Keithley 149 Service manual

INSTRUCTION MANUAL

MODEL 149

MILLI-MICROVOLTMETER

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 assist­ance and shipping instructions.

REPAIRS AND RECALIBRATION

Keithley Instruments maintains a complete re­pair service and standards laboratory in Cleve­land, and has an authorized field repair facility
in Los Angeles and in all countries outside the
United States having Keithley field repre­sentatives.

To insure prompt repair or recalibration serv­ice, 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 Bu­reau of Standards are available upon request.

MODEL 149 MILLI-MICROVOLTMETER CONTENTS

TABLE OF CONTENTS

Section Page

INTRODUCTION . l-1

1.
Specifications . 1-2
OPERATION. . 2-1

2.
Operating Controls . . . 2-l

Preliminary Set-Up . 2-l
General Precautions. . . 2-2

Measuring Voltage. . 2-3
Other Applications 2-4

CIRCUIT DESCRIPTION. 3-1

3.
Input Circuit.

AC Amplifier

DC Amplifier . . . . . .

.

3-l

. . 3-l

3-l

Zero Suppression . . . . 3-2

Other Controls .
Power Supply . .

. . . .

. . . .

3-2

3-2

Section

MAINTENANCE. . . . . . . . .

4.

Trouble-Shooting . . . . .

Excessive Output Noise . . . .

Output not Zero with Input

Terminals Shorted . . .

220-Volt Operation . . . . . . .

REPLACEABLE PARTS. . . . 5-l

5.
Replaceable Parts List . .

How to Order Parts . . .

Model 149 Replaceable Parts List
Models 1483, 1484 Replaceable

Parts List. . . . . . . . 5-6

Model 1491 Replaceable Parts List 5-7
Model 1501 Replaceable Parts List 5-7
Model 1502 Replaceable Parts List 5-7

Voltage and Resistance Chart . 5-9

Schematic Diagram. . . . . .

Green Repair and Calibration Form

*

Change Notice . . . . . Last

Page

4- 1
4-l

4- 1
4-2

4-3

5-1
5-l
5-2

5-11
5-13

Page

K

Yellow Change Notice sheet is included only for instrument modifications
affecting the Instruction Manual.

1068R

MODEL 149 MILLI-MICROVOLTMETER

INTRODUCTION

SECTION 1. INTRODUCTION

l-l. GENERAL.

The Model 149 Milli-Microvoltmeter is a stable, versatile instrument for measuring

a.

low-level dc signals. It functions as a voltmeter from 100 nanovol s full scale to 100

millivolts.

It also operates as a dc amplifier with gains up to 10 ' for driving recor-

ders.

The low noise level of the Model 149, together with its long-term stability, makes

b.

it ideal for many measurements requiring extreme power sensitivity.

Typical applications include measuring the output from strain gages, thermopilcs,

C.

thermocouples, bolometers, phototubes, ionization chambers, scintillation counters, and

barrier layer cells. Other applications are found in cell studies, measurement of elec-

trochemical potentials, electrolytic corrosion studies, molecular weight analysis and

Hall effect studies.

In addition to its use as a direct indicator of minute potentials and currents, the

d.

Model 149 may also be used as a null detector in Wheatstone or Mueller bridges.

e. An important feature of the instrument is zero suppression up

to

100

times

full
scale in place of the usual more limited meter zero. This permits measurements of small
signals in the presence of large thermal emf's or other masking dc signals.

FIGURE 1.

Keithley Instruments Model 149 Milli-Microvoltmeter.

l-l

INTRODUCTION

l-2. SPECIFICATIONS.

RANGE: 0.1 micro"olt (10 x 10-8 volt) full scale to 100 millivolts on zero-center meter.

13 overlapping ranges in lx and 3x steps.

ACCURACY: *2% of full scale on all ranges exclusive of noise and drift.

ZERO DRIFT: Less than 10 nanovolts per hour or less than 30 nanovolts in any *-hour per­iod after approximately Z-hour warm-up with reasonably constant ambient temperature.
Long-term drift is non-cumulative.

INPUT NOISE (with input shorted): Less than 0.6 nanovolt rms (3 nanovolts peak-to-peak)
on most sensitive range.

INPDT CHARACTERISTICS:

MODEL 149 MILLI-MICROVOLTMETER

Input Resistance

Greater than,

m

0.1 P"

0.3 ,I"

1.0 &I"

3.0 ,1"

10.0 1"

30.0 )A"

100 pv and above

Note:l

LINE FREQUENCY REJECTION: Greater than 5O:l on the most sensitive r~ange. (Ratio of im­pressed peak-to-peak line frequency voltage at input to indicated dc voltage.)

