Welch Allyn Masted Scales 5002 User manual
Welch Allyn Masted Scales
Service Manual
Model 5002 Mobile Stand-On Scale
Model 5122 Stand-On Scale
Model 5702 Mobile Bariatric Scale
Model 6002 Wheelchair Scales
Model 6702/SP/W Mobile Bariatric Scale
© 2016 Welch Allyn. All rights are reserved. To support the intended use of the product described in this
publication, the purchaser of the product is permitted to copy this publication, for internal distribution only,
from the media provided by Welch Allyn. No other use, reproduction, or distribution of this publication, or any
part of it, is permitted without written permission from Welch Allyn. Welch Allyn assumes no responsibility for
any injury to anyone, or for any illegal or improper use of the product, that may result from failure to use this
product in accordance with the instructions, cautions, warnings, or statement of intended use published in this
manual.
Software in this product is Copyright 2016 Welch Allyn or its vendors. All rights are reserved. The
software is protected by United States of America copyright laws and international treaty provisions
applicable worldwide. Under such laws, the licensee is entitled to use the copy of the software incorporated
with this instrument as intended in the operation of the product in which it is embedded. The software may not
be copied, decompiled, reverse-engineered, disassembled, or otherwise reduced to human-perceivable form.
This is not a sale of the software or any copy of the software; all right, title, and ownership of the software
remain with Welch Allyn or its vendors.
For information about any Welch Allyn product, contact your local Welch Allyn representative:
http://www.welchallyn.com/en/other/contact-us.html
DIR 80021524 Ver. D
Revision date: 2018-01
Welch Allyn, Inc.
4341 State Street Road
Skaneateles Falls, NY 13153 USA
www.welchallyn.com
Customer Support
Placing orders, checking on pricing, billing, or repairs
scale-tronix@welchallyn.com
1.800.535.6663, Option 2, 1, 5
9am - 5pm EST
Mon - Fri
CONTENTS
Scale setup .................................................................................................................................................... 4
Enter custom setup mode ......................................................................................................................... 4
Set the options .......................................................................................................................................... 4
Enter Advanced Service Mode .................................................................................................................. 6
Menu List for Advanced Service Mode ..................................................................................................... 6
Support services ............................................................................................................................................ 7
Warranty ................................................................................................................................................... 7
Repairs ...................................................................................................................................................... 8
Returning products ................................................................................................................................... 8
Warranty service ....................................................................................................................................... 9
Non-warranty service ................................................................................................................................ 9
Technical Descriptions .................................................................................................................................. 9
Scale Technical Description ....................................................................................................................... 9
Printer General Description .................................................................................................................... 15
Maintenance of Scales ................................................................................................................................ 20
Battery replacement ............................................................................................................................... 21
Change the printer paper ........................................................................................................................ 21
Calibration of Scale ................................................................................................................................. 23
Calibration Procedure ............................................................................................................................. 24
Masted Scales Disassembling Guide ........................................................................................................... 26
Required tools: ........................................................................................................................................ 26
5002, 6002, 6702, 6702SP, 6702W MAST Disassembling ....................................................................... 27
5002, 6002, 6702, 6702SP, 6702W Disassembling ................................................................................. 40
5122 Disassembling ................................................................................................................................ 51
Troubleshooting ......................................................................................................................................... 59
Scale Troubleshooting ............................................................................................................................. 59
Printer Troubleshooting .......................................................................................................................... 61
Symptoms and solutions ......................................................................................................................... 63
Repair Parts List .......................................................................................................................................... 72
5002/5702/6002 Base Components ....................................................................................................... 72
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6702/6702W/6702SP Base Components ................................................................................................ 72
6702/6702W Base Components ............................................................................................................. 72
6702SP Base Components ...................................................................................................................... 73
5122 Scale Components .......................................................................................................................... 74
MAST Components for 5002, 5702, 6002, 6702 ..................................................................................... 75
Options .................................................................................................................................................... 75
Schematics & Part Diagrams ....................................................................................................................... 76
Scale setup
You can customize the scale to best suit your needs. Configurations that can be selected
include the automatic shut-off time, weighing units, weight resolution, and more.
Enter custom setup mode
1. Make sure the scale is powered down.
2. Press and hold ST while pressing ON.
3. Once the scale powers up, press and release ST five times. The scale displays SEt-UP and produces a series
of four long beeps.
4. The "RE CALL" pushbutton is used to select the particular option value.
5. Once this value is selected the scale can be advanced to the next option by again pressing the “ST”
pushbutton or it can be shut-off by pressing and holding the "ON" pushbutton.
Set the options
Enter the custom setup as described.
1. Press ST to scroll through the custom setup options.
Note Options indicated with an asterisk (*) have multiple selections.
2. Press RE CALL once a value is displayed to change the value. When the desired value appears, press ST to
set the new value.
3. You can exit the menu at any time by pressing ON to power down the scale. The scale saves all changes.
The options displayed with each press are as follows:
Option displayed Feature
SOFt This option displays the software version of your scale.
dAtE This option displays the release date of the software. The format is MM/DD/YY.
SCALE This option displays the model number of the scale.
AutOFF* This displays the number of seconds before the scale turns off. The Cont value prevents the
scale from turning off automatically.
AC con* This option causes the scale to remain on when plugged in to AC power.
rES* This option allows you to change the resolution of weight. Pressing ST switches between the
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following options:
LBS Kg
0.1 0.05
0.1 0.1
0.2 0.1
0.5 0.2
1.0 0.5
UnitS* This option allows you to change the displayed weight unit. Press RECALL to choose between the following
options:
Note Do not change the scale units if you have purchased the kilogram-only option.
KILOS* This option allows the weight to display in kilograms.
POUNDS* This option allows the weight to display in pounds.
bEEPEr* This option determines the audible signal that occurs when a front panel button is pressed.
rECALL* This option allows you to turn ON or OFF the RECALL button functionality.
rS-232 Detailed instructions for this feature are available from Welch Allyn. Go to
http://www.welchallyn.com/en/other/contact-us.html to find your local representative.
OutPut* This option displays the current data port output option. Detailed instructions for this feature are
available from Welch Allyn. Go to http://www.welchallyn.com/en/other/contact-us.html to find
your local representative.
Set the value to OFF to disable the optional internal printer or the RS-232 port.
PrtUnt* This option determines whether pounds or kilograms are displayed on a printout.
