Keithley 155 User manual

1*
INSTRUCTION MANUAL
MODEL 155
NULL DETECTOR
MICROVOLTMETER
K E I T Pi L E Y I IST S T P^ U AP E IST T S

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,
ODD 216-248-0400. You will be given assist
ance and shipping instructions.
re,-
/ .-'S
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 Standards are available upon request.

MODEL 155
CONTENTS
CONTENTS
Section
Page
j-CAiiui\s
2. OPERATION---
5
8
4. SERVICING---- - - --------
5. GALIBRATION-- ----------
18
6« ACCES SORIEb------- - — - -- ---- -- --
7. REPLACEABLE PARTS
0373

SPECIFICATIONS
MODEL 155
SPECIFICATIONS
RANGE: ±1 microvolt full scale to ±1000 volts.
ACCURACY: ±1% of full scale at recorder output, ±2%
of full scale at meter, exclusive of noise and drift.
ZERO DRIFT: Less than 0.5 microvolt per 24 hours, typi
cally less than 0.1 microvolt per °C. Long-term drift is
non-cumulative.
METER NOISE: Less than 0.03 microvolt rms (0.15 micro
volt peak-to-peak) on most sensitive range with input
shorted.
INPUT RESISTANCE:
100 megohms — 3-volt to 1-kilovolt ranges;
10 megohms — 300-millivolt to 1-volt ranges;
1 megohm — 1-microvolt to 100-millivolt ranges.
NORMAL MODE REJECTION: An applied 50-60 Hz signal
which is 80 dB greater than full scale peak-to-peak will
not affect reading on most sensitive range (equivalent
to 100 dB NMRR).
COMMON MODE REJECTION: Common mode voltage —
dc or 50-60 Hz—120 dB greater than full scale up to
1200 volts peak will not affect reading (equivalent to
140 dB CMRR).
ISOLATION: Greater than IO12 ohms shunted by 0.01
microfarad between chassis ground (case) and input low.
RISE TIME (10%-90%): Less than 1 second on 10-micro-
volt range and above, increasing to 5 seconds on 1-
microvolt range.
ZERO SUPPRESSION: ±25 microvolts.
RECORDER OUTPUT: ±1 volt at up to 1 milliampere.
OVERLOAD: Up to 1200 volts peak may be applied on
any range. Recovery from overload 106 times full scale
for 1 second with 10-kilohm source is within 5 seconds
on the 30-microvolt and higher ranges.
CONNECTORS: Output: Barrier Strip. Input: Binding
Posts.
POWER: Four internally mounted zinc-carbon batteries
(2N6) provide more than 1000 hours continuous opera
tion. Barrier strip provided for external power supply
(+ and -15 volts unregulated).
DIMENSIONS, WEIGHT: 51/4" high x 8V4" wide x 6%"
deep; net weight, 6 pounds.
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0472

MODEL 155 MICROVOLTMETER
GENERAL DESCRIPTION
SECTION 1. GENERAL DESCRIPTION
1-1. GENERAL. The Keithley Model 155 is a com
pletely solid state, rugged, battery-operated com
bination Null Detector-Microvoltmeter. It measures
from 1 microvolt full scale to 1000 volts in 19 Ix
and 3x steps and has 150 nanovolts resolution. The
recorder output, accurate to 1% of full scale exclu
sive of noise and drift, extends the versatility of
the instrument.
1-2. FEATURES.
a. Excellent immunity to ac interference allows
the Model 155 to detect dc signals in the presence
of large ac voltages. The Microvoltmeter-Null De
tector has greater than 140 dB CMRR and 100 dB NMRR
(refer to specifications in Table 1) . Also, hook
up to source is simple and quick. Unshielded leads
may generally be used without degrading performance.
b. The Model 155 Null Detector can recover from
100-volt overloads within 5 seconds on the 30-micro-
volt range. Up to 1200 volts peak may be applied
momentarily on any range without damaging the in
strument.
c. Stability is better than 0.5 microvolt per 24
hours after warm-up with a reasonably constant am
bient temperature. The long-term drift is non-cu
mulative .
d. The ten-turn ZERO Control permits easy adjust
ment of instrument zero. It also provides up to at
least ±25 microvolts suppression, which allows mea
suring submicrovolt changes in signals up to the
limit of the suppression.
e. Zero Check position on the Power Switch allows
convenient zeroing of the instrument by shorting
the input. This means the measuring circuit need
not be disturbed by disconnecting and shorting the
input cables.
f. High input resistance of 1 to 100 megohms cou
pled with less than 10"^^ volt per ohm zero shift
with source resistance permits measurement accuracy
even with high resistance sources.
g. The Model 155 is designed for battery operation
to minimize ground loop and high frequency pibk-up
problems in sensitive voltage measurements. Four
internally mounted zinc-carbon batteries provide
greater than 1000 hours operation. The Model 155
may also be operated from external supplies. These
may be connected to the barrier strip on the rear
panel. Power requirement is ±15 to ±25 volts at 2
milliamperes.
h. For line operation, the Model 1554 Power Sup
ply is a convenient accessory. It attaches to the
rear panel of the instrument and its output may be
connected to the barrier strip provided also on the
rear panel. A switch on the supply provided select
able line or battery operation. Line operation
with the Model 1554 maintains excellent floating
characteristics and negligible coupling to line.
i. Four binding posts on the front panel provide
fast and convenient input connection. A ±1 volt at
1 milliampere output is provided on the rear panel
for convenient connection to recorders or other
readout devices. Accuracy is ±2% of full scale at
the meter and ±1% at the recorder output. A large
4-1/2 inch taut-band meter is provided for ease of
readout.
j. The Model 155 is completely solid state, uti
lizing a MOS-FET chopper in its input circuit. The
solid-state chopper requires little power to drive,
which gives long battery life and permits the use
of inexpensive batteries. It also has low noise
and increased sensitivity.
k. Another significant design characteristic of
the Model 155 is its electrical and mechanical rug-
gedness. All components except for the meter, bat
tery , input and output connectors are mounted on a
single printed circuit. For calibration or servic
ing the circuit board may be conveniently and
quickly removed from the instrument.
1. Circuit isolation from chassis ground is
greater than 10±2 ohms shunted by 0.01 micro
farad. This high isolation generally eliminates
the need for guarding the Model 155.
1-3.
APPLICATIONS .
a. As a null detector the Model 155 can be used
with potentiometers, bridges, ratio devices and
comparator circuits.
b. As a microvoltmeter it is ideal for measuring
semiconductor resistivity, thermopile and thermo
couple potentials. Hall-effect potentials, contact
resistances, biologically generated emfs, electro
chemical potentials, and strain guage outputs.
c. Other applications include monitoring power
supply stability, DTA measurements, resistance
thermometry and thermal conductivity measurements.
d. The Model 155 is useful as a general purpose
instrument in the research laboratory as well as in
electronic development work and process control ap
plications .
e. Its excellent sensitivity limits the need for
expensive potentiometer systems to measure micro
volt level signals accurately. It can measure a
signal of 30 microvolts or less as accurately as
some complex potentiometer systems.
0472R

