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TM 9-4935-282-34 DEPARTMENT OF THE ARMY TECHNICAL MANUAL
FIELD MAINTENANCE MANUAL
VOLTMETER, ELECTRONIC JOHN FLUKE MODEL 803D FSN 6625-072-4303
This copy is a reprint which includes current pages from Change 1.
HEADQUARTERS, DEPARTMENT OF THE ARMY MARCH 1964
TM 9-4935-282-34 C1
WARNING
RADIATION HAZARD This equipment contains the following radioactive items: Nomenclature OA2 Vacuum tube OA2 Vacuum tube OA2 Vacuum tube OG3-2 Vacuum tube FSN 5960-503-4880 5960-503-4880 5960-503-4880 5960-912-7630 Isotope UO 2 Ni63 Co60 Unknown Amount (Microcuries) 0.10uc 0.50uc 0.20uc Unknown
Refer to TM 3-261, TM 38-250, and TB 750-237 for information relative to shipping, storage, handling, and disposal of radioactive material. FIRST AID FOR RADIOACTIVE CONTACT The following first aid procedure for wounds caused by anything coated with a radioactive material represents the only reasonable first aid treatment which would possibly be available. a. Stimulation of mild bleeding by normal pressure about the wound and by use of suction cups. WARNING: Do no suck the wound by mouth. The wound must be washed with soap and flushed with plenty of clear water. b . If the wound is of the puncture type, or the opening is quite small, an incision should be made to promote free bleeding and to facilitate cleaning and flushing of the wound. c. Evacuate patient to a medical facility where monitoring of the wound can be accomplished. All such wounds should be examined by a medical officer. d. For wounds involving the extremities, pending medical attention, place a lightly constricting band (tourniquet) 2 to 4 inches closer to the heart than the site of the wound. The band should be tight enough to halt the flow of blood in superficial blood vessels but not tight enough to stop the pulse arterial flow). CLEANING SURFACES ON WHICH TUBES HAVE BEEN BROKEN Wet Method. Put on rubber or plastic gloves. Pick up large fragments with forceps; then, using a wet cloth, wipe across the area. Make one wipe at a time and fold cloth in half, using the clean side for wiping each time. When cloth becomes too small, discard and start again with a clean piece of cloth. Care must be taken not to rub the radioactive particles into the surface being cleaned by using a back and forth motion. All debris and cloths used for cleaning should be sealed in a container such as a plastic bag, heavy waxed paper, ice cream carton, or glass jar for disposal.
TM 9-4935-282-34
ERRATA AND ADDENDUM Model 803D/AC This manual was intended for use with the 803D AC/DC Differential Voltmeters. The 803D/AC, however, is very nearly electrical identical to the 803D/AG. Physical and electrical differences include: 1. 2. 3. The Model 803D/AC is mounted in a combination instrument-transit case made of aluminum. This case meets all environmental requirements of MIL-T-945A. The Model 803D/AC does not have recorder output provisions. Line power input for the Model 803D/AC is by way of an MS3102A-10SL-3P connector on the front panel. A power input cable with a mating connector (MS3102A-10SL-3S), a three-prong polarized connector, and a threeprong to two-prong grounded adapter is furnished. Storage space for the input cable is provided in the lid o the combination case. One set of universal test leads (part no. 699 - Hermin H. Smith, Inc., Brooklyn, New York) is provided with the Model 803D/AC. These test leads are stored in the lid with the power input cable. Heater resistors R6 and R7 are not used in the Model 803D/AC because the instrument is completely enclosed by the transit case. However, a Zener oven containing a heater and a thermostat must be used to maintain the Zener diode reference element at a constant temperature. The Zener diode plugs into the inside of the Zener oven and the Zener oven plugs into a chassis mounted tube socket. The line fuse is located on the front panel of the 803D/AC. The 803D/AC uses a specially enclosed transformer to prevent leakage at higher operating temperatures. The converter output diodes of the 803D/AC are selected for low leakage to provide for proper operation at the higher operating temperatures reached within the transit case.
4. 5.
6. 7. 8.
(a)
TM 9-4935-282-34
To create a parts list for the 803D/AC, change the 803D list of replaceable parts as follows: Delete:
CR501, 502 J4, J5 R1 R6, R7 T1
Front Panel Assembly Converter/AC Range Switch Assembly Diode, silicon, 60 ma Binding post, red Binding post insulator, dual, black Resistor, variable wirewound, 10K, +10%, 2W Resistor, wirewound, 6K +5%, 5W Transformer, power Line cord, 3 wire Rubber foot Case Assembly (cabinet model) Handle, flexible black vinyl, 5-1/2" (cabinet model)
*803D/AG-435 *803D-438 RE22 X2231 *801B-830 P10KA R6KW *803C-651 X27H X224 *803B-271 407-101
Add:
CR501, 502 T1
Front Panel Assembly Converter/AC Range Switch Assembly Diode, silicon, 60 ma, 40 PIV (selected RE22) Transformer, power Case, instrument-transit Connector (mates with line cord) Line cord Test lead set Tube socket (for Zener oven) Zener diode oven (includes heater and thermostat)
*803D/AC-403 *803D/AC-404 *803D/AC-402 *821A-651 *803D/AC-271 M3102A-10SL-3P *803D/AC-401 X2322 X52 X2259
(b)
TM 9-4935-282-34
To create a schematic diagram for the 803D/AC, change the 803D schematic as follows:
c
TM 9-4935-282-34 803D
Material in this manual is used by permission of John Fluke Manufacturing Company, Inc. TABLE OF CONTENTS Section I Title INTRODUCTION AND SPECIFICATIONS 1-1. 1-2. 1-3. II Introduction .......................................................................................... Damage in Shipment ......................................................................... Specifications ....................................................................................... Page 1-1 1-1 1-1 1-1 2-1 2-1 2-3 2-3 2-3 2-3 2-4 2-4 2-5 2-6 2-6 2-7 2-7 3-1 3-1 3-1 3-3 3-4 3-4 4-1 General ................................................................................................ Periodic Maintenance ......................................................................... Corrective Maintenance..................................................................... Calibration ............................................................................................ 4-1 4-1 4-1 4-5 5-1 6-1 Precision Voltage Dividers ................................................................. Potentiometric Recorder ................................................................... 6-1 6-2
OPERATING INSTRUCTIONS 2-1. 2-2. 2-3. 2-4. 2-5. 2-6. 2-7. 2-8. 2-9. 2-10. 2-11. 2-12. Controls, Terminals, and Indicators ................................................ Zeroing Instructions ........................................................................... Preliminary Operation......................................................................... Operation as a DC Differential Voltmeter ....................................... Operation as an AC Differential Voltmeter ..................................... Operation as a Conventional VTVM................................................. Measurement of Voltage Excursions About a Nominal Value.............................................................................. Use of 803D with a Recorder............................................................ Measurement of High Resistances .................................................. Notes on Measuring AC or DC Voltages ........................................ Notes on Measuring DC Voltages ................................................... Notes on Measuring AC Voltages ....................................................
III
THEORY OF OPERATION 3-1. 3-2. 3-3. 3-4. 3-5. General ................................................................................................ DC Vacuum Tube Voltmeter ............................................................. 0 to 500 Volt Reference Supply ....................................................... AC to DC Converter ............................................................................ AC - DC Polarity Switch......................................................................
IV
MAINTENANCE 4-1. 4-2. 4-3. 4-4.