If the pounds and kilograms indicators are illuminated on the control panel, the printed output is
determined by whichever unit is selected on the control panel.
If the kilogram indicator is illuminated, it will only print in kilograms, regardless of the unit is
selected on the control panel.
If the pounds’ indicator is illuminated, it will only print in pounds, regardless of the unit is
selected on the control panel.
PrtOPT* Press RECALL to choose between the following options:
PnlPrt: The current weight is printed when PRINT is pressed on the control panel.
AutPrt: The current weight is automatically printed when a weight reading occurs.
bAud* This option allows you to set the baud rate. Values are 1200, 2400, 4800, 9600, and
19200.
pArity* This option allows you to change the parity between the following options:
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Off: No parity, 8 data bits
Odd: Odd parity, 7 data bits
EuEn: Even parity, 7 data bits
StPbit* This option allows you to change the stop bits between the following options:
1 bit: One stop at the end of a word
2 bits: Two stops at the end of a word
rts.cts* This option allows you to turn On or Off the handshaking hardware for serial transmissions.
Prtbtn* This option allows you to turn On or Off the front panel PRINT button.
rtEPrt* This option allows you to turn On or Off the enabling of printing from a remote location by connecting the
receive line to the ground.
PC brd This option displays the model number of the printed circuit board. A second press of ST
displays the revision of the printed circuit board.
SEt-UP This option is displayed when you have cycled through all the options. Press and hold ON to
power down the scale.
Enter Advanced Service Mode
1. To enter the "service" mode start with the scale turned off.
2. Note the small "ST" logo ("ST") located directly above the "RE CALL" pushbutton, and below the “Zero”
pushbutton on the front panel of the readout.
3. This "ST" contains a small hidden pushbutton.
4. Next; press and hold this "Zero" pushbutton on the front panel while turning the power on with the "ON"
pushbutton.
5. As soon as the scale turns on release the "Zero" pushbutton, then quickly press and release "ST" logo
pushbutton 5 times.
6. The scale will display "Srvice" and produce a series of four long beeps.
7. Service mode has now been entered.
8. Pressing the "ST" pushbutton will advance the display to the next item.
9. The scale will exit the service mode automatically after 3 minutes, or shut-off by pressing and holding the
"ON" pushbutton for 3 or 4 seconds.
10. When the scale is turned back on, normal scale operation will resume.
Menu List for Advanced Service Mode
1. “A-d”, and next window on display will show the raw A/D reading. As you press on the Platform the A/D
reading will chance. The weight chance reading should be positive proportionally with the applied weight
in pounds.
2. “tESt”, and the next window will start displaying the display test, Number will show across from 0 to 9 and
repeat w/ DP, from right side to left side.
3. “Pr-OnS” = Power on start-up, next display will show the number of times the “ON” pushbutton has been
depressed.
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4. “Pr-rcl” = Powered by Recall, next display will show the number of times the “RECALL” pushbutton has
powered-up the scale (from an off state).
5. “COUntS” = Weighing Counts, next display will show the number of times a weight reading has been
achieved.
6. “rEcntS” = Reweigh counts, next display will show the number of times “Reweigh” pushbutton has been
depressed.
7. “rECLLS” = recalls, next display will show the number of times the “RECALL” pushbutton has been
depressed.
8. “OFFS” = Turned Off, next display will show the number of times the scale has been turned off by holding
the “ON” pushbutton down.
9. “PrintS” = Prints, next display will show the number of times the “Print” pushbutton has been depressed.
10. “SEtUPS” = set-up mode, next display will show the number of times the unit has entered into the set-up
mode.
11. “SEruES” = service mode, next display will show the number of times the unit has entered into the service
mode.
12. “CALS” = Cal mode, next display will show the number of times the unit has entered into the Cal mode.
Support services
If you have a problem with the device that you cannot resolve, call the Welch Allyn Technical Support Center
nearest you for assistance. A representative will assist you in troubleshooting the problem and will make every
effort to solve the problem over the phone, potentially avoiding an unnecessary return.
Technical support is available 9am-5pm EST.
Welch Allyn offers the following technical support services:
• Telephone support
• Replacement service parts
• Product service
For information on any of these services, go to www.welchallyn.com/en/servicesupport.html.
Warranty
Welch Allyn will warranty the weight scale to be free of defects in material and workmanship and to perform in
accordance with manufacturer specifications for the period of one year from the date of retail purchase.
The warranty period shall start on the date of purchase. The date of purchase is:
the invoiced ship date if the device was purchased directly from Welch Allyn
the date specified during product registration
the date of purchase of the product from a Welch Allyn authorized distributor as documented from a
receipt from said distributor.
This warranty does NOT cover damages caused by misuse or abuse, including but not limited to:
• Failure caused by unauthorized repairs or modifications
• Damage caused by shock or dropping during transportation
• Damage caused by improper use of the power supply
• Failure caused by improper operation not consistent with the instructions stated in
this Directions for use
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Should this device require maintenance (or replacement at our option) under warranty, contact your local Welch
Allyn representative: http://www.welchallyn.com/en/other/contact-us.html
Repairs
A Welch Allyn Service Center or Authorized Service Provider must perform all repairs on products under warranty
unless you are a properly trained technician.
CAUTION Unauthorized repairs will void the product warranty.
Qualified service personnel or a Welch Allyn Service Center should repair products out of warranty.
If you are advised to return a product to Welch Allyn for repair or routine maintenance, schedule the repair with
the service center nearest you.
Returning products
When returning a product to Welch Allyn for service, ensure that you have the following information:
• Product name, model number, and serial number. This information may be found on
the product and serial number labels.
• A complete return shipping address.
• A contact name and phone number.
• Any special shipping instructions.
• A purchase-order number or credit-card number if the product is not covered by a
warranty.
• A full description of the problem or service request.
1. Obtain an RMA number. Contact Welch Allyn and request.
Note: Welch Allyn does not accept returned products without an RMA.
2. Ship the device to Welch Allyn, observing these packing guidelines:
Remove from the device the battery, all hoses, connectors, cables, sensors, power cords, and other ancillary
products and equipment, except those items that might be associated with the problem.
Dispose of damaged or leaking batteries in an environmentally safe manner consistent with local regulations.
Note: To ensure safe receipt of your device by the service center and to expedite processing and return of the
device to you, thoroughly clean all residue from the device before you ship it to Welch Allyn. For cleaning
requirements, see the Cleaning instruction in the Directions for Use.
