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INSTRUCTION MANUAL
MODEL 662
GUARDED DC
DIFFERENTIAL VOLTMETER
.
WARRANTY
We warrant each of our products to be free
from defects in material and workmanship. Our
obligation under this warranty is to repair or
replace any instrument or part thereof which,
within a year after shipment, proves defective
upon examination. We will pay domestic
surface freight costs.
To exercise this warranty, call your local
field representative or the Cleveland factory,
DDD 216-248-0400. You will be given assist-
ance and shipping instructions.
REPAIRS AND RECALIBRATION
Keithley Instruments maintains a complete re-
pair service and standards laboratory in Cleve-
land, and has an authorized field repair facility
in Los Angeles and in all countries outside the
United States having Keithley field repre-
sentatives.
To insure prompt repair or recalibration serv-
ice, please contact your local field representa-
tive or the plant directly before returning the
instrument.
Estimates for repairs, normal recalibrations,
and calibrations traceable to the National Bu-
reau of Standa'rds are available upon request.
c
MODEL 662 DIFFERENTIAL VOLTMETER CONTENTS
_I
TABLE OF CONTENTS
section Page Section Page
1. GENERAL DESCRIPTION ........ 1 5-5. Adjusting Hum Bal Control . . 19
5-6. Zener Diode Replacement . . . 19
l-1. Description ........ 1 5-7. Meter Adjustment . , . . . . 19
1-2. Operating Modes ....... 1
1-3. Applications ........ 1 6. CALIBRATION . . . . . . . . . . . . 21
1-4. Specifications ....... 2
l-5. Accessories ......... 3 6-l. General . . . . . . . . . . . 21
1-6. Equipment Shipped ...... 3 6-2. Calibration Schedule. . . . . 22
6-3. Kelvin-Varley Divider . . . .
2. OPERATION . .. . . . . . . . . . . . 5 Verification . . . . . . . . 22
2-1. Front Panel Controls 6-4. Range Calibration . ... . . . 23'
and Terminals. . . . . . . . 5 6-5. Reference Voltage Supply
2-2. Rear Controls and Stability Test . . , . . . . 25
Terminals . . . . . . . . . 5 6-6. Oscillator Adjustment . . . . 27
2-3. Preliminary Procedures. . . . 6
2-4. Operating Procedures. . . . . 6 7. ACCESSORIES . . , . . . . . . , . . 35
2-5. Recorder Output . . . . . . . 8
2-6. Effects Due to Kelvin- 7-l. Model 6601A High Voltage
Varley Divider Out- Divider , . . . . . . . . . 35
put Resistance . . . . . . . 9 7-2. Rack Mounting . . . . . . . . 35
2-7. Effective Input Resis- 7-3. Placing in Rack , . . . . . . 36
tance . , . . . . . . . . . 9
2-8. Thermal EMF Precau- 8. REPLACEABLE PARTS . . . . . . . . . 37
tions . . . . . . . . . . . 10
2-9. AC Effects on Measurement . . 10 8-l. Replaceable Parts List. . . . 37
8-2. How to Order Parts. . . . . . 37
3. APPLICATIONS . . . . . . . , . . . 11 Model 662 Replaceable
Parts List . . . . . . . . . 38
3-l. Procedures for Measuring Model 6601A Replaceable
Resistances . . . . . . . . 11 Parts List . . . . . . . . . 44
3-2. Theory of Measuring Resis- Model 662 Schematic Diagram
tances . . . . . . . . . . , 11 185513 , . . . . . . . . . . 47
Model 6601A Schematic Diagram
4. CIRCUIT DESCRIPTION , . . . . . , . 13 16321B . . . . . . . . . . . 49
4-l. General . , . . . . . . . . . 13 e Change Notice . . . . . . . . . . .Last
4-2. Reference Voltage Supply. . . 13 Page
4-3. Kelvin-Varley Divider . . . . 14
4-4. Null Detector . . . . . . . . 14
4-5. Guarding . . . . . . . . . . 15
5. SERVICING . . . . . . . . . . . . . 17
5-l. General . , . . . . . , . . . 17
5-2. Servicing Schedule. . . . . . 17 * Yellow Change Notice sheet is in-
5-3. Parts Replacement . . . . . . 17 cluded only for instrument modifi-
5-4. Troubleshooting . . . . . . . 17 cations affecting the Instruction
MaIlLlal.
1165R
MODEL 662 DIFFERENTIAL VOLTMETER GENERAL DESCRIPTION
SECTION 1.. GENERAL DESCRIPTION
1-1. DESCRIPTION.
a. The Keithley Model 662 Guarded dc Differential Voltmeter is an extremely precise
self-contained, guarded potentiometer. It measures from 100 millivolts to 500 volts with
0.01% limit of error, and below 100 millivolts within 10 microvolts. Since the limit of
error includes all stability considerations, the unit need not be periodically calibrated
or manually restandardised for a full year. The 500-volt reference supply permits infinite
impedance at null for measurements from 0 to 500 volts. Six dials assure that every volt-
age setting has at least 5-dial resolution.
b. Features for convenient use include: six in-line readout dials with automatically
lighted decimal points; 10 to 25-millivolt recorder output; input polarity switch; floating
operation up to 500 volts off chassis ground; line frequency rejection greater than 45 db.
Also, full guarding minimizes leakage problems.
l-2. OPERATINGMODES. The Model 662 can be used as a potentiometer or as a conventional
vacuum tube voltmeter. As a potentiometer, it measures from 100 millivolts full scale to
500 volts with 20.01% limit of error and from 100 microvolts full scale to 100 millivolts
within 10 microvolts. As a VTVM, the Model 662 measures from 1 millivolt full scale to
500 volts with an accuracy of r3% of full scale. It can also measure resistances from 1
megohm to 100,000 megohms within 'sS%.
