7.3.1 Power Supply
Schematic 2105-070 sheet 1.
S1 is the front panel toggle switch that connects one side of the power line to one end of the
primary windings of transformer T1. The other end of the primary windings is permanently
connected to the other side of the line. T1 has two primary windings that are connected in
parallel by S10 for operation at 115 volts and in series for operation at 230 volts.
The voltage across one secondary winding of T1 is rectified by D3 and D4 to provide a positive
DC input to voltage regulator U1 which de5 volts at its output. The same secondary is
also rectified by D1 and D2 to provide the negative DC input to voltage regulator U2 which
develops -5 volts at its output. The other secondary is rectified by D7 and D8 to provide a
positive DC input to voltage regulator U3 which de15 volts at its output. The same
secondary is also rectified by D5 and D6 to provide the negative DC input to voltage regulator
U4 which develops -15 volts at its output.
The neutral input terminal is connected to the internal common bus (ground). T1 provides
isolation between the common bus and the power line. The plastic case and other insulators
isolate the operator from the input terminals. However, when taken apart for service,
personnel should verify that the neutral terminal is at the same potential as the power line
ground or serious injury could occur.
7.3.2 Voltage Amplifier and Scaling
Schematic 2105-070 sheets 3 & 4
The neutral line of the input (measured) voltage source is connected to the common internal
bus (ground). The other line is connected to the input of the voltage amplifier circuit through a
600KΩ
resistor, R100, on the rear panel assembly.
Figure 7-2 is a simplified diagram of the operational amplifier circuit U9 and RMS-DC voltage
converter U11. The four switches connected to the input of U9 are sections of the voltage range
relays. The number adjacent to each switch indicates the selected range for which it is closed.
The gain of U9 is determined by the closed section of the relay, which selects the resistance
value for the feedback loop of U9. RV9, RV10, RV11 and RV12 provide fine adjustments of gain
in each range. The output of U9 for full-scale input (30, 150, 300 or 600 volts) is 5 volts. RV8
provides the offset adjustment for U9.
The output of U9 passes through switch RL8, which selects the output of the voltage or current
amplifiers as an input to U11. When the "Volts" mode is selected, the output of U11 is a DC
voltage directly proportional to the RMS value of the input voltage and will be 5 volts full-scale.
It appears across the voltage divider composed of R38 through R41.
31