OPERATION
MODEL 155 MICROVOLTMETER
TABLE 2. Model 155 Front Panel Controls (Figure 1) .
The Table briefly describes each control and indicates the paragraph which contains instructions on the use
of the control.
Control
Functional Description
Paragraph
Power Switch
Turns instrument off, checks zero, sets instrument for
normal operation, checks battery condition.
2-3, 2-4
ZERO Control
Allows input zeroing.
2-4, 2-9
+ Terminal (input hi)
Connects input to signal source.
2-1, 2-4
- Terminal (input lo)
Connects input to signal source.
2-1, 2-4
GUARD Terminal
Provides partial guarding for circuitry
2-1
CASE Terminal
Connects instrument case to ground.
2-1
RANGE Switch
Selects full scale voltage sensitivity.
2-4
The Table briefly describes
of the terminal.
TABLE 3. Model 155 Rear Panel Terminals (Figure 2) .
each terminal and indicates the paragraph which contains instructions on the use
Control
Functional Description
Paragraph
GUARD
Provides alternate connection to ground.
2-2
OUTPUT HI
Provides output voltage proportional to input voltage
between this terminal and OUTPUT LO. For recording.
2-2, 2-6
OUTPUT LO
Reference point for output voltage. Common to INPUT LO.
Common tie point for use with external supplies.
2-2, 2-3, 2-6
+ POWER INPUT
Application of positive voltage to this terminal powers
instrixment circuits.
2-2, 2-3
+ BATTERY
Provides direct access to positive voltage from internal
battery.
2-2, 2-3
- POWER INPUT
Application of negative voltage to this terminal powers
instrument circuits.
2-2, 2-3
- BATTERY
Provides direct access to negative voltage from internal
battery.
2-2, 2-3
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INPUT TERMINALS
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FIGURE 1. Model 155 Front Panel Controls.
FIGURE 2. Model 155 Rear Panel Terminals.
0469R

MODEL 155 MICROVOLTMETER
OPERATION
SECTION 2.
OPERATION
2-1. INPUT CONNECTIONS.
a. The Model 155 uses four binding posts on the
front panel for all input signal connections: +,
GUARD and CASE.
1. The + (red) and - (black) Terminals are the
Model 155 input high and low respectively. The
voltage to be measured is applied differentially
between these two terminals.
2. The blue GUARD Terminal is provided for use
with guarded potentiometers. When no circuit guard
is available, short the GUARD Terminal to the
- Terminal.
3. The green CASE Terminal provides easy connec
tion to the instrument case. It should normally
be tied to the building ground to provide electro
static shielding for the instrument's circuits.
b. If the signal to be measured is less than 1
millivolt, use copper wires to connect the source to
the Model 155 input. This minimizes the error due
to thermoelectric voltages that may develop due to
temperature differences in the measurement circuit.
c. Use shielded input leads when the source resis
tance is, high, above 1 kilohm, or when long cables
are necessary. Tie the shield to building ground.
Also, shield the source being measured.
d. For low impedance measurements (under 100 ohms)
shielded input cable is usually unnecessary due to
the extremely high ac rejection of the Model 155.
2-2. BARRIER STRIP CONNECTIONS. A seven terminal
Barrier Strip Connector is mounted on the rear panel.
It provides 1) connection for the power supplies to
drive the Model 155 circuits, 2) an output voltage
related to the signal being measured, and 3) an
alternate connection to the circuit guard.
a. The rear panel GUARD Terminal is electrically
identical to the front panel GUARD Terminal (para
graph 2-la) .
b. The OUTPUT HI and OUTPUT LO Terminals provide
an output voltage equal to the input voltage divided
by the RANGE Switch setting. For example, a 15 micro
volt signal being measured on the 30 microvolt range
would produce an output signal of 15 pV/30 pV = 0.5
volt. These two terminals may be used for recording
the output of the Model 155 (Refer to paragraph 2-6) .
c. The OUTPUT LO Terminal is common to the front
panel - Terminal (Input Low). However, the OUTPUT
LO Terminal should not be used as an input connection
because 1) the power supply current that flows in the
output leads generates a voltage due to the wire re
sistance and 2) the output circuitry has not been
designed for low thermally developed voltages in the
leads.
d. The positive voltage from the internal batter
ies is connected directly to the + BATTERY Terminal.
The negative voltage is connected directly to the
- BATTERY Terminal. The common between the positive
and negative internal supplies is connected directly
to the, OUTPUT LO Terminal. Power to operate the
Model 155 must be applied to the OUTPUT LO, + POWER
INPUT and - POWER INPUT Terminals. The Model 155 is
supplied with barrier strip shorting links to accom
plish this function. (Refer to paragraph 2-3 for
operation from power supplies, internal or external) .
2-3. POWER SUPPLIES OPERATION AND CONDITION CHECK.
The Model 155 internal circuitry may be powered
either by its internal battery supply or by an ex
ternal supply.
NOTE
Refer to paragraph 2-2d for internal elec
trical connections of the terminals used
for power supplies.
a. To power the Model 155 circuits with the inter
nal supply, attach the +BATTERY Terminal to the
+POWER INPUT Terminal and the -BATTERY Terminal to
the -POWER INPUT Terminal on the rear panel Barrier
Strip Connector with the provided shorting links.
b. To power the circuits with an external supply,
attach the positive external supply to the +POWER
INPUT Terminal, the negative external supply to the
-POWER INPUT Terminal and the external supply com
mon to the OUTPUT LO Terminal. Make sure that
there are no shorting links connected between the
+ and - BATTERY Terminals and the + and - POWER IN
PUT Terminals.
c. In order for the Model 155 to function prop
erly the supplies used to power the circuits must
be of sufficient strength. To check the state of
the supplies, internal or external, set the Power
Switch to the BATT CHK position. Check the positive
supply by setting the Switch to the + position, and
the negative supply by setting it to the - position.
In each case the meter needle should deflect to
within the green strip on the meter face. If the
meter needle does not deflect to within the strip
for each supply, then that supply is not providing
enough power for proper operation.
1. If an external supply is being used and the
battery check shows a low reading, correct the
indicated supply.
2. If the Model 155 internal battery supply is
being used and the battery check shows a low read
ing, replace the indicated batteries immediately
to prevent corrosion. Table 4 shows the internal
batteries checked for the + and - position of the
Power Switch. It is recommended, however, that
if the reading in either + or - position is low,
all of the internal batteries be replaced.
3. The battery supplies consist of 4 inexpen
sive disposable zinc-carbon batteries (2 for the
positive supply and 2 for the negative) . The bat
teries are 9 volt 2N6 Mallory (246 Eveready) or
equivalent. Replacements may be obtained at most
drug and variety stores. When used continuously,
a new battery compliment should provide well over
2000 hours of operation if the recorder output is
TABLE 4.
Power Switch Position
Batteries Checked
BATT CHK +
BTlOl
BATT CHK -
BT102
0472R

