4-1. OVERALL DESCRIPTION.
4-2. The Model 410C includes an input network, a modulator - amplifier- demodulator, and a meter circuit. A block diagram of the Model 410C is shown in Figure 4-1.
4-3. Signals to be measured are applied through the
appropriate input lead to the input network. AC voltages are detected in the AC probe, and therefore all signals to the input network are DC. The input network attenuates the DC signal to a level determined by RANGE and FUNCTION SELECTOR settings. The attenuated DC voltage is applied to the modulator which converts the DC to AC for amplification. The amplified AC signal is converted back to DC voltage inthe demodulator and coupled to cathode follower VIB. The cathode follower output to the DC AMPLIFIER OUTPUT connector and meter circuit is a DC voltage proportional to the amplitude of the signal applied to the input. A portion of the voltage to the meter circuit is returned to the modulator as feedback. When the feedback voltage and attenuated DC voltage are nearly equal, the meter stabilizes.
4-4. CIRCUIT DESCRIPTION.
4-5. INPUT NETWORK.
4-6. The input network includes a precision voltage divider, which by means of the FUNCTION SELECTOR and RANGE switches, providesa maximum of 15 millivolts at the modulator input regardless of the range set and signal applied. The ± DCA, ±DCV, OHMS, and ACV modes of operation are discussed below.
4-7. DC CURRENT MEASUREMENTS: Refer to Figure 4-3, throughout this explanation. The purpose of the input network is to provide proper attenuation of currents applied. Currents from 1.5 µa to 150 ma full scale are applied with input impedance decreasing from 9K ohms on the 1.5 µa range to approximately 0.3 ohms on the 150 ma range.
4-6. Tbe change in input impedance is varied by using DC current shunts in conjunction with RANGE switch A2S2. The DC voltage developed across these shunt resistors, when applied through the modulator-amplifier -demodulator network to the meter, provide a deflection on the meter proportional to the DC current being measured.
4-9. DC VOLTAGE MEASUREMENTS. R e f e r t o Figure 4-4throughout this explanation. The purpose of the input network is to accurately attenuate the input signal to a maximum of 15 millivolts at the modu-
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later input. The network presents an input impedance of 10 megohms on the three most sensitive ranges and 100 megohms on all other ranges.
4-10. The resistor R1 (located in the DCV probe) in conjunction with resistors A2R10 through A2R26, provides the 10 megohm input impedance required for the three most sensitive DCV ranges. Resistors A2R4 and A3R30 are shunted out of the circuit by the RANGE switch on the three most sensitive DCV ranges.
4-11. When using the eight less sensitive ranges, A2R4 and A3R30 are placed in series with Rl and A2R10 through A2R26 to present more than 100 megohm impedance to the input.
4-12. A3R30 is used to calibrate full scale on the 1500 volt range. (See Paragraph 5-35.)
4-13. RESISTANCE MEASUREMENTS. The purpose of the input network shown in Figure 4-5is to place
approximately 0. 6 volt DC source in series with a known (reference) resistance. The resistance to be measured is ptaced in parallel with the known resis- tance, which changes the voltage proportionally. The maximum changes in voltage applied to the modulator is 15 mv because of attenuation provided by A2R4, A3R30, and A1R2.
4-14. A DC current of approximately 60 ma is supplied at the junction of A2R22 and A2R23 through A7R10, R2, A2R2 and A2R1 to the input network. The OHMS 
ADJ., R3, sets the meter for full scale
Resistor A2R1 is shorted out in the XIM position of the RANGE switch; resistors A2R1 and A2R2 are shorted out in the X10M range. The resistors A2R2 and/or A2R1 are electrically removed from the circuit to increase the voltage at the junction of A2R22 and A2R23. This is done to compensate for tbe loading of the attenuator (A2R4, A3R30, and A1R2) on these ranges.
4-15. AC VOLTAGE MEASUREMENTS. Refer to Figure 4-6throughout this explanation. Voltage at the AC probe is converted to DC and applied to the input network. The input signal is attenuated to produce a maximum of about 15 millivolts at the modulator input . AC zero adjustment of meter pointer is made with the AC ‘ZERO control.
4-16. MODULATOR-DEMODULATOR.
4-17. Refer to the Amplifier Schematic, Figure 5-10 ,
and to the Mechanical Analogy Schematic, Figure 4-2throughout this explanation.
4-18. The input network applies approximately 15 millivolts DC, for full scale meter deflection (posi- tive or negative, depending on the polarity of the
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