Keithley 414 User manual

INSTRUCTION MANUAL
MODEL 414, 414C
MICRO-MICROAMMETER

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, transistors, fuses, and batteries, which,
within a year after shipment to the orig-
inal buyer, proves defective on examination.
DAMAGE IN SHIPMENT
Be sure to include the instrument model num-
ber and serial number in all communications,
if the instrument is damaged when received,
or fails to operate properly, a claim should be
filed with the carrier. Upon receipt of the claim
agent’s report, we will inform you regarding
repair or replacement.
REPAIRS
When returning an instrument for repair or
recalibration, it should be securely packed
against shipping damage and sent to the fac-
tory, freight prepaid. A brief letter describing
the difficulty should accompany the instrument.

CONTENTS
INTRODUCTION
SPECIFICATIONS
Ranges
Accuracy
Input Drop
Grid Current
Zero Drift
Response Speed
Zero Check
Tube Complement
Power Requirements
Accessories Supplied
output
Noise
Connectors
Accessories Available
Cabinet
OPERATION
Operating Controls
Operation
CIRCUIT DESCRIPTION
Basic Theory
Circuit Description
MAINTEUANCE
Factory Adjustments
Trouble Shooting
Voltage Resistance Diagram
Circuit Schematic Diagram Model klh
Circuit Schematic Diagram Model LlhC
Replaceable Parts List
414
KEITHLEY INSTRUMENTS
SECTION
I
II
III
IV
V
CLEVELAND, OR10

SECTION I - INTRCDDCTION
Model klk
----
The Keithley Model lilh Micro-microammeter is a line operated
vacuum tube electrometer for measuring currents in the range
of 2 micro-microamperes to 10 milliamperes. Features include
a response speed of greater than 0.5 seconds on all ranges,
an input drop of less than 5 millivolts at full scale, a
zero drift of less than 2% per day on any range, good accu-
racy and calibration stability, negligible noise, and
simplicity of operation. For recording, an output signal
of five volts at 1 milliampere is available.
This model is identical to the klh except that a meter-relay
replaces the panel meter. An adjustable conta& pointer is
built into the meter-relay, When the indicating pointer
rises to the value set with the adjustable contact, the
contacts lock and actuate an internal relay. The relay's
SPDT s-ampere contacts are brought out through an AN con-
nector at the back of the chassis.
A manual reset button is supplied on the front panel. Reset
connections are also brought out through the AN connector for
rem&e resetting of the locked-up meter-relay.
414
I-l

SECTION II - SPECIFICATIONS
Ranges:
Accuracy:
Input Drop:
Grid Current!
Zero Drift:
Response
Speed:
Zero Check:
Tube
Complement:
Power
Requirements:
Accessories
Supplied:
output:
Noise :
Connectors:
Accessories
Available:
Cabinet:
17 ranges in overlapping 1 and 3 sequence, from 10
milliamperes to 0.1 milli-microamperes full scale.
Within 3% of full scale on all ranges from 10 milli-
amperes to 10 milli-microamperes, and within k% of
full scale from 3 milli-microamperes to 0.1 milli-
microamperes.
Less than five millivolts for full scale on any
range.
Negligible.
Less than 2% in any eight hour period after a five
minute warm-up if the source voltage is not less
than one volt.
Greater than 0.2 second for 67% of full scale on
any range with up to 5000 micro-microfarads of
input capacity.
Zero check switch allows zeroing of the instrument
without disturbing the test circuit.
1 5886, 1 613~8, 1 6CM6, 1 082.
100-130 or 200 to 260 volts SO-60 cps, 33 watts.
Mating connectors, two wire adapter for power cord.
Five volts at up to one milliampere is developed for
full scale deflection.
Noise at output and on indicating meter is less than
1% of full scale peak to peak on any range if the
source voltage is not less than one volt.
Input: teflon insulated UHF (Amphenol 83-798)
Output: binding posts on front panel, Amphenol SOPC2F
at rear.
Model 6031 end frames for bench mounting.
Dimensions 19" x 5i” high by 10" deep. Net weight
16 pounds.
414
II - 1