(.. 13 .
- APPLICATIONS.
a. The Model 662 meets exacting requirements in research, development and production
for accurate voltage measurements. It can be used with a recorder to detect source in-
stabilities down to 25 ppm over weeks of continuous measurements.
FIGURE 1. Keithley Instruments Model 662 Guarded dc Differential Voltmeter.
1065R 1
GENERAL DESCRIPTION MODEL 662 DIFFFRRNTIAL VOLTMETER
b. Typical applications include calibrating power supp.lies and meters; monitoring noise,
transients and drift in precise voltage sources , such as sener diodes and electro-chemical ~'3
cells.
c. The null-detector output permits use with potentiometric recorders and digital volt-
meters equipped with automatic printout. The Model 662 is useful in quality control,
product development, inspection and production.
L-4. SPECIFICATIONS.
AS A POTENTIOMETER
:
LIMIT OF ERROR: TO.Ol% of reading or 10 microvolts, whichever is greater, after 30-minute
warm-up.
LONG-TEPX STABILITY: Will operate within stated limit of error for one year.
TEMPERATURECOEFFICIENT: Does not exceed 0.001% per OC.
REPEATABILITY: Within 0.0025%.
KAXIHUMNULL SENSITIVITY: 100 microvolts fuL1 scale with 3-microvolt resolution.
INPUT RESISTANCE: Infinite at null, from 0 to 500 volts.
FLOATING OPERATION: 500 volts maximum off chassis ground. .\,
./
INPUT ISOLATION: Circuit ground to chassis ground: LOS ohms shunted by 0.05 microfarad.
RFSOLUTION CHART:
Maximum Usable
Input Voltage Maximum Dial Full-Scale Null Meter
Range, Resolution, Sensitivity, Resolution,
volts millivolts millivolts microvolts
50 - 500 1 1 10
5 - 50 0.1 0.1 3
0.5 - 5 0.01 0.1 3
0 - 0.5 0.01 0.1 3
AS A VACLUJMTUBE VOLTMRTER:
VOLTAGE RANGES: 0.5 volt full scale to 500 volts in four decade ranges,
NULL RANGES: 100 microvolts full scale to LOO volts in seven decade ranges.
TCVM ACCURACY: i3% of fulb scale on all ranges, except 5.5% on 100-microvolt range, exclu-
sive of noise and drift.
ZERO DRIFT: Less than 10 microvolts per 24 hours, non-cumulative, after 30-minute warm-up.
INPUT RESISTANCE:: 50 megohms, 0.5 to SOO-volt ranges; -l'
10 megohms, O.l-volt range;
1 megohm, 0.1 to lo-millivolt ranges.
2 1065R
MODEL 662 DIFFERENTIAL VOLTMETER GENERAL DESCRIPTION
-`., LINE FREQUENCYREJECTION: Greater than 45 db.
!'
\ !
GENERAL:
LINE STABILITY: Better than 5 ppm for 10% change in line voltage.
RECORDER OUTPUT:
output: Adjustable 10 to 25 millivolts dc for full-scale meter deflection.
Output Resistance: 300 ohms maximum.
Noise: 2 microvolts peak-to-peak referred to input up to 1 cps.
Note: Recorder used must have fully isolated input, 1010 ohms minimum to ground.
POIARITY: Positive or negative, selectable by switch.
CONNECTORS: Input: Binding posts. output: Banana jacks.
POWER: 105-125 or 210-250 volts (switch selected), 50-400 cps, 50 watts.
DIMENSIONS, WEIGHT: 5-l/2" high x 17-l/2" wide x 13-l/2" deep; net weight, 25 pounds.
1-5. ACCESSORIES.
a. Model 6601A High Voltage Divider is a 1OO:l divider which extends the range of the
Model 662 to 5000 volts. The divider accuracy is 'rO.Ol% and its input resistance is 10
megohms. The overall limit of error of the Model 662 with the Model 6601A is 20.02%.
Section 7 gives operating instructions for the Divider.
b. Model 4000 Rack Mounting Kit, containing two brackets and a top cover, converts the
Model 662 to fit standard 19-inch racks. Rack mounted, the Model 662 is 5-l/4 inches
high x 19 inches wide x 13-l/2 inches deep. Section 7 has assembly instructions.
l-6. EQUIPMENT SHIPPED. The Model 662 Guarded dc Differential Voltmeter is factory-cali-
brated and is shipped with all components in place. All units are shipped for bench use.
Model 4000 Kit may be ordered for rack mounting; refer to Section 7 for assembly instruc-
tions. The shipping carton also contains the Instruction Manual.
1065R
GENERAL DESCRIPTION MODEL.662 DIFFERENTIAL VOLTMETER
FIGURE 2. Model 662 Front Panel Controls and Terminals. Circuit designations refer to
the Replaceable Parts List and the Schematic Diagram. Newer Model 662's have a toggle
switch in place of the OVEN Pilot Light to turn on the instrument.
FIGURE 3. Model 662 Rear Controls and Terminals.