Welch Allyn thoroughly cleans all returned devices on receipt, but any device that cannot be adequately cleaned
cannot be repaired.
3. Write the Welch Allyn RMA number with the Welch Allyn address on the outside of the shipping carton.
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Warranty service
All repairs on products under warranty must be performed or approved by Welch Allyn.
Refer all warranty service to Welch Allyn Product Service or another authorized Welch Allyn Service Center.
Obtain a Return Material Authorization (RMA) number for all returns to Welch Allyn Product Service.
CAUTION Unauthorized repairs will void the product warranty.
Non-warranty service
Welch Allyn Product Service Centers and Authorized Service Providers support non-warranty repairs. Contact any
Welch Allyn regional service center for pricing and service options.
Welch Allyn offers modular repair parts for sale to support non-warranty service. This service must be performed
only by qualified end-user biomedical/clinical engineers.
Technical Descriptions
Scale Technical Description
Welch Allyn Scales utilize the latest developments in electronic scales and microcomputer technology to provide
weighing scales, engineered to be easy to use. This section describes the technical aspects of these scales.
LOAD CELL TRANSDUCERS
The function of the load cell transducer is to convert the weight applied to the weighing platform into an electrical
signal for further processing and subsequent display by the readout. Welch Allyn uses proprietarily designed load
cells in most of the scales to optimize performance and reliability. One of three types of load cell transducers may
be used in your Welch Allyn Scale depending on the model number.
MODELS 5002, 5702, 6002
For many years these scales utilized a configuration called a “gauged-plate”. It is a rectangular, flat aluminum plate
with a bending element in each corner. The bending elements are an integral part of the aluminum plate formed by
cutting relief areas in the four corners. Each bending element has two resistive strain gauges bonded to it in order
to form an electrical “half-bridge” configuration. The four bending elements are further interconnected to form an
equivalent “Wheatstone bridge”. Additional compensating resistors are contained with the gauged-plate’s wiring
to correct for temperature effects.
It is called a “gauged-plate” because it is a flat plate with resistive strain gauges installed on beams that have been
machined out in the corners. The beams were mechanically tuned and matched with small files. This is a unique
and special large flat load cell that allowed us to make a very low platform that is easy for unstable patients to step
up on. It has served us well for over 35 years. We then invented the planar wingback bending beam with four strain
gauges (full bridge).
This “gauged-plate” has been replaced in most scales with four discrete planar beam type load cells designed to
accomplish the same performance as the single gauged plate. This was done in order to maintain costs. A further
advantage is that a single load cell may now be replaced if necessary rather than the complete set of four. These
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beams are electronically tuned and matched with precision resistors. Thus it is possible to replace one load cell
without having to recalibrate the scale—another Welch Allyn unique feature.
Use of four bending elements or matched load cells sums the total weight applied to the weighing platform and
produces an accurate result regardless of the patient’s position on the platform.
READOUT ELECTRONICS
The Welch Allyn second generation scales employ the model 23005 computer instrument board and the 22DSDP
display board. NOTE: Different revision levels of these boards may be incorporated in your scale depending on its
manufacturing date. Differences between these boards will be noted in the description where applicable.
Readout electronics consist of the following:
Differential signal amplification.
Additional amplification and signal filtering.
Analog-to-Digital (A/D) converter and Clock circuit.
Battery and support circuitry, voltage regulators, power supplies, etc.
Microcomputer and support circuitry.
Display board.
Printer assembly.
Printer interface board.
Printer controller board.
DIFFERENTIAL SIGNAL AMPLIFICATION
The weight dependent output signal produced by the load cell transducers in the weighing frame is a “differential
signal”, meaning it is the voltage difference between the “+ Signal” and “- Signal” leads. Integrated circuit U4, an
instrumentation amplifier, is used to interface to this differential signal and amplify it.
The output signal from the load cells is applied to the protection network consisting of diodes CR4/CR5/CR6/CR7.
These diodes prevent destructive high voltages caused by static discharges from damaging U4. A high frequency
filter, formed by L1/L2/C9, C10 couples the weight signal to the input of U4. In U4 the differential signal is amplified
by a factor of 100, and converted to a “ground-referenced” voltage for further processing.
Capacitors C16/C17/C24 provide local bypassing of the power supplies used by instrumentation amplifier U4.
Capacitor C18 furnished compensation of U4 by reducing amplification at higher frequencies.
ADDITIONAL AMPLIFICATION AND SIGNAL FILTERING
Components for an optional second gain stage may be added on the printed circuit board. If included, operational
amplifier U5 is used to provide additional gain and signal filtering. U5, together with capacitors C14/C15 and
resistors R17/R18, forms an active low-pass filter. These components help to remove fluctuations in the weight
signal caused by movement of the patient on the scale. U5, like U4, is “chopper-stabilized” to correct internal offset
and drift errors.
Resistors R24/R25 (optional) are used to increase gain; resistors R26/R27 (optional) are used to gain reduction. An
additional low-pass filter stage furnished by resistor R34 and capacitor C23.
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ANALOG-TO-DIGITAL (A/D) CONVERSION
Integrated circuit U6 is the analog-to-digital converter. Included on this integrated circuit are auto-zero functions,
auto-polarity, and the digital and analog functions necessary to perform dual slope integration conversion to 20,000
counts (4-1/2 digits). The weight signal voltage is applied to the analog input (pin 10) of U6.
A reference voltage for the conversions is applied to pin 2 of U6. The reference voltage, nominally 1 Volt, is derived
from the load cell transducer excitation voltage, by the divider network consisting of resistors R29, R30 and
potentiometer P1. Adjusting p1 sets the “span” or weight calibration of the scale.
The system clock, applied at pin 22 of U6, is used to precisely time and control the phases of the dual slope
conversion process. Refer to the converter time diagram when reading the following description.
A/D CONVERTER TIMING DIAGRAM PHASE 1, AUTO ZERO
During auto zero, the errors in the analog components (offset voltages of buffers, comparators, etc.) will be
automatically nulled out. This action is performed by internal logic that disconnects the input pins (9 & 10) from the
applied analog signal, connects them to ground, then closes an internal feedback loop such that offset error
information is stored in the “auto zero” capacitor, C21. Also during this phase, “reference capacitor” C22 is charged
to the voltage present on “Vref” (pin 2 of U6).
