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78
Service Manual
166 kJ for the 400-mV range, and Z1-10 connects to 16 kJ for the 40-mV range. Z1-11
is the summing node of the integrator circuit. The autozero capacitor (C11) stores op
amp and comparator offsets. The integrator capacitor is C12.
The system reference voltage (1.23 V) is generated by VR1 and R23. The 1.000 V
reference voltage for the a/d converter is supplied through U1-1 (REFI). This voltage is
adjusted by R26, the dc calibration potentiometer in conjunction with R24 and R25. In
addition to generating the a/d reference, the VR1 voltage is used for power supply
reference, voltage comparator offset generation, the ohms source voltage, and the open
thermocouple detect source voltage.
2-23. Beeper
Devices Q5, Q6, R27, R28, and R29 make up the beeper drive circuit.
2-24. Power Supply
The power supply consists of two regulators, one shunt and one series, which set Vdd at
+3.1 V and Vss at -3.2 V for all battery voltages down to about 7 V. The shunt
(common) regulator sets |Vdgnd - Vss| (Vdgnd = COM = 0V) and consists of an op amp
and current shunt devices integrated on U1. Resistors R34 and R35 provide voltage
division. The series (Vdd) regulator, which sets |Vdd-Vdgnd|, is made up of another on-
chip op amp, along with devices Q7, R32, and R33. Q7 is the series regulator element,
and R32 and R33 are for voltage sensing. Capacitors C16 and C17 provide circuit
compensation and power supply decoupling for the shunt and series regulators,
respectively. Q8 provides base drive for Q7 at turn on. Voltage level information is
presented in Table 2-1.
2-25. Display
The liquid-crystal display (LCD) operates under control of the microcomputer. Segments
are driven by the computer and displayed on the LCD. Both digital readings and an
analog bar-graph display are presented in conjunction with annunciators and decimal
points. Refer to the Fluke 78 Users Manual for information about the display.
VBT +
VDD
VSS
VBT-
REFH
PS0
PS1
VOA
VOB
2-10
Table 2-1. Typical Voltage Levels and Tolerance (Referenced to Common)
3.7 ±0.2
3.1 ±0.2
-3.2 ±0.2
-5.3 (battery at full 9 V charge of 6.9 V)
-3.2 (battery at low charge of 6.9 V)
1.23 ±0.04
1.23 ±0.15
0 ±0.15
2.2 to 1.7 (referenced from VSS)
1.07 to .85 (referenced from VSS)
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Table of Contents
Troubleshooting