4 1165R
MODEL 662 DIFFERENTIAL VOLTMETER OPERATION
SECTION 2. OPERATION
2-l. FRONT PANEL CONTROLSAND TERMINALS. (See Figure 2.)
a. POLARITY Switch. The POLARITY Switch selects the input polarity by reversing the
polarity of the internal reference voltage supply; thus, both positive and negative volt-
ages may be measured. The Switch does not reverse meter polarity. On older models, the
Polarity Switch also turns the instrument on. Newer models have a toggle switch.
b. NULL Switch. The NULL Switch sets the null detector sensitivity for seven decade
ranges from 0.1 millivolt full scale to 100 volts. When the Switch is in the VTVM posi-
tion, the Model 662 operates as a conventional vacuum tube voltmeter for the four ranges
of the RANGE Switch.
c. RANGE Switch. The RANGE Switch adjusts VTVM sensitivity in four steps: 0.5, 5, '
50 and 500 volts full scale. It also determines the voltage across the Kelvin-Varley
divider and the position of the decimal point light - which also serves as a pilot
light - between the six Reference Voltage Dials.
d. Reference Voltage Dials. Six in-line dials at the top of the front panel set the
reference voltage when the Model 662 is used as a potentiometer.
e. METER ZERO Control. The METER ZERO Control adjusts the meter needle to zero. The
Control is needed on only the 0.1 and l.O-millivolt null ranges; on the other ranges, the
needle will normally be on zero without adjustment. The Control has a range of approxi-
mately 230 microvolts.
f. Input Terminals. The red HI Post is for connections to the high impedance terminal
and the black LO Post is for connections to the low impedance terminal of the unknown
voltage. A second set of binding posts marked LO and GND is provided for grounding the
LO input terminal to the chassis when desired. The LO terminals are connected together
internally.
NOTE
Older models have an OVEN Pilot Light which indicates when the oven controlling
the temperature for the zener diode in the reference voltage supply is on. The
slight clicking sound is characteristic of the oven cycling.
2-2. REAR CONTROLSAND TERMINALS. (See Figure 3.)
a. Fuse. For 105-125 volt operation, the Model 662 uses a l-ampere 3 AG fuse. For
210-250 volt operation, the Model 662 uses a 0.5-ampere 3 AG fuse.
b. Power Cord. The 3-wire power cord with the NEXA approved 3-prong plug provides a
ground connection for the cabinet. A 3:2 prong adapter is also provided.
C. NULL DETECTOROUTPUT. Two terminals, marked + and -, supply a dc signal from the
null detector.
/
d. OUTPUTADJUST. A screwdriver control next `to the OUTPUT terminals adjusts the null
detector output between 10 and 25 millivolts full scale.
106SR 5
OPERATION MODEL 662 DIFFEREXTIAL VOLTMETER
e. 117-234 Switch. The screwdriver-operated slide switch sets the blodel 662 for 117
or 234~volt ac power lines. -3
2-3. PRELIMINARY PROCEDURES.
a. Check the 117-234 Switch and the Fuse'for the proper ac line voltage. Connect the
power cord.
b. Set the Model 662 as follows:
BARGE Switch 500
NULL Switch VTVM
POLARITY Switch +
Reference Voltage Dials Zero
The decimal light between the third and fourth Dials will light. Allow the instrument to
warm up for 30 minutes to meet the specified accuracy on all ranges.
c. With the input terminals open, set the NULL Switch to 0.1 MV and zero the meter with
the METER ZERO Control. Then return the NLTLL Switch to VTVM. The stability of the Model
662 is such that no adjustment should be required after a 30-minute warm-up;
2-4. OPERATING PROCEDURES.
a. The Model 662 is used first ,as a VTVM to determine the approximate value of the
unknown voltage, It is then used in the potentiometric mode to determine the voltage to
20.01%.
[7] 000000
E" 0000
Potential -
A B
FIGURE 4. Input Connections to Hodel 662. The two diagrams show the input circuit for
measuring at ground and for floating.
In A, the unknown voltage has one terminal at ground. The shorting link is between
the LO and GND Posts of the Model 662.
In B, the unknown voltage has both terminals off ground potential. Note this floating .-*
or off-ground potential must be Less than 500 volts. Also note the shorting link is n&
used.
6 0665
MODEL 662 DIFFERENTIAL VOLTMETER OPERATION
Cl NOTE
The Model 6601A High Voltage Divider extends the Model 662 range to 5000 volts.
Refer to Section 7 for operating instructions.
b. VTVM Operating Procedures.
1. Eleven full-scale ranges are available for VTVM operation. When the NULL Switch
is at VTVM, the RANGE Switch determines one of four full-scale ranges. By putting the
six Reference Voltage Dials at Zero, the Model 662 can then operate as a VTVM on the
seven null ranges.
2. Connect the unknown voltage to the input terminals, using the LO Post for the low
impedance terminal of the unknown. Refer to Figure 4.
3. Switch the RANGE Switch to the most sensitive range for an on-scale meter deflec-
tion.
c. Potentiometric Operating Procedures.
NOTE
Avoid large overload voltages on the null detector. No permanent damage will
occur even with 500-volt overloads, but some open circuit offset will be caused
in the null detector. The offset, due to the polarization of the input filter
capacitors, will disappear after about five minutes.
1. Leave the RANGE Switch at the last setting used in the VTVM operation. If the
VTVM reading is negative, reverse the POLARITY Switch position.
2. Set the first two.Reference Voltage Dials to the first two digits of the unknown
voltage found in the VTVM operation.
3. Set the NULL Switch to the initial RANGE Initial Most Sensitiv
null setting shown in Table 1, Adjust Switch NULL Switch NDLL Switch
the Voltage Reference Dials progressively Setting Setting Setting
for zero meter deflection while increasing
the null detector's sensitivity with the 500 v 100 v 10 MV
NULL Switch. Deflections to the right 50 v 10 v 1.0 Mv
indicate the voltage being measured is 5v 1v 0.1 MV
more positive than the Reference Voltage 0.5 v 100 MV 0.1 MV
Dial setting.
