Ohmeda 3000 User manual

EDB/ ECI
Infant Warmer Systems
Model 3000 and 3300
Service Manual
BOC Health Care

Ohmeda
Service Manual
Infant Warmer System
Model 3000 and 3300
Copyright 1986 Ohmeda
This document is not to be reproduced in any manner, nor are the contents
herein to be disclosed to anyone, without the express authorization of the
_Service Department, Ohmeda, Madison, Wisconsin.

Table of Contents
Εις ιο ee eee ee eee
Technical Competence .
5/Disassembiy and Repair _..................
5.1 Heater Module Repairs ........ …
Repair Policy .....................44........ A. Heater Housing Disassembly ... ..
Precautions iv B. Heater Replacement ....................
Torres assesses cee secs sess teres C. Heater Housing Assembly ...............
1/Functional Description .................... 1-1 D. Alarm Lamp Replacement ........
A. Power Supply Board .................... 1-1 E. Examination Lamp Replacement .. ..
B. Control Board ......................... 1-2 5.2 Control Module Repairs .................
C. Display Board ......................... 1-8 A. Control Module Removal ................
B. Control Board and Power Supply Board
2/Specifications に で に た と た だ とまと すすま たま たま と た まま まま まま 2-1 Replacement .. ον εντ νωνν ενω κε ων εως
2.1 Electrical... -. 24 C. Display Module Disassembly
2.2 Controller ............................. 2-1 D. Display Module Assembly ...............
2.3 Alarms ............................... 2-1 E. Control Module Replacement ...
2.4 Environmental ................ 2-2 F. Battery Replacement .........
2.5 Mechanical (without accessories) 22 G. Circuit Breaker Reset ..
2.8 Accessories ................... 2-2 5.3 Bed Platform Repairs ....
3/Setup and Checkout Procedure .... . 34 A. Side Panel Replacement.
3.1 Setup...................... ... 34 3. Side Panel Repairs .......
3.2 Checkout Procedure .................... 34 5 Bed Flatform Disassembly -
i . Be form Asse yo...
μα
B. Control Unit Checks .................... 31 ,F. Hydraulic System Installation
5.4 Caster Replacement ................-.. 5-16
4/Calibration and Adjustments .............. 4-1 5.5 Yoke Manifold Repairs ...:............. 5-16
A. Control Unit Access .............. . 41 A. General ............. 5-16
B. Power Supply Board Voltage Checks ... 4-1 B. Gauge Replacement ..... 5-16
C. Display Brightness Check ........... . 4-1 C. Gauge Lens Replacement . 5-16
D. Alarm Volume Adjustment ...... . 41 D. Strainer Replacement ..... 5-19
E. Analog to Digital Converter (ADC) . 41 E. Check Valve Replacement ..... 5-19
F. Line Voltage Sensing ........... 4-2 F. High Pressure Regulator Repair . 5-19
G. Triac Safety Circuit Test . 42 G. Pneumatic Troubleshocting ... 5-20
H. Test Loop ............................ 4-3 。 。 .
L Oxygen Regulator Check and Adjustment . 4-3 6/Control Unit Troubleshooting Guide .... . 6-1
y. Electrical Safety Check .................. 7/Illustrated Parts and Parts List ............. 71
K. Ground Resistance Check ...
L. Leakage Current Tests .................. 8/Schematics and Wiring Diagrams ........... 31
Appendix.................. „A1
A. Test Eguipment and Special Tools. . А-1
B. Temperature Conversion Chart ..... . A-1
C. Patient Probe Characteristics .. - A-1
D. Drawer Kit Accessory ....... .. A2
E. O2 Yoke and Regulator Assembly Kit ...... A3

Important
The information contained in this service manual
pertains only to those models of products which are
marketed by Ohmeda as of the effective date of this
manual or the latest revision thereof. This manual
‘was prepared for exclusive use by Ohmeda service
personnel in light of their training and experience
and the availability to them of proper tcols and test
equipment. Consequently, Ohmeda provides this
manual to its customers purely as a business con-
venience and for the customer's general information
only without warranty of the results with respect to
any application of such information. Furthermore,
because of the wide variety of circumstances under
which maintenance and repair activities may be per-
formed and the unique nature of each individuals
own experience, capacity, and qualifications, the
fact that customer has received said information
from Ohmeda does not imply in any way that
Ohmeda deems said individual to be qualified to
perform any such maintenance or repair service.