PHASE 2, SIGNAL INTEGRATE
The input signal is reconnected and then integrated for exactly 10,000 clock pulses. On completion of the integration
period, the voltage V is directly proportional to the input voltage, corresponding to the weight applied to the scale.
Capacitor C20 is the integration capacitor, with resistor R32 setting the integration current. At the end of this phase
the input signal polarity is determined.
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PHASE 3, REF. INTEGRATE, SIGNAL DE-INTEGRATE
The input to the integrator is switched from the input signal to reference capacitor C22. Internal switches connect
capacitor C22 to the integrator input so that its polarity is opposite that of the previously applied input signal. This
causes the integrator to discharge back towards zero. The number of clock pulses counted between the beginning
of this cycle and the time when the integrator output passes through zero is a digital measure of the magnitude of
the input signal. This count is stored in an internal latch on U6 for output to the microcomputer.
ZERO INTEGRATOR PHASE
One minor additional phase is included to insure that the integration capacitor C20 is fully discharged to zero volts.
This typically lasts 100-200 counts.
CLOCK CIRCUIT
A clock is required for the A/D converter, integrated circuit U6.
23005 R01 board: Integrated circuit U2 is a dual timer circuit. One half (pins 8, 9, 12, and 13) is connected as an
astable oscillator and used to generate a system clock for the A/D converter. Resistors R6, R7 and capacitor C2 set
the clock frequency to approximately 150 KHZ.
23005 R02 board: A clock signal is generated internally in microcomputer U7 and appears on port pin “P1.0”. The
frequency is internally set by the microcomputer’s software and is nominally 120 KHz.
POWER SWITCHING, VOLTAGE REGULATION AND SUPPORT CIRCUITRY
Depending upon their configuration, WELCH ALLYN second generation scales may contain disposable primary cells.
(Note that some models may be specially ordered to utilize a different power supply than is normally provided; refer
to the technical description that best matches the particular scale.) Additional circuitry is included to switch the
battery supply, provide voltage regulation and detect low battery voltage conditions.
BATTERY SWITCHING
In order to conserve battery life, the battery supply is switched on and off as needed by the scale.
Main board: Transistor Q1 is a series switch which applies battery voltage to the remainder of the circuitry. Q1 is
controlled by transistor Q2 which, in turn, is controlled by “watchdog timer” circuit U11.
To initiate power-on Q1 is turned on through momentary closure of membrane switch S9 (“ON”) and diode CR10;
diode CR13 is used to signal input pin “PB.7” of port expander U7 that the “ON” switch is pressed.
A secondary turn-on circuit occurs through diode CR12 and switch S3 (“RE CALL”) to allow display of the previously
stored weight if the scale is presently turned “off”. The switch closure is also coupled through diode CR14 to signal
input pin “PB.1” of port expander U7 that the “RE CALL” switch is pressed.
Once Q1 is on and voltage is applied to the circuit, watchdog timer U11 will keep transistor Q2 on through output
line /WDO and resistor R5, subsequently keeping transistor Q1 energized. If no further action occurs, an internal
timer contained within watchdog timer U11 will time-out after approximately 1.6 seconds and switch off Q2, causing
Q1 to turn off and remove power from the scale’s circuity.
Once energized and properly running, microcomputer U10 will keep resetting watchdog timer U11 by periodically
pulsing U11’s input line, labeled “WDI”. Should the scale’s operating program call for shut-off, or a
hardware/software failure of microcomputer U10 occurs, the reset pluses to U11 will no longer occur and 1.6
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seconds later U11 will time-out and cause the circuit power to switch off. The model 5102 has a jumper added at
location J7; this causes U11 to reset U10 in the event of a hardware/software failure.
Resistors R1, R2, R3, R5 and R14 are included for proper circuit biasing. Capacitor C6 is used as an output filter.
VOLTAGE REGULATION
Voltage regulators VR1 and VR2 render regulated sources of +5 Volts DC for operation of the analog (VAA) and digital
(VCC) circuits, respectively. Use of two separate +5V regulators helps to prevent noisy digital signals from entering
the sensitive analog circuits. Capacitors C3 and C8 are used to insure regulator stability.
+9.5V/-7.5V SUPPLY
Integrated circuit U2 is used to convert +5 Volts DC to +9.5V and -7.5 Volts DC for use in the analog circuits. It contains
an internal oscillator (operating at approximately 8 KHz) and a series of switches. During one half of the cycle
capacitor C25 is connected between VAA and ground, charging C25 to VAA’s potential of +5 Volts. During the other
half cycle capacitor C25 is reconnected between the VAA and pin 8 (negative lead of C25 to VAA) so that its voltage
adds to VAA and charges filter capacitor C26 to approximately twice VAA or 9.5 to 10 volts.
The remainder of U2 is used to generate a negative supply voltage. Capacitor C28 is connected between ground and
the +9.5 Volt source on pin 8 during one half cycle of the internal oscillator. During the other half cycle, it is
reconnected between ground and pin 4 such that its negative lead is connected to pin 4. This produces a negative
voltage. Diodes Cr8 and CR9 reduce the voltage slightly to obtain the desired -7.5 volts.
BATTERY MONITOR
Integrated circuit U3 is included to monitor the voltage of the battery and provide an indication to the scale’s
operator when battery replacement is required. Two states of weak battery operation are detected; “low-battery”
(battery is usable but will soon need replacing” and “low-low battery” (battery is too weak to properly operate the
scale.)
Pins 1, 2 and 3 of U3 are connected to a voltage divider network consisting of R8, R9 and R10, to form the “lowbattery” detector. The output of this circuit (pin 1) is normally low when the battery is good and switches high when
the battery is low. It is coupled to the busy signal of U6 (pin 21) through resistor R36 to the driver for the front panel
“LO BATT” indicator. This causes the “LO BATT” indicator to flash when the battery is low.
The remaining half of U3, pins 5, 6 and 7, are connected to resistors R11, R12 and R13 to form the “low-low” battery”
detector. The output on pin 7, which is normally “high” with a good battery, goes “low” when the battery is too
weak to reliably operate the scale. This output is connected to an I/O pin on microcomputer U10 to signal the
microcomputer the “low-low” battery has been recognized. The microcomputer U10 will process the “low-low”
battery signal and cause “bAttrY” to appear on the scale’s front panel display, in addition to the flashing “LO BATT”
annunciator.