TABLE 1. Recommended Null Sensitivities
and Settings.
NOTE
The most accurate resistors in the Kelvin-Varley divider are in the first two
Reference Voltage Dials. Therefore, to obtain the most accurate readings, use
the first two dials as much as possible.
4. The value of the unknown voltage is read directly from the Reference Voltage Dials.
a) The Dial reading will be within the specified limit of error if the NDLL Switch
1065R 7
OPERATION MODEL 662 DIFFERENTIAL VOLTMETER
is at the most sensitive setting (Table 1) for the range used and if the meter indi-
cates as close to null as possible. Null does not have to be reached.
b) Readings can be made from only five Reference.Voltage Dials to be within spec-
ifications (20.01% of reading or 10 microvolts). Using the sixth Dial allows the
instrument to approach null closer, further reducing loading effects upon the source.
2-5. REcoBsm OUTPUT.
a. Recommended recorders for use with the Model 662 are the F. L. Moseley Autograf
680 series recorder and the Minneapolis Honeywell recorder (LOmv-0-1Omv scale, 50 kilohms
input resistance). Any recorder used must be able to float 500 volts off ground and its
input must be fully isolated (1.010 ohm minimum leakage resistance to ground).
b. Before attaching the recorder , set all Reference Voltage Dials to zero. Disconnect
the unknown voltage and short both Model 662 input terminals. Set the NULL Switch to
10 MV. Connect the recorder to the OUTPUT terminals on the Model 662 rear panel.
c. Set the Reference Voltage Dials to LO millivolts to apply an accurate LO-millivolt
potential to the null detector on the lo-millivolt null range. This will provide a full-
scale recorder output which can be matched to the recorder's range between 10 and 25 milli-
volts by adjusting the OUTPUT ADJUST Control.
d. To obtain accurate results and/or to prevent damage to the instruments, the recorder
must be able to float off-ground with the Model 662. Leakage and pickup between the two
instruments should also be minimized. .
j
/
1. wake sure neither recorder terminal
is grounded. Use a 3-wire grounded power
line for the recorder. If a 2-wire line
is used, connect the recorder chassis and 1 Red
the Model 662 chassis with a separate lead. Model
662
2. Minimize all sources of leakage be- DEigj =zf + ;:j:::
tween the output terminals, the recorder
and ground. Use polystyrene or Teflon-
insulated wire where possible. If the
connecting wires are shielded, connect 1 Black
the shield to the LO Post.
3. Avoid long leads between the Hodel .
662 and the recorder. FIGURE 5. Recorder Filter. A filter be-
tween the Model 662 and the recorder may
4. If difficulty is encountered in off- be necessary when using the O.l-millivolt
ground measurements, such as unsW7le read- null detector range,
ings , connect a lo-microfarad ca:?scitor
between the LO and GND terminals .rn the
Model 662 front panel.
NOTE
Do not short either Node1 662 ,.,`:tput terminal to the case; this may damage the
../
Kelvin-Varley divider.
0665
:
MODEL 662 DIFFERENTIAL VOLTMETER OPERATION
i-:: e. If there is substantial recorder jitter on the O.l-millivolt null range, place a
filter between the Model 662 and the recorder. Refer to Figure 5 for this connection.
Note the filter must also be insulated from ground,
2-6. EFFECTS DUE TO KELVIN-VARLEY OUTPUT RESISTANCE.
a. When the Model 662 is used for nulling on the O.l-millivolt range, the last Ref-
erence Voltage Dial may appear to be inaccurate. The apparent error is due to a voltage
drop across the Kelvin-VarLey divider, This effect involves only the null detector sen-
sitivity and not the accuracy of the Dial setting. When the Model 662 is as near to null
as possible, the Reference Voltage Dial setting is correct within the instrument's speci-
fied limit of error. There is no effect present at null.
b. The effect is most apparent on the 0.1, 1 and lo-millivolt null ranges. For example,
on the O.l-millivolt range a O.l-millivolt off-null setting of the reference voltage will
not produce a full-scale meter deflection. This is because the Kelvin-Varley divider out-
put resistance is significant compared to the shunt resistance across the null detector 5
meter. The IR drop across the divider will cause the meter to be off up to 6%, depending
upon the Reference Voltage Dial settings. On the lOO-millivolt range the maximum error
is 1%. The effect cannot be observed on the other null ranges of the Model 662.
c. The amount of deflection on the meter is equal to the ratio
R*
%-I + NW
where R,, is the shunt resistance across the meter (50 megohms for the 100 to l-volt null
ranges, 10 megohms for the lOO-millivolt range, and 1 megohm for the 10 to~O.l.-
millivolt ranges);
Rkv is the output resistance of the Kelvin-Varley divider, which is a maximum of
62.4 kilohms at Reference Voltage Dial settings of 2 4 5 4 5 5 and 2 5 4 5 5 5
and a minimum of 100 ohms at settings of 4 9 9 9 9 8 and 0 0 0 0 0 2.
NOTE
'For a fuller treatment of the effect source resistance has on measurements,
send for the Keithley Instruments Product Note, "The Effective Input Resis-
tance of Potentiometric Voltmeters."
2-7. EFFECTIVE INPUT RESISTANCE.
a. The input resistance of the voltmeter for the seven null ranges varies from 50 to
1 megohm as given in Table 2. This, however, is not the Model 662's effective input re-
sistance. Its input resistance is considerably higher due to the potentiometric princi-
ple of operation. When the reference voltage (Reference Voltage Dial setting) is much
greater than the meter reading, the value is:
&I % Equation 1
Fin" V
where R. is the effective input resistance of the Model 662;
Eh*is the setting of the Reference Voltage Dials in volts;
Rn is the shunt or input resistance of the null detector meter in ohms;
V is the null detector meter reading in volts.