Moreover, it should not be assumed that every ac-
ceptable test and safety procedure or method, pre-
caution, tool, equipment or device is referred to
within, or that abnormal or unusual circumstances
may not warrant or suggest different or additional
procedures or requirements.
This manual is subject to periodic review and cus-
tomers are cautioned to obtain and consult the
latest revision thereof and suggestions are invited
from our customers for consideration by Ohmeda
with these periodic reviews.
WARNING: After completing a repair of the Infant
Warmer System the appropriate calibration proce-
dure must be performed to make sure the Infant
Warmer System is in proper operating condition.
In addition a final electrical safety check and leak-
age current test must be performed. Record the in-
formation for future reference.
WARNING: After completing any portion of the
calibration and adjustments procedure for the In-
fant Warmer System the checkout procedure must
be performed to make sure the Infant Warmer
System is in proper operating condition. In addi-
tion a final electrical safety check and leakage cur-
rent test must be performed. Record the informa-
tion for future reference.
CAUTION: Servicing of this product in accordance
with this service manual should never be underta-
ken in the absence of proper tools, test equipment
and the most recent revision of this service man-
ual which is clearly and thoroughly understood.
This static control precaution symbol appears
throughout this manual. When this symbol ap-
pears next to a procedure in this manual, static
control precautions must be observed. Use the
static control work station (Part No. 0175-2311-
000) to help ensure that static charges are
safely conducted to ground and not through sta-
tic sensitive devices.
This document is not to be reproduced in any man-
ner, nor are the contents herein to be disclosed to
anyone, without the express authorization of the
Ohmeda Product Service Department, Madison, Wis-
consin.
iii
Repair Policy and Procedure
Note: Service must be performed by a “Technically
Competent” individual.
Do not use malfunctioning equipment. Make all nec-
essary repairs, or have the equipment serviced by
an Authorized Ohmeda Service Representative.
After repair test the equipment to ensure that it is
functioning properly, in accordance with the man-
ufacturer's published specifications.
To ensure full reliability, have all repairs and service
done by an authorized Ohmeda Service Representa-
tive. If this cannot be done, replacement and
maintenance of those parts listed in this manual
may be undertaken by a competent, trained indi-
vidual having experience in the repair of this type of
equipment.
CAUTION: No repair should ever be undertaken or
attempted by anyone not having such qualifica-
tions.
Replace damaged parts with components manufac-
tured or sold by Ohmeda. Then test the unit to as-
certain that it complies with the manufacturer's
published specifications.
Contact the nearest Ohmeda service office for ser-
vice assistance. If you send the unit to the Ohmeda
Service Center, package it securely in the original
shipping container, if possible, and ship it prepaid.
Enclose a letter with the unit describing in detail
any difficulties experienced and the repairs felt nec-
essary. In all cases, other than where Ohmeda's
warranty is applicable, repairs will be made at
Ohmeda's current list price for the replacement
part(s) plus a reasonable labor charge.
CAUTION: Detailed drawings and procedures for
more extensive repairs are included herein solely
for the convenience of users having proper knowl-
edge, tools, and test equipment, and for service
representatives specially trained by Ohmeda.
Technical Competence
The procedures described in this service manual
should be performed by trained and authorized per-
sonnel only. Maintenance should only be underta-
ken by competent individuals who have a general
knowledge of and experience with devices of this
nature.
Genuine replacement parts manufactured or sold by
Ohmeda must be used for all repairs.
Read completely through each step in every proce-
dure before starting the procedure; any exceptions
may result in a failure to properly and safely com-
plete the attempted procedure.
Copyright 1986 by Ohmeda, Ohmeda Drive, Madi-
son, Wisconsin 53707-7550.

