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78
Service Manual
2-3. Circuit Descriptions
Each of the functional blocks in Figure 2-1 is discussed in more detail in the following
paragraphs. Refer to the schematic diagrams in Chapter 5 for circuit details not provided
in this chapter.
2-4.
Input Overload Protection
The zinput is protected from overload by a network consisting of metal-oxide
varistor (RV1), three current-limiting resistors (R1, R2, and RT1) and spark gap E1.
Under extremely high energy conditions, R1 will help limit overload current until RT1
heats up. Thermistor RT1 rises to a high impedance during a sustained voltage overload
in the ohms or temperature mode. Transistors Q1 and Q2 form a voltage clamp network.
This clamp performs a current-limiting function on the overload current to U1 at 10 mA
during ohms and temperature overloads. Power supply regulation and system operation
is maintained during any of these overloads. The 10 A input is protected from overloads
by F1 (15 A/600 V). R40 limits the input current in RPM, while Q11 and Q12 clamp the
voltage seen by U4.
2-5.
Rotary Knob Switch
Input signals are routed from the overload protection circuits to a double-sided switch
wafer. This switch wafer provides the necessary connections to implement signal
conditioning and function-encoding for U1.
2-6.
Input Signal Conditioning Circuits
Each input signal is routed through signal conditioning circuitry before reaching IC U1.
Incoming signals received through thez input are routed to precision resistor
network Z1. This divider network precisely scales the input for the various voltage
ranges and provides precision reference resistors that are used for the ohms and
capacitance functions.
Input divider Z1 is used in two modes, series and parallel. In volts functions, a series
mode provides four divider ratios. In the ohms function, a parallel mode provides five
reference resistors.
2-7.
Volts Functions
During the following discussion of the volts function, refer to schematic and signal flow
diagrams in Chapter 5. In volts functions, signal flow for input divider Z1 begins with a
voltage that appears at the z input. (See Figure 2-2, 4V Range Simplified
Schematic.) This input is connected to the high end of the 9.996-MJ resistor (Z1-1)
through R1 and RT1. If the AC volts function is selected, dc blocking capacitor C1 is
also connected in series. If the DC volts function is selected, C1 is shorted by S1
(contacts 5 and 6).
Internal switches connect the 9.996-MJ and 1.1111-MJ resistors (Z1-2 and -3). The
low end of the 1.1111-MJ resistor (Z1-7) is connected to the COM input through S1
contacts 11 and 12. This produces the divide-by-10 ratio needed for the 400 mV dc, 4000
mV dc, 4 V dc, and 4 V ac ranges. The 4 V ac range requires frequency compensation,
which is supplied by C2.
For the 40 V range, internal switches connect the Z1-4 (101.01 kJ) resistor to provide a
divide-by-100 ratio. In the 300.0 V range, Z1-5 (10.01 kJ) produces a divide-by-1,000
ratio. And in the 300 V range, the Z1-6 (1.0001kJ) resistor provides a divide-by-10,000
ratio.
2-4
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