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Technical Manual
4-4 Welch-Allyn, Inc.
Reset/Self Tests
Upon battery installation, (assuming that the electronics have been discharged
sufficiently by pressing a user button with batteries removed) the microprocessor
receives a power up reset signal from the components associated with the reset line at
U1-12. When power is applied continuously, C3 is charged slowly through R7,
providing an active low reset to the microprocessor.
When the reset signal is complete, the microprocessor launches a series of self
checks which include RAM test, ROM test, instruction set test, self calibration tests
(electronics accuracy test, hi cal, low cal), probe warmer circuitry tests, probe test,
battery voltage test and ambient temperature test. Any failures here will cause a
specific error code to be displayed to assist debugging.
Microprocessor Clock
The clock for the microprocessor is generated by X1 and capacitors C26 and C27,
which form a 4.9152 MHz. oscillator circuit. The microcontroller is running at
approximately 2.5 MHz, which is achieved by internally dividing the frequency of
oscillation by two.
Temperature Measurement and Display
The thermometer probes incorporate negative temperature coefficient thermistors.
When the temperature of the probe is increased, its electrical resistance decreases.
Model 678 and 679 use “20K” thermistors, so they are at approximately 20 Kohms at
room temperature. At 37
°
C (98.6
°
F) they are near 12 Kohms. The change in
resistance is nonlinear with temperature, and an equation describing this curve is
programmed into the thermometer.
Temperature Measurement A/D Converter
The primary function of the temperature measurement A/D converter circuit is to
convert a measured resistance into a 16-bit word, which is then used by the
microprocessor to calibrate itself and calculate a temperature.
Theory of Operation
The A/D circuit is made up of 4 major parts:
•
The resistance to be measured (either the probe thermistor, the PTBCAL resistor,
or the HICAL resistor).
•
The timing capacitor (C21, 0.33uF) and associated driver circuitry
(Q1-Q4, Q9, R15, and R30).
•
A low bias dual voltage comparator (U2, TLC555).
•
A 16 bit timer internal to the microprocessor.
- SureTemp 1
- Technical Manual 1
- TABLE OF CONTENTS 3
- LIST OF FIGURES 5
- WARRANTY 6
- SPECIFICATIONS 7
- TERMINOLOGY 8
- Basic System Description 9
- 1-2 Welch-Allyn, Inc 10
- Self Tests 11
- Battery Life 12
- Normal Mode 13
- Monitor Mode 14
- Biotech Mode 15
- 1-8 Welch-Allyn, Inc 16
- Operational Characteristics 17
- Model 678/SureTemp 17
- Model 679 1-9 17
- 1-10 Welch-Allyn, Inc 18
- 2. PREVENTIVE MAINTENANCE 19
- 2-2 Welch-Allyn, Inc 20
- 3. CALIBRATION TESTING 21
- 3-2 Welch-Allyn, Inc 22
- 4. THEORY OF OPERATION 23
- Probe Switch 24
- Power Supply 25
- 4-4 Welch-Allyn, Inc 26
- Rthermistor 27
- ()PWthermistor ( )Rhical 27
- Probe Identification Logic 28
- Theory of Operation 29
- Model 679 4-7 29
- Other Components 30
- Model 679 4-9 31
- 4-10 Welch-Allyn, Inc 32
- 5. TROUBLESHOOTING 33
- Equipment Required 34
- Troubleshooting Table 35
- 5-4 Welch-Allyn, Inc 36
- 70898-0000A 36
- Troubleshooting 37
- Model 679 5-5 37
- 5-6 Welch-Allyn, Inc 38
- Model 679 5-7 39
- Field Serviceable Repairs 40
- Field Serviceable Parts 40
- Model 679 5-9 41
- 5-10 Welch-Allyn, Inc 42
- Model 679 5-11 43
- Thermometer Disassembly 44
- Model 679 5-13 45
- Figure 8 - Main PCA 46
- Model 679 5-15 47
- WELCH ALLYN 48
- 5-16 Welch-Allyn, Inc 49
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