Precautions
Warnings
After completing a repair of the Infant Warmer Sys-
tem the appropriate calibration procedure must be
performed to make sure the Infant Warmer System
is in proper operating condition. In addition a final
electrical safety check and leakage current test
must be performed. Record the information for fu-
ture reference.
After completing any portion of the calibration and
adjustments procedure for the Infant Warmer Sys-
tem the checkout procedure must be performed to
make sure the Infant Warmer System is in proper
operating condition. In addition a final electrical
safety check and leakage current test must be per-
formed. Record the information for future reference.
Overloading the shelves can affect the stability of
the unit.
Do not perform the Check-Out Procedure while a
patient occupies the Infant Warmer System.
Never
oil or grease oxygen equipment unless a lu-
bricant that is made and approved for this type of
service is used. Oils and grease oxidize readily, and
in the presence of oxygen, will burn violently. Vac
Kote* is the oxygen service lubricant recommended
(Order No. 0220-0091-300).
When replacing gauges, be sure to use identical
pressure ranges.
Do not use oil or oil bearing materials on or near the
regulator. Oils and greases oxidize readily and, in
the presence of oxygen, they will burn violently. All
metallic parts of the regulator must be discarded if
contaminated with oil or grease.
* Vac Kote is a trademark of the Ball Corporation.
Cautions
Servicing of this product in accordance with this
service manual should never be undertaken in the
absence of proper tools, test equipment and the
most recent revision of this service manual which is
clearly and thoroughly understood.
This static control precaution symbol appears
throughout this manual. When this symbol ap-
pears next to a procedure in this manual, static
control precautions must be observed. Use the
static control work station (Part No. 0175-2311-
000) to help ensure that static charges are
safely conducted to ground and not through sta-
tic sensitive devices.
No repair should ever be undertaken or attempted
by anyone not having such qualifications.
Detailed drawings and procedures for more exten-
sive repairs are included herein solely for the con-
venience of users having proper knowledge, tools,
and test equipment, and for service representatives
specially trained by Ohmeda.
For the Model 3000 Infant Warmer System the
height of the bed should be 27 inches +2 inches
from the bottom of the heater module for proper
heating and temperature control.
Insulation on electrical wiring can deteriorate with
age. Check for brittle or deteriorated insulation on
power cord and all other electrical wiring.
Use the Static Control Work Station (Part No. 0175-
2311-000) to help ensure that static charges are
safely conducted to ground. The Velostat material is
conductive. Do not place electrically powered circuit
boards on it.
The back panel and display panel may drop down
when the bottom cover mounting screws are re-
moved. Be sure to secure the panels with tape be-
fore disassembly.
Disconnect the Infant Warmer System power cord
and allow the unit to cool before replacing the
examination or alarm light.
Disconnect the Infant Warmer System power cord
and allow the unit to cool before replacing the
examination light. The lamp normally operates at a
high temperature.
Take static precautions for these procedures.
For safety have at least 2 people available to re-
place a caster. Remove all accessory equipment.
iv

1/Functional Description
A. Power Supply Board
This is a functional description for the Infant
Warmer System Power Supply Board Part No. 0631-
5032-700. Réfer to Schematic No. 0676-0327-000 for
a detailed circuit diagram.
The power supply board contains circuitry for the
control and monitoring of voltage devices. The
board also provides power to the control board and
the display board. Also found on the board is a line
voltage sensing circuit that provides an indication of
line voltage to the microcontroller.
. The control circuits for each line voltage device on
the power supply board are functionally identical
with a logic “High” signal from the control board
switching "On” the desired device. This is per-
formed with an opto-isolator triac driver so low/line
voltage circuits can interact but remain electrically
isolated (2500 volt dielectric).
The heater is controlled from the supply board with
an opto-isolator triac driver and triac. There is also a
relay contact connected in series with the neutral to
the heater triac. This is used to switch “Off" the
heater if the heater triac fails or there is a failure on
the control board.
The regulator circuits provide a +5 Vdc supply to
the display board and +5 Vdc, and +9 Vdc,
supplies to the control board. A ni-cad battery
supplies the § Vdc supply and a de-rated 9 Vdc sup-
ply for standby power, in the case of a power loss.
