TM 11-6625-1614-15
Section IV
Paragraphs 4-19to 4-31
voltage or current being measured) to the neon-photo- conductor chopper. Also applied to the opposite side of the chopper is the amplifier feedback voltage, which is of the same polarity and approximately 5 micro- volts lower in amplitude than the input voltage. The modulator-chopper consists of two photoconductors, A4V1 and A4V2, which are alternately illuminated by two neon lamps, A4DS1 and A4DS2, respectively. The neon lamps are part of a relaxation oscillator, whose frequency is controlled by A3R5. The oscillator fre- quency is nominally set to 100 cps for operation from 60 cps power line, or to 85 cps for 50 cps line. This frequency is selected so that it is not harmonically related to the power line frequency, precluding pos- sible beat indications on the meter.
4-19. As the photoconductors are alternately illumi- nated by the neona, their respective resistances are low (conductive ) when illuminated and high (non-con- ductive) when darkened. Therefore the input voltage and feedback voltage are alternately applied to the input amplifier. The amplitude of the resultant signal to the amplifier is the voltage difference between the input and feedback voltages.
4-20. The chopped DC signal is amplified by a three
stage RC amplifier, consisting of A3V1A, A3Q1 and A3Q2. The amplified signal to the input of the de- modulator-chopper is 180° out of phase with the out- put of the modulator-chopper.
4-21. The demodulator - chopper consists of two photoconductors, A4V3 and A4V4, which are alternately illuminated by neon tamps A4DS1 and A4DS2, respecti- vely. Approximately 150 millivolts square-wave is
applied to the demodulator from the amplifier. Since the same neon lamps illuminate both the modulator
and demodulator photoconductors, operation of the two chopper is synchronous. Therefore, when A4V1 is sampling the input voltage, A4V3 is clamping the amplified and inverted difference voltage to ground. Alternately, when A4V2 is sampling the feedback vol- tage, A4V4 is charging capacitors A3C13 and A3C14 to the peak value of the square-wave. These capaci- tors maintain this charge so long as the input voltage remains constant by virtue of having no discharge path and because they are being repetitively recharged by the demodulator.
4-22. Therefore, a DC potential, proportional to the difference between the input and feedback voltages, is
applied to the grid of the cathode follower and subse- quently to meter circuit and DC AMPLIFIER OUTPUT connector. A portion of the meter circuit voltage is fed back to the modulator. The meter stabilizes when the feedback voltage and input voltages are nearly equal.
4-23. THE FEEDBACK NETWORK.
4-24. The feedback network drives the meter and determines the DC gain of the amplifier. The feed- back is varied depending on the position of the FUNC- TION and RANGE selectors. The different feedback configurations are discussed below.
4-2
Model 410C
4-25. FEEDBACK NETWORK FOR ±DCA. OHMS, AND ±DCV. Figures 4-3, 4-4 and 4-5 show the feed- back configuration for ail positions of the FUNCTION SELECTOR except ACV. The meter is electrically inverted for ±DCV and ±DCA modes of operation. The DC OUTPUT ADJ., A6R20 sets the output voltage. The DC pot, A6R18 determines the amount of feedback to the modulator. The resistor A2R30 is in the circuit in the ± .015 DCV and ±1.5 µa modes of operation, to decrease feedback and thus increase amplifier gain to compensate for the decrease in input signal to the modulator on these ranges.
4-26. FEEDBACK CIRCUIT FORAC VOLTAGE MEA- SUREMENTS: Figure 4-6 shows the feedback confi- guration for the ACV position of the FUNCTION SEL- ECTOR switch, A2S2. The resistors that are placed in the circuit by the RANGE switch program the am- plifier gain to compensate for the non-linear response of the AC probe. A6R16 and A6CR1 compensate the non-linear response of the AC probe to the linear calibration of the upper meter scale on the 5 volt range.
4-27. POWER SUPPLY.
4-28. PRIMARY POWER. Either 115 or 230 volt ac power is connected through fuse R1 (0.25 amp slo-blo) and switch S3 to the primary of power transformer T1. Switch S4 connects T1 primaries in parallel for l15 volt operation of in series for 230 voit operation.
4-29. UNREGULATED AND ZENER REGULATED POWER SUPPLY. Full wave rectifier CR1 and CR2 produces unregulated +270 volts, which is used to drive the photochopper neons. Unregulated +175 volts and +140 volts are tapped off and are used to provide B+ to the plates of A4V1B and A4V1A, respectively. Zener regulators A7CR6 and CR7 provide regulated +38 volts and -9 volts to bias A3Ql and A3Q2. Filtering of the outputs is provided by the RC network consisting of A7R1 through A7R3 and C5A through C5D.
4-30. SERIES REGULATED POWER SUPPLY. The output of the full wave rectifier CR3 and CR4 is re- gulatedbytransistor Ql, which is connected in series with the output. Zener diode A7CR8 provides reference voltage to the base of Q1. Regulated +6 volts is supplied to the filaments of A3VlA/B and the AC Probe diode A6V1. +0.6 volts is provided through A7R10 to R3, the OHMS
ADJ, control. Filtering of the outputs is provided by C6A and C6B.
4-31. STANDBY FILAMENT SUPPLY. The filament tap (Tl, Pins 1 and 2) provides 6.0 volts actothe filament of the AC probe diode, A8V1, so that the filament remains warm when the Modei 410C is being used in modes of operation other than ACV. W h e n FUNCTION selector A1S1 is switched to ACV, 6.0 volts AC is removed from the filament and 6 volts DC is applied. Therefore, the ACV mode is ready for imrnediate use, without waiting for the filament to warm up.
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