Tektronix 168 Instruction Manual
INSTRUCTION MANUAL
MODEL 168
AUTORANGING DMM
KEITHLEY INSTRUMENTS. INC
CONTENTS
‘rifle page
content3
List Of illuarrarions.
Dimensional “ata
Specificarions
1. General Informutio,,
2. Initial Prepururian.
3. operating InSrruCLionS
4. n,eol;y Of operation.
5. MainLenance.
6. Replacevble Partr.
7. Schematic Diagrams
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I-I. T,“tr”duction
1-2. Warranty Inf”rmation
1-3. Change Notice.
2-1. General.
L-2. Inspfciion
1-3. Preparation for “xe.
3-1. General. ...
1-2. HOW to Select Power.
3-3. HOW f” Make Input Cannccrions.
3-4. ii”W to Select Funcri~o”
1-5. HOW to Measure “olttlge .................
3-6. HUWLOMeaSureCurrent
3-7. How 1” Measure Resistance.
4-1. General.
4-2. halOg circuitry
4-1. DC Voltage Operario”
4-4. nc “oltvge operation
4-5. “HMS OperaLion
4-6. current Operation.
4-7. Power supply
4-8. Digital circuitry.
5-1. General.
5-2. Required Test Equipment.
5-3. Performance “eliflcarlon ................
5-4. AdjusrmentiCalibrarion Procedure ............
5-5. 'rrouhleshooting.
6-1. General.
6-2. Ordering IniormziLion .................. 72
6-3. cross Reference.
6-4. PariS List 74
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MODEL 168
ILLUSTRATIONS
F
LOClfi”” Of current Fan
M,
ILLUSTRATIONS
I
SPBCIFIcATIONS
ml-
MODEL 168
12.75
(324)
iv
DIMENSIONS IN INCHES (MM)
FIGURE 1.
Dimensional m.ta.
:i
0374
MODEL 168
SPECIFICATIONS
I SPECIFICATl”NS
MODEL
168
FIGURE 2. n-ant Panel
0374
MODEL 168 GENERAL INFORMATION
I
SECTION 1.
l-1. INTRODUCTION. The Model 168 is a versatile autoranging digital multimeter useful for meaeurement of ac and dc voltage, ac and dc curre"t.
and resistance.
volts dc or .OOOlvolr to 500 volts ac. current meaSureme"es can be made
from .OOOl milliampere to 1 ampere BC and dc in two spans.
measurements can be made from 0.1 ohm to 20 me&m in two overlapping
spans. Range and polarity is automatically selected. In addition to the
display of digits, the 168 indicates decimal point, function (AC or DC),
and measurement unit (IDA, A, kn, MR).
WARRANTY INFORMATION. The Warranty is give" on the inside front
1-2.
cover of this Instruction Manual.
Warranty, contacf the Keiehley Representative I" your area to de~emine
the proper action co be r&en. Keirhley maintains service facilities in
England, west Germany, as we11 as in the United states. Check the inside
front cover of this Instruction Manual far addresses.
1-3. CHANGE NOTICBS.
occur after printing of the Instruction Manual will be explained on a
Change Notice sheet attached to the inside back cover.
Voltage meaS"remenfs can be made from f.OOO1 volt to t1000
1mprovemenrs or changes to the instrument which
GENERAL INFORMATION
RCS3fSta"CEY
If there is a need to exercise the
I
0374
1
‘i
,,
CONDENSED OPERATING INSTRUCTIONS
(N&M, 11'7)
LINE SWITCHES SET FOR
105 TO 125 VAC OPERATION
4
0374
SECTION 2.
2-l. GENERAL. This section provides information needed for incoming inspection and preparation for use.
INITIAL PREPARATION
2-3. PREPARATION FOR "SE.
strument may be powered from line voltage or from rechargeable nickel-cad-
mium batteries (when the optional Model 1688 Rechargeable Battery Set is
installed).
a. HOW to operate From Line Power.
wire line cord which mates with third-wire grounded receptacles. The permanently installed line cord is stored by wrapping the cord around the
base of the instrument aa shown in Figure 4.
1. liow to Set Line Switches. The Model 168 has two rear panel line
switches which are used to select line voltage ranges of 90-llOV, 105125", 195-235", or 210-ZSOV.
,234V (5102) and LOW/NORM (5101).
been defermined, then the line voltage range should be selected from
the four ranges available on the Model 168. For example, when the line
voltage to be used is within the range from 105 fo 125 volts, then ihe
line switches should be set to
voltage to be used is within either of two overlapping ranges, such as
107 volte, then either range may be selected (117V, LOW or 117V, NORM,
for this pareicu1ar example).
anyone of the four ranges are not useable. After the line voltsw
switches are set, connecf rhe line cord and depress the LINE pushbutton
to operate.
Line Fuse Requirements.
2.
co protect the line-operated power supply. The fuse types are l/8
ampere, 3AG slo-blo. Replacement instructions are given in section 5.