Standby power of 9 volts is used to activate the
transducer alarm, while the 5 volt supply provides
power to the microcontroller and associated IC's for
memory retention purposes.
5 Volt LEDS
A nominal 8 Vac is input to the power supply board
at J11 pins 3 and 4. The line frequency is also con-
nected to the control board via J12 pin 2. The
bridge rectifier CR2 and capacitor C11 provide a fil-
tered unregulated 8 Vdc to the relay, opto-isolators,
and the regulator VR12. The 8 Vdc unregulated sup-
ply can be measured at TP-1. The unregulated sup-
ply must be a minimum of 7.32 volts for proper op-
eration of the relay circuit.
The output of regulator VR2 is nominally +5 Vde
and supplies power to drive the LED displays on the
display board. The output is measurable at J12 pin
12 (TP-10). When the supply voltage is within 10%
of nominal, the output voltage should be between
4.8 and 5.2 volts dc with a maximum load of 500
ma. The maximum allowable ripple voltage is 150
millivolts.
Line Voltage Sensing
A voltage of approximately 11 Vac from the trans-
former secondary is input to the board at J11 pins 1
and 2. Bridge rectifier CR1 and capacitor C12 pro-
vide a full wave, filtered voltage of approximately 12
Vdc. Resistor R3 is preset to produce an output of
approximately 0.6 volts at J12 pin 11 (TP-11) when
the line voltage is at the nominal value for the unit.
The analog voltage signal at J12 Pin 11 connects to
the control board and is fed into the A/D Converter,
ADC 3711 (U6), via the multiplexer, MC14051B (U-
13). The digital output of the A/D converter is input
to the microcontroller where the software then ad-
justs the power to the heater to compensate for var-
iations in line voltage.
9 Volt Standby
The output of regulator VR3 is adjusted by R4 to
provide 9.0 + 0.2 volts (TP12). This voltage is used
for charging the NI-CAD battery, and supplying the
input voltage to the +5 standby regulator.
5 Volt Standby
When line voltage is available, current flows from
the output of VR3 and through CR5 to provide 9.0
+ 0.2 volts to the input of VR4, and to J12 pin 3
(TP-12). In tum, regulator VR4 outputs a voltage of
5.0 + 0.2 volts to J12 pin 14 (TP-9) with a maximum
ripple voltage of 150 millivolts.
If power loss occurs with the unit switched ON, the
7.2 volt NI-CAD battery maintains a de-rated output
voltage of approximately 6.5 volts to pin 3 of J12
(TP-12). It also provides input to VR4. The output of
VR4 only regulates to approximately 5.0 volts as the
input voltage drops below 7.0 volts.
Observation Light, Alarm Lights, Motor Control
The control circuits for the observation light, and
the alarm light are similar. The alarm light is a resis-
tive load and does not have a snubber circuit in
parallel with the load. The snubber circuit on the
observation light supply protects the triac from
large voltage spikes characteristic of inductive
loads. If the control lines are logic “Low", less than
0.45 volts, this keeps the FET, triac driver, and triac
switched “Off”. The triac acts as a switch to the
line voltage circuit, removing voltage from the load.
When a device should be switched “On”, a logic
“High” of 2.4 volts minimum is output to the corres-
ponding FET. The FET switches “On” causing the
LED ot the isolator-driver to switch on. The isolator/
driver output drives sufficient current to the triac
gate, switching the triac "On" allowing the selected
device to switch “On”.
Heater Status and Control
The heater circuitry consists of a monitoring circuit,
a controller for the heater, and à relay to switch
“Off" the heater in the event of a triac or system
failure.
The full wave bridge rectifier CR6 takes a sample
(through R 13) of the ac signal supplied to the hea-
ter and provides rectified dc to the opto-isolator U3.
If the heater is “On” the de output switches “On”
the LED in the opto-isolator, except at voltage levels
below the forward bias voltage. When the LED is
“On” the transistor goes into saturation causing the
output at J12 pin 1 to go “Low” (about 0.3 volts).