OPERATION
MODEL 155 MICROVOLTMETEH
not used. If the recorder output is used, the
batteries will normally provide more than 1000
hours of operation. When the Model 155 is used
intermittently the battery life is limited by the
shelf life of the batteries.
2-4. OPERATING PROCEDURES.
a. With the Power Switch set to OFF, check the
meter zero. If necessary, adjust with the meter
mechanical zero.
b. Turn the Power Switch to the BATT CK positions
and check the battery condition per paragraph 2-3c.
c. Set the front panel controls as follows:
Power Switch
RANGE Switch
ZERO CK
as necessary
d. Follow the input connection precautions out
lined in paragraph 2-1. Connect the unknown voltage
differentially between the + and - Terminals on the
front panel. Set the Power Switch to ON and increase
sensitivity with the RANGE Switch, rechecking zero on
each range sensitivity increase. Continue to increase
sensitivity until the greatest on scale deflection is
obtained. Read the voltage as the percentage of full
scale that the meter reads times the RANGE Switch
setting, positive or negative scale.
e. For sensitive measurements, measurements below
10 millivolts, see paragraphs 2-7 through 2-10.
2-5.
FLOATING OPERATION.
a. The Model 155 may be connected between two po
tentials, neither of which is at ground. It can be
floated up to 1200 volts off ground.
b. In this mode, the Barrier Strip Connector is
floating at the input potential. Therefore, be care
ful to keep the Connector from shorting to any low
voltage point.
c. The Model 155 is excellent for measuring sig
nals off ground because of the extremely high resis
tance between the input terminals and the case. How
ever, the lO-*-^ ohm isolation specification can be
maintained only as long as the front panel binding
posts and the area around the Barrier Strip Connec
tor are kept clean.
d. Except for the above outlined precautions, op
eration for the Model 155 in floating configuration
is the same as outlined in paragraph 2-4.
2-6. RECORDING.
a. The Model 155 has &o output of ±1 volt at up
to ±1 milliampere for recording. It can be used
directly with 1 volt and 1 milliampere recorders.
If the Model 155 is used for floating measurements,
the recorder must also be floating.
b. To record the Model 155 output connect the OUT
PUT HI and OUTPUT LO Terminals on the Barrier Strip
Connector to the respective input high and low ter
minals on the recorder. Adjust the recorder sensi
tivity and zero as desired. The system is now
ready to record.
c. The actual isolation of the recording system
is the parallel combination of the isolation of the
Model 155 and the recorder. Thus the 10l2 ohm is
olation of the Microvoltmeter may be compromised by
the recorder low-to-ground isolation..
2-7. ACCURACY CONSIDERATIONS. For sensitive meas
urements, other external considerations besides the
Model 155 will affect the accuracy. Effects not
noticeable when working with higher voltages are
very important with microvolt signals. The Model
155 reads only the signal received at its input;
therefore, it is important that this signal be prop
erly transmitted from the source. The following
paragraphs indicate factors which affect accuracy:
noise and source resistance, thermal emfs and stray
pickup.
2-8. NOISE AND SOURCE RESISTANCE.
a. The limit of resolution in measuring voltages
with the Model 155 is determined by the noise pres
ent. The input noise of the Microvoltmeter is 150
nanovolts peak-to-peak. This noise is inherent in
the Model 155 itself and will be the minimum amount
present in all measurements. The 150 nanovolts of
noise i,s due to the instruments voltage noise. The
noise at the Model 155 input increases beyond this
minimum when the noise current passes through a
source resistor and thereby generates a voltage
noise. Thus the total noise becomes a function of
the source resistance and is given by the equation
2
n =
equation 1.
+ (i Rp)^
n \ n SJ
where n is the total input noise;
e^^ is the input voltage noise of the Model 155;
in is the input current noise;
Rg is the parallel combination of source re
sistance and input impedance;
b. Even on the most sensitive range, the noise
due to the current is not appreciable until Rs
reaches approximately 10 kilohms. Thus, for an Rg
of zero ohms to 10 kilohms the noise at the input
is effectively the inherent 150 nanovolts peak-to-
peak. Beyond 10 kilohms the noise due to Rs be
comes evident and a limiting factor in the measure
ment. Therefore, it is recommended that for sen
sitive measurements Rs be kept relatively low and,
if possible, below 10 kilohms.
c. The input impedance of the Model 155 is at
least one megohm as long as the instrument ampli
fiers are not saturated, i.e., as long as the meter
needle is on scale. On higher ranges it is even
greater (see specifications) . Therefore, the Model
155 can measure signals with a large amount of
source resistance on the input without degrading
performance. Note, however, that if the source re
sistance is high, the signal seen at the input of
the Model 155 will be reduced by the voltage divi
sion between the source resistance and the Model
155 input resistance.
2-9. THERMAL EMFS.
a. Thermal emfs (thermo-electric potentials) are
generated by thermal differences between two junc
tions of dissimilar metals. These can be large
compared to the signal which the Model 155 can
measure.
b. Thermal emfs can cause the following problems:
1. Instability or zero offset much higher than
expected.
2. The instrument is sensitive to and responds
to temperature changes. This is seen by touching
the circuit, by putting a heat source near the
circuit, or by a regular pattern of instability,
corresponding to heating and airconditioning sym-
tems or changes in sunlight.
0469R

MODEL 155 MICROVOLTMETER
OPERATION, CIRCUIT DESCRIPTION
c. To minimize the drift caused by thermal emfs
use copper leads to connect the circuit to the Model
155. The input terminals of the Model 155 are of a
copper alloy, and using the same metal or metals
having the same thermo-electric power as the input
will result in minimal generation of thermal emfs.
The leads to the input may be shielded or unshielded,
as necessary (See paragraph 2-10) .
d. Widely varying temperatures within the circuit
can also create thermal emfs. Therefore, maintain
constant temperatures to minimize these thermal emfs.
e. The ZERO Control can be used to buck out
constant offset voltages.
2-10. SHIELDING.
a. The Model 155 is quite insensitive to ac volt
ages superimposed upon a dc signal at the input ter
minals. However, ac voltages which are extremely
large compared with the dc signal may erroneously
produce a dc output. Therefore, if there is ac
interference, the circuit should be shielded and
the shield connected to the Microvoltmeter ground,
particularly for low-level sources.
b. Improper shielding can cause the Model 155 to
reach in one or more of the following ways:
1. Unexpected offset voltages.
2. Inconsistent readings between ranges.
c. To minimize pickup, keep the voltage source
and the Microvoltmeter away from strong ac magnetic
sources. The voltage induced due to magnetic flux
is proportional to the area of the loop formed by
the input leads. Therefore, minimize the loop area
of the input leads and connect each shield at only
one point.
SECTION 3. CIRCUIT DESCRIPTION
3-1. GENERAL.
a. The Keithley Model 155 Microvoltmeter is basic
ally composed of a variable gain chopper amplifier,
an offset voltage suppression circuit, an ac atten
uator filter, input attenuators, an output monitor
ing circuit and power supplies (See Figure 3) .
b. The main signal flow path is as follows:
An input signal is applied through the Power Switch
to the Range Switch where it is divided to a deter
mined ratio by the Range Switch Resistors. A MOS
FET chopper converts this dc input signal to an ac
signal. The ac signal is amplified, demodulated, dc
amplified and applied to the meter and the output.
A feedback network samples the signal at the output
and compares it to the input. The dc input signal
and the feedback signal are compared at the input of
the chopper and the voltage-difference signal between
the two is increased. The ac amplifier amplifies
the difference signal. The ac signal is then demod
ulated and enters a dc amplifier. The dc amplifier
output is connected to the meter, the output termin
als and the feedback network. The feedback resistors
determine full-scale range.
c. The power source for the Model 155 is derived
from disposable batteries.
NOTE
The circuit designations referred to in this
section are for Schematic Diagram 22354E
found at the back of the manual.
3-2. CHOPPER AMPLIFIER. The basic chopper ampli
fier consists of a chopper (sometimes called a modu
lator) which switches the input dc signal on and off
to produce an ac output. This ac is then amplified
and demodulated to regain the dc signal. Further
amplification is then achieved with a dc amplifier.
The negative feedback is employed around the total
amplifier to achieve gain accuracy and gain stabil
ity. Synchronous demodulation is obtained by syn
chronizing the demodulating switch with the chopper.
The individual circuits in the chopper amplifier
are described as follows.
a. MOS FET Chopper.
1. The field-effect transistor when used as a
chopping device provides low offset currents, low
offset voltages, low noise and low drive power.
A series shunt chopping configuration provides
low noise and high input impedance.
2. Transistors QlOl and Q102 are the chopper.
Resistor R184 and Capacitor C104 are used to mini
mize the problem of the chopper drive feeding into
the signal channel.
b. AC Amplifier. The ac amplifier is composed
of a low noise amplifier and a variable gain ampli
fier.
1. A bi-polar transistor, Q103, biased for op
eration at low current levels is the input de
vice. Transistors Q103 and Q104, and associated
0472R

