Drive
Page iii
8-15
8-16
8-16
8-17
9-1
A-1
B-1
C-1
D-1
E-1
F-1

Section 2-THEORY OF OPERATION
2-1. Overall Theory
The VIC-1541 cons is ts mainly of three sub-assemblies:
controller PC Board, drive unit and frame.
disk
The frame sub-assembly provides the power supply on the drive
controller PC 13oard with +9 Vrms and +16 Vrms, which are
derived from the 120 VAC power input. (Refer to Section 3
for selection of input power.)
The drive unit sub-assembly is capable of reading or writing
to or from a floppy disk. The drive unit is also capable of
rotating the floppy disk, changing read/write head location,
detecting write protect status, and magnetically reading or
writing data.
The disk controller PC Board sub-assembly includes the power
supply, read/write circuitry, track select circuitry, timing
circuitry, and a 6502-based computer. The computer operates
the drive unit as well as managing the floppy disk. The
program which this computer uses is cal led the DOS ( Disk
Operating System).
The floppy disk consists of a mylar disk with a magnetic
coating inside a jacket (Refer to Figure 2-1). The jacket
has several cutouts in it. The index cutout (Item 1) is not
used in the VIC-1541 since sector information is written on
each block. The slot (Item 2) allows the head to touch the
magnetic coating. A similar slot is cut on the opposite side
of the floppy disk. The write protect slot (Item 3) is
provided as a means of protecting a disk against accidental
erasure or overwriting. If the write protect slot is left
uncovered, writing to the floppy disk is permitted. If the
write protect slot is covered with opaque tape, writing is
disabled.
The recording principles involved are identical to those of
magnetic tape. The shape of the disc is more convenient than
magnetic tape since each piece of information passes the
vicinity of the head three hundred times per minute. This
allows random access of the information on the disk. Data is
physically stored in rings on the face of the disk. These
rings are called tracks. There are 35 tracks on each disk.
Each tracK is further divided into sectors. Each sector
contains sync, ID, track, sector and checksum information
along with 254 bytes of data. Track #18 is used for
housekeeping purposes (i.e., the directory and the block
availability map). Track #18 is automatically managed by the
DOS. Any information to be written or read is received and
transmitted by the disk controller to the VIC-20/COMMODORE 64
over the serial bus.
Page 2-1

2-3. Electrical Theory (Refer to Figures 2-5 and 2-6)
2-3-1. Frame Electrical Theory
AC LINE
POWER
SWITCH
FUSE
X-FMR
9V RMS
16V RMS
Figure 2-5. Frame Assembly, Block Diagram.
The electrical function of the frame assembly is to condition
and convert the AC line voltage before applying it to the
power supply on the disk controller PC Board.
The AC line voltage enters the disk drive at J9, which is
both a connector and an RFI filter. After passing through
the filter, the AC voltage is applied to the SPST power
switch, Sl. The output of the power switch is applied to Fl,
which provides ever-current protection. Fl is a !A, 250V
Slow Blow fuse. The output of Fl is fed to the transformer,
Tl. Tl steps down the AC line voltage into +9 Vrms and +16
Vrms. Both of these outputs have their own secondary
windings and are isolated from each other.
It should be noted that the AC return line for the primary of
Tl may be connected to either tap on the primary winding.
The black tap must be connected for use with Disk Controller
P.C.Bd. 1540001-xx (long board version typical of
VIC-1541(1540)). The red tap must be connected for use with
Disk Controller P.C.Bd. 1540048-xx (short board version
typical of 1541). Either Board may be used for replacement
of the other provided the correct tap of Tl is used. See
Section 3 for AC power configuration.
Page 2-7

J9,AC LINE
AC HI
GND
AC LO
J9,AC LINE
AC HI
GND
AC LO
2
3
Tl
OR
Fl
1.0A
16V
OR
BU
For use with P.C.Bd. 1540048-xx
Tl
Sl,POWER Fl
OR
LOA
16V
OR
BU
For use with P.C.Bd. 1540001-xx
Figure 2-6. Frame Assembly, Schematic.
Page 2-8
Jl,AC INPUT
16V 01
4 16V 02
2 9V 01
9V 02
Jl,AC INPUT
16V
01
4 16V
02
2 9V
01
9V
02

Section 3-INITIAL CONFIGURATION
3-1. General
This section provides step-by step procedures for setting the AC
line voltage (selectable between 100 VAC and 120 VAC) and the
device number (selectable between 8,9,10 or 11).
3-2. AC Power
When replacing the Disk Controller P.C. Bd., the AC input power
may require changing. If Disk Controller P.C.Bd. 1540001-xx (long
board version typical of VIC-1541 (1540)) is replaced with Disk
Controller P.C.Bd. 1540048-xx (short board version typical of
1541), or vice versa, the input power will require
re-configuration. To re-configure, perform the following steps:
1. Disconnect AC power cord from J9.
2. Disconnect serial bus cable(s) from J3 and/or J4.
3. Remove top cover. (Ref er to Section
6-DISASSEMBLY/REASSEMBLY)
4. Remove RFI shield. (Refer to Section
6-DISASSEMBLY/REASSEMBLY)
5. Remove Disk Controller P.C. Board.
6-DISASSEMBLY/REASSEMBLY)
(Refer to Section
6. Desolder wire from pin 1 of J9. (Refer to Figure 3-1)
7. Solder desired tap of Tl to pin 1 of J9:
a. Solder black tap of Tl to pin 1 of J9 for 1540001-xx
P.C.Bd. (long board version typical of VIC-1541(1540)).
(Refer to Frame Schematic)
b. Solder red tap of Tl to pin 1 of J9 for 1540048-xx
P.C.Bd. (short board version typical of 1541). (Refer
to Frame Schematic)
!WARNINGI
• TO PREVENT A POSSIBLE SAFETY HAZARD,
INSULATE PIN 1 OF Tl WITH HEAT SHRINK
TUBING OR EQUIVALENT. USING HEAT SHRINK
OR ELECTRICAL TAPE, INSULATE UNUSED TAP
OF Tl AS SHOWN IN FIGURE 3-1.
Page 3-1

5-5. Head Alignment
5-5-1. General
This procedure aligns the read/write head to the physical tracks
on a floppy disk. A truly accurate alignment requires the use of
an alignment reference disk and a test program disk. The
procedure which follows allows the user to "get by" without such
software. However, the degree of success achieved will depend on
the accuracy of the disk which is used. Consequently, the
alignment may cause compatibility problems when using software
recorded on other drives or when using other drives to read disks
recorded on a drive aligned with this procedure. This problem may
be reduced by selecting a pre-recorded disk which has not been
written to since it was recorded at the factory. Although this
procedure is not 100% accurate, it will usually suffice for the
home user. For an accurate alignment of the drive unit, one of
the following alignment systems is recommended:
1. COMMODORE System
This system is available from:
Commodore Business Machines,Inc.
Customer Service Dept.
1200 Wilson Dr.
Westchester, Pa. 19380
Alignment disk
Test program disk
Service manual
970160-01
970154-01
9900445
Total=
$130
$ 26
$ 25
$181
The above items, plus a dual trace oscilloscope, are
required to align a disk drive using the COMMODORE
system.
2. Peltier Industries System
This system is available from:
Peltier Industries,Inc.
735 N. Doris
Wichita, Ks. 67212
Disk Alignment System (DAS-1541)
The system includes:
Alignment reference disk
Control disk
Instruction manual
Video detector
$39.95
The above items, plus a 3 1/2 digit, 100 Kohm digital
voltmeter, are required to accurately align a disk drive
using the Peltier Industries System.
Page 5-4