Keithley 660A User manual

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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 (except
tubes and batteries) 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 factory, DDD 216.
795-2666. You will be given assistance 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.
To insure prompt repair or recalibration serv-
ice, please contact your local field representa-
tkve or the plant directly before returning the
instrument.
Estimates for repairs, normal recalibrations,
and calibrations traceable to the National Bureau
of Standards are available upon request.

MODEL 660A DIFFERENTIAL VOLTMETER CONTENTS
TABLE OF CONTENTS
Section Page
1. GENERALDESCRIPTION ........ 1
1-1. Description ......... 1
1-2. Operating Modes ....... 1
l-3. Applications ........ 1
l-4. Specifications ....... 2
1-5. Accessories ......... 3
1-6. Equipment Shipped ... ... 3
2. OPERATION. ............ 5
2-l.
2-2.
2-3.
2-4.
2-5.
2-6.
2-7.
2-8.
2-9.
Front Panel Controls
and Terminals . . . . . . . 5
Rear Controls and
Terminals . . . . . . . . . 5
Preliminary Procedures . . 6
Operating Procedures . . . . 6
Recorder Output . . . . . . . 8
Effects Due to Kelvin-
Varley Divider Out-
put Resistance . . . . . . . 9
Loading and Off-Null
Resistance . . . . . . . . . 9
Thermal EMF Precau-
tions . . . . . , . . . . .lO
AC Effects on Measurement . .lO
3. APPLICATIONS . . . . . . . . . .lL
3-1. Procedures for Measuring
Resistances . , . . . . . .lL
3-2. Theory of Measuring Resis-
tances . . . . . I . . . . .ll
4. CIRCUIT DESCRIPTION . . . . . . . .13
4-l. General . . . . , . . . . .13
4-2. Reference Voltage Supply . .13
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
5-3. Parts Replacement . . . , . .17
5-4. Troubleshooting . . . , . .L7
Section Page
6. CALIBRATION . . . . . . . . . 21
6-l. General . . . . . . . . . . 21
6-2. Calibration Schedule . . . 22
6-3. Kelvin-Varley Divider
Verification . . . . . . . 22
6-4. Range Calibration , . . . . 23
6-5. Reference Voltage Supply
Stability Test . . . . . . 25
6-6. Oscillator Adjustment . . . 27
7.
8.
ACCESSORIES , . . . . . , . . . . 35
7-L. Model 660LA High Voltage
Divider . , . . . . , , . 35
7-2. Rack Mounting . . . . . . . 35
7-3. Placing in Rack . . . . . 36
REPLACEABLE PARTS , . . . . . . : 37
5-5. Adjusting Hum Bal Control . 19
5-6. Zener Diode Replacement . . 19
5-7. Meter Adjustment . . . . . 19
8-l. Replaceable Parts List . . 37
8-2. How to Order Parts . . . . 37
Model 660A Replaceable
Parts List . . . . . . . . 38
Model 6601A Replaceable
Parts List . . . . . . . . 44
Model 66OA Schematic Diagram
182693. . . . . . . . . . 47
Model 6601A Schematic Diagram
1632LB . . . . . . . . . 49
$0Change Notice
-'- Yellow Change Notice sheet is in-
cluded only for instrument modifi-
cations affecting the Instruction
Manual.
1065R

MODEL 660A DIFFERENTIAL VOLTMETER
SECTION 1. GENERAL DESCRIPTION
GENERAL DESCRIPTION
l-1. DESCRIPTION.
a. The Keithley Model 660A Guarded dc Differential Voltmeter is a convenient, self-
contained potentiometer. It measures from 100 millivolts to 500 volts with 0.02% limit
of error, and below 100 millivolts within 20 microvolts. Since the limit of error includes
all stability considerations, the unit need not be periodically calibrated or manually
restandardized for a full year. The 500-volt reference supply permits infinite impedance
at null for measurements from 0 to 500 volts.
b. Features for convenient use include: five 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. OPERATING MODES, The Model 660A can be used as a potentiometer or as a conventional
vacuum tube voltmeter. As a potentiometer, it can measure from 100 millivolts full scale
to 500 volts with i-0.02% limit of error and from 100 microvolts full scale to LOO milli-
volts within 20 microvolts. As a VTVM, the Model 660A measures from 1 millivolt full
scale to 500 volts with an accuracy of -f3% of full scale. It can also measure resis-
tances from 10 megohms to 100,000 megohms within i-5%.
1-3. APPLICATIONS.
a. The Model 660A is used for measurements over a wide range. Typical applications in
the microvolt region are the matching of semiconductors and the monitoring of noise, tran-
sients and drift. High level voltage applications include power supply and meter calibra-
tion, and tube potential measurements.
FIGURE I.. Keithley Instruments Model 660A Guarded dc Differential Voltmeter.
1.065R 1