SECTION III - OPERATION
A. OPERATINGCONTROLS
1. Zero Check: This is a push-button located at the extreme left
of the panel. Its function is to allow the zero of
the instrument to be checked while the current source
is attached. To uss, depress and set the zero with
ZEROCONTROL. It is a feature of the instrument that
the external circuit is not disturbed during the test.
2. Range Switchr The RANGESWITCHis the dial next to and to the right
of the ZEROCHECKbutton. The range in use is always
at the top of the dial for convenient operation.
3. Power, Meter
Reverse Switch: The knob to the right of the RANGESWITCHis the
Ir. Zero I
B. OPERATION8
1.
2.
Precautions:
Operation:
power and meter polarity switch. This control per-
mits positive or negative meter readings. The out-
put voltage at the recorder terminals is not re-
versed by this switch.
This knob, next to the meter, is used in conjunction
with the ZEROCHECKswitch as described above to set
the instrument zero.
If the instrument is to be used at its extreme sen-
sitivity it is necessary to shield all input leads.
The lead-in cable should be polyethylene or teflon
insulated coaxial cable; and a low noise, graphits-
coated, dielectric cable such as Amphenol 21-537 is
recommended. At low sensitivities some of these
precautions may be disregarded.
(a) Connect to a power line of proper voltage. The
instrument is shipped for 117 volt SO-60 cps opera-
tion. For 230 volt operation see oircuit diagram
at the rear of the manual.
(b) Turn the RANGESWITCHto 10 milliamperes.
(c) Turn on power. After 30 seconds the instrument
is ready to operate.
(d) Depress ZEROCHECKbutton and set meter to zero.
CAUTION1 Do not use control to set meter to any
other point than zero.
(e) Instrument is now ready to operate. Attach
current source and turn RANGESWITCHto proper
range.
III - 1

3.
Special Instructions for Contact Meter Models:
(a) The operation of the instrument is identical to the non-
contact meter models with the exception that, on ttrn-on,
the meter pointer will strike the meter contact and lock.
Therefore, after the instrument is on for about one minute
press RESET button to release pointer and zero the meter.
At this point proceed as in Sections 1 and 2.
(b) Special Output Connection: The output connector at the
rear of the contact models is an AN connector which contains
the output terminals, the control relay contacts and the re-
setting circuit. It is necessary that ccntacts A and B in the
resetting circuit be shorted if the panel reset button is to
be operative. If remote resetting is desired terminals A and
B may be connected through a remote normally closed switch.
The wiring of thi~e connector is shown in detail in DR12&9C
AT THE REAR of the manual.
(c) The SPDT relay contacts are rated at 5 amperes at 110
volts AC or 2L volts DC.
IL. Recording:
The Model hlh is provided with binding posts at the front of
the instrument and a connector at the rear for connection to
a recorder. A mating plug is provided for the rear connector.
The wiring of the connector is shown in the schematics at the
rear of the manual and is different for the hlh and the UC.
The output is five volts and 1 milliampere for full scale
meter deflection. The terminals may be connected directly
to high impedance recording devices. A series resistor is
necessary when driving a milliampere recorder or a mirror
galvanometer. The exact value of resistance varies from
recorder to reccrder. For a one milliampere recorder, ap-
proximately 3.S kilohms is required. The exact sensitivity
may be set by means of a potentiometer in series with the
output if desired.
5. Speed of Response:
The speed of response of the instrument is specified as not slower
than 0.2 seconds for 67% of full scale with 5000 micro-microfarads
of input capacity. Capacities greater than 5000 micro-microfarads
may cause instability on some ranges. However, the above speci-
fications allows up to 500 feet of 10 mmf per foot coaxial cable.
III - 2
414

SECTION IV - CIRCUIT DESCRIP'TION
i in --
FIG. I
A. BASIC 'THEORY:
Figure 1 shows the basic circtiit. The input current 1 flows into
the input termj.nal. If we neglect Cl and C2 for the moment, the
current i flows throuph R the feedback resistor. Since A is an
amplifie? of gain k and the feedback is negative.
-
ein = co/k (1)
and since all the current must flow through R,
i q -e, + e, /k
--.-_,-~_ ~_--
R
(2)
now if &, the loop gain is large, the current flowing through
the input is e,/R and may be read from a properly calibrated
meter at the output. ei,, is the input drop of the ammeter.
The input resistance Rin is given by e-in/i and is,
Since in the Model hlh, the amplifier gain k is about 1000, the
input drop, ein will be about 5 millivolts for a five volt output
and the effective innut resistance Rin will be about l/1000 of
the range resistor being used.
IV - 1

