Keithley 200A, 200 Service manual
INSTRUCTION MANUAL
MODELS 200,200A
ELECTROMETER VOLTMETERS
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 plant directly, 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 authorized field repair facilities
in Los Angeles and Albuquerque.
To insure prompt repair or recalibration serv.
ice, please contact your local field representative, or the plant directly, before returning
the instrument.
Estimates for repairs, normal recalibrations,
and calibrations traceable to the National Bu-
reau of Standards are available upon request.
MODEL 200, 2OOAELFLYl'ROMEXl'ER
TARLE OF COl4TEN'B
Title
Section I. General .................. L-1
Table of Specificationa ................ l-2
section II. Description ............... 2-l
Section III. Operation ................
EeE!
3-l
Connections ...................... 3-1
Range ......................... 3-l
Zero
Measurement ......................
Increasing Voltage Sensitivity ............
Alternative Methods of Recording ...........
..........................
3-l
3-1
3-3
3-3
Uses ......................... 3-3
Section IV.
Battery Replacements .................
Tube Replacement ................... 4-l
Recalibration Instructions
Insulation Resistance .................
Schematic Diagrams
Appendix - Uses
Maintenance ...............
..............
4-l
4-l
4-l
4-2
0762~
MODEL 200, 200A EI.,EX!TRoMETER
GENERAL
SECTION1 -GENERAL
The Keithley Vacuum Tube Electrometers provide rapid and accurate measurements
of dc voltage in circuits having extremely high internal resistance, and for dc
applications where no current can be drawn from the voltage source.
They are also
widely used as micromicroammeters, megohmmeters, and static detectors. The in-
struments contain a subminiature electrometer tube which operates from batteries
within the case.
Highly stable circuits insure long battery life, very low drift,
instantaneous warmup, and accurate calibration over long periods of time. Principal characteristics are:
A. Ranges, Model 200:
The two ranges are 2 volts and 20 volts full scale;
the pointer can be electrically zeroed at any point on the scale to read positive or negative inputs.
Ranges, Model 2oOA: The two ranges are 2 volts and 8 volts full scale;
the pointer can be electrically zeroed at any point on the scale to read positive or negative inputs.
B. Input Impedance: The input circuit has a resistance higher than 1014 ohms
in parallel with a capacitance of 8 micro-microfarads.
C. Grid Drift Current, Model 200: The open grid drift current is less than
5
x 10eL* ampere on the 2-volt range, and 5 x 10-l-3 ampere on the 20-volt range.
This means that with the RI input terminal floating, the meter pointer
will
drift across the scale on either range in approximately three minutes. The
drift rate is reduced when capacitance is connected across the input
and no drift or zero displacement is observable if a resistance of
10
or less is connected across the input terminals.
Grid Drift Current, Model ZOOA:
range is less than 5 x 10-l" ampere, just as in the Model 200. On the
The open grid drift current on the 2..volt
8-volt
range the current is less than 2 x lo-13 ampere.
05633
l-l
MODEL 200, 200A ELECTROMETER
TABLE OF SPEKIFICATIONS
RANGES: Model 200:
Zero to 2 Volts; Zero to 20 Volts.
Model 2OCA: zero to 2 Volts; Zero to 8 Volts.
AccuRAcy:
ZERO DRIFT:
INPUT IMPEDANCB:
GRID CURRENT: 5 X lo-l4
2% of full scale.
Less than 1% per hour after a 2-hour warmup.
More than 1014
ampere maximum.
OUTPUT RANDWIDTH: DC to 20 kc.
MAXIMCM NOISE LZVRL:
0.5%
of full scale.
INPUT PROTECTION: 1 megohm.
OUTFUTGAIN:
About
OUTHJT IMPEDANCE:
.65
on each range.
50K
(Loads under 5 megohms affect meter reading).
POWER REQUIREMENTS: Batteries:
D cell life, 200 hours; others, 800 hours.
"IWE COMPLEMENT: one
5886.
ohms, 8 micro-microfarads.
One D cell, two type 4l.l, seven penlight cells.
DIMENSIONS:
WZIGRT:
8 in. high X
5
pounds.
5*
in. wide X 4 in. deep.
1-2
0563~
MODEL 200, 200A ELEXXROMEX!FB
DFixxtpmON
SECTION II - DESCRII'TION
The equipment layout and circuit schematic diagrams are enclosed at the back of
the bulletin.
A.
Meter:
The large meter on the face of the instrument indicates the dc poten-
tial inlts across the input terminals.
B. Selector switch, Model 200 (OFF-20-2):
The right hand knob on the panel turns
the instrument off or to the 20-or 2-volt r-es.