GENERALDESCRIPTION MODEL 660A DIFFERENTIAL VOLTMETER
b.
Its Long-term stability makes the Model 660A useful in measurements of extended
duration. Measurements of Long-term power supply stability, Long-term drift runs, and
monitoring during environmental and reliability tests are possible uses.
c. The null-detector output permits use with potentiometric recorders and digital volt-
meters equipped with automatic print-out. The Model 660A is useful in quality control,
product development, inspection and production.
d. Floating operation to 500 volts is provided for measurements such as plate potential
differences of balanced amplifiers.
l-4. SPECIFICATIONS.
AS A POTENTIOMETER:
LIMIT OF ERROR: +0.02% of reading or 20 microvolts, whichever is greater, after 30-minute
warm-up.
LONG-TERM STABILITY: Will operate within stated Limit of error for one year.
TFXPERATURECOEFFICIENT: Does not exceed 0.002% per OC.
REPEATABILITY: Within 0.005%.
MAXIMUMNULL SENSITIVITY: LOO microvoltb full scale with 2-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.
RESOLUTION CHART:
Input Voltage
Range >
volts
i
50 - 500
5 - 50
0.5- 5
0 - 0.5
Maximum Dial
Resolution,
millivolts
Maximum Usable
Full-Scale Null
Sensitivity,
millivolts I
Meter
Resolution,
microvoLts
10
1
0.1
0.1
LOO
LO
2
2
AS A VACUUMTUBE VOLTMETER:
VOLTAGE RANGES: 0.5 volt full
scale to 500 volts in four decade ranges.
NULL RANGES: 100 microvolts full scale to 100 volts in seven decade ranges.
VTVM ACCURACY: t3% of full scale on all ranges, exclusive of noise and drift.
ZERO DRIFT: Less than 10 microvolts per 24 hours, non-cumulative, after 30-minute warm-up.
2 0665R

MODEL 660A DIFFERENTIAL VOLTMETER GENEPaL DESCRIPTION
INPUT RESISTANCE: 50 megohms, 0.5 to 500-volt ranges;
10 megohms, O.l-volt range;
1 megohm, 0.1 to lo-millivolt ranges.
LINE FREQUENCYREJECTION: Greater than 45 db.
GENERAL:
LINE STABILITY: Better than 5 ppm for 10% change in Line voltage.
RECORDEROUTPUT:
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.
POLARITY: 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, 45
watts.
DIMENSIONS, WEIGHT: 5-l/2 inches high x 17-l/2 inches wide x 13-l/2 inches deep; net
weight, 24 pounds.
l-5. ACCESSORIES.
a. Model 6601A High Voltage Divider is a LOO:L divider which extends the range of the
Model 660A to 5000 volts. The divider accuracy is ?O.Ol% and its input resistance is
10 megohms. The overall limit of error of the Model 660A with the Model 660lA is ?0.03%.
Section 7 gives operating instructions and Section 8 contains the Replaceable Parts List
and the Schematic Diagram for the Divider.
b. Model 4000 Rack Mounting Kit, containing two brackets and a top cover, converts the
Model 660A to fit standard 19-inch racks, Rack mounted, the Model 660A is 5-l/4 inches
high x 19 inches wide x 13-l/2 inches deep. Section 7 has assembly instructions.
1-6. EQUIPMENT SHIPPED. The Model 660A Guarded dc Differential Voltmeter is factory-
calibrated 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
instructions. The shipping carton also contains the Instruction Manual.
1066R 3