1065R 9
UPERATION MODEL 662 DIFFERENTIAL VOLTMETER
b. To find the loading effect the Model 662 will have on a circuit, use equation 1 to
compute the effective input resistance. At null, where V=O, the input resistance is in- '2
finite. Off null, the input resistance is usually high compared to the.internal resis-
tance of the unknown voltage, and loading will not be enough to affect the measurement
accuracy. The graph in Figure 6 shows the Model 662 effective input resistance for the
three most sensitive null ranges as a percent of meter reading off null (V/Ed).
2-a. THRlWAL KMF PRECAUTIONS. Observe standard thermocouple techniques to reduce
thermal emf errors for measurements using the most sensitive null ranges. Since the
Model 662 can read to 3 microvolts, thermal emf's can introduce considerable errors into
the measurements. In general, use pure copper leads throughout the system when measuring
fn the microvolt rsnge. For extensive measurements in the microvolt region, request the
article, DC Microvolt Measurements, from'Keithley Instruments, Inc., or its representative.
2-9. AC EFFECTS ON i%ASDREMENTS. To minimize errors from ac signals present in the
unknown voltage, the Model 662 employs a chopper-stabilized null detector operating at a
42-cps chopping rate with a 3-section R-C filter at the input. Very large ac components ,
on the measuring lines, however, may reduce off-null sensitivity. Also, heavy 60-cps '
pickup will be observed as needle quiver. If ac components affect measurements by the
Model 662, additional filtering is required. For a single-frequency ac signal, a twin-
T filter is effective. For a variable frequency signal, use an ordinary low-pass filter.
Voltage Input
Range Resistance
300 volts 50 megohms
50 volts 50 megohms
5 volts 50 megohms
0.5 volts 50 megohms
Input
Null Resistance
Range ( Skwing)
100 volts 50 megohms
10 volts 50 megohms
1 volt 50 megohms
100 millivolts 10 megohms
10 millivolts 1 megohm
1 millivolt 1 megohm
0.1 millivolt 1 megohm
TABLE 2. Model 662 VTVM Input Resistance FIGURE 6. Model 662 Off-Null Effective In-
by Ranges. Resistance for Null Ranges is put Resistance. The graph shows the effec-
when the Reference Voltage Dials are set tive input resistance for the 0.1 to lo-
to zero. millivolt ranges. Percent off null is V/Ed "
as defined in Equation 1.
IO 1265R
MODEL 662 DIFFERENTIAL VOLTMETER APPLICATIONS
SECTION 3. APPLICATIONS
3-l. PROCEDURRS FOR MEASURINGRRSISTANCES. The Model 662 can be used to rapidly measure
resistances from 1 megohm to 100,000 megohms with an accuracy of 25%. To measure resis-
tance, connect the unknown resistor across the Model 662 RI and LO terminals. Use a short
isolated lead to the RI terminal to prevent measuring leakage between the leads. Set the
RANGE Switch to 500. Then determine the value of the resistor as follows:
a. For resistances between 1 megohm and 1000 megohms, set the NULL Switch to 10 volts;
adjust the Reference Voltage Dials to obtain a full-scale meter deflection. Subtract
10.000 from the Dial setting and multiply the difference by 5 to obtain the value of the
resistor in megohms.
b. For resistances between 1000 megohms and 10,000 megohms, set the NULL Switch to 1
volt; adjust the Reference Voltage Dials to obtain a full-scale meter deflection. sub- '
tract 1.0000 from the dial setting and multiply the difference by 50 to obtain the value
of the resistor in megohms.
c. For resistances between 10,000 megohms and 100,000 megohms, set the NULL Switch to
1 volt; adjust the Reference Voltage Dials to obtain a convenient deflection on the meter.
Calculate the value of the resistor using,
Rx = 50 Ed megohms Equation 2
v
Where Rx is the unknown resistance;
Ed is the Reference Voltage Dial setting in volts;
V is the meter reading in volts.
3-2. THEORY OF MEASURINGRESISTANCES. The
above method for determining the value of an
unknown resistor is based upon the equation
for the circuit. (See Figure 7;) If an un-
known resistance is across the Model 662
input terminals, then
Ed = i (Rx + Rn) Equation 3
Where Ed is the Reference Voltage Dial set-
ting in volts;
i is the current in the circuit; Ed
Rx is the unknown resistance;
Rn is the input resistance of the null
detector meter in ohms.
The current can be written i = V/R,,, where I
V is the null detector meter reading in
Equation 3 now becomes FIGURE 7. Simplified Model 662 Circuit for
volts. Measuring Resistances. Rr is the unknown
Equation 4 resistance. Rn is the input resistance of
the null detector; V is the null detector;
Ed is the buckout voltage.
1065R 11
APPLICATIONS MODEL 662 DIFFERENTIAL VOLTMETER
If measurements are made on the 1 to LOO-volt null ranges, the input resistance, Rn, is .'-J
50 megohms. Equation 4 becomes ~2
Rx"5x107 Equation 5
This is the basis for the simplified calculations in paragraph 3-1.
7
12 1065R
MODEL 662 DIFFERENTIAL VOLTMETER CIRCUIT DESCRIPTION
.-.