CIRCUIT DESCRIPTION
MODEL 155 MICROVOLTMETER
'» "■«» 'I"""
SMITCHING
FIGURE 3. Block Diagram of Model 155 Circuits.
components, are a low noise amplifier with a gain
of 34, as fixed by the feedback resistors R129
and R131.
2. The low noise amplifier is followed by a
variable gain ac amplifier consisting of transis
tors Q105, Q106 and Q107 and associated components.
It is necessary to have highgain when measuring
very low voltages, and less gain when the total
chopper amplifier is to be used at lower gain to
prevent oscillations. For this reason, the gain
of the second amplifier is varied by switching
the feedback resistor. Resistor R147 and R148
along with capacitor C128 provide a high frequen
cy cutoff for the attenuation of the spikes gen
erated in the chopper by the chopper drive.
c. Demodulator. Field-effect transistor Q118 acts
as a switch which is synchronized with the input
chopper and thus provides synchronous demodulation.
The average value of the signal obtained at the junc
tion of capacitor C112, and resistor R149, is pro
portional to the dc input signal. Because of the
switching action of Q118, the signal at this junc
tion is shorted to ground for half of each chopping
cycle. Consequently this dc signal has a large
chopper frequency component.
d. DC Amplifier.
1. The function of this amplifier is twofold:
It gives additional amplification to the relative
ly small signal seen at the output of the demodu
lator, and it integrates the output of the demod
ulator, thus removing most of the chopper frequen
cy ripple which appears there. Complimentary
symmetry output is used for the amplifier to meet
the requirements of low idling power while still
being capable of providing 1 milliampere of out
put current.
2. The dc amplifier is composed of three differ
ential amplifiers and emitter follower. The sig
nal from the output of the demodulator is applied
to the first differential dc amplifier, composed
of transistors Q116 and Q108, and amplified. The
second amplifier, transistors Q109 and QUO, amp
lifies the output signal from the first amplifier
and applies it to a third differential amplifier,
Qlll and Q112, for further amplification. Emitter
followers Q113, Q114 and Q115 are an impedance
changing circuit to provide low output impedance.
e. Total Loop DC Feedback.
1. The Model 155 uses negative feedback to
achieve gain accuracy and stability and assure
high input impedance. The resistors are switched
into the feedback current in such a way as to
maintain low feedback current and avoid excessively
high value resistors in the feedback loop.
2. The feedback network, composed of resistors
R118 through R126 and R185, is formed from the
output of the dc amplifier to the input of the
chopper amplifier. The Range Switch, SlOl, se
lects the feedback ratio used for each range.
f. Multivibrator.
1. The multivibrator circuit generates the
drive voltage for the chopper and demodulator.
2. Transistors Q120 through Q123 and their
associated components are an astable multivibrator.
Output voltages are taken at the emitters of Q120
and Q123. These output voltages are opposite
phase square waves and are used directly as the
chopper drive. The output at the emitter of Q120
is also used as the demodulator drive.
3-3. OFFSET SUPPRESSION CIRCUITS.
a. When measuring signals in the microvolt reg
ion it is often desirable to suppress the zero
voltage level so that small changes may be readily
observed. For this reason a front panel ZERO Con
trol, R173, is provided. This control is non-lin
ear so that for normal operation (suppression of
less than ^5 yV) accurate zeroing may be easily
0469R

MODEL 155 MICROVOLTMETER
CIRCUIT DESCRIPTION
achieved, while still having available suppression
of at least ±25 V.
b. Offset current is suppressed by the circuit
consisting of potentiometer R104 and resistors RllO
and R186.
3-4. HIGH FREQUENCY ATTENUATE INPUT FILTER. The
frequency attenuating filter at the input of the
Model 155 provides approximately 50 dB of ac rejec
tion at 60 Hz. The filter is a 3-section RC ladder
filter consisting of resistors, R107, R108, Rill
and R112 and capacitors ClOl, C102 and C103.
3-5. INPUT ATTENUATION.
a. The chopper amplifier has a minimum gain of
100 and a maximum output voltage of ±1 volt. This
means it is necessary to attenuate signals larger
than 10 millivolts to prevent saturation of the
chopper amplifier. The input attenuator resistors
R102, through R106 and R113 through R117, are switch
ed by the RANGE Switch, SlOl. One decade of atten-
ation is necessary to handle signals as high as 100
millivolts. Two decades of attentuation are necessary
to handle signals as large as 1 volt, and so on.
Input attenuation is switched in a decade at a time,
and the gain of the chopper amplifier is alternated
between 333 and 100 for all ranges above 10 milli
volts .
b. Because of potential instability in the re
sistance value of high value resistors, potenti
ometers have been placed in series with all high
value resistors in the attenuator string. Thus,
the instrument can be accurately calibrated even
if the high value resistors drift.
3-6. POWER SUPPLIES.
a. Power for the Model 155 is provided by four
9 volt zinc-carbon batteries. The idling current
of the Model 155 is approximately 500 microamperes.
Thus the battery life for most applications will be
the same as the battery shelf life. If the record
er output is used, a maximum of 1 itiA may be drawn
from it, so the battery life will still be normally
in excess of 1000 hours.
b. Because the battery noise may increase and
terminal voltage will decrease with battery age,
and because it is necessary to have fixed voltage
for the offset suppression circuits, zener supplies
consisting of transistors D106, D107 and associated
components provide a regulated ±6 V. The low noise
amplifier supplies are isolated from the other sup
plies by filters consisting of resistor R167 and
R168 and capacitors C116 and C117.
TABLE 5.
Equipment Recommended for Calibrating and Troubleshooting the Model 155. Use these instruments or their
equivalent.
Instrument
Use
Fairchild Instruments 7050 DVM, 1.5 to 1000 volts
full scale, 0.1% accuracy, 1.5 kfl to 15 MJ2 input
resistance
Hewlett Packard 200CD Oscillator
Hewlett Packard 5210A Frequency Meter
Keithley Instruments 241 High Voltage Supply
Keithley Instruments 260 Nanovolt Source
Keithley Instruments 370 Recorder
Keithley Instruments 500 Megohmmeter
Keithley Instruments 662 Differential Voltmeter
Tektronix dc coupled Model 503 Oscilloscope
5 pF Polystyrene Capacitor
1000:1 Voltage Divider
General Calibration and Troubleshooting
Normal Mode and Common Mode Rejection Checks
Multivibrator Adjust
General Calibration
Rise Time Check and Range Accuracy Verification
Drift Check
Initial Adjustment
DC Amplifier Balance Adjust
General Calibration and Troubleshooting
Normal Mode and Common Mode Rejection Checks
Normal Mode and Common Mode Rejection Checks
0472R