V VI
LIST OF REPLACEABLE PARTS ACCESSORIES 6-1. 6-2. WARRANTY CIRCUIT DIAGRAM
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TM 9-4935-282-34 803D
LIST OF ILLUSTRATIONS Figure Frontispiece 2-1 2-1 2-2 2-3 2-4 2-5 2-6 2-7 3-1 3-2 4-1 4-1 4-2 4-3 4-4 4-5 5-1 5-1 5-2 5-2 5-3 5-4 5-5 5-6 5-7 5-7 5-8 Title Model 803D AC/DC Differential Voltmeter ..................................................... Controls, Terminals, and Indicators (sheet 1 of 2)......................................... Controls, Terminals, and Indicators (sheet 2 of 2) ........................................ Recommended Null Settings ............................................................................. VTVM Ranges .................................................................................................... Full Scale AC Deflections .................................................................................. Recorder Recommended Connection Diagram ............................................ Percent Error Due to Harmonic Distortion ..................................................... Signal Voltage with Converter Noise ............................................................... 803D AC/DC Differential Voltmeter Block Diagram ....................................... Function of Polarity Switch................................................................................. Troubleshooting (sheet 1 of 2) .......................................................................... Troubleshooting (sheet 2 of 2) ......................................................................... Tube Voltage Chart ............................................................................................ Calibration Equipment ....................................................................................... Adjustment Locations ......................................................................................... AC to DC Converter Calibration Set-up........................................................... Final Assembly (sheet 1 of 2)............................................................................ Final Assembly (sheet 2 of 2)............................................................................ Front Panel Assembly (sheet 1 of 2) ............................................................... Front Panel Assembly (sheet 2 of 2) ............................................................... Reference Supply Board Assembly ................................................................. Null Detector Chassis Assembly....................................................................... Range Resistor Board Assembly...................................................................... Kelvin-Varley Resis tor Board Assembly ......................................................... Converter/AC Range Switch Assembly (sheet 1 of 2) ................................. Converter/AC Range Switch Assembly (sheet 2 of 2) ................................. Chopper Drive Assembly .................................................................................. Page iv 2-1 2-2 2-3 2-4 2-5 2-5 2-7 2-8 3-1 3-5 4-2 4-3 4-4 4-5 4-6 4-7 5-3 5-4 5-6 5-7 5-9 5-12 5-14 5-15 5-18 5-19 5-20
iii
TM 9-4935-282-34
iv
TM 9-4935-282-34 803D
SECTION I INTRODUCTION AND SPECIFICATIONS 1-1. INTRODUCTION a. This instruction manual is for use with the 803D series AC/DC Differential Voltmeters. These are available in either a cabinet model or a standard 19 inch rack model, and with either a standard cell or a Zener diode as the reference element. Model designations are: 803D (cabinet model, standard cell); 803DR rack model, standard cell); 803D/AG (cabinet model, Zener diode); and 803DR/AG (rack model, Zener diode). b. The high accuracy, portability, and compactness of the 803D make this instrument ideal for the precise measurement of almost any AC or DC voltage. Ease operation, inherent protection from accidental overload, and high reliability contribute to the outstanding performance that assures suitability for both production line testing and precision laboratory measurements. This instrument is capable of being used as a vacuum tube voltmeter, as a precision potentiometer, and as a megohmmeter for measurement of high resistance. It can also be used to measure the excursions of a voltage about some nominal value. One feature that should be emphasized is that no current is drawn from the unknown source for DC measurements when balance is attained. Thus the determination of the unknown DC potential is independent of its source resistance. The 803D is basically used for measurements of DC voltages from 0 to 500 volts and AC voltages from 0.001 to 500 volts. However, high accuracy measurements to 30,000 volts DC are possible with precision voltage dividers especially designed for use with the Fluke Model 800 Series Differential Voltmeters. As additional features, this instrument contains a polarity switch for equal convenience in measuring positive or negative DC voltages and an adjustable recorder output which makes the 803D particularly useful for monitoring the stability of almost any AC or DC voltage. Furthermore, the 803D meter has taut-band suspension which eliminates problems due to meter stickiness. c. When used as a DC differential voltmeter, the 803D operates on the potentiometric principal. An unknown voltage is measured by comparing it to a known adjustable reference voltage with the aid of a null detector. An accurate standard for measurements is obtained by setting the reference supply with a standard cell for the 803D and 803DR and with a Zener reference diode for the 803D/AG and 803DR/AG. The known adjustable reference voltage is provided by a 500 volt DC power supply and five Kelvin-Varley decade resistor strings that are set accurately by five voltage readout dials. In this way, the 500 volts can be precisely divided into increments as small as 10 microvolts. The unknown voltage is then simply read from the voltage dials. When used as an AC differential voltmeter, the 803D operates essentially the same as for DC differential measurements. The AC input voltage is converted to a DC voltage and this DC voltage is measured by comparing it to a known adjustable reference voltage. In the highest recommended null sensitivity range, potential differences between the unknown and the reference voltage of only 0.001 volts for DC and 0.001 volts for AC will cause full scale meter deflection. d. The instrument is normally supplied ready for use on 115 volts. Upon request, instruments are supplied for 230 volts operation. If it becomes desirable to convert from one mode of operation to the other, refer to the instruction decal on the back of cabinet models or on the cover in rack models. 1-2. DAMAGE IN SHIPMENT Immediately upon receipt, thoroughly inspect for any damage that may have occurred in transit. If any damage is noted, follow the instructions outlined on the warranty page at the back of this manual. 1-3. SPECIFICATIONS
AS A PRECISION POTENTIOMETER DC ACCURACY: +0.02% of input voltage from 0.1 to 500 VDC +(0.02% +25 uv) below 0.1 VDC
1-1
TM 9-4935-282-34 803D
AC ACCURACY:
Input Voltage
Basic AC Accuracy 30 cps to 5 kc 20cps to 10kc +0.15% +(0.15% + 25uv) -
Low Frequency AC Accuracy 10cps to 20cps +0.5% +(0.5%+25uv) 5cps to 10cps +2% +(2% + 25uv) -