MICROCOMPUTER AND SUPPORT CIRCUITS
To attain various additional features such as automatic zero tare, pounds/kilograms conversion, weight lock-in,
previous weight memory, etc., a microcomputer is employed to additionally process the data supplied by the A/D
converter. This microcomputer system consists of U10, a microcomputer; U7, a peripheral port expander to furnish
additional input/output lines; U9, a non-volatile memory which stores the previous weight reading; and U11 (23005
R02 only), a device to generate reset conditions for the microcomputer.
During operation of the scale the microcomputer continually receives the weight readings from the A/D converter.
This data is received in a “multiplexed” format (one digit at a time) from the output of the A/D converter
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(microcomputer input lines P1.0 through P1.6). The microcomputer also continually scans the keyboard (using U7)
looking for closed switches. If a key press is sensed, the microcomputer executes whatever action is called for in its
program. After processing the A/D data, the microcomputer assembles it for viewing and transfers it to the front
panel display.
U10 is a complete microcomputer, containing a software program stored in read-only memory, read/write memory
for temporary storage of program variables, an arithmetic logic unit, input/output and other control lines, etc.
Crystal XTAL1 and capacitors C29/C30 form the clock oscillator which controls the internal timing of the
microcomputer.
PORT EXPANDER/KEYBOARD/BEEPER
Integrated circuit U7 is included to supplement the I/O (input/output) of the microcomputer system. U7 contains
additional I/O lines (referred to as “ports”). Microcomputer U10 reads (from input lines) or writes (to output lines)
data to U7 periodically by use of the data bus (lines DB0 through DB7) and the /RD and /WR lines.
The front panel keyboard is attached to some of U7’s input port pins (PB.0 through PB.7, and PC.4). Resistor
networks R42/R43 serve as “pull-ups” and keep the input pins at a “high” state (+5V) until a key switch is pressed;
this pulls the respective input pin “low” (0V). The microcomputer will recognize this key press when it reads the
input pins from U7 and if the key remains closed for number of milliseconds the microcomputer will execute that
key’s function.
A four position “DIP” (dual in-line package) switch, SW1, may be optionally included. It is connected to U7’s “C”
port, lines PC.0 through PC.3. It is used to select software contained in U7.
A small audio annunciator is driven by transistor Q3, which in turn is controlled by U7’s output pin PA.7. The
annunciator gives a short beep as audible recognition of a key being pressed. The length of the beep and its various
sequences are controlled by U10’s software. Additional output lines of U7 are used to control the driver for
annunciator lamps on the display.
NON-VOLATILE MEMORY
The internal memory of microcomputer U10 does not retain data when the power is switched off. Because some
features of the scale may require lasting data retention (such as last weight recall) integrated circuit U is included.
This device, called an “electrically erasable programmable read only memory”, or “EEPROM” will store selected
information for periods of up to 100 years.
Information needed to be stored to or retrieved from U is sent in serial form using the lines SCL (serial clock) and
SDA (serial data). These are controlled by microcomputer U10. A data bit (a high or low level) is sent and received
on SDA when the SCL line provides a pulse.
Resistors R54/R55 are provided as pull-ups on the SCL/SDA lines to insure the data and clock pulses are properly
shaped. Capacitor C36 improves power supply bypassing.
RESET GENERATION
In order for microcomputer U10 to properly execute its software instructions it must be initialized to the start of the
program when power is first turned on. Reset pin 9 of U10 will accomplish this when it is set “high”.
Main board: A reset pulse of approximately 200 mS is automatically generated by “watchdog timer” U11 when the
Vcc level rises above 4.65 volts. If Vcc is below 4.65 volts, the reset line stays “high”, keeping the microcomputer
U10 in an inactive state. The reset pulse is also connected to port expander U7.
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DISPLAY BOARD
Presentation of the weight information is performed by the model 700027W display board. It incorporates LED
(light-emitting diode) digits and annunciators to provide a clear, bright, easy-to-read display.
The weight value is displayed on six, 0.43” high common cathode digits. These are driven in a multiplexed fashion
(one digit on at a time) by LED driver U1. U1 receives the digit display information from the microcomputer’s parallel
data bus (DB0-DB7) and automatically performs the multiplexing function. Resistor package RP1 sets the operating
current level for the displays.
A variety of LED annunciator lamps are contained on the front panel to indicate “POUNDS”, or “KILOGRAMS”,
“PRIOR WEIGHT”, and “LO BATT”. These annunciator lamps contain multiple LED’s to provide an evenly illuminated
surface. They are driven by integrated circuit U2. U2 receives the on/off information for the annunciators from
microcomputer U10 via port expander circuit U7. Resistor packs RP2 and RP3 provide current limiting for the
annunciators. Display boards 22DSP R01, 22DSP R02 and 22DSP R03 are all equivalent in operation with some minor
changes in board layout to improve spacing. Some minor differences in connections to the terminals of the resistor
networks were also made to improve spacing.
Printer General Description
SETUP
The optional printer assembly will normally be installed at the factory when your scale is produced and require no
further attention during initial set-up.
PRINTER
On various model scales a digital paper tape printer can be supplied. The printer provides a convenient printed
record of the weight. Space is provided on the tape printout to write in the patient's name or ID, Room number,
and date. See the sample weight ticket printed below:
The printer will only print a valid weight. It will not print when the scale is reading "0.0" or is empty. It will Print the
prior weight if that feature is currently activated. Press the front panel "PRINT" pushbutton to activate the printer.
A single beep should be heard, then the printer will power-up and begin printing. A series of four (4) short beeps
signifies that an invalid print request was made (weight not displayed on scale or a weight ticket already being
printed). One long beep signals an internal problem with the printer, such as an out of paper condition, open print
head or disconnected cable.
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Printing may occur after the patient has left the scale. To print the previous patient's weight, press the "RE CALL"
pushbutton followed by the "PRINT" pushbutton. The "PRIOR WEIGHT" annunciator will flash, and the scale's
readout will display the previous patient's weight for a short time while the printing continues. The scale will not
automatically shut-off until the printing is completed.
Various set-up options are available for the printer. These include "auto" print (weight is printed automatically on
display), print only in selected units regardless of front panel display, etc. See the section pertaining to "SET-UP" in
your scale manual for further information.
The printer is a modern thermal type using a specially coated paper. The paper should be 2¼" wide by
approximately 1 7/8" in diameter. One roll of paper will produce about 500 weight tickets. No ink ribbons are
required.
Paper is loaded through a removable access door on most scale models. Be sure to install the paper as shown in
the illustration.