SECTION 4. CIRCUIT DESCRIPTION
4-l. GENERAL. The Model 662 Differential Voltmeter measures voltage by the potentiomet-
ric (null) method. The variable known voltage is an ultra-stable 500-volt reference sup-
ply used in conjunction with a precision multi-dial Kelvin-Varley divider. Electronic
referencing of the 500-volt output to a zener diode standard maintains the reference sup-
ply's stability and accuracy. This method eliminates repeated manual standardization. The
difference between the divider output and the unknown voltage is indicated by the null
detector, a chopper-stabilized vacuum-tube voltmeter. At null the unknown voltage can be
read directly from the in-line dials of the Kelvin-Varley divider. The input and null
detector are fully guarded to avoid leakage.
NOTE
Refer to Schematic Diagram 185513 for circuit designations,
INPUT GUARDED B-DIAL REFERENCE
TERMINALS NULL KELYIN-"ARLEY VOLTAGE
DETECTOR 0l"lOER SUPPLY
FIGURE 8. Simplified Model 662 Circuit Diagram.
4-2. REFERENCEVOLTAGE SUPPLY.
a. Unregulated voltage from transformer T3001 is rectified by a silicon half-wave rec-
tifier, D3001, and is filtered by capacitors C3001 and C3002. The voltage then is applied
to the regulator series pass tube, V3004. Regulator tube V3005 is used to keep the screen
of V3004 at a constant potential.
b. To obtain a stable, accurate voltage, the 500-volt output of V3004 is sampled by a
divider network of wirewound resistors, R3023 to R3028. The divider network ratio is ad-
justed with potentiometer R3025 to better than 0.01%. Light modulator E3002 compares the
) sample voltage from the divider network to the voltage across zener diode D3003. Any dif-
ference between the two voltages is chopped by E3002 and amplified by a two-stage ac-coupled
amplifier, V3001. The amplified output of V3001 is converted to a dc signal by light mod-
1065R 13
CIRCUIT DESCRIPTION MODEL 662 DIFFERENTIAL VOLTMETEX
ulator E3001 and then is amplified by the two-stage differential dc amplifier, V3002 and 1>
v3003. The amplifier output is applied to the grid of the series tube, V3004, to nullify
input variations. Capacitor C3004 is used in the ac feedback circuit.
c. An ultra-stable aener diode, D3003,is used as the basic reference; typical variations
are limited to less than 10 ppm/OC. Thus, a highly stable reference which eliminates man-
ual standardization is provided with respect to both time and temperature. The zener diode
will also withstand shock and vibration.
d. The regulated 500-volt output of V3004 is either applied directly to the Kelvin-Var-
ley divider or Ft is dfvided to 50, 5 or 0.5 volts by very stable wirewound resistor'net-
works. The'XANGE Switch, S3010, determines which network is used. The 50-volt range divl-
der consists of resistors R3030, R3032 and R3033; the S-volt, of R3030, R3035 and F.3036;
and the 0.5-volt of R3030, R3038 and R3039. Using potentiometers R3032, R3035 and B3038
accurately sets the voltage division on each range.
4-3. KELVIN-VART.EY DIVIDER.
a. The Kelvin-Varley divider precisely divides the reference voltage for nulling an
unknown voltage. It is, in effect, a constant input impedance decade potentiometer, con-
sisting of resistors R3040 through R3099. The resistors within each decade are matched;
the decades are matched for each instrument.
b. Each decade of the Kelvin-Varley divider, except the first, R3040 through R3045,
parallels two resistors of the preceeding string. Between the two contacts of the first
Reference Voltage Switch, S3003, the total resistance is 40 kilohms (80 kilohms in paral- y>,
lel with the 80 kilohms total resistance of the four remaining strings). With the 2'
RANGE Switch set at 500, 100 volts dc will appear across the contacts of Reference Voltage
Switches S3004, 10 volts across S3005, 1 volt across S3006, 0.1 volt across S3007 and 0.01
volt across S3008.
4-4. NULL DETECTOR. The Model 662 uses a null detector with a chopper stabilized, feed-
back amplifier. The input signal is attenuated, if necessary, and sent through a three-
stage R-C filter. The signal is then amplified and applied to the meter.
a. The null detector has three full-scale sensitivities, O.l;l and 10 millivolts.
Above the LO-millivolt range, the input is divided by resistors R1004 through R1009 to
the 10 millivolts. The full-scale sensitivities are determined by one of three feed-
back resistors, RLO4C to R1042, in the circuit. A three-stage R-C filter, consisting of
Rl.010, RlOll, C1003, RL012, Cl004, RlOl8 and C1005, decreases the ac input components.
b. The light modulators ElOOl'and El002 convert the difference between the filtered
input voltage and the output of the Kelvin-Varley divider into an ac voltage, which is
fed to a four-stage ac coupled amplifier, VlOOl and VlOO2. The amplifier output is then
demodulated by light modulator El003 and filtered by capacitor C1019. The null detector
meter, MlOOl, indicates the value of the filtered signal. A 42-cps oscillator drives the
light modulators. Using a drive source harmonically unrelated to the standard line
frequency minimizes 60-cps pickup effects.
c. One arm of the feedback network is formed by resistors R1043 to R1046, and one of
the feedback range resistors, R104C to R1042. Resistor R1039 forms the second shunt arm.
The feedback is applied to light modulator E1002.
d. A zero-control network is used to buck out thermal emf's at the input on the two
14 1065R
MODEL 662 DIFFERENTIAL VOLTMETER CIRCUIT DESCRIPTION
most sensitive ranges. The network consists of a 1.34~volt mercury battery, BTlOOl, and
resistors R1013 thrnugh R1016. The zerc control on the front panel, R1016, has approxi-
mately a 60-microvolt span.
e. The null detector output is obtained acrcss resistors R1045 and R1046, which are
in the feedback network. Potentiometer R1045 adjusts the output from 10 to 25 millivolts
at full scale. The output voltage is proportional to the full-scale meter reading.