High Frequency AC Accuracy 10kc to 20 kc +0.3% +(0.3% + 25uv) 20 kc to 100 kc +1%
0.5 to 500 0.001 to 0.5 0.05 to 50
+0.1% +(0.1% + 25uv) -
RANGE: For DC Measurements
RESOLUTION: Voltage Readout Dial Resolution
Input Voltage Range
Recommended Null Range
Input Resistance Per Volt of Input Voltage At Null Infinite Infinite Infinite Infinite Infinite Infinite Infinite Infinite 1% of Full Scale Off Null 100 1,000 1,000 10,000 10,000 100,000 100,000 100,000 Meg Meg Meg Meg Meg Meg Meg Meg
Range Setting 500 50 5 0.5
Null Setting any any any any
Resolution 10 mv 1 mv 0.1 mv 0.01 mv
50-500 5-50 0.5-5 0-0.5
10-0-10 1-0-1 1-0-1 0.1-0-0.1 0.1-0-0.1 0.01-0-0.01 0.01-0-0.01 0.001-0-0.001
Meter Resolution (1/4 of a small scale meter division)
For AC Measurements Input Voltage Range 50-500 Recommended Null Range 100-0-100 10-0-10 1-0-1 10-0-10 1-0-1 0.1-0-0.1 1-0-1 0.1-0-0.1 0.01-0-0.01 0.1-0-0.1 0.01-0-0.01 0.001-0-0.001 Polarity Setting Input Impedance 1 Meg, 35 uuf 1 Meg, 35 uuf 1 Meg, 35 uuf 1.1 Meg, 35 uuf 1.1 Meg, 35 uuf 1.1 Meg, 35 uuf 1 Meg, 50 uuf 1 Meg, 50 uuf 1 Meg, 50 uuf 1 Meg, 50 uuf 1 Meg, 50 uuf 1 Meg, 50 uuf DC DC DC DC DC AC AC AC AC AC AC AC AC AC AC AC AC Range Setting any any any any any 500 500 50 500 50 5 50 5 0.5 5 0.5 0.5 Null Setting 10 1 0.1 0.01 0.001 1 0.1 1 0.01 0.1 1 0.01 0.1 1 0.01 0.1 0.01 Resolution
50 mv 5 mv 0.5 mv 0.05 mv 0.005 mv 500 mv 50 mv 50 mv 5 mv 5 mv 5 mv 0.5 mv 0.5 mv 0.5 mv 0.05 mv 0.05 mv 0.005 mv
5-50
0.5-5
0.001-0.5
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TM 9-4935-282-34 803D
AS A VACUUM TUBE VOLTMETER ACCURACY: 3% of full scale RANGE: GENERAL SPECIFICATIONS REGULATION AND STABILITY OF 500 VOLT SUPPLY: +0.0025% for a ±10% line change +0.005% per hour after 30 min. warmup STABILITY OF METER ZERO: +0.25% of full scale for a +10% line change TERMINAL LINEARITY OF KELVIN-VARLEY DIVIDER: ±0.0005% (5 ppm) from 0 to 0.1 of full scale ±0.0005% to +0.005% (5 ppm to 50 ppm) from 0.1 full scale to full scale OPERATING TEMPERATURE RANGE: Model 803D (Standard cell reference element) Within DC accuracy specifications from 55°F to 95°F, derated at 0.001%/°F outside these limits to 35°F and 110°F Within AC accuracy specifications from 55°F to 95°F, derated outside these limits to 35°F and 110°F as follows: +0.003%/°F up to 10 kcps +0.005%/°F above 10 kcps Model 803D/AG (Zener diode reference element) Within DC accuracy specifications from 55°F to 95°F, derated at 0.001%/°F outside these limits to 0°F and 125°F Within AC accuracy specifications from 55°F to 95°F, derated outside these limits to 0°F and 125°F as follows: +0.003%/°F up to 10 kcps +0.005%/°F above 10 kcps RELATIVE HUMIDITY: Within specifications from 0 to 80% relative humidity SHOCK AND VIBRATION: Meets requirements of MIL-T-945A INPUT POWER: 115/230 VAC ±10%, 50 - 400 cps, 80 watts (803D), 85 watts (803D/AG) SIZE: Cabinet: 13" high x 9-1/2" wide x 16" deep Rack: 7" high x 19" wide x 15-1/2" deep WEIGHT: Cabinet: Approximately 28 lbs. Rack: Approximately 26 lbs.
Voltage Ranges DC: 500-0-500 50-0-50 5-0-5 0.5-0-0.5 10-0-10* 1-0-1* 0.1-0-0.1* 0.01-0-0.01* 0.001-0-0.001* 0-500 0-50 0-5 0-0.5 0-100** 0-10** 0-1** 0-0.1** 0-0.01** 0-0.001**
Input Impedance 50 Meg 50 Meg 50 Meg 50 Meg 10 Meg 10 Meg 10 Meg 10 Meg 1 Meg 1 Meg, 35 uuf 1 Meg, 35 uuf 1 Meg, 50 uuf 1 Meg, 50 uuf 1 Meg, 35 uuf 1 Meg, 35 uuf 1 Meg, 35 uuf 1 Meg, 35 uuf 1 Meg, 50 uuf 1 Meg, 50 uuf
AC:
*These ranges are obtained by using Null ranges with all five voltage readout dials set to 0. **These ranges are obtained by the proper setting of the range and null switches with all five voltage readout dials set to 0.
1-3
TM 9-4935-282-34 803D
SECTION II OPERATING INSTRUCTIONS 2-1. CONTROLS, TERMINALS, AND INDICATORS The location, circuit symbol, and a functional description of external controls, terminals, and indicators on the 803D Precision AC/DC Differential Voltmeter may be found in figure 2-1.
CONTROLS TERMINALS AND INDICATORS Input terminals Chassis ground terminal
LOCATION
CIRCUIT SYMBOL
FUNCTION DESCRIPTION
Front panel Front panel
J1, J2 J3
Provided for connecting AC or DC voltage to be measured. Provided for grounding purposes. A 0.47 uf capacitor is connected from the lower input binding post to the chassis ground post. The upper input post should never be connected to the chassis ground post. Since the instrument is equipped with a three-wire line cord with the third wire fastened to the chassis, the external circuit should be checked for conflicts in grounding before connecting lower input binding post (middle post) to the chassis post. Applies AC line voltage to the primary circuit of the power transformer. Remains in the OPERATE position at all times except when it is necessary to calibrate the internal 500 volt DC reference supply. When held in the CALIBRATE position, a representative sample of the reference voltage is compared to the voltage of an internal standard cell, or Zener reference diode for the /AG models, and any difference is indicated on the meter. Varies the output of the 500 volt DC reference supply. When the OPERATE-CALIBRATE switch is held in the CALIBRATE position, the reference supply is accurately set by adjusting the CALIBRATE control for zero meter deflection. Select desired voltage rage. Full scale voltage ranges of 500, 50, 5, and 0.5 volts are available. indicates the AC NULL MULT for each range position.
Power toggle switch OPERATECALIBRATE switch
Front panel
S1
Front panel
S4
CALIBRATE control
Front panel
R2
RANGE switch
Front panel
S2
It also
Figure 2-1. CONTROLS, TERMINALS, AND INDICATORS (sheet 1 of 2)
(Figure 2-1 Sheet 1 of 2) (Figure 2-1 Sheet 2 of 2)
2-1
TM 9-4935-282-34 803D
CONTROLS TERMINALS AND INDICATORS NULL switch
LOCATION
CIRCUIT SYMBOL
FUNCTIONAL DESCRIPTION
Front panel
S3
Set to VTVM for determining the approximate value of unknown voltage prior to differential measurements. Five null switch ranges of 10, 1, 0.1, 0.01, and 0.001 volts are used for differential measurements. For the DC mode, the null ranges represent full scale differences between the unknown voltage and the amount of precision internal reference voltage that is set on the voltage readout dials. For the AC mode, the null range used times the applicable AC Null Multiplier, indicated by the range switch, represents the full scale difference between the unknown voltage and the amount set on the voltage dials. Provide an in-line readout of the amount of internal reference voltage necessary to null the unknown voltage. Serve as decimal points for the voltage readout dials. A different light illuminates for each position of the RANGE switch. Selects the AC, + DC, or - DC mode of operation. With this switch in the positive position, the polarity of the upper input binding post is positive with respect to the lower input binding post (middle post). Provided for attaching a recorder to monitor voltage excursions.
A, B, C, D, and E voltage readout dials Decimal lights
Front panel
S6, S7, S8, S9, S10 DS1, DS2, DS3, DS4
Front panel
AC - DC polarity switch
Front panel
S5
Output terminals
Rear panel of cabinet models, front panel of rack models. Rear panel of cabinet models, front panel of rack models. Front panel
J4, J5
Output level control
R1
Varies the output level of the output binding post from 0 to at least 18 millivolts full scale deflection.