PRINTER TECHNICAL DESCRIPTION GENERAL
This internal printer employs a thermal type print mechanism, which heats a specially coated paper to produce the
resulting weight ticket. The use of a thermal printing mechanism eliminates the need for ribbons or ink cartridges,
and provides quiet, maintenance-free operation. The total printing system consists of two assemblies:
1.) Printer mechanism assembly with stepper motor and thermal print head
2.) Printer interface board assembly
PRINTER MECHANISM:
The printer mechanism employs a thermal print head consisting of a horizontal row of 384 small heater resistors.
The thermal print head also contains driver electronics that receive the formatted print data and control the 384
heater resistors.
When any of the heater resistors are momentarily energized (powered for about 3 milliseconds) it will produce a
small rectangular black dot on the thermal paper. Each printed character composition is 10 dots wide by 24 dots
high. The paper width allows a maximum of 24 characters per line, which includes spaces between characters and
paper edge margins. Printed characters are formed as a series of
horizontal lines. A stepper motor in the printer mechanism advances the paper after printing a horizontal line of
dots, to print the next line of dots, or to advance the paper on blank lines. The printer mechanism also contains an
optical paper sensor consisting of an infrared light emitting diode and matching phototransistor. Light from the
diode is reflected off the surface of the paper and sensed by the phototransistor.
An optional switch is also included to sense when the thermal print head is in the open, paper loading position.
The printer mechanism is serviced as a complete assembly and is generally not repairable.
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PRINTER INTERFACE ASSEMBLY
The printer interface assembly is used to control the printer mechanism. It contains all the circuitry needed to
interface to the printer mechanism, scale’s electronics, and battery/power supply. For technical explanation
purposes it can be divided up into the following sections:
1.) Printer voltage regulation
2.) Microcontroller & support circuitry
3.) Interface to scale electronics
4.) Interface to thermal print head
5.) Interface to stepper motor
6.) Thermal print head temperature measurement
7.) Paper sensing and head position.
8.) Dip switch options
PRINTER VOLTAGE REGULATION
The printer mechanism and its electronics require a source of +5Vdc for operation. To increase battery life, the
printer is only powered up when needed. Integrated circuit voltage regulator U201, an LTC1963, is
employed to regulate the varying battery voltage to a constant +5V dc.
Input voltage to U201 is applied at connector J201. U201’s output voltage is set by the resistor divider R210 and
R211. U201 has internal current limiting and reverse battery protection, and has a switchable output controlled by
an on/off terminal. Capacitors C201, C202, C205, and C206 provide filtering and insure regulator stability.
Additional capacitors C207 and C210 are included to provide distributed filtering.
A series transistor switch network consisting of Q202 and Q203 is used turn the U201 regulator on when printer
operation is required. Transistor Q202 is turned on when the scale’s instrument board powers up and the RXD
raises to a “high”; it remains on the entire time that the scale is on. Transistor Q203 is turned on when the scale’s
instrument board sets the RTS line “high”, it is activated only when printing is desired.
Resistors R204, R205, R206, and R207 bias Q202 and Q203. Capacitors C203 and C204 are included to form time
delays to prevent the printer electronics from inadvertently turning on as the scale powers on or off. R209 is
included to keep U201’s on/off terminal at ground potential until switched on.
An additional circuit to turn the printer on is the manual paper advance switch S201. One set of S201’s contacts is
used to apply battery voltage to U201’s on/off terminal through resistor R208. The other set of S201’s contacts is
connected to microcontroller U202 to indicate that a paper advance is requested. On the 500057-B printed circuit
boards, R220 is included to provide additional pull-up bias.
PRINTER MICROCONTROLLER & SUPPORT CIRCUITRY
Microcontroller U202 is included to perform all the data manipulation and timing required to make the printer
mechanism work. It contains RAM (random access memory), a flash ROM (read only memory) with program
software, I/O (input/output) ports, a UART (universal asynchronous receiver/transmitter)
and the needed arithmetic logic, clock generating, and associated internal logic.
U202’s internal software is loaded into the device through the ISP (in-circuit serial programming) port at J205
during manufacture.
A clock circuit for U202 is formed from crystal XTAL201 along with capacitors C208 and C209.
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Reset supervisor U205 is provided to initialize microcontroller U202 on power up. U205 measures the circuit
power supply and keeps U202’s reset line high until the proper operating voltage has been reached. C212
determines the width of U205’s reset pulse. R218 keeps U205’s reset input biased high until pulled low for in
circuit programming.
An 8-position dip switch assembly is attached to one port of U202 to allow operating options to be set.
D202, a bi-color (red/green) LED is used to indicate printer status. Internal software in U202 determines the
operation of the LED. Resistor R212 sets the operating current for D202.
PRINTER INTERFACE TO SCALE ELECTRONICS
The printer electronics are powered “on” when the scale’s electronics set the “RTS” line high on J202. The “RXD”
will have been set high prior to the RTS line. This gates on the series transistor network of Q202 and Q203, turning
on voltage regulator U201.
Data to be printed is transmitted to the printer through J202’s “TXD” line. Data is sent in ASCII serial format at
9600 bauds. A data bit is about 104uS in length. Word length is 8 bits, no parity, at TTL levels (0 to 5V). Data is
buffered by sections of U206, a Schmitt trigger inverter. Resistors R201 and R202 are included for protection and
circuit bias.
Printing begins once U202 receives a carriage return (“CR”, ASCII 13) byte. While busy printing U202 will halt data
transmission between the scale and printer by setting the “CTS” (clear to send) line of J202 high. The scale’s
electronics will recognize this and stop transmission until CTS goes low again. Diode D201, resistor R203 and
sections of U206 are used to interface to the CTS line.
PRINTER INTERFACE TO THERMAL PRINT HEAD
Microcontroller U202 takes the character print data (letter, number, etc.) from the scale and then converts it into
the appropriate dot pattern to form letters and numbers on the paper. This is a complex process executed by
U202’s internal software. Dots that need to be printed on the paper are set to a “1” level; blank dots are set to a
“0” level.
Once converted, the resulting dot data string is transmitted in serial format through the “DI” (data in) line as the
“CLK” (serial clock) line is pulsed. Once all 384 bits (1 per dot) have been sent, the “/LAT” (latch) line is pulsed to
load the data into the thermal print head. The process is similar to manipulating a digital
logic shift register. The thermal print head contains the appropriate logic interface and current drivers to operate
the heater resistors.