When the heater is “Off” the de signal is in the re-
gion of zero potential and there is insufficient for-
ward bias voltage for the LED. This switches “Off”
the transistor allowing capacitor C10 to charge and
causes J12 pin 1 to go high (5 volts), When the hea-
ter is switched “On" the LED switches the transis-
tor “On” again, and the capacitor discharges. The
low output shows small glitches caused by the
charge/discharge of the capacitor at every half
cycle. The glitches are acceptable provided they do
not exceed the trigger voltage of 1.4 volts for the
74LS132 on the Control Board.
1-1

1/Functional Description
The heater control circuit uses a zero crossing opto-
isolator triac driver to isolate the line voltage from
the low voltage circuits. Operation of the heater
control and other line voltage controls differ only in
the type of isolator used and the use of snubber cir-
cuits. When a logic “High” signal is sent to the hea-
ter control circuit from J12 pin 9 the output of the
isolator will not switch “On” until the ac signal of
the heater crosses the zero potential from a nega-
tive voltage. After the input line from the microcon-
troller goes “Low”, heat will not switch “Off” until
the first zero crossing preceded by the negative half
cycle. This provides zero crossing control of the hea-
ter switching. The time that the heater is “On" de-
pends on the percent heat desired (controllable in
5% increments).
The microcontroller also monitors the line voltage
and
adjusts the number of ac cycles that the heater
is switched “On”. This provides heater compensa-
tion. If the line voltage is not at the nominal value
these two functions result in 60 levels of heat.
Relay
The relay circuit is used to switch “Off" the heater
in the event of a triac or microcontroller failure.
Under normal conditions the input line from J12 pin
10 is a logic “High”, 2.4 volts minimum. A logic
"High" signal on the input from the control board
switches “On” the FET causing the relay coil to en-
ergize and close the switch. If the FET input is a
“Low” from the control board, (0.5 volts max.) the
FET switches “Off” and the relay contacts open.
The signal at J12 pin 10 comes from U1 on the con-
trol board which is a part of a logic/timing circuit in-
dependent of the microprocessor.
A minimum voltage of 7.2 volts is reguired to ener-
gize the relay coil. Therefore the minimum allowable
voltage for the 8 volt unregulated supply is 7.32
volts since the FET has an internal voltage drop of
0.12 volts.
B. Control Board
This is a functional description for the Infant
Warmer System Control Board Part No. 0631-5033-
700. Refer to Schematic No. 0676-0326-000 for a de-
tailed circuit diagram.
The control board contains electronic circuitry invol-
ved with the measurement, control, computation,
memory, logic, and decision making functions of the
Infant Warming System. The principle IC on this
board is the 8031 single component, 8-bit microcon-
troller. The 8031 has: an internal read/write memory
(RAM)of 128 bytes, 32 I/O lines configured as four 8-
bit parallel ports, two 16-bit timers, a five source
two priority nested interrupt, a programmable serial
VO port, and an on-chip oscillator with clock cir-
cuitry. The program memory is stored in a 2764 64k
bit (8k x 8) UV EPROM. An octal transparent latch
(74LS373) is connected to address inputs of the
EPROM to permit the use of the bi-directional data
bus port of the microcontroller for addressing the
EPROM and receiving program instructions.
Four ICs with a network of precision resistors are
used to interface the microcontroller. The tempera-
ture sensor, calibration resistors, or line voltage
1-2
scaler are selected by an MC14051B 8 Channel Mul-
tiplexer. An LM-10 precision reference with adjusta-
ble reference buffer, and on-board operational
amplifier furnishes a stable reference supply. This is
reguired by the temperature measurement circuits
and the ADC 3711 Analog to Digital Converter. An
8243 I/O expander is used to interface the microcon-
troller with the multiplexer and the A/D converter.
The control board is also equipped with several IC's
that form the triac watchdog circuit, watch-dog
timer, and the audio alarm tone generator. The
audio transducer for the alarm signals and its driver
circuit are also included on the control board.
The operation of the circuits listed in the preceding
paragraphs will be explained in detail below.