Meter
M1
Indicates approximate voltage when 803D is in VTVM mode and difference between unknown and internal reference voltage when 803D is in differential mode. The upper scale, 500-0-500, is used when the NULL switch is set to VTVM at all other times, the lower scale, 1-0-1, is used. Sets meter to zero mechanically. This adjustment should be used only after instrument has been turned off for at least three minutes or when the internal meter terminals have been shorted. Fuse holder protrudes from instrument to provide easy access to the fuse. The fuse is a 1.5 ampere slow blowing type for 115 volt operation and a 0.75 ampere slow blowing type for 230 volt operation.
Mechanical zero control
Meter case
None
Fuse
Rear panel of cabinet models front panel of rack models.
F1
Figure 2-1. CONTROLS, TERMINALS, AND INDICATORS (sheet 2 of 2)
2-2
TM 9-4935-282-34 803D
2-2. ZEROING INSTRUCTIONS From time to time, it may be necessary to adjust the internal meter zero control. This will normally be done a somewhat more frequent intervals than complete instrument calibration. Proceed as follows: a. Mechanically zero the meter with the adjustment screw on the front of the meter case. If the instrument is in the case, it must be shut off for at least three minutes prior to this adjustment. If out of case, another method would be to short out the internal meter terminals prior to zeroing. b. Set power switch to ON and allow a 20 minute warmup period. c. Set RANGE switch to 5 or 0.5 volts, voltage readout is to zero, and NULL switch to VTVM. d. Adjust R232, null detector ZERO ADJ, for zero meter deflection. This control may be reached through the ventilation holes in the cabinet model (see figure 4-3). In the rack model, an identified access hole is provided on the rear panel. 2-3. PRELIMINARY OPERATION The following procedure prepares the voltmeter for operation: a. Connect power plug to a 115 volt AC power source. If instrument has been wired for 230 volt operation, connect to 230 volts AC. NOTE The round pin on polarized three-prong plug connects instrument case to power system ground. Use three-to-two pin adapter supplied with instrument when connecting to a two-contact receptacle. For personnel safety, connect short lead to a suitable ground. b. Set controls on 803D voltmeter as follows: RANGE NULL AC - DC polarity all voltage readout dials power 500 volts VTVM + (positive) 0 (zero) ON 0-0.5 0.01 then 0.001 b. Turn RANGE switch to lowest range that will allow an on-scale reading and note approximate value of unknown voltage as indicated on upper meter scale. c. If meter reads to the left, turn AC - DC polarity switch to the negative position. The meter needle will deflect to the light. This is because polarity of unknown voltage is negative. d. Noting the position of illuminated decimal light, set five voltage readout dials to approximate voltage determined in step b. For example, if approximate voltage is 35 volts, the decimal light between the B and C voltage readout dials will be illuminated. Therefore, set A dial to 3, B dial to 5, and C, D, and E dials to 0. e. Set NULL switch to successively more sensitive null ranges, as indicated in figure 2-2, and adjust voltage readout dials for zero meter deflection in each null position. When the meter needle indicates to the right, the voltage under measurement is greater than the voltage set on the voltage readout dials. When the indication is to the left, the voltage is less than that set on the readout dials. f. Read unknown voltage directly from the five voltage readout dials. INPUT VOLTAGE RANGE 50-500 *RECOMMENDED NULL SETTING 10 then 1 (then 0.1 for voltages from 50 to 100 volts) 1 then 0.1 (then 0.01 for voltages from 5 to 10 volts) 0.1 then 0.01 (then 0.001 for voltages from 0.5 to 1 volt)
5-50
0.5-5
*Any null range may be used with any input voltage range; recommended settings are those most useful. Figure 2-2. RECOMMENDED NULL SETTINGS 2-5. OPERATION AS AN AC DIFFERENTIAL VOLTMETER a. After completing preliminary operation set AC-DC polarity switch to AC. b. Connect unknown AC voltage to input binding posts. c. Turn RANGE switch to lowest range that will allow an on-scale reading and note approximate value of unknown voltage as indicated on upper meter scale. d. Noting the position of illuminated decimal light, set five voltage dials to approximate voltage determined in step c. For example, if approximate voltage is 35 volts, the decimal light between the B and C voltage dials will be illuminated. Therefore, set A dial to 3, B dial to 5, and C, D, and E dials to 0.
c. After a warmup period of at least ten minutes, advance OPERATE-CALIBRATE switch against spring tension to CALIBRATE, and adjust internal 500 volt DC reference supply with CALIBRATE control for zero meter deflection. Release OPERATE-CALIBRATE switch. 2-4. OPERATION AS A DC DIFFERENTIAL VOLTMETER a. After completing preliminary operation, connect unknown voltage to input binding posts. If one side is grounded, always connect it to the lower input post (middle post).
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TM 9-4935-282-34 803D
e. Set null switch to 0.1 and then 0.01 volt, adjusting voltage dials for zero meter deflection in each position. f. Read unknown voltage from the five voltage readout dials. 2-6. OPERATION AS A CONVENTIONAL VTVM If it is desired to use the instrument as a VTVM on additional ranges can be made available by converting the NULL ranges to VTVM ranges. This is made possible by setting the voltage readout dials to zero. Proceed as follows: a. Perform preliminary operation procedures as stated in paragraph 2-3. b. Consult figure 2-3, and select full scale voltage deflection desired. If the approximate value of the voltage to be measured is unknown, select the 500 volt range initially. c. Set AC - DC polarity switch, RANGE switch, NULL switch, and voltage dials as indicated for the range selected. d. Connect voltage to be measured to input binding posts. Deflection to the right indicates that an unknown DC voltage is of positive polarity. An unknown AC voltage will always cause deflection to the right. e. Read voltage from upper or lower scale as listed in figure 2-3. 2-7. MEASUREMENT OF VOLTAGE EXCURSIONS ABOUT A NOMINAL VALUE a. After completing preliminary operation as stated in paragraph 2-3, set AC - DC polarity switch to desired position. b. Connect voltage to be observed to input binding posts. If meter reads to the left, the voltage being measured is negative DC; set polarity switch to the negative position in this case. This will cause meter pointer to defect to the right. c. Set RANGE switch to lowest range which will give an on-scale meter indication and note nominal value of voltage indicated. d. Set the five voltage readout dials to nominal voltage. e. Turn NULL switch to lowest position that will allow voltage excursions to remain on scale. f. Read voltage excursions from meter. Note that full scale right and left meter deflections are equal to the NULL voltage setting for DC measurements. For AC measurements, full scale right and left meter deflections are equal to the NULL voltage setting multiplied by the AC NULL MULT indicated by the RANGE setting. Figure 2-4 shows full scale AC deflections for the recommended settings of the range and null switches.
