Welch Allyn SureTemp 679 - Page 4

ii Welch-Allyn, Inc.
4. THEORY OF OPERATION................................................................................................ 4-1
Technical Overview ........................................................................................................... 4-1
Probe Enhancements................................................................................................. 4-2
Probe Switch .....................................................................................................................4-2
Normal Mode.....................................................................................................................4-2
Power Supply ....................................................................................................................4-3
Low Battery Detection ................................................................................................ 4-3
Microcontroller............................................................................................................ 4-3
Reset/Self Tests......................................................................................................... 4-4
Microprocessor Clock................................................................................................. 4-4
Temperature Measurement and Display..................................................................... 4-4
Temperature Measurement A/D Converter................................................................. 4-4
Theory of Operation ................................................................................................... 4-4
Circuit Description ...................................................................................................... 4-5
Probe Identification Logic .................................................................................................. 4-6
Basic Function............................................................................................................ 4-6
Circuit Operation ........................................................................................................ 4-7
Probe Warming ( Oral probes only)................................................................................... 4-7
Theory of Operation ................................................................................................... 4-7
Circuit Operation ........................................................................................................ 4-8
Other Components............................................................................................................ 4-8
Liquid Crystal Display................................................................................................. 4-8
LCD Backlight (Model 678 Only) ................................................................................4-8
Probe Switch.............................................................................................................. 4-9
Mode Button............................................................................................................... 4-9
Timer Button (678 Only)............................................................................................. 4-9
Serial Communications Port....................................................................................... 4-9
Horn ........................................................................................................................... 4-9
5. TROUBLESHOOTING....................................................................................................... 5-1
Error Codes....................................................................................................................... 5-1
Equipment Required.......................................................................................................... 5-2
Troubleshooting Table.......................................................................................................5-3
Field Serviceable Repairs.................................................................................................. 5-8
Field Serviceable Parts...................................................................................................... 5-8
Model 678/679 Replacement Parts.................................................................................... 5-9
Replacement Parts-Circuit Board Assemblies ................................................................. 5-10
Thermometer Disassembly.............................................................................................. 5-12