Oki ML420 Series User manual

Oki Data CONFIDENTIAL
ML420/421
Maintenance Manual
For Rev "E0" and Higher
032411A

Copyright © 2008 by Oki Data. All Rights Reserved
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contained herein shall be construed as conferring any license or right under any Oki
Data copyright.
Oki Data has taken care to insure that the information which follows is complete,
accurate and up-to-date. However, Oki Data assumes no responsibility for errors or
omissions which may occur. All the information provided is subject to change from time
to time at the sole discretion of Oki Data.
All publications may include technical inaccuracies or typographical errors. We reserve
the right to make periodic changes, additions, and deletions to publications without
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The most up-to-date drivers and manuals are available from the web site:
http://www.okiprintingsolutions.com
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Contents
1. CONFIGURATION ...............................................................................................6
1.1 Standard Printer Configuration ............................................................................................ 6
1.2 Options................................................................................................................................. 7
2. THEORY OF OPERATION ..................................................................................9
2.1 Electrical Operation.............................................................................................................. 9
2.1.1 Summary................................................................................................................. 9
2.1.2 SOC and the Peripheral Circuit............................................................................... 9
2.1.3 Initialization ........................................................................................................... 12
2.1.4 Interface Control ................................................................................................... 13
2.1.5 Print Control .......................................................................................................... 16
2.1.6 SP/LF Motor FAN Control ..................................................................................... 18
2.1.7 Operation Panel .................................................................................................... 21
2.1.8 Alarm Circuit ......................................................................................................... 23
2.1.9 Power Supply Unit ................................................................................................ 26
2.2 Mechanical Operation ........................................................................................................ 28
2.2.1 Printhead Mechanism and Operation ................................................................... 28
2.2.2 Spacing Operation ................................................................................................ 31
2.2.3 Head Gap Adjusting.............................................................................................. 32
2.2.4 Ribbon Drive ......................................................................................................... 34
2.2.5 Paper Feed Operation .......................................................................................... 35
2.2.6 Paper Detection Mechanism................................................................................. 45
2.2.7 Automatic Sheet Feed .......................................................................................... 47
2.2.8 Paper Park Function (Continuous paper) ............................................................. 49
3. ASSEMBLY/DISASSEMBLY..............................................................................50
3.1 Precaution for Parts Replacement ..................................................................................... 50
3.2 Service Tools ..................................................................................................................... 51
3.3 Disassembly/Reassembly Procedure ................................................................................ 52
3.3.1 Printhead............................................................................................................... 54
3.3.2 Ribbon Protector ................................................................................................... 55
3.3.3 Pull-up Roller Assy................................................................................................ 56
3.3.4 Upper Cover Assy, Access Cover Assy and Sheet Guide Assy ........................... 57
3.3.5 Gear Case Assy.................................................................................................... 58
3.3.6 PC Connector ....................................................................................................... 59
3.3.7 Space Motor, Guide Roller Assy........................................................................... 60
3.3.8 Space Rack........................................................................................................... 61
3.3.9 Carriage Cable...................................................................................................... 62
3.3.10 Backup Roller Holder Assy ................................................................................... 63
3.3.11 Platen Assy ........................................................................................................... 64
3.3.12 Main Control PCB ................................................................................................. 65
3.3.13 LF Motor................................................................................................................ 66
3.3.14 Operation Panel PCB............................................................................................ 67
3.3.15 Power Supply Unit, Sensor PCB........................................................................... 68
3.3.16 AC inlet and AC Switch......................................................................................... 69
3.3.17 Change Lever and Gears...................................................................................... 70
3.3.18 Carriage Shaft....................................................................................................... 71
3.3.19 Paper Pan ............................................................................................................. 72