SERVICING
MODEL 155 MICROVOLTMETER
SECTION 4. SERVICING
4-1. GENERAL. Section 4 contains the maintenance
and troubleshooting procedures for the Model 155
Microvoltmeter. Follow these procedures as closely
as possible to maintain the performance of the in
strument .
4-2. SERVICING SCHEDULE. Periodically check the
condition of the batteries, using the convenient
battery check as described in paragraph 2-3. Ex
cept for battery replacement, the Model 155 requires
no periodic maintenance beyond the normal care re
quired of high quality electronic equipment.
4-3. PARTS REPLACEMENT. The Replaceable Parts List
in Section 7 describes the electrical components of
the Microvoltmeter. Replace components only as nec
essary. Use only reliable replacements which meet
the specifications.
4-4. TROUBLESHOOTING.
a. The procedures which follow give instructions
for repairing troubles which might occur in the
Model 155. Use the procedures outlined and use only
specified replacement parts. Table 5 lists equip
ment recommended for troubleshooting. If the trou
ble cannot be readily located or repaired, contact
Keithley Instruments, Inc., or its representative.
b. Table 6 contains the more common troubles
which might occur. If the repairs indicated do not
clear up the troi:ible, find the difficulty through a
circuit-by-circuit check, such as given in paragraph
4-6.
c. Refer to the circuit description in Section 3
to find the more critical components and to deter
mine their function in the circuit. The complete
circuit schematic, 22345E, is given in Section 7 at
the back of the manual.
4-5. PRELIMINARY PROCEDURES.
a. Before initiating any troubleshooting proce
dures, double-check the system to make sure that the
Model 155 is indeed faulty. Once this is determined,
turn the Microvoltmeter Power Switch to OFF and gath
er the tools and instruments that may be necessary
to disassemble, troubleshoot, repair and reassemble
the instrument. Table 5 lists equipment recommended
for troubleshooting.
b. If the trouble is such that the Model 155 must
be disassembled (i.e. other than battery check,
etc.) , then disassemble the instrument to the point
where the circuits are access able and the power may
be safely turned on.
c. If the user is quite familiar with the instru
ment, he may be able to deduce what circuit is most
likely to be faulty from the symptoms of the fault.
In such a case, time may be saved by checking out
TABLE 6.
Trouble
Probable Cause
Solution
Excessive Zero Offset
Input transistors may be defec
tive
Check QlOl and Q102 (paragraph
4-6e); replace if faulty.
Batteries failing
Replace batteries.
DC Amplifier Balance potentiom
eter, R151, out of adjustment
Adjust per paragraph 4-6h or
5-5.
Mechanical meter zero out of
adjustment
Adjust correctly.
Excessive Offset Current
Input transistors may be defec
tive
Check QlOl and Q102 (paragraph
4-6e); replace if faulty.
Offset Current Suppress poten
tiometer, R109, out of adjust
ment
Adjust per paragraph 5-8.
Instrument inaccurate
on all ranges
Meter Calibrate potentiometer,
R183, out of adjustment
Adjust per paragraph 5-6.
Instrument inaccurate
on 300 mV and IV ranges
Accuracy Set potentiometer,
R104, out of adjustment
Adjust per paragraphs 5-7 and
5-13.
Instrument inaccurate
on 3V and higher ranges
Accuracy Set potentiometer,
RlOl, out of adjustment
Adjust per paragraphs 5-7 and
5-13.
Apparent Oscillation in
Output
Chopper frequency beating
with line frequency
Adjust Multivibrator Frequency
Set potentiometer, R178.
(Paragraphs 4-6e and 5-4) .
Multivibrator Frequency Set
potentiometer, R178, out of
adjustment
Check per paragraph 4-6e and
adjust per paragraph 5-4.
0472R

MODEL 155 MICROVOLTMETER
SERVICING
hunches. Otherwise, it is best to proceed in the
manner given in paragraph 4-6.
d. Before starting a step-by-step check, inspect
the circuit visually. Solid-state circuitry usual
ly has a very low failure rate. Consequently, a
high percentage of the problems which arise will be
due to such things as broken wires, dirt between
switch contacts, loose battery clips, etc.
e. Turn the Model 155 Power Switch to ON and
check out the circuit according to paragraph 4-6.
When the trouble is located, turn the Power Switch
to OFF, make the repair and reassemble the instru
ment. Also, after the repair has been made, a final
check should always be made to make sure that the
instrument is working properly.
4-6. PROCEDURES TO GUIDE TROUBLESHOOTING.
a. If the instrument will not operate, check the
condition of the batteries. If these are found to
be defective, replace them.
b. If the batteries are satisfactory, set the
Range Switch to 1000 VOLTS, Power Switch to ZERO
CHK and check the voltage at the plus and minus
battery check points (these are points 1 and 2 given
in Figure 4). The voltage at each point should be
approximately +16.2 and -16.2 volts respectively.
1. Check for battery current of less than 3
milliamperes if the plus and minus 16.2 volts
cannot be obtained. If the 3 milliamperes is
present, then there is a short circuit between
the battery leads in the Power Switch.
2. If the 3 milliamperes cannot be obtained,
then there is a shorted component loading the
supplies. Replace the faulty component.
c. If the plus and minus battery supplies are
found to be satisfactory, then check the filtered
plus and minus battery supplies (these are points 3
and 4 given in Figure 4) . Check these supplies for
approximately plus and minus 14.9 volts respectively.
If unobtainable, then the fault is in the associated
circuitry components. Find the component and re
place it.
d. If the filtered battery voltages are satisfac
tory, check the regulated plus and minus 6 volt sup
plies for +6 volts +2V and -6 volts ±2V respectively.
Check these voltages at points 5 and 6 shown in Fig
ure 4. If these values are unobtainable, then the
fault lies in the associated circuitry. Find the
faulty component and replace it.
e. If the regulated voltage supplies are satis-
facto^, then the troxable may lie in the multivibra
tor circuit. Check the multivibrator waveform with
a do coupled Model 503 Oscilloscope at the gates of
FET QlOl and Q102 (test points 7 and 8, Figure 4) .
Set the Oscilloscope to 2 volts per division verti
cal and 1 millisecond per division horizontal. The
waveform should be near symmetrical 7 to 12 volts
peak-to-peak (Figure 5) A little overshoot may be
observed on some units.
11
NfJPNf
m
tljaL?!
%
... .....
-s jHm
iHKf-
fji liliiiiif
WI
FIGURE 4. Test Points Within Model 155 For Troubleshooting Procedures.
0969R