Analog to Digital Converter
Temperatures are measured using a negative tem-
perature coefficient thermistor that is calibrated for
specific resistance values and interchangeability.
Analog voltage signals inversely proportional to
temperature are derived from a voltage divider net-
work consisting of a 5.76k + 0.1% resistor in series
with the temperature sensor. The voltage source for
the measuring circuit is obtained from the LM-10's
internal precision reference source of 200 mV
amplified to a nominal 1.0 volts by the reference
buffer of the LM-10. The Op-amp portion of the LM-
10 provides an adjustable reference of 2.0 volts
nominally, which is required by the A/D converter,
U6. In addition to the patient probe, there are three
other
voltage divider networks on the control board.
Two have fixed output and are used for calibration
check points of the A/D system at 25.0 and 37.9 de-
grees C. The third divider network is unused.
A separate input to the control board A/D circuit
comes from the line voltage monitor network lo-
cated on the power supply board.
The outputs of all the voltage dividing networks are
connected to individual switch input terminals of
U13, the MC14051B Analog Multiplexer. The
MC14051B contains eight normally open switches
with a common output terminal. The common out-
put of the MUX (pin 3) is tied directly to the analog
input (pin 9)of the A/D converter. The microcontrol-
ler selects which sensor is to be measured by toggl-
ing the control lines, pin 11A, pin 10B, and pin 9C of
the MUX via the 8243 #2, US. The following table
shows the digital codes used to select the individual
switches of the MUX:
Control On Pin
Inputs Switches Number
ABC
000 χο 13 calibration value 25C
001 xt 14 calibration value 37.9C
010 X2 15 line voltage monitor
011 X3 12 unused
100 X4 01 patient probe
101 X5 05 unused
110 X6 02 unused
111 X7 04 unused
Note: Inhibit terminal (pin 6) of the MUX has no ef-
fect on the switch selection because it is tied "Low"
through R19 (200 ohms).

1/Functional Description
The ADC 3711, U6, uses a pulse modulation analog
to digital conversion technique. The conversion rate
is set by the frequency of an internal oscillation
whose frequency is determined by the external com-
ponents R4 and C14. The exact oscillator frequency
is not critical and may vary by + 15% from the
nominal 400 kHz. The oscillator frequency may be
measured on pin 18 of U6. With a nominal 400 kHz
clock frequency, conversions within the ADC 3711
will take place at an approximate rate of 3 per sec-
ond.
The ADC 3711 will output BCD data on demand in
accordance with the coded digital signals applied to
the digit select inputs DO and D1, pins 20 and 21 re-
spectively. The data latch enable is tied “Low”,
therefore, the BCD data of the A/D converter will be
output to the microcontroller through 8243 #2 in
conformance to the following codes that are applied
to the digit select inputs:
Do D1 Selected Digit
L L Digit 0 LSD
L H Digit 1
H L Digit 2
H H Digit 3 MSD
Note: The magnitude of the selected digit is present
at pins 23 and 24,
The ADC 3711 is continuously converting the analog
voltage present at its input to a number of counts
between 0 and 3989 (BCD format). Therefore, the
start conversion, input at pin 7, and the conversion
complete, output at pin 6, are misnomered. The
start conversion input only controls the transfer of
information from the internal counter to the digital
latches. The conversion complete output goes to a
logic “Low” on the rising edge of the start conver-
sion pulse which is issued by the microcontroller.
The conversion complete will go to a logic “High”
sometime later when the new conversion informa-
tion has been transferred to the display latches. The
start conversion pulse may occur at any time in the
conversion cycle because the microcontroller is run-
ning asynchronously to the A/D clock. Therefore,
the amount of time from the start to finish will vary.
The maximum time difference between the start
conversion and conversion complete pulses in this
application is about 300 msec.
The operation of the temperature and line voltage
Measurement circuits can be summarized as fol-
lows: The analog voltage signal derived from a vol-
tage divider network and a precision reference
source is directed to the input of the A/D converter
through an eight channel analog multiplexer. For
the line voltage measurement, the voltage source is
obtained from the rectified, filtered, and unregulated
output of the power transformer. Switch selection is
software controlled by the microcontroller which
toggles the A, B, and C input lines of the multi-
plexer.