If the capacities Cl and C2 are considered, the time constant
of the circuit is given by,
T = RCl/k + RC2
Thus the input time constant is reduced by a factor equal to
the loop gain. Capacitor C2 is placed across R to damp the
response and prevent ringing.
B. CIRCIJIT DiQXRIl'TION: Refer to DR127580 at the rear of the manual:
The input circuit consists of Vl, a type 5886 electrometer tube,
followed by a 6BH8 and a 6CM6 cathode-follower. This arrange-
ment provides a loop gain of about 1000. A high decree of zero
stability is assured by regulation of the filament supply of Vl
and B+ supply by Vlr the OA2 voltage regulator. The negative
bias supnly is regulated to the necessary degree by an NE81
neon lamp. The amplifier is a simple single ended direct-
coupled amplifier. Vl and V2 are directly coupled and the
coupling between V2 and V3 is by means of a NE81 neon lamp.
RlZh, the zero control, operates by varying the screen poten-
tial of Vl.
The current sensitivity is changed by either changing the
feedback resistor (RlOO through R113) or by changing the
feedback voltage by means of resistors Rll6 and R117. From
10 milliampers to 10 millimicroamperes a feedback voltage
of five volts is used and one resistor is used per range.
From 3 millimicroamperes to 0.1 millimicroamperes one resisLor
is used per decade and the feedback voltage is either one or
three volts depending on whether a 3x or 10x range is being
used.
The power supply consists of a conventional rectifier-filter
circuit for the positive and negative supplies.
IV - 2

SECTION V - MAINTENANCE
Except for occasional tube replacement very little maintenance is
required for the model hlh. Components are operated well below
rating, and high quality components have been used throughout.
1. FACTORYADJlJSTMENTS
Only one factory adjustment is made. This is Rl32, the meter
calibration potentiometer. This adjustment is only necessary
if the panel meter is changed. To calibrate set the instrument
on the 1 or 10 milliampere range anti set the meter to the cor-
rect reading with a calibration current sufficient to deflect
the panel meter to at least 75% of full scale.
2. TROUBLESHOOTING
Refer to DR12758-C and the voltage-resistance diagram at therear
of the manual.
Follow this general procedure in trouble shooting if the fault
is not apparent from the tabulated list below:
1. Check the B+ and J?- potentials: The E+ poten,tial can be
measured between the rectifier string and R201 and should
be about 320 volts. The ripple should not exceed 3 volts
rms. The B- potential may be measured between the negative
rectifier string (RFh,5) and R2O5 and should be about 2irO
volts. The ripple again should not exceed 3 volts RMS. If
either potential is no't correct the trouble is probably due
to a defective rectifier or filter component although in
some cases the transformer may be defective. Refer to the
schematic diagram to locate the defective component.
2. SPECIFIC FAULTS
a. Instrument will not balance: If no defect has been
foundT%x%zpection suggested in l., check the volt-
age at pin 1 or 5 of Vlr. This should be 150 volts. If the
voltage is not 15Ov, check R2O3, 202, and 201 for continuity
and check Vh. If the voltage is correct, check the voltage
at pin 3 of Vl (The red dot is next to lead 1) the reading
should be 3.2 volts. If the voltage at this point has risen
to 150 volts, the filament of Vl is open and this tube should
be replaced. If this voltage checks correctly, the following
procedure should be started: Short circuit the input to
ground with the range switch on 1 microampere full scale.
This removes the negative feedback and allows the operating
points of the individual tubes to be checked. It will usually
be found that the amplifier will become very sensitive and
difficult to balance, however it is only necessary that the
voltage swing through the correct operating value to confirm
that the circuit is operating correctly. Then with the input
V-l