4-5. GUARDING. Guarding is accomplished by floating the null detector and the input
circuitry at a voltage equal to the input voltage from a low impedance source. This full
guarding eliminates leakage between the input terminal and ground. Such leakage in an
unguarded circuit is difficult to avoid, even under laboratory conditions, and can result
in sizeable errors. For example, in an unguarded circuit with a l-megohm source, leakage
of 108 ohms will introduce 1% error. A guarded circuit eliminates this element of error.
1065R 15
MODEL 662 DIFFERENTIAL VOLTMETER SERVICING
SECTION 5. SERVICING
5-1. GENERAL. Section 5 contains the maintenance and troubleshooting procedures for the
Model 662. Follow these procedures as closely as possible to maintain the accuracy and
stability of the instrument.
5-2. SERVICING SCHEDULE. The Model 662 needs no periodic maintenance beyond the normal
care required of high-quality electronic equipment. No part should need replacement un-
der ordinary use except a pilot lamp, fuse or, occasionally, a vacuum tube.
5-3. PARTS REPLACEMENT,
a. The Replaceable Parts List in Section 8 describes the electrical components in the
Model 662. Replace components only as necessary, and use only reliable replacements
which meet the specifications.
b. Replace resistors within any one of the first three Kelvin-Varley divider decades
only as an assembly. Refer to the Replaceable Parts List for the part number for resis-
tors R3040 to R3099. Resistors R3023, R3024 and R3026 and resistors R3033, R3036 and
R3039 are also parts of assemblies. Reorder using the Keithley part number (see Section
8) and replace all resistors in the assembly. Because of its importance in maintaining the
power supply stability, order sener diode D3003 only from Keithley Instruments, Inc., or
its representative. Refer to paragraph 5-6 for instructions to replace the zener.
5-4. TROUBLESHOOTING.
The following procedures are for repairing troubles which might occur in the Model
66;: Use these procedures to troubleshoot and use only specified replacement parts.
Table 3 lists equipment recommended for troubleshooting. If the trouble cannot be readi-
ly located or repaired, contact Keithley Instruments, Inc.,or its representative.
b. Paragraph 7-2 describes how to remove the Model 662 cover. Before proceeding with
the troubleshooting, check the vacuum tubes. Normally, replacing tubes will clear up the
difficulty. All tubes can be readily tested on a grid-modulated tube tester. If repla-
cing a tube does not correct the trouble, continue the procedures. Replacing tubes does
not necessitate recalibration of the instrument.
C. Table 4 contains the more common troubles which might occur. If the repairs indica-
ted in the Table do not clear up the trouble, the difficulty will have to be found through
Instrument Use
Tektronix Type 503 Oscilloscope Check wave forms
dc voltmeter, 10% accuracy, minimum lOO- Circuit checking
megohm input resistance, 1 volt to 500 volts
Grid-modulated tube tester Test vacuum tubes
TABLE 3. Equipment Recommended for Model 662 Troubleshooting. Use these instruments or
their equivalents.
1065R 17
SERVICING MODEL 662 DIFFERENTIAL VOLTMETER
teference voltage supply
lrifts after 30-minute
ram-up, requiring fre-
Luent adjusting of the One of the divider resis- Return the instrument for
:ange controls, R3025, tom, R3023 to R3028, is factory check-out
L3032, R303.5, R3038 value rapidly dur-
Return the instrument for
easurements ou
:olerance on all ranges
R3023 to R3028 is fault
;pecifications on one divider network is the resistors used for each
:ange other than 500- Try to bring the
rolt range ranges within specifications
by calibrating with the pot-
entiometers in the network.
See paragraph 6-4. If this
does not work, check indivi-
easurements are out 0 ty resistor
specifications on all
:anges other than SOO-
rolt range
TABLE 4 (Sheet 1). Model 662 Troubleshooting.
18
MODEL 662 DIFFERENTIAL VOLTMETER SERVICING
,,-
Difficulty Probable Cause Solution
Measurements are out of One of the Kelvin-Varley See paragraph 6-3
specifications on any divider resistors is faulty
range when the Reference
Voltage Dials are at any
setting other than
49 99 9 10
Instrument is out of Resistor R1003 is faulty Check resistor; replace if
specifications on all faulty
null ranges above 10
millivolts
TABLE 4 (Sheet 2). Model 662 Troubleshooting.
a point-by-point check of the circuits. Refer to the circuit description in Section 4 to
find the more crucial components and to determine their function in the circuit. The
complete circuit schematic, 185513, is found in Section 8.
5-5. ADJUSTING RUM BAL CONTROL.
a. Potentiometer ~1048 (Figure 20) min-
imizes 60-cps pickup in the null detector.
Misadjustment will reduce sensitivity and
cause needle quiver.
b. To adjust the potentiometer, set the
Reference Voltage Dials to zero and short
the input terminals. Attach an oscillo-
scope to the junction of capacitor Cl016
and resistor R1035 (from pin 6 of V1002).
Figure 9 shows the wave form if potentio-
meter R1048 is adjusted for minimum output.
5-6. ZONERDIODE REPLACXMENT. Zener diode,
D3003, is a reference for the voltage divider, FIGURE 9. Wave Form with R1048 Adjusted.
resistors R3023 to R3028. The values of re- A Type 503 Oscilloscope was used; horison-
sistors R.3027 2nd R3028 (Figure 22) are de- tal sweep was 20 msec/cm; vertical, 10
termined by the reference voltage across di- V/Cm.
ode D3003. When the zener is replaced, the
value of these two resistors may have to be
changed.