The analog voltage is converted in the ADC 3711 to
a digital signal in four digit BCD format (0 to 3999
counts). The microcontroller sends a start conver-
sion pulse to the ADC 3711 which then starts to up-
date the digital data in the output latches. When all
of the counts have been internally transferred, the
A/D converter toggles the conversion complete out-
put line. The microcontroller then reads the indi-
vidual BCD digits using coded signals to the digit
select lines of the A/D converter.
ADC Calibration
The A/D converter is calibrated by connecting a
5900 + 0.1% ohm resistor to the patient probe jack
and placing the DIP switch on the control board in
the following position:
Swritch 41 Open (Off)
Switch 42 Open (Off)
Switch #3 Open (Off)
Switch #4 Closed (On}
Potentiometer R44 on the control board is then ad-
justed until the elapsed time display reads exactly
1122. With the DIP switches in the given position,
the patient temperature display will read out the ac-
tual patient temperature, even if it is outside of the
normal range and the control temperature display
will read out the percent of nominal line voltage.
During operation, the calibration of the A/D conver-
sion system may be checked by pressing and hold-
ing the hidden switch located above the alarm si-
lence switch on the control panel. After 2 seconds,
the patient temperature display should read 25.0
and the control temperature display should be 37.9.
The elapsed timer should read the nominal line vol-
tage + 2%.
Microcontroller
The control system is located in the 8031 microcon-
troller. It operates at a clock speed of 6MHz and can
he verified by measuring the frequency at the Ad-
dress Latch Enable (ALE) pin to be 1 MHz ("On" =
0.33 usec and “Off” = 0.67 usec). Grounding the EA
pin enables the 8031 to execute instructions from an
external memory device.
When the microcontroller performs a read instruc-
tion from EPROM, the low order address (8 bits) is
output from Port 0 while the high order address (6
bits) outputs from Port 2. (Note: Bit 6 is configured
only to provide expansion compatibility with a
27128 EPROM). The ALE pin goes “High” allowing
the LS373 to appear transparent between the
EPROM and the microcontroller. After the ALE out-
put goes “Low”, the low order address is latched to
the outputs of the D flip flops within the LS373.
This allows the 2764 to remain addressed by the
microcontroller, and return 8 bits of data while
using only two ports.
Port 1 of the 8031 is used to communicate to the
three 8243 I/O expanders. Bits 5-7 are connected to
the Chip Select (CS) line of the first, second, and
third respective 1/O expanders. Providing a “Low”
signal on one and only one of the outputs activates
the corresponding IC. Bits 0-3 hold the instruction
to be carried out by an 8243 when the enable bit 4
transitions between “High” and “Low".

1/Functional Description
Port 3 is used to perform remaining tasks required
by the control system. Connections 3.0 and 3.1, (re-
ceive and transmit respectively), are used in con-
junction with the serial interface chips so that com-
munication to an external microcomputer is possi-
ble. Connection INTO/P3.2 is a line frequency inter-
rupt line that is used to aid in timing subroutines
found within the system software. Connection T0/
P3.4 sends serial data to the display driver while
connection T1/P3.5 provides clocking to the driver.
Line Frequency
The line frequency circuit converts a 60 or 50 Hz
sinusoidal signal into a square wave signal. The
output of the circuit is used to clock the 4020B
counter and to provide a low frequency clock source
for the system software. The 1N4001 diode haif-
wave rectifies the 8 Vac (nominal) signal for use
with the Schmitt trigger NAND gate. With one line
tied “High”, the output of the trigger will be inver-
ted. Since the gate will not respond until the input
exceeds 1.9 volts minimally, the duty cycle of the
output will be slightly more than 50%.
Heater Status
The Heater Status function signals the microcontrol-
ler and the safety. circuitry as to whether or not the
heater is “On" or “Off”. The input to the Schmitt
trigger is “High” if the heater is “Off” and “Low” if
the heater is “On”. Small glitches appear when the
heater is “On". Consult the Functional Description
of the power supply board for further explanation.