Full-Scale Deflection DC: 500-0-500 50-0-50 5-0-5 0.5-0-0.5 10-0-10 1-0-1 0.1-0-0.1 0.01-0-0.01 0.001-0-0.001 AC: 0-500 0-50 0-5 0-0.5 0-100 0-10 0-1
AC-DC Polarity Switch
Range Switch
Null Switch
Voltage Readout Dials
Meter Scale
+ + + + + + + + +
500 50 5 0.5 No effect No effect No effect No effect No effect
VTVM VTVM VTVM VTVM 10 1 0.1 0.01 0.001
No effect No effect No effect No effect All zero All zero All zero All zero All zero
Upper Upper Upper Upper Lower Lower Lower Lower Lower
AC AC AC AC AC AC AC
0-0.1 0-0.01 0-0.001
AC AC AC
500 50 5 0.5 500 500 50 500 50 5 50 5 0.5 5 0.5 0.5
VTVM VTVM VTVM VTVM 1 0.1 1 0.01 0.1 1 0.01 0.1 1 0.01 0.1 0.01
No effect No effect No effect No effect All zero All zero All zero
Upper Upper Upper Upper Lower Lower Lower
All zero All zero All zero
Lower Lower Lower
Figure 2-3. VTVM RANGES
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TM 9-4935-282-34 803D
Range Switch Setting 500 Null Switch Setting 1 0.1 0.01 1 0.1 0.01 1 0.1 0.01 1 0.1 0.01 AC Null Multiplier X100 Full Scale AC Deflection (in volts) 100 10 1 10 1 0.1 1 0.1 0.01 0.1 0.01 0.001 CAUTION Do not ground either of the 803D recorder output terminals or either of the A-70 input terminals. If any of these terminals are grounded, current will be drawn from the 803D Kelvin-Varley voltage divider and the accuracy of the 803D will be completely destroyed. b. After connecting the recorder, perform preliminary operation as stated in paragraph 2-3. c. Check for excessive leakage as follows: (1) Connect a DC voltage to the input of the 803D and differentially measure its potential in a recommended null range. (2) Alternately connect and disconnect the recorder leads from the output terminals of the 803D while notting the meter needle deflection. More than one-quarter of a small scale division deflection indicates that excessive leakage has been introduced by the recorder. This will impair the accuracy of the 803D voltmeter. (3) Disconnect the voltage. d. After the leakage test has been successfully completed, short the input terminals of the 803D. e. Set switches on the 803D voltmeter as follows: AC - DC polarity RANGE NULL Voltage readout dials + (positive) 50 10 10.000
50 5
X10
X1
X0.1
0.5
Figure 2-4. FULL SCALE AC DEFLECTIONS 2-8. USE OF 803D WITH A RECORDER Recorder output binding posts and an output level control are provided on the 803D for monitoring the excursions of an unknown voltage from the voltage indicated by the voltage dial settings. If the leakage resistance between the recorder and ground is less than 10,000 megohms, the accuracy of the 803D will be impaired. Also, both input posts of the recorder should be able to stand 500 volts DC to ground. Therefore, the John Fluke Model A-70 Potentiometric Recorder (manufactured by the Texas Instrument Co.) is recommended for this application. Set up the recorder as follows: a. Connect A-70 recorder to 803D voltmeter with teflon leads as shown in figure 2-5.
The 803D meter will indicate full scale deflection. This provides up to at least 18 millivolts at the output terminals depending on the setting of the output level control. f. Turn the output level control until the recorder deflection obtained is that desired to correspond to full scale deflection of the 803D. g. Remove the short from the input terminals of the 803D. The 803D and the recorder are now ready for combined use. Proceed as instructed under paragraph 2-7.
Figure 2-5. RECORDER RECOMMENDED CONNECTION DIAGRAM
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2-9. MEASUREMENT OF HIGH RESISTANCES One of the important features of the 803D voltmeter is its ability to be used as a megohmmeter for rapidly measuring high resistances from 1 megohm to 250,000 megohms. The following equation may be used to compute the resistance in megohms of an unknown connected to the input binding posts: Rx = 10( E - 1) megohms, Em unknown resistance in megohms voltage indicated by voltage readout dials voltage indicated on meter megohms of input resistance of VTVM circuit on 10, 1, and 0.1 null range. deflection. (5) Multiply the amount set on the voltage readout dials by 100 to find the resistance of the unknown in megohms. d. FROM 50,000 MEGOHMS TO 250,000 MEGOHMS. To determine the value of an unknown resistance between 50,000 megohms and 250,000 megohms, proceed as follows: (1) Perform preliminary procedure as stated in paragraph 2-3. (2) Set RANGE switch to 500 and NULL switch to 0.1. (3) Connect unknown resistance between input terminals. (4) Adjust voltage readout dials for a convenient meter deflection. (5) The resistance in megohms may be calculated by substituting the meter reading in volts (Em , 0 to 0.1 volt on bottom scale) and this voltage readout dial setting (E) into the following equation: Rx = 10 E Megohms Em
where: Rx E Em 10
= = = =
When connecting the unknown resistance to the input terminals, use short isolated leads to prevent measuring leakage resistance between the leads. a. FROM 1 MEGOHM TO 500 M EGOHMS. For rapid measurement of resistances from 1 megohm to 500 megohms, proceed as follows: (1) Perform preliminary procedure as stated in paragraph 2-3. (2) Set RANGE switch to 500 and NULL switch to 10. (3) Connect unknown resistance between input terminals. (4) Adjust voltage readout dials for full scale meter deflection. (5) Subtract 10.00 from the amount set on the voltage readout dials to find the resistance of the unknown in megohms. b. FROM 500 MEGOHMS TO 5,000 MEGOHMS. For rapid measurement of resistances from 500 to 5,000 megohms, proceed as follows: (1) Perform preliminary procedure as stated in paragraph 2-3. (2) Set RANGE switch to 500 and NULL switch to 1. (3) Connect unknown resistance between input terminals. (4) Adjust voltage readout dials for full scale meter deflection. (5) Subtract 1.00 from the amount set on the voltage readout dials and multiply the result by 10 to find the resistance of the unknown in megohms. c. FROM 5,000 MEGOHMS TO 50,000 MEGOHMS. For rapid measurement of resistances from 5,000 to 50,000 megohms, proceed as follows: (1) Perform preliminary procedure as stated in paragraph 2-3. (2) Set RANGE switch to 500 and NULL switch to 0.1. (3) Connect unknown resistance between input terminals. (4) Adjust voltage readout dials for full scale meter
2-10. NOTES ON MEASURING AC OR DC VOLTAGES a. ADJUSTMENT OF 500 VOLT REFERENCE SUPPLY. The 500 volt DC reference supply may be adjusted (paragraph 2-3, step c) at any time deemed necessary without heed to the position of the switches and without removing any input or output connections. However, until the instrument has warmed up to an equilibrium temperature (about 1/2 hour), it should be adjusted prior to each specific measurement for best accuracy. When making prolonged measurements, allow one hour warmup time to insure that 500 volt reference supply does not shift during the final warmup phase. b. GROUND LOOP PRECAUTIONS. Ground loop currents should be avoided to assure accuracy when making measurements. Potential differences are often found at different points on power system grounds. When this is the case, current may flow from the power system ground through the 803D and the equipment under measurement and back to the power system ground. To avoid this when system being measured is grounded, do not connect shorting link from lower input binding post to chassis ground post. c. USE OF SHORTING LINK. A 0.47 uf capacitor (C1) is connected from the lower input binding post to the chassis ground post to reduce the effect of circulating AC currents from the transformer. In some cases, it is possible for C1 to acquire a charge. For example, C1 will become charged through leakage resistance over a period of time if there is no external connection to the input terminals and the controls are set as follows: range to 500, null to any null range, polarity to +, and voltage readout dials to several hundred volts. This condition may cause an error on low level measurements (under 5 volts) due to C1 discharging through the Kelvin-Varley divider. Connecting the shorting link from the lower input post to the ground post will discharge C1 and thus prevent an inaccurate indication.