ISOLATION: Circuit ground to chassis ground: Approximately 10'
crofarad.

RISE TIME (10% to 90%):

O.l-microvolt Range:

maximum; 4 seconds using maximum source resistance.

0.3-microvolt to lOO-millivolt Ranges: Less than 1 second when source resistance is
less than 10% of maximum; 2 seconds using maximum source resistance.

Source resistances higher than the recommended maximum will increase noise

and rise time.

Circuit ground may be floated up to *400 volts with respect to chassis ground.

mess than 2 seconds when source resistance is less than 10% of

ohms

10 k

30 k
100 k
300 k

1M
3M

10 M

Maximum Source1

Resistance,

ohms

100

300

lk
3k

10 k
30 k
30 k

ohms shunted by 0.05 mi-

-

ZERO SUPPRESSION: Up to at least 1 millivolt on the microvolt ranges and up to at least

10 millivolts on the millivolt ranges. Stability is such that 100 times full scale may

be suppressed.
RECORDER OUTPUT:

output: *lO volts dc at up to 5 milliamperes for full-scale meter deflection.
Resistance:

Gain:

1-2 1167R

Range setting in volts

Less than 10 ohms within the amplifier pass band,

10 volts

MODEL 149 MILLI-MICROVOLTMETER

INTRODUCTION

Noise:

Modulation Products:

CONNECTORS:

Input noise times gain plus modulation products.

Less than 2% peak-to-peak of full scale with input shorted.

Input:

Special connector.

Front Output: Binding posts.

Rear Output:

Amphenol 80-PC2F.
POWER:

105-125 or 210-250 volts, 60 cps, 50 watts.

DIMENSIONS, WEIGHT:

7 inches high x 19 inches wide x 13 inches deep; net weight, 24 pounds.

SO-cps models available.

ACCESSORIES SUPPLIED: Model 1501 Low Thermal Input Cable with alligator clips; mating

output connector; length of low-thermal solder.*

-'t The solder is screwed to the right side of the copper input chassis, located inside the
Model 149.

Remove the top cover to reach the solder.

1-3

MDDEL 149 KLLI-MICROVOLB

2-1

OPERATION

SECTION 2 - OPERATION

A.

OPERATING CONTROLS

The controls of the Model 149 are simple and conveniently

placed.

Their functions are as follows:

ON-OFF switch is located to the right of the panel

meter.

FUNCTION switch selects the function which Is to be

used:

MilliVOltS, Or

!dCrOVOltB.

2AN'm switch selects the full scale multiplier of

the function selected by the FUNCTION switch.

ZERC SUPPiWSS controls consists of the ZERO RANGE

switch which selects the coa.188 range of sup­pressing voltage in discreet steps and the ZERO
SET potentiometer which gives continuously

variable fine control Including settings through

TieI-0.

PRELIMINARY SET-UP

B.

Connect the instrument to the power line. Unless otherwise
marked the unit may be used on 117 volt, 60 cps line.

vert to 220-volt operation,

refer to the

MAIN'lXK4NC!E

To con-

section.

A three-wire line cord is furnished, which grounds the cabinet.

If a three-wire receptable is not available, use the two-pin
adapter furnished, and ground the third lead to an external

ground.

Set controls *a followa:

Function:
Range :

Millivolts

100

zero suppress: OFF

Input:

Short the input leads together.

OPE3ATION

C.

GENERAL PRECAUTIONS

1.
sistance SpeCifiCatYiOg the maximum source resistance for use with

each voltage range is specified.
with source resistance up to ten times greater than those specified,
however, the measurement will suffer from a considerable decrease in
speed of response,
left completely open-circuited,
scale on any range.

Shielding - Since the instrument operates with a modulator

2.

frequency of 120 cps,
up unless it is large enough to overload the amplifier. The pickup

may be a source of difficulty when using the amplifier with high

impedances on ~the more sensitive voltage ranges. In these cases it

is desirable to shield the leads and the sources as completely as
possible. In some cases a simple low-pass filter at the input to
eliminate frequencies of about 1 cps and above will be helpful.

No use is made of an input filter in this instrument since any input

series impedance due to the filter will increase the input noise and

the thermal drift. When operating above ground, the case of the in­strument must be grounded.