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3.3.20 Rear Tractor Assy ................................................................................................. 73
3.3.21 Rear Pressure Assy .............................................................................................. 74
3.3.22 Switch Lever ......................................................................................................... 75
4. ADJUSTMENT ...................................................................................................76
5. CLEANING AND LUBRICATION .......................................................................85
5.1 Cleaning............................................................................................................................. 85
5.2 Lubrication ......................................................................................................................... 86
6. TROUBLESHOOTING AND REPAIR ................................................................93
6.1 Items to Check Before Repair............................................................................................ 93
6.2 Troubleshooting Table ....................................................................................................... 93
6.3 Lamp Display ..................................................................................................................... 94
6.4 Connection Circuit Check for Printhead and SP/LF Motor................................................. 98
6.5 Troubleshooting flow chart............................................................................................... 100
APPENDIX A PCB LAYOUT...............................................................................111
APPENDIX B RS-232C SERIAL INTERFACE BOARD (OPTION)....................112

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1. CONFIGURATION
1.1 Standard Printer Configuration
This printer consists of the following assemblies:
Figure 1-1 Configuration
Upper cover
Access cover assy
Platen knob
Sheet guide assy
Main control PCB
Power supply unit
Pull-up roller assy
Main chassis assy
Operation panel assy

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1.2 Options
(1) Cut sheet feeder unit (CSF)
(Narrow and wide versions available)
Single-bin CSF
(2) Pull-tractor assy
Attachment assy

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(3) Bottom push tractor unit
(4) Roll paper stand (Narrow only)
(5) Option I/F Board
• RS232C Serial interface board • OKI LAN board

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2. THEORY OF OPERATION
2.1 Electrical Operation
The electrical operation of the printer circuit is described in this section.
2.1.1 Summary
Figure 2-1 shows the block diagram of the printer.
The Main Control PCB is made up of the SOC (SYSTEM ON CHIP), peripheral circuits, drive
circuits, sensors and interface connectors.
The power to the Main Control PCB is supplied by the power supply unit through the connector
cord.
The power to other electrical parts is also distributed through the connectors within the Main
Control PCB.
2.1.2 SOC and the Peripheral Circuit
(1) SOC
SOC to be CPU and past LSI function, building SRAM into, and the outline of use is as
follows.
• 3.3V single power supply (1.2V in internal core voltage)
• Operation frequency 48MHz
• Built-in CPU core (ARM7TDMI)
• SPAM 4M bit
• With built-in USB controller
• With built-in UART function
• AD converter x 4ch DA converter x 2ch
OKI of other peripheral circuitry is original though CPU core uses ARM7TDMI.

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RESET-N
IEEE1284
DERIVER IC
FLASH
ROM
OSC
48Mhz
OPTION
CSF or
ROLL
PAPER
STAND
OPERATION
PANEL PCB
HEAD ALARM
CIRCUIT
ALARM-P
ALARM-P
12V
PRINT
HEAD
HEAD DRIVE
CIRCUIT
SPALARM
CIRCUIT
SP
Photo
Sensor
MOTOR
DRIVER
IC
DC-DC Switching
Converter
40V
40V
40V
FUSE
FUSE
HD TEMP
φA/φB
SP TEMP
ADDRESS
DATA
USB
I/F
PARARREL
I/F
RESET
IC2
RESET
IC1
SEQUENCE
CIRCUIT
3.3V
REGULATOR
IC
TRANSISTOR
AC INPUT AC SW AC FUSE
POWSAVE-P
RESET-N
VOLTAGE LOW
DETECTION
CIRCUIT
40V
40V
5V
5V 5V
3.3V
3.3V
3.3VD1
ALARM-P
POWSAVE-P
POWR SUPPLY
UNIT
3.3VD2
5VD2
5V
OPTION I/F
RS232C
OR
NIC
SOC
(SYSTEM ON CHIP)
(CPU+LSI+RAM)
Release SW
Bottom SW
PE SENSOR
FAN
MOTOR
DRIVER
CIRCUIT
12V
FUSE
FAN ALARM
Figure 2-1

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(2) Program ROM (external FLASH ROM)
The program to make the printer work is stored in FLASH ROM as external ROM.
FLASH ROM that can be installed is 64Mbit(16 x 4096k) or less. The access timing of
the outline to FLASH ROM is shown in the following.
A0~A21
CS-N
RD-N
WR-N
D0~D15
WR-N
RD-N
CS-N
D0~D15
A0~A21
SOC ROM
(3) RAM
In RAM, the memory capacity built into SOC is 256K x 16bit (4Mbit).
(4) USB Controller
The USB Controller detects and controls the USB interfacee.
The USB controller is built into SOC. The figure below shows the outline circuit.
0V
USB ON-N
USB ON-N
USB LEV-P
DP-P
DM-N
SOC USB Connector
+3.3V
D+
D-
VCC
0V
FG
0V