SERVICING
MODEL 1,55 MICROVOLTMETER
IHEBBIB
FIGURE 5. Multivibrator Output Signal. Scale is
2V/div. vertical and 1 msec/div. horizontal.
1. Are both chopper drives present? If not,
taking care to prevent damage to QlOl and Q102,
remove the gate lead from the standoff.
a) Are both chopper drives now present? If
not, repair the multivibrator.
b) If the chopper drives are present, re
place QlOl and Q102.
2. After either repairing the multivibrator or
replacing the FETs, and if both chopper drivers
are present, check for a signal at the output of
the FETs (point 9, Figure 4) .
a) If there is no signal, the fault lies in
the input harnessing, switches or the input fil
ter.
b) If there is a signal, replace QlOl and
Q102.
c) After repairing the fault and/or replac
ing QlOl and Q102, replace the gate leads.
NOTE
Defective input FETs QlOl and Q102 may be
the cause of ac amplifier unbalance or
large current offset.
f. Check for a square wave at the demodulator
test point, (point 10, Figure 4) . If there is no
square wave, then the fault lies in the ac amplifier
consisting of transistors Q103 to Q107. Find and
replace the defective transistor(s) . To localize
collector Q107
-l.lv
(6V p-p)
emitter Q107
-7.0V
collector Q105
-6.2V
(,3V p-p)
base q106
-1 IV
I|ff J V
emitter Q104
+6.3V
''v.,' '
emitter Q105 emitter Q106
-6.5V -1.2V
collector Q104
-.81V (1.5V p-p)
collector Q103
— +5.7V
emitter Q103
- -0.51V
60mV
(p-p)
FIGURE 6. Test Points Within the AC Amplifier Circuit and Indicated Nominal Voltages to be Expected at the
Test Points.
10 0969R

MODEL 155 MICROVOLTMETER
SERVICING
the trouble in the ac amplifier refer to Figure 6.
It shows test points within the ac amplifier cir
cuit and indicates nominal voltages to be expected
at the test points. Nominal voltage measurements
are made with the feedback test point (point 11 in
Figure 4) jumpered to low, the Model 155 Range
Switch set to 1000 VOLTS and the Power Switch at
ZERO CHK. Voltages may be slightly higher or lower
than the nominal voltage listed.
g. If there is a square wave at the demodulator
test point, short the test point to low and check
for a square wave again. If there is no square
wave then the fault lies in the demodulator circuit.
Find and repair the fault.
h. If the demodulator is found satisfactory,
keep the demodulator test point shorted to low and
check the dc amplifier operation.
^1. Turn the DC Amplifier Balance potentiometer,
R151, completely clockwise. The meter should peg
in the minus direction. Then turn R151 complete
ly counterclockwise. The meter should peg in the
plus direction.
2. If the meter does not peg in both cases,
then the fault lies in the dc amplifier circuit.
Find the fault and repair it.
3. To localize the trouble in the dc amplifier,
refer to Figure 7. It shows test points within
the dc amplifier circuit and indicates nominal
voltages to be expected at the test points.
a) Nominal voltage measurements are made with
the Model 7050 Digital Voltmeter. The demodula
tor test point is jumpered to low, the Range
Switch at 1000 VOLTS and Power Switch to ZERO
CHK. The Model 7050 common must be above ground.
b) Connect the Model 7050 across thecollec
tion of transistors Q109 and QUO. Turn DC
Balance potentiometer, R151, to achieve a read
ing near 0 (balance) . When balanced, the volt
age to low should be near nominal values listed.
i. If the dc amplifier circuit is found to oper
ate satisfactorily, then the trouble is in the out
put switching or meter circuits. Locate and repair
the trouble.
•raitt«rs Qlll & qU2 •mitter Q103
4.4V * +0.45V *
collactor Q109
-3.5V to
-5.2V
when balanced
collector Q116
+2.5V to
+4.7V
when balanced
collector Q108
+2.5V to
+4.7V
when balanced
m
collector QUO collector Qlll
-3.5V to -5.2V +0.95V *
when balanced
* With Q109 & QUO
FIGURE 7. Test Points Within DC Amplifier Circuit and Indicated Nominal Voltages to be Expected at Test
Points.
OSfilR
11

CALIBRATION
MODEL 155 MXCROVOLTMETER
SECTION 5.
CALIBRATION
5-1. GENERAL. The function of the calibration
section is to provide a method of checking the
Model 155 to make sure that it operates within the
specifications given in Table 1, page ii.
a. The following procedures are recommended for
calibrating and adjusting the Model 155. Use the
equipment in Table 5. If proper facilities are not
available or if difficulty is encountered, contact
Keithley Instruments, Inc. , or its representatives
to arrange for factory calibration.
b. If the Model 155 is not within specifications
after the calibrations and adjustments, refer to
the troubleshooting procedures in Section 4 or con
tact Keithley Instruments, Inc. , or its nearest
representative.
NOTE
Figure 4 shows the location of internal test
points used in calibrating the Model 155.
TABLE 7.
Model 155 Internal Controls.
Control
Circuit
Desig.
Paragraph
Accuracy Set
RlOl
4-7, 4-13
Accuracy Set
R104
4-7, 4-13
Offset Current Suppress
R109
1
w
t
00
DC Amplifier Balance
R151
4-3, 4-5
Multivibrator
Frequency Set
R178
4-4
Meter Calibrate
R183
4-6
5-2. INITIAL ADJUSTMENTS.
a. Set the Model 155 Power Switch to OFF and
RANGE Switch to 1000 VOLTS.
b. Check the Barrier Strip Connector on the rear
panel of the Model 155. Make sure that shorting
links are placed between the +POWER SUPPLY and
+BATTERY Terminals and between the -POWER SUPPLY
and -BATTERY Terminals.
c. Connect a Model 500 Megohmmeter between Model
155 front panel -(low) and CASE Binding Posts.
Check to make sure the isolation between these two
posts is greater than 10l2 ohms. Connect the
ground lead of the Model 500 to the Model 155 CASE
Binding Post to minimize pickup.
d. Adjust the Model 155 meter for zero with the
Mechanical Zero.
e. Check the battery condition by setting the
Power Switch to BATT CHK + and - positions. For
each polarity the meter needle should indicate 70%
to 100% of full scale (green area) . New batteries
typically indicate greater than 86% of full scale
(18.5 volts or more) . After checking batter con
dition, set the Power Switch to OFF.
NOTE
See paragraph 2-3 also for checking the
condition of the batteries.
5-3. PRELIMINARY CALIBRATION PROCEDURES.
a. Make sure that Offset Current Suppress Poten
tiometer, R109, is at least one turn from either
end. Jumper the center tap of potentiometer R109
to the low end of resistor R186. Do not remove this
jumper until specifically stated in paragraph 5-6.
b. Turn the Power Switch to ZERO CHK. Within a
few moments the meter needle should come to zero
indication. If necessary, zero the meter with the
ZERO Control. Increase the Model 155 sensitivity
to 100 microvolts and zero the meter.
c. If the Model 155 is inoperative, that is if
the meter pins, etc., then check the voltage at the
test points given in Table 8 to the values indicated
in TalDle 8. If these voltages are found satisfac
tory, check the multivibrator per paragraph 5-4. If
all the above checks are satisfactory, localize the
trouble to the ac or dc section of the amplifier by
shorting the demodulator test point (point 10, Fig
ure 4) to low and adjusting DC Amplifier Balance
Potentiometer, R151, from one end to the other. If
potentiometer R151 can swing the meter full scale
from + to - and vice versa, the problem is in the
ac section of the amplifier. If it cannot, the
problem is in the dc section. (Refer to the trou
bleshooting procedures 4-6) .
TABLE 8.
Test Points within the Model 155. Table refers to
the test points called out in Figure 4 , paragraph
4-6, and gives the voltage expected at each point.
Test Point
Voltage
1
approximately
+ 16.2 volts
2
tl
-16 .2 volts
3
It
+14.9
volts
4
II
-14.9 volts
5
+6V ±2V
fi
-6V ±2V
5-4. MULTIVIBRATOR ADJUST.
a. Connect the Model 5210A Frequency Meter be
tween the multivibrator test point (point 11, Fig
ure 4) and low. Adjust the Multivibrator Frequency
Set Potentiometer, R178, for a reading of 220 Hz
±3 Hz.
b. Then connect a dc coupled Model 503 Oscillo
scope between the multivibrator test point and low,
and observe the waveform. The Oscilloscope should
be set at 2 volts per division vertical and at a 1
millisecond sweep. The wave form should be near
symmetrical 7 to 12 volts peak-to-peak square wave
(refer to Figure 5 in paragraph 4-6e).
5-5. DC AMPLIFIER BALANCE ADJUST.
a. Connect the Model 155 output to the Model 662
12
0472R