GENERALDESCRIPTION MODEL 660A DIFFERENTIAL VOLTMETER
PIGURE 2. Model 660A Front Panel Controls and Terminals. Circuit Designations refer
to Replaceable Parts List and the Schematic Diagram.
J
FIGURE 3. Model 660A Rear Controls and Terminals.
4 1065R

MODEL 660A DIFFERENTIAL VOLTMETER
SECTION 2. OPERATION
OPERATION
2-1. FRONT PANEL CONTROLSAND TERMINALS. (See Figure 2.)
a.
Power Switch. A toggle switch turns the instrument on when it is set to the ON
position.
b. POLARITY Switch. The POLARITY Switch selects the input polarity. The Switch rever-
ses the polarity of the internal reference voltage supply so both positive and negative
voltages may be measured; it does not reverse the meter polarity.
c. 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 position,
the Model 660A operates as a conventional vacuum tube voltmeter for the four ranges of the
RANGESwitch.
d. RANGE Switch. The RANGESwitch adjusts the sensitivity of the VTVM 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 five Reference Voltage Dials.
e. Reference Voltage Dials. Five in-line dials at the top of the front panel set the
reference voltage when the Model 660A is used as a potentiometer. The last dial is con-
tinuously variable.
f. 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 approx-
imately i30 microvolts.
25. Input Terminals. At the lower left front panel, the black LO Post is for connections
to the low impedance terminal of the unknown voltage and the red HI Post is for connections
to the high impedance terminal. 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.
2-2. REAR CONTROLSAND TERMINALS. (See Figure 3.)
a. Fuse. For 105-125 volt operation, the Model 660A uses a l-ampere 3 AG fuse. FOK
210-250 volt operation, the Model 660A uses a 0.5-ampere 3 AG fuse.
b. Power Cord. The Model 66OR is designed for a 105-125 volt, SO-&O0 cps line source,
unless otherwise specified on the rear panel. The 3-wire power cord with the NEMA approved
3-prong plug provides a ground connection for the cabinet. An adapter for operation from
2-terminal outlets is provided.
c. NULL DETECTOROUTPUT. Two terminals, marked + and -, supply a dc signal from the
null detector.
d. OUTPUT ADJUST.
+4 screwdriver control next to the OUTPUT terminals adjusts the null
detector output between 10 and 25 millivolts full scale.
1066R 5

OPERATION MODEL 660A DIFFERENTIAL VOLTMETER
e. 117-234 Switch. The screwdriver-operated slide switch sets the Model 660A for 117
or 234-volt ac power lines.
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 660A as follows:
Power Switch ON
RANGESwitch 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 NULL Switch to VTVM. The stability of the Model
660A is such that no adjustment should be required in eight hours after a 30-minute warm-up.
2-4. OPERATING PROCEDURES.
a. The Model 66OA is used first as a VTVM to determine the approximate value of the un-
known voltage. It is then used in the potentiometric mode to determine the voltage to iO.OZ"/,.
LH LG
0 0000
I I ,
Unknown
Voltage -
Off-Ground,
Potential -
A
B
m
I
FIGURE 4. Input Connections to Model 660A. 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 660A.
In 13, 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 not
used. -
6 1066R

MODEL 660A DIFFERENTIAL VOLTMETER OPERATION
NOTE
The Model 6601A High Voltage Divider extends the Model 660A range to 5000 volts.
Refer to Section 7 for operation 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
five Reference Voltage Dials at zero, the Model 660A 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 deflection
c. Potentiometric Operating Prbcedures.
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 5 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 un-
known voltage found in the VTVM operation.
Initial Most Sensitive
NULL Switch NULL Switch
Setting
3. Set the NULL Switch to the initial
null setting shown in Table 1. Adjust
the Voltage Reference Dials progressively
for zero meter deflection while increasing
the null detector's sensitivity with the
NULL Switch. Deflections to the right TABLE 1. Recommended Null Sensitivities
indicate the voltage being measured is and Settings.
more positive than the Reference Voltage
Dial setting.
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 unknwon voltage is read directly from the Reference Voltage Dials.
a) The Dial reading will be within the specified limit of error if the NULL Switch
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.
1066R 7