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2.1.3 Initialization
This printer is initialized when the power is turned on or when the I-PRIME-N signal is input
from the host side via the parallel interface.
For the initialize operation, the RST-N signal is first output from the reset circuit to reset the
SOC and Flash ROM. When resetting ends, the program starts. Reset operation by I-PRIME
starts program to initialize, but does not reset the SOC.
The program here sets the mode of the SOC, checks the memories (ROMs and RAMs), then
carries out carriage homing, and determines the LF motor phase.
Finally, the program establishes the interface signals (P-I/F: ACK-P signal sending, and S-I/F:
BUSY-N signal off) and lights the SELECT lamp to inform the ready state for receiving to the
host side and ends the initialize operation.
After USB I/F control I/O initialization and USB I/F bus opening.
End
I/F BUSY OFF
Serial
I/F
I/F ACK Send
Parallel
I/F
LF Motor Phase
Initialization
Carriage Homing
USB I/F Bus Open
RAM Clear
ROM CHECK
Internal RAM
CHECK
SOC
Initial Setting
SOC RESET
Start

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2.1.4 Interface Control
(1) Parallel Interface
The parallel data input from the host to the interfaced SOC is latched to its internal register
at the falling edge of the STROBE-N signal.
At the same time, the SOC sets the BUSY signal to the high level to inform the host that
the data is being processed, and outputs the RXD signal to inform the SOC of data
reception. The data is read upon receiving the RD-N signal from the SOC.
When the data processing ends, the BUSY signal is set to off and the ACK-N signal in
sent to request the next data. When reception is impossible because the buffer is full, the
BUSY signal is sent to request stopping of data transmission.
* The STROBE-based timing for the BUSY signal is adjustable from the Maintenance
menu.
ACK signal timing and BUSY signal timing can be adjusted from the Maintenance menu.
Receive Data
SOC IEEE1284
DRIVER IC
BSY
ACK
NSTB
STB-N
BSY-P
ACK-N
Receive Data
CN1
BUSY-P
ACK-N
STROBE-N
Receive Data
1 to 8
STROBE-N
BUSY
ACK-N
RXD-N
2~8µs
500ns max.

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(2) Universal Serial Bus (USB)
Universal Serial Bus Specification Revision 2.0 (Full speed) compliance.
1) Connector
•Printer Side : “B”Receptacle (Upstream Input to the USB Device)
•Cable Side : Series “B”Plug
2) Cable
•Cable Length : Approx 1.8m (A cable must be met USB Spec Rev 1.1 for
normal operation)
Note: Cable is not supplied.
3) Table of USB I / F signals
Contact Signal Name Typical Wiring
Number Assignment
1 Vbus Red
2 D - White
3 D + Green
4 GND Black
Shell Shield Drain Wire
4) Connector pin arrangement
2
34
1
5) Mode & Class of Device
•Full - speed Driver
•Self - powered Device
6) Data Signaling Rate
•Full - speed function - 12Mb/s ±0.25%(2500ppm)
7) Interface circuit
SOC
+3.3V
D+
D-

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8) Signal Level
•Input / Output Level
Parameter
Input Levels :
High (driven)
High (floating)
Low
Output Levels :
Low
High (driven)
Output Signal Crossover Voltage
Symbol
V IH
V IHZ
V IL
OL
OH
V CRS
Min.
2.0
2.7
0.0
2.8
1.3
Max.
3.6
0.8
0.3
3.6
2.0
Units
V
V
V
V
V
V

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HEAD DRIVE TIMING CHART
DT1
DT2
HEAD
DRIVE CURRENT
2.1.5 Print Control
Print data is transmitted as parallel data (HD01~HD09) from SOC to print head. SOC generates
print timing and drive time.
Print Data
SOC
DRIVER Print Head
HD01-P~
HD09-P
Main Control PCB
Print Data
HEAD1-N~
HEAD9-N