MODEL 155 MICROVOLTMETER
CALIBRATION
Differential Voltmeter. Set the Model 155 Power
Switch to ZERO CHK and read zero from the Model 662.
1. Set the Model 155 RANGE Switch to 100 MI
CROVOLTS and adjust the ZERO Control for 0±2 mV
at the output exclusive of noise (typical noise
is from 2 to 5 mV peak-to-peak) .
2. Set the RANGE Switch to 1000 VOLTS and ad
just the DC Amplifier Balance Potentiometer, R151,
for 0±0.5 mV at the output.
3. If necessary, repeat steps 1 and 2.
b. Once adjusted, step the RANGE Switch from 100
MICROVOLTS through 1000 VOLTS. Large zero shifts
(8 mV or more) between ranges generally indicates
that input FETs QlOl and Q102 may be defective.
5-6. OFFSET CURRENT SUPPRESS CALIBRATION.
NOTE
Make sure the Model 155 cover is on during
this test procedure. Diodes DlOl through
D105 may be sensitive to light and the ad
justment is void without the cover on.
a. Remove the jumper from between the center tap
of potentiometer R109 and the low end of resistor
R186.
b. Shield the Model 155 input. (The input may
be shielded by affixing banana plugs inside a metal
case and covering the four front panel binding posts
with the case, being careful to insert the banana
plugs into the GUARD and CASE Terminals) . Shield
ing is necessary to reduce pickup.
c. Set the Model 155 Power Switch to ZERO CHK
and the RANGE Switch to 100 MICROVOLTS.
1. Adjust the ZERO Control for zero meter in-
dication.
2. Open the input by setting the Power Switch
to ON and adjust the Offset Current Suppress Po
tentiometer, R109, for near zero meter indication.
d. Set the Power Switch to ZERO CHK and the
RANGE Switch to 30 MICROVOLTS.
1. Adjust the ZERO Control for zero meter in
dication.
2. Set the Power Switch to ON and adjust po
tentiometer R109 for less than a ±5 )jV shift (0±5
minor divisions on the lower meter scale) .
3. If necessary, repeat steps 1 and 2 to obtain
less than 5 pV shift on the meter when Switching
the Power Switch between ZERO CHK and ON Posi
tions. (The Model 155 can be readily adjusted
for less than 2 pV shifts) .
e. With the Power Switch set to ON step the
RANGE Switch from 30 MICROVOLTS to 1 VOLT. Offset
on the 100 and 300 microvolt ranges should be less
than 5 microvolts decreasing to a negligible offset
on the-1 millivolt through 1 volt ranges.
5-7. METER CALIBRATION.
a. Connect the Model 241 Voltage Supply to the
■Model 155 input and connect the output to the Model
7050 DVM.
b. Set the Model 155 RANGE Switch to 1 VOLT and
apply ±1 volt to the input with the Model 241.
1. Adjust the ZERO Control and/or the input
voltage to obtain a +1.000 volt at the output.
2. Adjust the Meter Calibrate Potentiometer,
R183, for a full scale positive deflection on the
Model 155 meter scale.
c. Apply -1 volt to the Model 155 input and ad
just the ZERO Control and/or the input voltage to
obtain -1.000 volt at the output.
1.
tion.
Note the negative full scale meter deflec-
2. If necessary, adjust potentiometer R183 to
split the difference between the positive and
negative full scale deflections.
d. Typical positive and negative full scale error
is less than 1% (1/2 minor division).
5-8.
ACCURACY SET CALIBRATION.
a. Keep the Model 155 connected as in above par
agraph 5-7.
b. Set the RANGE Switch to 1 VOLT and the Power
Switch to ZERO CHK.
1. Adjust the ZERO Control for 0.000 volts at
the output.
2. Apply 1.000 volt to the input and adjust
Accuracy Set Potentiometer RlOl for 1.000 volt at
the output.
c. Set the RANGE Switch to 10 VOLTS and the Power
Switch to ZERO CHK.
1. Adjust the ZERO Control for 0.000 volts at
the output.
2. Apply 10.000 volts to the input and adjust
Accuracy Set Potentiometer R104 for 10.00 volts
at the output.
NOTE
Always adjust potentiometer RlOl before po
tentiometer R104 because RlOl affects R104.
5-9. NOISE CHECK (Keep Model 155 cover on to min
imize noise pickup) .
a. Set the Model 155 Power Switch to ZERO CHK
and the RANGE Switch to 1 MICROVOLT. Zero the in
strument with the Zero Control. After zeroing, ob
serve the meter noise for less than 150 nanovolts
peak-to-peak (7 minor divisions on the upper meter
scale). Observe the meter for a period of 15 sec
onds .
b. Next, observe the meter noise in the same
manner on the 3 microvolt and 100 microvolt ranges.
The noise on the 3 microvolt range should be ap
proximately the same as that on the 1 microvolt
range decreasing to less than 1% (1/2 division on
the upper meter scale) on the 100 microvolt range.
5-10. RISE TIME CHECK (Keep Model 155 cover on to
minimize noise pickup).
a. Connect the Model 260 Nanovolt Source to the
Model 155 input and a dc coupled Model 503 Oscillo
scope to the output. The vertical scale of the
Oscilloscope should be set at 0.2 volt per division.
0969R
13