The output of the NAND gate is inverted because
one input is tied “High”.
Hardware Triac Test
The 4020B 14 bit binary counter, U9, counts at a
rate equal to the line frequency and responds to the
negative edge of the clock pulse. The clock signal is
received from a Schmitt trigger NAND gate, pin 6 of
U8. The counter resets when the 7418123 retrigger-
able one shot flip flop outputs a “High” level pulse
on the O output Ime. With CLR tied “High” and A
tied “Low”, the counter will reset when B of the
7418123, US, is “High” at a time equal to (04)
+(013) +(014) or after 12296 counts (04 = 8, 013
= 4096, 014 = 8192). Approximately 6.19 usec.
later the output of the one shot will return to its in-
itial “Low” state.
Q13 and Q14 of the 4020B are tied to a 2 input
“And” gate which will go “High” after 12288
counts. On odd numbered counts Q1 of the counter
goes “High”. Q1 is tied to the CLR pin of D flip flop
Ui, which when “High” allows the output O to
equal the input D on the next positive edge of the
clock pulse. Therefore the output at pin 5 will up-
date on even counts. After 12288 counts (3.4133
minutes on 60Hz units, or 4.096 minutes for 50Hz
units) the signal at the D input of the flip flop goes
“High”. This signal is also input to the microcontrol-
ler through the 1/0 expander U4. The software will
then switch “Off” the heat. Two counts later the
“High" input on D is clocked to the output Q. The
heater status (“Off"-“Low", “On"-“High") sent from
the Schmitt trigger NAND gate pin 11 of U8 is al-
ways present at the input of U2 pin 5. If the heater
is still “On” after the 2 counts, the output of the
“And” gate pin 6 of U2 will clock the second D flip
flop. The outputs of the flip flops switch ~O goes
“High” and “Not Q” goes “Low”. A “Low" on “Not
O" sets off the audio alarm and drops out the non-
1-4
resettable safety relay causing the heater to switch
“off”.
Heater Status LED
A heater status LED is located on the control board
for troubleshooting. The LED can be seen through
the rear of the controller assembly cover. When the
status line from the Schmitt trigger is “High”, (hea-
ter “On”) the transistor O2 switches “On” causing
the LED to emit light. If heat is “Off”, the LED is
“Off”.
Watchdog Timer
A watch dog timer is used to “check” that the
microcontroller is working properly. After every
cycle through the system software the microcontrol-
ler sends a “Low” pulse to the A input of U3, a
74L$123. The RC network connected to the RxCx
and Cx pins create a time constant, t = 0.45xRxC
= 0.263 seconds. If a pulse is not received at the
input before the time constant expires, the output
will go “Low". The high priority alarm will then be
activated due to the microcontroller failure. Note
when the microcontroller detects a high priority
alarm condition, pulses to the watch dog circuit
stop.
Alarm Tone Generator and Control Circuits
The alarm circuit consists of an alarm tone
generator and control circuitry for high or low prior-
ity alarm conditions. Under a no alarm condition the
7556 timers are both inactive, (reset lines low).
Low Priority Alarm
Under normal operating conditions the input to U8
pin 9 is “High”. When the microcontroller detects a
low priority alarm a 1 Hz square wave is output to
U8 pin 9. The timer activates, causing a 2 kHz audio
output. This results in a one second “On”, one sec-
ond “Off" audio alarm. The 2 kHz signal is adjusted
within + 100 Hz by R38. The volume of the audio
alarm is adjusted by R37. This should be adjusted
fully CCW for maximum volume.
High Priority Alarm
The high priority alarm is activated if the microcon-
troller quits sending pulses to the watchdog timer.
This occurs when a high priority alarm condition is
detected or if the microcontroller fails. The high
priority alarm is also activated if the hardware triac
test circuitry detects a failed triac. Both timers be-
come active with one timer feeding a 1 Hz signal to
the control line of the second. The 1 Meg resistor
changes the output frequency of the second timer
to produce a warbling effect (two tone alternating
alarm). If high and low priority alarms are both
“On", the output of the “And” gate overrides the
low priority signal, keeping both timers active.