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2-11. NOTES ON MEASURING DC VOLTAGES a. RECOMMENDED NULL RANGES FOR DC MEASUREMENTS. Certain null ranges are recommended for use with each setting of the RANGE switch for the following reasons. With the RANGE switch at 500 volts the last voltage readout dial (E) changes the reference voltage in steps of 0.01 volt. Therefore, the unknown voltage would have to be an exact multiple of 0.01 volt if a null is to be obtained on the 0.1 or 0.01 volt NULL range. Furthermore, it is unlikely that an unknown voltage of a few hundred volts will be stable within 10 millivolts. Finally, the regulation of the reference supply is approximately +0.0025% (+0.0125 volt) for a 10% change in line voltage. Although this is more than adequate when the instrument is used in the recommended way, a badly fluctuating line voltage may cause the 803D to meter the regulation of its own 500 volt reference supply. For example, when measuring 500 volts a line change of 10% may cause the 500 volt reference supply to change as much as 12.5 mv. Although this is small, the 803D will indicate full scale for a change as little as 100 mv or 10 mv if attempting to use the 0.1 or 0.01 null ranges. b. EFFECT OF AC COMPONENTS ON DC MEASUREMENTS. An AC component of several times the unknown DC may be present on the unknown and the 803D will always indicate well within the specifications for frequencies over a few hundred cycles. An AC component may have an adverse effect if it is of a low frequency or if it has a frequency that is a multiple or submultiple of the chopper frequency. A double section low pass filter (R201, C201, R202, and C202) is used at the input of the null detector to reduce any AC present on the DC being measured. At lower frequencies, this low pass filter is less effective and the magnitude of the AC component is more significant. If this frequency is below 100 cycles, the accuracy may no longer be with specifications. For example, a 60 cycle AC voltage that is 10% of the input voltage will cause an error of approximately 0.01% which is well within the specifications. Also, since the input attenuation is less for the more sensitive null settings, the accuracy may be affected only on the more sensitive null settings. When the frequency is very close to a multiple or submultiple of the chopper frequency (approximately 83 cycles), the meter needle will oscillate at the difference frequency. If AC components that affect the accuracy are ever encountered, additional filtering will be required. For an AC of a single frequency, a twin-T filter is effective and has the advantage of low total series resistance. For an AC variable frequency, an ordinary low pass filter may be used. In either case, high quality capacitors of high leakage resistance should be used. c. MEASUREMENT OF NEGATIVE DC VOLTAGES. Because of a polarity switch, voltages which are negative with respect to ground as well as the more commonly encountered positive voltages may be measured with equal facility. If the upper input post is connected to the metal case or line ground, either at the 803D or at the source under measurement, the accuracy of the voltmeter may be reduced. However, with the polarity switch the upper input post never has to be connected to ground. If the unknown voltage is grounded, always connect the grounded side to the lower input post (middle post) and use the polarity switch to obtain the proper result. 2-12. NOTES ON MEASURING AC VOLTAGES a. ERRORS DUE TO DISTORTION. The AC to DC converter in the 803D is an average measuring device calibrated in RMS. The converter will put out a DC voltage that is proportional to 1.11 times the average value of the AC input voltage. Thus, if the input signal is not a true sinusoid, the 803D reading is probably in error because the ratio of RMS to average is usually not the same in a complex wave as in a sine wave. The magnitude of the error is dependent on magnitude of the distortion and on its phase and harmonic relationship with respect to the fundamental. Figure 2-6 indicates how the accuracy will be affected by various harmonics for different percentages of distortion. If the distortion present in the signal is composed of even harmonics and is less than 2%, the error between the 803D reading and true RMS is fairly minor. A larger error can occur if the distortion is composed of odd harmonics, especially the third harmonic. Note that for 2% of third harmonic distortion the error in the reading could range from 0 to 0.687%. % Distortion % Error From True RMS* Maximum Maximum Positive Negative 0.000 0.000 0.000 0.000 0.033 0.167 0.328 0.667 0.020 0.000 0.0001 0.005 0.020 0.033 0.168 0.338 0.687 0.020
Harmonic
Any even harmonic
0.1 0.5 1.0 2.0
Third harmonic
0.1 0.5 1.0 2.0
Fifth harmonic
0.1
0.5 0.099 0.101 1.0 0.195 0.205 2.0 0.380 0.420 *Error depends upon phase relationship between harmonic and fundamental, i.e. error can be any value between maximum positive and maximum negative, including zero. Figure 2-6. PERCENT ERROR DUE TO HARMONIC DISTORTION
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b. ERRORS DUE TO GROUNDING. In the 803D there is a 0.47 uf capacitor connected from the lower input terminal (middle post) to chassis ground. If it is desired to make measurements where the voltage to be connected to the lower input terminal is not at ground potential, a line cord adapter must be used to isolate the 803D chassis from line ground. Otherwise, the 0.47 uf capacitor would place an AC load on the circuit being measured. c. INTERNAL CONVERTER NOISE. When the instrument is shorted in the AC mode, the converter may produce a residual noise output of approximately 100 uv. This noise voltage will cause an insignificant error as long as AC input signals of 1 mv or larger are applied to the instrument. Figure 2-7 shows a typical half wave of the signal voltage at the output of the converter amplifier. It is easily seen that the noise contributes very little to the average value of the signal and is well within the 2.6% accuracy of the instrument at 1 mv. d. 0.001 VOLT NULL RANGE - DC ONLY. The 0.001 volt null range is not recommended for use when measuring AC voltages for several reasons. One reason is that most AC sources are not stable enough. For example, if 0.5 volt is measured with the range set to 0.5 and the null switch set to 0.001, the effective null detector sensitivity is 100 microvolts full scale. Since 100 uv is 0.02% of 0.5 volts, an AC source with a stability worse than 400 parts per million will cause the 803D meter pointer to swing from one end of the meter to the other. Another reason for not using the 0.001
Figure 2-7. SIGNAL VOLTAGE WITH CONVERTER NOISE volt null range for AC is the converter noise discussed in paragraph 2-12c. This noise will cause the meter pointer to move about erratically on the 0.001 volt null range. Although it is difficult to determine how much pointer movement is due to converter noise and how much is due to AC source stability, the AC source stability is usually the major source of erratic movement.