CALIBRATION
MODEL 155 MICROVOLTMETER
b. Set the Model 155 RANGE Switch to 1 MICROVOLT
and the Power Switch to ON.
1. Zero the Model 155 Microvoltmeter with ZERO
Control then apply +1 microvolt with the Model 260
to the input and observe the Model 155 10% to 90%
rise time on the meter. This rise time must be
less than 5 seconds and typically it is less than
3 seconds.
2. Repeat this test with a -1 microvolt signal.
c. Set the Model 155 RANGE Switch to 100 MICRO
VOLTS and the Power Switch to ON.
1. Zero the Microvoltmeter with ZERO Control.
2. Apply +100 microvolts with the Model 260 to
the input and observe the 10-90% rise time on the
Model 503 Oscilloscope. This rise time must be
less than 1 second and typically is less than 1/2
second. (Figure 8 shows a typical rise time of
the Model 155 on the 100 microvolt range).
5-11. OVERLOAD RECOVERY CHECK.
a. Place a 10 kilohm resistor across the Model
155 input and then connect the Model 241 to the in
put.
b. Set the Model 155 RANGE Switch to 30 MICRO
VOLTS and the Power Switch to ON. Zero the meter
with the ZERO Control.
c. Apply 30 volts to the input for approximately
one second. The Microvoltmeter should recover from
this overload within five seconds.
d. Set the RANGE Switch to 1 MICROVOLT and apply
1 volt to the input for approximately one second.
The instrument should recover within 20 seconds.
e. Remove the 10 kilohm resistor from across the
input.
5-12.
DRIFT CHECK.
FIGURE 8. Typical 10-90% Rise Time on 100 Micro
volt Range. Scale is 20 mV/cm vertical and 0.1
sec/cm horizontal.
a. Connect the Model 370 Recorder to the Model
155. Set the Microvoltmeter RANGE Switch to 1 MI
CROVOLT and the Recorder attenuator to 1 volt. Re
corder calibration is now 1 microvolt full scale.
b. After a half-hour warm-up, re-zero the instru
ment. Using zero as a reference, the Model 155 must
not drift more than 0.5 microvolt either side of
zero in 24 hours. (In that 24 hour span the in
strument will wander about thezero reference but
should never deviate more than 0.5 microvolt from
rei^rence) . Figure 9 shows a typical Model 155
drift.
5-13. RANGE ACCURACY VERIFICATION.
a. Check the 1000 volt through 100 microvolt
ranges for 1 volt ±1% at the Model 155 output and
±2% of full scale (1 minor upper scale division) on
the meter. Check the 30 microvolt through 1 micro
volt ranges for ±2% of full scale exclusive of
noise and drift.
1. To check the 1000 volt through 3 volt ranges
use the Model 241 to apply the voltages to the Mo
del 155 input. Monitor the output with the Model
7050 DVM.
2. To check the 1 volt through 1 microvolt
ranges, use the Model 260 to apply the voltage to
mmm
mMm
m
iii
1
FIGURE 9. Typical 24-Hour Drift Chart for Model 155. This particular drift was run at 1 microvolt full
scale and 0.75 inch per hour. Notice that it is well below the specified Model 155 drift. The user may,
if desired, perform the drift check at a faster rate and on a less sensitive scale as long as the specified
drift can be resolved.
14
0969R

MODEL 155 MICROVOLTMETER CALIBRATION
the Microvoltmeter input. Monitor the output on
the 1 volt through 100 microvolt ranges with the
Model 7050.
3. Check the 1000 volt and 10 microvolt ranges
for both positive and negative polarity. All
other ranges may be checked using only one polar
ity.
b. If necessary, adjust the Accuracy Set Poten
tiometer, R104, to bring in all ranges from 3 volts
to 1000 volts within tolerance. Also, the Accuracy
Set Potentiometer, RlOl, may be adjusted to bring
the 300 millivolt and 1 volt ranges within toler
ance. Note, however, that re-adjusting potentiom
eter RlOl will require rechecking the 3 volt through
1000 volt ranges (refer to note of paragraph 5-8).
1. Set the Model 155 Power Switch to ON, RANGE
Switch to 1 MICROVOLT and zero the Microvoltmeter
with the ZERO Control. Due to thermals on the in
put, it should require approximately one minute
for the instrument to stabilize.
2. Increase the Oscillator output to 10 volts
peak-to-peak. There should be no shift in the
meter reading. (Do not confuse noise and drift
for a shift in meter reading) .
5-15. COMMON MODE REJECTION CHECK.
5-14. NORMAL MODE REJECTION CHECK.
a. Use the same setup as in paragraph 5-14 ex
cept apply the signal between the +(high) and CASE
Terminals and connect the + (high) and - (low) Termi
nals together.
b. Check Model 155 zero.
a. Set the Model 155 Power Switch to ZERO CHK.
b. Set up the rejection check equipment as fol
lows: apply a signal from the Model 200CD Oscilla
tor through a 5 yF capacitor to a 1000:1 divider
and connect the divider output to the Model 155 in
put. Connect the Model 155 -(low) , GUARD and CASE
Terminals together. Monitor the Signal Generator
with the Model 503 Oscilloscope.
c. Set the Oscillator frequency to 50 Hz and the
output to minimum.
c. Set the Oscillator frequency to 50 Hz and out
put to minimum.
1. Set the Model 155 Power Switch to ON, RANGE
Switch to 1 MICROVOLT and zero the instrument with
the ZERO Control. Allow time for the unit to sta
bilize (approximately one minute).
2. Increase the Oscillator output to 1 volt
peak-to-peak. There should be no shift in the
meter reading. (Do not confuse noise and drift
for a shift in meter reading).
BTlOl
& •
BT102
SlOl
fircw 45'
i!
(
PC-242
(Figs. U & 12)
■FIGURE 10. Top View Model 155 Chassis. Front panel faces up. View shows location of batteries and PC-242.
See Figures 11 and 12 for Model 155 component locations.
0969R
15

CALIBRATION
MODEL 155 MICROVOLTMETER
D104'
Q123'
Q120'
C120"
C108
D105
C121
D103
C123
Q122
Q121'
(
Q115'
(
Q112
C
Qlll
Q
Q
C
107
U4"
117'
Q113
C113
C115
QUO'
Q109'
cm
C106
Q104 ci05
C107
Q103
I
Iti'i Ari.-*.
6
'
•QlOl
-Q102
C122
■C104
C103
-C102
Q106
■C109
ClOl
-Q105
Clio
-C116
Did
■D102
C118
-D107
C112
Q117
Q116
Q118 Q119
C114 Q108 C119 Dice
FIGURE 11. Component Layout for PC-242. Figure shows locations of capacitors, diodes, and
transistors. For resistor locations, refer to Figure 12.
16
1069