Because of the current required per dot (about 35mA) only a limited number of dots can be energized at one time
to avoid overloading the scale’s power supply. This is accomplished by dividing the thermal head’s heater resistors
into segments by use of the STB1, STB2, STB3, and STB4 (strobe 1, strobe 2, etc.) lines.
Note that STB2 and STB3 are joined together to divide the heater resistors into three, 128 heater resistor
segments. U202 also controls the maximum number of heater resistors energized by assemblies using limiting the
number of “on” dots in the dot data string sent to the thermal head.
With the dot data now loaded into the thermal print head, microcontroller U202 controls the length of time the
heater resistors are on, typically around 3-4 mS, to form the dot image on the paper. Microcontroller U202
calculates this based on the temperature of the thermal print head, which is measured by a thermistor embedded
in the head. U202 will pulse mosfet transistor Q201 to provide power for the heater resistors to match the interval
required. This is repeated as necessary with the various STB sections until all the required dots on a single line are
formed.
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Resistors contained in RN201 provide pull-up bias on the various logic interface lines to insure a fast transition.
PRINTER INTERFACE TO STEPPER MOTOR
The printer mechanism contains a stepper motor to advance the paper during printing and paper feed. The
stepper motor is driven by a separate IC designed for that purpose, U203, and is connected to lines OUT1, OUT2,
OUT3, and OUT4. The stepper motor requires various combinations of high and low logic signals from U203 in
order to rotate.
U202’s operating software generates these combinations. A section of Schmitt trigger inverter U206 is used to
enable U203 and insure the stepper motor is off while the microcontroller is reset.
To provide manual paper advance for paper loading, switch S201 is included. One section of S201 is used to turn
on regulator U201 and provide circuit power; the other section is connected to microcontroller to U202 to request
a paper advance. U202 will then provide the correct signals to U203 to operate the stepper motor and advance the
paper.
PRINTER THERMAL PRINT HEAD TEMPERATURE MEASUREMENT
The temperature of the thermal print is measured to determine the correct “on” timing of the heater resistors.
An NTC thermistor is embedded in the thermal print head and changes its resistance based on the print head’s
temperature. U204, a 10 bit (1 part in 1024) A/D (analog to digital) converter is included to measure the voltage
developed by the divider formed of R217 and the thermistor connected at “TM”.
U204 converts the thermistor voltage from an analog value to a digital number, and passes it to microcomputer
U202.
Assemblies employing the 500057-A printed circuit board: The converted temperature value is passed to
microcontroller U202 in I2C serial format via use of data (SDA) and clock (SCL) lines. Resistor R219 and
capacitor C211 provide filtering of the temperature signal. Resistors contained in RN201 provide pull-up bias for
the SDA and SCL lines.
Assemblies employing the 500057-B printed circuit board: The converted temperature value is passed to
microcontroller U202 in SPI serial format via use of data (Dout) and clock (CLK) lines. When U204’s chip selects
(/CS) line is pulled low by U202 the conversion process begins. Capacitor C211 provides filtering of the
temperature signal.
Resistors contained in RN201, along with resistor R220, provide pull-up bias for the Dout, CLK, and /CS lines.
PRINTER PAPER SENSING AND HEAD POSITION
The printer mechanism contains an optical paper sensor consisting of an infrared diode emitter and a
complimentary phototransistor. Light from the infrared emitter reflects off the paper and is detected by the
phototransistor at connector J207. Microcontroller U202 uses this signal to determine the presence or
absence of paper and take action accordingly. The infrared emitter is pulsed on by U202 to conserve battery and
emitter life.
One section of U206 is used to process the detected signal before application to U202. Resistor R214 sets the
operating current of the emitter. Resistor R215 provides the phototransistor load.
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A snap action switch is incorporated within the printer mechanism to determine whether the thermal print head is
open for paper insertion or closed for printing. Resistor R206 is included for circuit protection. A
section of S202 parallels this switch so that mechanisms not equipped with this switch will operate properly.
PRINTER DIP SWITCH OPTIONS
An 8-position dip switch is included to allow troubleshooting and some adjustments to operating parameters. Note
that the settings below may vary slightly depending on the current software revision.
For normal default operation all switches should be “OFF”. Use a fine tip ballpoint pen to actuate the switches as
follows:
S202-1: Puts printer in test mode. When “ON” and the paper advance switch is pressed, the printer will display the
software version, date, and other dip switch options. If the paper advance is held long enough the printer will
produce a variety of printable characters. When S202-1 is “OFF” paper advance operation is normal.
S202-2: Sets maximum number of simultaneous dots to be printed. With S202-2 “OFF” the maximum number of
dots is 12. With S202-2 “ON” the maximum number of dots is 30. While increasing the maximum number of
simultaneous dots produces faster print speeds, it also increases power
consumption and reduces battery life. S202–2 “ON” should only be used with scales that have rechargeable
battery packs.
S202-3 & S202-4: Increases resistor heater on time to improve printing contrast. May need to be adjusted
depending on paper type. Adjustment as follows
S202-3 “OFF” & S202-4 “OFF” : 100%
S202-3 “OFF” & S202-4 “ON” : 115%
S202-3 “ON” & S202-4 “OFF” : 133%
S202-3 “ON” & S202-4 “ON” : 150%
Note that increasing the on time will slow the printing slightly and decrease battery life. Setting should be at the
minimum consistent with good print results.
S202-5: Bypasses head temperature sensor circuitry and provides fixed resistor heater pulse width. Used for
troubleshooting only.
S202-6 & S202-7: Not used
S202-8: Bypasses thermal head position switch. Used for printer mechanisms without head position switch or for
troubleshooting.
Maintenance of Scales
Routinely perform the following preventive maintenance to keep your scale in working
order.
1. Check the calibration annually
2. Inspect the scale for cracks or loose mounting hardware. Replace or repair as necessary.
3. Visually inspect the scale enclosure for damage or loose or missing hardware.
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4. Replace or repair as necessary.
5. If equipped, inspect the AC line cord for abrasions or other signs of wear.
6. Do not expose the scale to excessive water or moisture.
7. Do not store the scale where heavy objects can be placed on it.
8. Replace the batteries annually or as required.
9. Do not service or perform maintenance while the scale is in use with a patient.
Battery replacement
CAUTION Use only size D disposable alkaline batteries.