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SECTION III THEORY OF OPERATIONS 3-1. GENERAL a. Figure 3-1 shows the block diagram for the 803D AC/DC Differential Voltmeter. As seen in this figure, the circuit consists basically of an AC to DC converter, a DC vacuum tube voltmeter (vtvm), and a 0 to 500 volt DC reference supply. Refer to the functional schematic following section VI for more detail. This schematic is designed to aid in the understanding of circuit theory and troubleshooting. The signal flow is from left to right and the components are laid out in a functionally logical manner. b. The overall operation of the 803D may be summarized as follows. To measure the approximate value of a DC voltage, the unknown voltage is connected directly to the DC vtvm. To accurately measure a DC voltage, the unknown voltage is connected across the series combination of the DC vtvm and the 0 to 500 volt DC reference supply. The reference supply voltage is then adjusted with the five voltage readout dials until it matches the unknown voltage as indicated by the vtvm. All AC measurements are made by first converting the AC input voltage to a DC voltage by means of the AC to DC converter. The 803D then operates essentially the same as for approximate and accurate DC measurements. c. In order to provide for a more complete understanding of the 803D voltmeter, the following paragraphs describe each section of the circuit in detail. 3-2. DC VACUUM TUBE VOLTMETER a. GENERAL. The DC vtvm is composed of an attenuator, a null detector, and a meter. The heart of the DC vtvm is the null detector in which the DC signal input is modulated by an electromechanical chopper, amplified by a convention four stage resistance-capacitance coupled amplifier, rectified by the chopper, and
Figure 3-1. 803D AC/DC DIFFERENTIAL VOLTMETER BLOCK DIAGRAM
3-1
TM 9-4935-282-34 803D
finally filtered to produce a DC output. The chopper amplifier has a high amount of negative current feedback. This makes the output current approximately equal to the signal voltage divided by the impedance of the feedback network, regardless of the amplifier characteristics. The high negative feedback also makes the amplifier relatively insensitive to the gain changes in individual tubes due to aging and replacement. The output current from the null detector is indicated on a meter that has taut-band suspension. This suspension does away with a friction associated with meter pivot stickiness. Thus, any tendency for the meter pointer to stick at one point of the scale and then jump to another point is completely eliminated. The attenuator is used to reduce the voltage span of each range to a common range usable by the chopper amplifier to produce proper meter deflection. b. NULL DETECTOR. At the input to the null detector, R201, C201, R202, and C202 form a double section low pass filter that reduces any AC component present on the DC voltage being measured. The difference between the voltage appearing at the output of the filter and the voltage developed across the feedback network is converted to an alternating voltage by G1, an 83 cycle chopper. This chopped voltage is amplified by V202A, V202B, and V03A before passing through cathode follower V203B. During half the chopper cycle the output of the amplifier is clamped to approximate null detector common potential by G1 while during the other half the output is filtered by C212 to provide a DC current for the meter. The voltage developed across feedback network R220, R221, and R222 is proportional to the meter (output current. When the chopper provides connection between contacts 5 and 7, this feedback voltage effectively reduces the magnitude of the voltage that is chopped and applied to the input of the amplifier. The impedance of the feedback network (R220, R221, and R222) is adjustable between 8.82 and 9.83 ohms. Since the output current is approximately equal to the signal voltage divided by the impedance of the feedback network, a 1 my signal voltage indicates an output current of 101.7 to 113.4 ua. However, there is a loss due to finite amplifier gain and filtering that leaves the output current around 100 ua which can be set accurately by means of the feedback network. Thus, current feedback makes the output current essentially proportional to the signal voltage. For full scale deflection, a 1 mv signal voltage will cause 100 ua to flow through the meter. c. INPUT ATTENUATOR. In the DC vtvm mode, four positions on the vtvm attenuator selected by range switch section S2C provide the necessary reduction of the 500, 50, 5, and 0.5 volt ranges for proper chopper-amplifier input. For this mode, the resistance of the attenuator and thus the input resistance of the 803D is 50 megohms (R301 through R310). In the DC differential mode, the voltage difference (unknown voltage minus reference voltage) is reduced by four positions on the vtvm attenuator selected by null switch sections S3C and S3D to give full scale deflections corresponding to inputs of 10, 1, 0.1, and 0.01 volts. For full scale deflection corresponding to 0.001 volt, the voltage across the attenuator is fed directly to the chopper amplifier. Although the resistance of the vtvm attenuator is 10 megohms (R305 through R310) for the 10, 1, 0.1, and 0.01 volt null ranges and 1 megohm (R306 through R310) for the 0.001 volt null range; this is not the input resistance of the 803D. The input resistance is determined by dividing the unknown terminal voltage by the current drawn from the unknown. The current drawn from the unknown is equal to the difference between the unknown terminal voltage and the internally known voltage divided by the resistance of the input attenuator. The equation for input resistance can be hence written as: Rin = Eu = Eu Ra = Es (Ra+ Rs ) -Rs where: Iu Eu-E Es -E Rin Eu Iu Es Rs Ra E = input resistance of voltmeter = Es - I u Rs = terminal voltage of unknown = current drawn from unknown = source voltage of unknown = source resistance of unknown = resistance of input attenuator = voltage indicated by voltage readout dials = absolute value (magnitude only)
Since the reference voltage (E) is equal to the unknown voltage (Eu) and the source voltage (Es) at null, no current is drawn from the unknown and the input resistance is therefore infinite. d. In the AC vtvm mode, null switch sections S3C and S3D and AC - DC switch section S5E provide connection to only one position on the vtvm attenuator regardless of where the range switch is set. This is because the output of the AC to DC converter is 5 volts DC for full input on each range. In the AC differential mode, the voltage difference (converter output voltage minus reference voltage) is reduced by the same positions on the vtvm attenuator as for DC differential measurements. Because of this and the fact that the converter puts out 5 volts DC for full input on each range, the null range used must be multiplied by the AC null multiplier indicated by the range switch to find the full scale difference between the unknown voltage and the reference voltage. The input impedance for the AC vtvm and AC differential mode depends on the input impedance of the AC to DC converter and its attenuator. The input impedance is thus dependent on the setting of the range switch and is 1 megohm 35 uuf for the 500 volt AC range, 1.1 megohm 35 uuf for the 50 volt AC range, and 1 megohm 50 uuf for the 5 and 0.5 volt AC ranges. e. NULL DETECTOR POWER SUPPLY. The B+ supply for the null detector is obtained from a half-wave rectifier consisting of diode CR201 and a filter network (C213A, R227, and C213B) that is regulated by an OA2 tube (V204) and series dropping resistor R230. Divider resistors R228, R229, R231, and R232 and diode CR202 provide a compensating voltage for the purpose of adjusting the null detector to zero with R232 when there is no signal input. Diode CR202 keeps one side of R232 at approximately -0.6 volts DC with respect to the null detector common.
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f. The filament of V202 and the chopper drive unit require a stable supply. This is obtained by using a filtered half-wave rectifier (CR203 and C214) that drives a Darlington connection of Q201 and Q202. A divider string R234 and R235 uses the B+ supply to obtain approximately 11.8 volts for pin-5 of V202 while pin-4 is maintained approximately 0 volts through the emitter-base junctions of Q201 and Q202. Any change in the half-wave rectifier voltage causes a corresponding change in the voltage across the emittercollector junction of Q201 so that the filament and chopper voltage remain stable. The filament supply for V203 obtained from an ordinary 6.3 volt AC winding. g. EFFECT OF AC COMPONENTS. The only AC voltage component that will reduce the accuracy of the 803D is one that either saturates the chopper amplifier or one that beats with the chopper frequency. Since the voltage required for saturation is greater than that required for beating, the null detector is most sensitive to an AC component with a frequency that is a submultiple or a low multiple of the chopper frequency. However, this is easy to detect because the meter will beat at the difference frequency. The low pass filter at the input of the chopper amplifier will attenuate any AC component. The magnitude of the AC voltage appearing at the output of the filter depends on both is amplitude and frequency before filtering. For all practical purposes, one should never encounter any trouble above a hundred cycles. Below this, the filter may not attenuate the AC component enough. However, this is not as bad as it appears. A 60 cycle AC voltage that is 10% of the input voltage will cause an error of approximately 0.01% which is well within specifications. If AC components that affect accuracy are ever encountered, additional filtering as set forth in paragraph 2-11 will eliminate the problem. h. GAIN AND ZERO ADJUSTMENTS. Variable resistor R232 in the null detector power supply provides a means of adjusting the output current of the amplifier to zero when there is no input signal. The gain of the amplifier is adjusted by means of R222 in the feedback circuit. i. RECORDER OUTPUT. The recorder output is picked off divider string R225, R1, and R226. Output level control R1 provides a means of adjusting the output voltage up to a maximum of at least 18 millivolts at full scale deflection. The voltage at the output terminals is proportional to the meter reading. 