Selector switch, Model 2OOA (OFF-8-2):
The right hsnd knob on the panel turns
the instrument off or to the &or a-volt ranges.
C. zero:
The knob at the left is the electrical zero adjustment; with it, the
meterpointer can be set to any point on the scale with the input terminals connected together.
The scale is marked for zero at the left and for zero at the
center.
D. Input Terminals:
E.
Output Terminals:
meters
are located on the panel to the right of the range switch. The white ter-
The input terminals are RI and ground.
The output terminals of the Models 200 and 2OOA electro-
minal is connected to the cathode (F-) of the electrometer tube; the black terminal
is marked I.0 and is connected to ground. Thus, the terminals permit the use of
the electrometer tube as a cathode follower. 10 megohms is the minimwn permissable load resistance for less than 2% change in the panel meter sensitivity. Table
I, below, gives the voltage gain and zero offset (the voltage between the two out-
put terminals with the electrometer input short-circuited and the panel meter set
to zero) for the three ranges
of the battery electrometers.
Table I
Range
Voltage Gain Zero Offset
0.54 2.6 volts
i:: vo1ts
20.0
0.68
0.66
4.6
7.6
F. Input Switch: The input switch 52, is depressed while setting the instrument zero.
It connects the tube grid to the low side of the circuit. The one-
megohm resistor, Rl, prevents short-circuiting the circuit under test.
G.
Calibration Controls:
accessible through holes in the back of the case.
The two range calibration controls, R7 and R8, are
CAUTTON -- W NOT AJNUST THFSE
CONTROLS hITROUT -G 'x) THE RFCALIBRATION INSTRUCTIONS ON PAGE 4-l.
H. Batteries and Tube: The batteries and tube are readily accessible by re-
moving the panel from the case.
0762R
2-l
MODEL 200, 2OOA EL?3CTROIWl'EX
SECTION III - OPERATION
The Model 200 and Model 2OOA VIXUUII Tube Electrometers arc shipped complete with
a fresh set of batteries, and are accurately calibrated at the factory.
applications and circuit possibilities for use of these instruments are discussed
in the appendix.
Connections:
A.
or hi-insulated terminal, of the circuit under test.
input terminal.
Range, Model 200:
B.
as required to read the voltage across the input terminals.
ials ,over 20 volts, the ground terminal can be raised in potential to approximate
the source voltage and the difference rea on the Electrometer, as discussed on
paQ;e U.
1OC:l and 1OOO:l Voltage Divi.ders,'Models 2006 and 2007 respectiveq may be used
to extend the'maxiium voltage to 20,000 volts.
Or, if a shunt resistance of 10
The HI terminal should be connected to the high resistance,
Ground is the other
Set the range switch at the Z-volt or 20-volt position
In measuring potent-
$2
ohms across the source is permissible,
OPEmON
NWIIWOUS
Range,Model ZOOA: Set the range switch at the 2-volt or O-volt position as
required to read the voltage across the input terminals.
over 8 volts the methods discussed on page IA nay be used, or, if a 10
is permissible, 1OO:l and 1OOO:l Voltage Dividers, Models
may be used to extend the maximum voltage to 8000 volts.
Be certain that the range switch is in the OFP position after using the instrument
to avoid unnecessary discharge of the batteries.
C. zero:
rotatzthe ZERO lmob to bring the meter pointer to the desired zero. If the
voltage to be measured is positive, use the zero point at the left of the scale
to take advantage of the full scale range;
negative, use the mid-scale zero; or zero at the right for large negative potent-
ials. Depressing the input switch connects a one-megohm resistance across the
input terminals. If charged capacitors are in the circuit under test, they will
be discharged or brought to the proper initial state of charge, as the case may
be,when the input switch is closed.
vents high surge currents from flowing.
Measurement:
D.
in mind the full-scale voltage corresponding to the position of the range switch
At this point, several precautions should be mentioned.
1.
usually be shielded adequately against stray electrostatic and power frequency
fields by keeping the HI lead short - no more than a few inches long.
The instrument should be zeroed b;r depre oning the input switch and
if the voltage changes sign or is
In any case, the one-megohm resistor pre-
Read the voltage directly on the appropriate scale, keeping
Shielding;:. Circuits with internal resistance less than lOlo ohms can
In measuring~otentials
ohm load
2006
and 2007 respectively,
It is sometimes necessary to enclose the highly insulated portion of the cir-
cuit and lead to the HI terminal in an electrostatic shield. Support insulators may be fashioned from polystyrene or polyethylene stock. The impor-
tance of rigid supports cannot be overemphasized, especially if the shield
is at a high potential with respect to the HI terminal. Small relative motions
0762~ 3-l
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