1. Using a screwdriver, remove the two fasteners on the access door located on the rear of the mast and remove
the door.
2. Install six new batteries in the battery holder. Make sure to follow the polarity instructions.
3. Re-attach the access door.
Change the printer paper
1. Use a screwdriver to loosen the screw on the printer access door.
2. Remove the spindle containing the empty printer paper roll from the printer paper well.
3. Replace the printer paper roll in the spindle. Make sure the new printer paper is advancing from the bottom of
the roll.
4. Lift the green knob to lift the print head lever.
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5. Insert the paper through the paper guide slot, and make sure it goes under and around the black roller bar.
6. Pull the paper under the print head lever and push the green knob down to push the print head lever down.
7. Press PAPER ADVANCE to move the paper through the printer.
8. Install the printer access door, and advance the new paper through the slot on the door.
9. Install the screw on the printer access door
Note: Never pull or force paper through the printer mechanism with the head lever in the "down" (or closed)
position. Never insert any object besides paper into the printer head assembly. Damage to the print head can
occur.
If problems occur with the printer, check the condition of the printer status light located on the printer assembly.
Press the paper advance switch to power up the printer and activate the lamp.
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Calibration of Scale
Your scale has been carefully calibrated at the factory. This calibration involves matching and tuning of the load cells
and readout electronics. The scale calibration should be checked annually. Only use calibrated, certified test weights
for this purpose. Traction or physical therapy weights are NOT acceptable since their actual weight can often be in
error as much as +/-10%. Calibration weights may be purchased from WELCH ALLYN or a local scale dealer. An
alternative to calibration weights is the weight comparison method. This requires a known accurate, calibrated
scale. A fixed weight is “weighed” on the calibrated scale then the same weight is placed on the scale for comparison.
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TEST CALIBRATION WEIGHTS ARE AVAILABLE FROM WELCH ALLYN.
THREE (3) 25 KILOGRAM TEST WEIGHTS ARE RECOMMENDED.
ORDER PART NO. 20021W. (25 KG TEST WEIGHTS)
Large changes in calibration often indicate a damaged load cell or faulty readout component. It is generally
recommended that if calibration is necessary for your scale it should be returned to the factory. Calibration
procedure follows for those situations where it is not desirable. Calibration should not be attempted by those not
having the proper tools or knowledge of electronic systems and their attendant shock hazards.
Calibration Procedure
Enter the “calibration mode” by following exactly the procedure outlined below:
1. Be sure scale is off.
2. Press and hold in the “RE WEIGH” pushbutton.
3. While pressing the “RE WEIGH” pushbutton, press and release the “ON” pushbutton. On scales that do not
employ the “ON” pushbutton, such as the model 5102, turn the power rocker switch on while pressing “RE
WEIGH”.
4. NOTE: The “ST” pushbutton is a special hidden programming and test pushbutton located under the WELCH
ALLYN logo (“ST”) on the left side of the front panel between the “ZERO” and “RE CALL” pushbuttons. After
the scale displays the test pattern of “888888” release the “RE WEIGH” button and press the “ST”
pushbutton five (5) times. This will cause the readout to enter the calibration mode. The display will
indicate “CAL”.
5. Press the “ST” pushbutton once more; the display will indicate “A-d”. This indicates the start of the “raw”
analog-to-digital converter data being inputted to the microprocessor.
6. Press the “ST” pushbutton one more time. The number displayed is now the raw analog to digital data.
7. The automatic turn-off timer has also been programmed for an extended “on” period to give you time to
calibrate the scale. This time period is three minutes. The scale may be turned off before this time period
by simply pressing and holding the “ON” pushbutton. Hold it in for several seconds until the power shuts
off. If additional time is needed to complete the calibration procedure, press the “ON” pushbutton briefly.
This will reset the timer for an additional three minutes.
The readout is displaying a number, which represents the zero offset value of the platform and load cell transducers,
in tenths of pounds (0.1 pound). Note and record this value. (Even though your scale may be “kilograms only” in
operation, pounds are used internally because of their finer resolution.) Units conversion is as follows:
1.0 kilogram = 2.2 (2.2046) pounds
5.0 kilograms = 11.0 (11.0231) pounds
10.0 kilograms = 22.0 (22.0462) pounds
25.0 kilograms = 55.1 (55.1156) pounds
50.0 kilograms = 110.2 (110.231) pounds
100 kilograms = 220.4 (220.462) pounds
Add the specified test weight to the platform. Note the new number displayed. Subtract the original zero offset
value from this new number to obtain the scale’s displayed value of the calibration weight.
NOTE: Early versions of the software did not display the decimal point in the a-d calibration mode, i.e.: 11.7 pounds
is displayed as “117”.
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Example: The zero offset value is “11.7” (representing 11.7 pounds). Adding the specified three 25-kilogram test
weights (equivalent to 165.3 pounds) to the platform produces a reading of “177.1” The difference is 177.1-
11.7=165.4 (equivalent to 165.4 pounds). This would indicate the calibration is 0.1 pound “high”.
Using the specified three 25-kilogram test weights, a difference of 165.3 +/-0.1 pound should be obtained. If
necessary, adjust potentiometer P1 (span adj.) on the instrument board until the correct value is obtained. Remove
the test weight and recheck the zero offset value. Note that adjusting P1 may also alter the zero offset value. Repeat
the process as necessary to obtain the correct difference.
Now you may turn the scale off by pressing the “ON” pushbutton and holding it in for a few seconds. That will force
it to turn off. You may also wait for it to time out and turn off by itself.
DATA OUTPUT (RS232) DETAILS
For detailed help and information regarding the data output capabilities, contact Welch Allyn. RS232 data output
has been made a standard feature on many of our scales in 2013. Existing scales can be easily modified to provide
data output when needed. Consult Welch Allyn for details.
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Masted Scales Disassembling Guide
Required tools:
- 5/32” Allen wrench
- 1/8” Allen wrench
- 9/16” Open end wrench
- ½” Open end wrench
- 5/16” socket
- 10 mm socket
- 11 mm socket
- Small slotted screwdriver
- Small Philips screwdriver
- #1 Phillip screwdriver
- ¼” Ratchet driver with extension
- 10 mm nut driver
- 5 mm nut driver
*Power down the scale and remove any A/C power*
CAUTION: Do not apply extra force to the base of the scales. The load cell capacity can overload and
cause damage to the load cells.
Note: Whenever the scale is opened for repair purposes a Calibration must be performed.
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