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Head Gap Range 1 2 3 4 5
Print speed 100% 90% 85% 85% 80%
Drive time Short Long
(Drive time lengthens at each step.)
(e) Print mode compensation
According to the thickness of the printing medium, the print mode is compensated
as shown in the table below:
As shown in the drawing left, the stroke length
up to the platen is different for each pin.
Print Head
Time
Pin 3~6
Pin 1, 2,
8, 9
Pin coil
current
Pin 1, 2
3~6
8, 9
Platen
Print Compensation Control
The print compensation can be made as shown below:
(a) Voltage compensation (See 2.1.8 “Alarm Circuit.”)
(b) Temperature compensation (See 2.1.8 “Alarm Circuit.”)
(c) Pin stroke compensation
(d) Simultaneous Compensation of the number of impact pins
The SOC is provided with the compensation table for each pin to make necessary
compensation.
Number of impact pins Few Many
Drive time Short Long

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SPU-P
SPV-P
SPW-P
UH out
VH out
WH out
UL out
VL out
WL out
SPU_OUT-P L L OP H H OP L L OP H H OP L L OP H H OP L L OP H H OP
SPV_OUT-P HOPL LOPH HOPL LOPH HOPL LOPH HOPL LOPH
SPW_OUT-P OP H H OP L L OP H H OP L L OP H H OP L L OP H H OP L L
BD6206FS
IC in
BD6206FS
IC out PWM PWM PWM PWM
PWM PWM PWM PWM
PWM PWM PWM PWM
HD1 Connector
out
2.1.6 SP/LF Motor FAN Control
(1) Space motor control
The SP motor driver (BD6206FS) drives the three-phase brushless motor based on the
phase signal (SPU-P, SPV-P and SPW-P) from the speed instruction data from the SOC.
The SOC can identify the current speed of the space motor by measuring through the pulse
length of the output (øA, øB) of the slit encoder included in the space motor.
By comparing the target speed for each print mode with the actual current speed to change
the speed instruction data, the motor speed is accelerated or decelerated to maintain the
specified speed for each print mode.

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1/720"
1/120"
SPU
SPV
SPW
PHASE-A
PHASE-B
1/144"
IPT 10 CPI
IPT 12 CPI
IPT 15 CPI
IPT 17 CPI
IPT 20 CPI
1/180"
1/206"
1/240"
(2) Encoder disk
In the operation of the spacing motor, the PHASE-A and PHASE-B signals are generated
when the encoder disk interrupts the photo sensor.
The SOC divides these edge pulse signals in accordance with the print pitch, and sends
the IPT signal to provide dot-on timing and carriage position detection timing.
•UTILITY MODE
LFSTEP-P
LFFOW-N
LFRST-N
LFENB-N
LFMS1-N
LFMS2-N
LFREF-P
SOC Driver
0V 0V
LF Motor
M
+40V
FUSE
SENSE1
SENSE2
OUT1A
OUT1B
OUT2A
OUT2B
3
4
1
2
A
nA
B
nB
(3) LF motor control
The motor that uses it is PM type stepping motor.
The outline circuit composition : as shown in the following figures.
The drive system is a bipolar fixed current drive method.
The LFSTEP-N signal is clock signals, OUT1A, 1B and OUT2A, 2B signals are turned on
synchronizing with the signal, the current flows to the motor each aspect, and the motor rotates.
The method of exciting the aspect by combining signals of LFMS1-N and LFMS2-N is set.
The direction of the rotation is set by the LFFOW-N signal.
The motor drive current value is set by the LFREF-P signal.
The platen rotates for one step 1/432 inches in case of two aspect excitation method and
the motor rotates.

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SOC MAINFANON-P
MAINFANALM-P
FUSE
0V
0V
+12V
FAN
MOTOR
(4) FAN control
The motor that uses it is DC brushless FAN motor.
The outline circuit composition is as follows.
On/off is controlled by control signal MAINFANON-P from SOC.
When the motor stops by some factors when the motor rotates, the FANALM-P signal is
sent from FAN, and this signal is recognized with SOC and it is assumed fan alarm.