Source Resistance -

and measuring accuracy.

it is not generally sensitive to 60 cps pick-

KNEL

1n

Section luncler the Input Re-

149

MILLI-MICROVOL-R

Reasonable operation is possible

If the instrument is

the meter will generally drift off

3.
the output of the AC amplifier at the point of the demodulator is

provided at the rear of the instrument. It is labeled DEMOD. OUTPUT.
If an inability to make consistent readings persists, it is possible
to check for the presence of excessive pick-up by observing the

wave-form at this point.
be approximately as shown in figure 2. If
served it will look as shown in figure

reasonably well as long as the wave-form
in figure 4.

Determination of Excessive AC Pickup - A terminal attached to

With the input shorted the picture should

excessive

3.

The circuit will operate

does

not clip as shown

At this point the operation will be erratic.

pickup is ob-

--I.-

lo v P-P

t

FIOUIlE2

c

40 v P-P

t

MODEL 149 MILLI-MICROVOLl'METI

Thermal EMF - Extreme precautions have been taken in the input

4.
circuit to minimize thermal EMF's so the residual RMF is usually less
than 0.5 uv.

The material used in the input circuit is pure copper.
Any other metal will generate a thermocouple potential.
particularly troublesome. Where thermal W's are a problem, soldering
should be done with the cadmium-tin solder supplied with the instrument.

Input Noise - The noise at the input is a function of input

5.
resistance and is approximately given by

E = 1.29 x 10-loum

OPERATION

FIGURE 4

Lead solder is

where E is the RMS noise, and R is the source resistaixe. It is assumed

that the bandwidth of the instrument is about 1 cps and the temperature
is 80° F.

If noise is observed, calculate the theoretical noise and

compare results. Also bear in mind that only wire-wound resistors

approach the ideal resistor. However, if Evanohm or Manganin resistors
are used,

a considerable thermal EMF of the resistor material against

copper will be observed.

Checking the Zero Point - At low levels, spurious W's may be

6.

generated

under test.

simply by contact between the input leads and the terminals

If possible,

always leave the instrument connected and

adjust the zero after establishing a zero reference in the apparatus

under test. For example, in bridge measurements, disconnect the bridge

exciting voltage;

Overloads - The current applied to the input circuit should be less

7.
than one milliampere dc steady state,

or with a phototube, shield the tube from light.

10 milliamperes dc short-term.

When the FUNCTION switch is on the MILLIVOLTS position, the off-scale

impedance can be as low as 1000 ohms.

On the MICROVOLTS position, it may

approach one ohm.
MEASURING VOLTAGE

D.

Direct Voltage Measurements - Place the FUNCTION switch at MILLIVOLTS

1.
or MICROVOLTS as necessary for the measurement to be taken.

Then turn
the RANGE switch to more sensitive ranges until the meter gives a usable
deflection,

Measuring Voltage Variations

2.

- To observe small variations in a large
steady signal, first set the FUNCTION and RANGE switches as described in
D 1.
the meter deflection to zero.

Then operate the ZERO SUPPRESS switch and potentiometer to reduce

Increase the meter sensitivity with the range

0667R

2-3

OPERATION

MODEL 149 MILLI-MICROVOLTMETER

switch.
suppression.

Model 149 after suppressing the meter deflection to zero, the RANGE may set at 100

microvolts.
after suppression variations of 2.5OC are seen full-scale.

3. Measuring Differential Voltages - When measurements are to be made in a circuit

where the LOW connection is above ground potential,

posts.
attempt to make such measurements where the side of the circuit being measured is

more than 400 volts above external ground potential.

If a recorder is being used with the instrument in this arrangement, the recorder must

not be grounded since the low side of the output is no longer being grounded.
The Keithley Model 370 Recorder is ideal for use with the Model 149 in recording

operations.

Milli-Microvoltmeter's entire range.

E. QTJEJ APPLICATIONS

Null Indicator - The Model 149 makes an extremely sensitive null indicator which

1.

may be used in a wheatstone or Mueller Bridge.
If the bridge is arranged so than one terminal of the detector is grounded, the Model

149 may be used as described in D 1.

the DISCONNECT LINK at the rear and observe the same precautions as in D 3.

The stability of the suppression voltage is adequate for 100 x full scale

Thus,

If the 10 millivolt signal corresponds to a temperature of 250°C then

This disconnects the instrument circuit ground from chassis ground.

The Model 370 maximizes the performance of the Mo$el 149 over the

if a thermocouple is suppling a signal of 10 millivolts to the

slide OUTPUT LINK from one of its

The Model 370 can float -500 off ground.

If the aet~ector must be used floating, remove

NOT DO

7-4

0667R