3-3. 0 TO 500 VOLT REFERENCE SUPPLY a. GENERAL. When the 803D is used for differential voltage measurements, an internal voltage is nulled or matched against the unknown voltage. An extremely accurate reference voltage is therefore required. This is obtained from the 0 to 500 volt reference supply. The 0 to 500 volt reference supply is composed of a well regulated 500 volt power supply, a range divider and a Kelvin-Varley five decade attenuator. The output of the 500 volt power supply is applied directly to the Kelvin-Varley attenuator for the 500 volt DC range. In the 50, 5, and 0.5 volt DC ranges, the range divider reduces the voltage to 50, 5, and 0.5 volts before it is applied to the KelvinVarley attenuator. For any AC range, the range divider always reduces the voltage to 5 volts. The Kelvin-Varley attenuator divides its input voltage (500, 50, 5, or 0.5 volts) into 50,000 equal increments any number of which may be selected by setting the five decades with the voltage readout dials. The output of the Kelvin-Varley Attenuator, therefore, provides an extremely accurate reference voltage. b. 500 VOLT POWER SUPPLY. The 500 volt power supply uses three diodes (CR101, CR102, and CR103) and a filter network (R109, R110, C101, and C102) to supply unregulated DC voltage to series passing tube V101. The voltage is regulated by comparing a sample of the output voltage tapped off a divider string (R123, R124, R121, and R2) with the voltage from reference tube V102 in differential amplifier V104. The output of V104 is fed to differential amplifier V105 which drives the grid of series passing tube V101. The control action of the differential amplifiers continuously adjusts the voltage drop across the series passing tube so as to keep the sample voltage equal to the voltage of the reference tube. Any difference in voltage, the error, is amplified by V104 and V105 and thus changes the voltage drop across V101 to maintain the output at 500 volts. c. For proper operation a highly stable and accurate balance of amplification must be maintained between the two halves of differential amplifier V104. The filament supply for V104 must be regulated to maintain this balance. Regulation is provided by a transistor regulator which supplies constant voltage to the filament of V104. The filaments of the AC to DC converter tubes (V501 and V502) are also supplied by this regulator. A filtered full wave rectifier (CR104, CR105, and C103) which is regulated by a three transistor network (Q1, Q2, and Q3) supplies the regulated DC filament voltage. One side of the filaments are connected to the reference supply common (0 volts) while the other side is maintained at approximately 5.9 volts through the emitter-collector junction of Q3. The output of Q1 drives a Darlington connection of Q2 and Q3. Any variation in the unregulated supply causes a corresponding change in the voltage across the emittercollector junction of Q3 so that the filament voltage of V104, V501, and V502 remains stable. d. RANGE DIVIDER. In the 500 volt DC range the output of the 500 volt supply is passed directly to the KelvinVarley attenuator by range section S2F. In the 50, 5, and 0.5 volt DC positions, range resistors (R320 through R330) selected by section S2E divide the reference voltage to 50, 5, and 0.5 volts before it is switched to the Kelvin-Varley attenuator by section S2F. With the AC - DC switch set to AC, AC - DC switch section S5E provides connection to the range resistors that divide the reference to 5 volts. This 5 volts is then passed to the Kevin-Varley attenuator by section S5D. The voltage applied to the Kelvin-Varley attenuator is always 5 volts for AC because the AC to DC converter always supplies up to a maximum of 5 volts to the vtvm attenuator. e. KELVIN-VARLEY ATTENUATOR. The five KelvinVarley decade resistor strings R401 through R449 and associated voltage dials A through E (S6 through S10)
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provide a means of making the four precision voltages (500, 50, 5, and 0.5) adjustable. Note that each string, with the exception of the first, parallels two resistors of the preceding string. Between the two wipers of S6 (voltage dial A) then, there is a total resistance of 40K (80K paralleled by 80K). With the range switch in the 500 volt position, a total voltage of 100 volts DC will appear across these two wipers. Also, there will be 10, 1, and 0.1 volts DC across the wipers of S7, S8, and S9, respectively. Voltage dial E (S10) picks increments of 0.01 volt DC from the last decade. These voltages are reduced by a factor of 10 for each lower voltage range. All resistors of each decade are matched and the three most critical decades (40K, 8K, and 1.6K are matched for each instrument, providing an overall accuracy of 0.005% absolute. With the null switch in any null range, the output of the Kelvin-Varley attenuator is connected in series with the vtvm attenuator thus providing the accurate 0 to 500 volts reference supply required. f. ADJUSTMENTS. Variable resistor R121 is used during calibration to set the 500 volt supply to 500 volts with calibrate control R2 set at its center of rotation. This allows the reference supply to be adjusted to 500 volts by means of the calibrate control at any time deemed necessary. With the operate-calibrate switch held at calibrate, a fixed percentage of the 500 volt supply is compared to the precise potential of an internal standard cell or Zener reference diode in the (AG models). Any difference in potential is fed to the null detector to give an indication on the meter so that the 500 volt supply can be set with the calibrate control. The fixed percentage of the reference supply is set accurately during calibration by means of R318. The 50, 5, and 0.5 volt range resistor networks are set accurately during calibration by means of R323, R326, and R329. 3-4. AC TO DC CONVERTER a. GENERAL. The AC to DC converter is composed of an attenuator, an operational amplifier, and a rectifier-filter circuit. A diode in the rectifier-filter circuit is used to convert the unknown AC into pulsating DC, which is then filtered to obtain a DC voltage that is proportional to the average value of the AC input voltage. The output however, is calibrated to indicate the rms value of a pure sine wave. An operational amplifier containing three resistance-capacitance coupled amplifier stages with high negative feedback is used to make the rectification characteristics of the diodes linear and stable. The amplifier achieves a midband loop gain of approximately 70 db with a virtually flat frequency response from 20 cps to 10 kc. The high negative feedback makes the amplifier practically noise free and relatively insensitive to gain changes in individual tubes due to aging and tube replacement. At the output to the amplifier, full wave rectification is used to return negative feedback to the grid of the first amplifier tube. The attenuator is used to reduce the AC input voltage by a factor of 10 or 100, as required to restrict the operational amplifier input to 5 volts maximum for full scale inputs of 50 and 500 volts respectively. b. OPERATION. All AC measurements are made by first converting the AC input voltage to a DC voltage. The converter provides a DC output of 5 volts when full range voltage is applied to the 803D in each AC range. In the 5 volt AC position, range switch sections S2G and S2H connect the input binding posts directly to the converter input. In this case, the converter feedback is of such a value that the DC output voltage is equal to the rms value of the converter input AC voltage. In the 500 and 50 volt AC positions, an input attenuator reduces the unknown AC by a factor of 100 and 10 respectively. The operation of the converter is then the same as for the 5 volt position. In the 0.5 volt AC position, range switch section S2I and section S2J provide connection to feedback resistor that allow the converter to produce an output that equal to ten times the AC input. Thus, an output of 5 volts DC is provided for full scale input on any AC range. c. NULL INDICATIONS. When making AC differential measurements, the null range used times the applicable AC null multiplier must be used to represent full scale on the 803D meter. This is due to the way that the converter is constructed. For the 500 volt AC range, the converter produces a DC output voltage equal to 1/100 of the AC input voltage. Thus, the AC null multiplier for the 500 volt range position is X100. For example, when the range switch is set to 500 and the null switch is set to 0.01, full scale meter deflection represents a one volt (100 x 0.01) difference between the unknown voltage and the amount set on the voltage readout dials. By similar reasoning, the multipliers for the 50, 5, and 0.5 volt AC ranges are X10, X1, and X0.1, respectively. d. ADJUSTMENTS. For 0.5 volt converter gain, R536 and R545 at the output of the converter are adjusted. The gain of the amplifier for the 5 volt range is adjusted by means of R542 and R541 in the feedback network. The high frequency response of the amplifier input is adjusted by means of C504 while C519 adjusts the high frequency response of the 0.5 volt feedback circuit. The attenuation of the 500 volt attenuator is adjusted with R535 and R544 and the attenuator high frequency response is adjusted with C501. The attenuation of the 50 volt attenuator is adjusted with R503 and R533 and the attenuator high frequency response is adjusted with C523. 3-5. AC - DC PO