5-7. METER .AD.TUSTMERT.
a. Potentiometer R1043 (Figure 20) is the internal meter sensitivity adjustment. It
sets the cLrrent through the meter to indicate a full-scale deflection for a full-scale
applied voLtage.
/ b. Warm -:i i-he Model 662 fcr 30 minutes. Set the NULL Switch on the O.l-millivolt
range and se;: the I. ~L'erence Voltage Dials to .OOOlOO. Adjust potentiometer R1043 until
the meter re'~is fui~ si~sle.
1065~ 19
CALIBRATION MODEL 662 DIFFERENTIAL VOLTMETER
FIGURE 10. Models 662 and 66OlA Traceable Chart to National Bureau of Standards. Other
Keithley instruments are also shown.
20
MODEL 662 DIFFERFXCIAL VOLTMETER CALIBRATION
SECTION 6. CALIBRATION
6-l. GENERAL.
a. The following procedures are recommended for calibrating the Model 662. Use the
equipment recommended in Table 5. If proper facilities - especially important for cal-
ibrating an -fO.Ol% instrument -'are not available or if difficulty is encountered, con-
tact Keithley Instruments or its,representative to arrange for' factory calibration.
b. Four procedures are covered: Kelvin-Varley divider verification, voltage range
calibration, reference voltage supply stability test and oscillator adjustment.
C. If the Model 662 is,not within specifications after the calibration, follow the
troubleshooting procedures or contact Keithley Instruments, Inc., or its representative.
Instrument use
Electra Scientific Industries Model SV194B Range voltage calibration
Voltage Calibrator, &0.005% accuracy with
corrections on certificate
Electra Scientific Industries Model RV722 Voltage divider for range calibration
Decade Voltage Divider; terminal linearity,
?-1 ppm; certificate corrected to CO.2 ppm
Electra Scientific Industries Model LC875B Range voltage calibration
Lead Compensator
Hewlett-Packard Model 200CD Oscillator Monitor oscillator frequency
Keithley Instruments Model 150A Micro- Null detector for range calibration
voltmeter
Keithley Instruments Model 241 Regulated Voltage supply for range calibration
High Voltage Supply
Keithley Instruments Model 662 Guarded dc Check voltages in Kelvin-Varley divider
Differential Voltmeter
Mosley Instruments. Model 680 Direct Reading Recorder for reference voltage supply sta
Recorder bility
Tektronix Type 503 Oscilloscope Check wave forms
Weston Instruments Model 3 Type 7 Saturated Range calibration and reference voltage
Standard Cell supply stability
Weston Instruments Model 66 Oil Bath Range calibration and reference voltage
supply stability
TABLE 5. Equipment Recommended for Model 662 Calibration. Use these instruments or their
equivalents.
1065R 21
CALIBP.ATION MODEL 662 DIFFERENTIAL VOLTMETER
circuit Fig. Refer to
Control Desig. Ref. Paragraph
Meter Calibrate R1043 20 5-7
Hum Balance R1048 20 S-5
Oscillator R200 7 22 6-6
500-volt Calibrate R3025 22 6-4
SO-volt Calibrate R3032 22 6-4
5-volt Calibrate x3035 22 6-4
0.5-volt Calibrate R3038 22 6-4
TABLE 6. Model 662 Internal Controls. The Table lists all internal controls, the fig-
ure picturing the location and the paragraph describing the adjustment.
6-2. CALIBRATION SCHEDULE. ,'
a. Recalibrate the Model 662 yearly. This normally means performing the voltage range
calibration (paragraph 6-4). The other verifications need not be done.
b. Verify the Kelvin-Varley divider (paragraph 6-3) only if trouble is suspected in it,
or if components in the divider have been replaced. Also make the voltage range calibra-
tion after verifying the divider accuracy.
c. Check the reference voltage supply stability (paragraph 6-5) only if trouble is
suspected in the supply or if some of its components have been replaced.
6-3. KELVIN-VARLEY DIVIDER VERIFICATION.
a. There is no in-field calibration for the Kelvin-Varley divider: its accuracy can
only be verified. The divider accuracy depends upon matching resistors and switches. At
manufacture, each resistor within the first two Reference Voltage switches, S3003 and
S3004, is matched to rO.O0125%. The resistors in the switches are checked as a set to an
accuracy of better than 0.0025%. Individual resistors cannot be replaced without remat-
ching the string at Keithley Instruments.
b. Kelvin-Varley Divlder~ Accuracy Verification Procedures.
1. Use another Model 662 Differential Voltmeter or equipment with better limit of
error to match the Model 662 under test. Connect the wiper arm of Switch S3008 of the
instrument under test to the RI terminal of the Model 662. Connect both LO terminals.
NOTE
Be careful of high voltages when working within the Model 662. up to 900 volts
dc is present at various points.,
2. Set the dials to random settings on both instruments. Settings should match to
20.01%. This procedure, however, only indicates the Kelvin-Varley divider accuracy,
The errors of the two instruments may be additive, causing a false verification. The
most accurate way is to use standard procedures for checking a Kelvin-Varley divider
or to return the Model 662 to Keithley Instruments, Inc., for checking. J
22 1065R
MODEL 662 DIFFERENTIAL VOLTMETER CALIBRATION
3. If any resistor fails to test out, the entire divider string will have to be re-
matched at the factory.
6-4. RANGE CALIBRATION.
a. The reference voltage supply has a 500-volt output which can be attenuated to SO, 5
or 0.5 volts. This voltage is then divided by the Kelvin-Varley divider to provide the
accurate buckout voltage. Each of.the four voltage ranges is set by internally adjusting
potentiometers R3025, R3032, R3035 and R3038 (Figure 22).
b. The ranges are calibrated by applyin