The Model 168 is shipped ready-to-use. The in-
The Model 168 provides a three-
The line swirches are idenrified as 117f
Once rhe line voltage to be used has
"117V" and "NORM" ,msitia"s. If the line
Line voltages which are not covered by
The Model 168 requires f"o line fuses
0374
5
I
INITIAL
PREPAiUTION
MODEL 168
FIGURE 6. Model 1688 Rechargeable Battery Pack.
0374
I
MODEL 168
INITIAL
PREPARATION
b. HOW to operate From Battery Power
Battery set). The Model 168 may be operated from rechargeable nickel-
.A
(Model 1688 Rechargeable
cadmium batteries when ehe optional Model 1688 Rechargeable Battery Set
is installed. The Model 1688 may be field installed at any time or may
be ordered factory installed. The Rechargeable Battery Set includes a
battery pack which mounts within rhe Modal 168. Wiring is accomplished
by a single plug-i" connector. Battery operation from fully-charged
NI-CAD hareeries is eypically 6 hours.
1. How to Install Model 1688 Rechargeable Bateery Set. The bae-
teriea furaiahed with the Model 1688 come already fnstal~led in the
battery pack. The battery pack includes 7 rechargeable "C" cells
(LZV, 2 AMP HR) and 2 rechargeable "button" cella (8.4V, .225 AMP
HR). If batteries need ea be replaced or re-installed, be certain
to observe the proper polarity of individual cells as shown in
Figure 6. TO insrall the Model 1688 Battery Pack, turn the instrument over 80 that the bottom co"er faces up.
BCT~WB on the bottom cover as shown in Figure 4.
Remove four slotted
(A chisel-blade
screwdriver is required to loosen the sloteed screws.) Turn o"er
the instrument with COP co"er facing up, taking,care to hold the top
and bottom covers together. Remove the top co"er to gain access to
the printed circuit board. Check to see ebat the four insulating
standoffs are in position on the printed circuit board. Place the
Model 1688 Battery Pack in position an the standoffs with the cable
oriented 88 shown in Figure 8.
Plug the 4-wire connector (5103) into the mating receptacle 0'103) taking care to oriene the connector
as shown in Figure 8. After the Battery Pack is inscalled, replace
the top cover. Turn o"er the instrument with bottom cover facing up
and inscall the four slatted-head screws.
TABLE 2-l.
Summary of Batteries Used in Model 1688.
0374
Rechargeable "C" cell,
1.2”. 4 Am-km
Rechargeable "Button" type
battery, 8.4V, .225 AMPHR (4 individual 1.2V
Cells)
Quanticv Keithley Part No.
I I
7
2
BA-30
BA-29
7
I
INITIAL PREPAR4TION
MODEL 168
s
h
FICUKE 7.
Exploded View of Model 168 with Model 1688.
0374
MODEL 168
INITIAL PREPARATION
MODEL 1688 BATTERY PACK
0374
MODEL 168 -".--_-
9
INITIAL PREPARATION
MODEL 168
10
FIGURE 9.
Bath-~ ‘feet ~ocatian.
0374
I
MODEL 168
2. How to Check Batteries. (Valid only in BAT mode). The Model 168
provides two fest points (A and 8) located on the bottom of the instrument as shown in Figure 9.
of the condition of the internal Battery Pack without need to remove
the Model 168 cover.
ured using the Model 168 or any other comparable voltage measuring
instrument.
BAT mode, connect rhe HI terminal of the Model 168 (for a self check)
or oeher voltmeter to fest points A or B and observe the measured voltage. (If a separate voltmeter is used, it is necessary to make a connection to the LO terminal of the Model 168 since both points A and B
are to be referenced to circuit low.)
voltages required at each teat point.
The instrument mutt be operated in the BAT made in order to obtain
a valid battery condition at test points A and B.
that the batteries are supplying power to the instrument.
voltages are measured when the Model 168 is operated in the LINE
made a different reading may be observed since the batteries are
not connected and therefore do not supply power to the instrument.
To check the voltages at test points A or B, select the
These test points permit a convenient check
The voltage at test points A or B may be meas-
Table 2-2 gives the battery
INITIAL PREPARATION
This will ensure
If the
Summary of Battery Voltage Levels (BAT mode)
Test Point Acceprable Battery Levels Recharge
A 2.9 - 9" 4.8V 2.w B&29
B
3. HOW fo Charge Batteries. The Model 168 provides built-i" recharging circuitry for recharging the Model 1688 Battery Pack. To recharge the internal batteries, connect the Model 168 to line power and
depress the CHG pushbutton. Recharging time is dependent on the condition of the batteries at the time of recharge. Typically, the recharge
time is l-112 hours per bow of discharge (or 9 hours of charging time
for every 6 hours of operating time in the battery mode).
The Model 168 may be operated while in the CHG mode. However,
if the Battery Pack is not installed, the Model 168 will not be
operable when the CHG mode is selected since the batteries are
connected in series with the line power supply.
0374
&+*g,e
7v - 10.5" 8.4V 7v
TABLE 2-2.
Batteries
Normal if below Tested _
RA-30
NOTE
-
11
OPERATING INSTR”CTIONS
'MODEL 168
SECTION 3.
GENERAL.
3-l.
Model 168 for measurmene of voltage, current, and resistance.
3-2. HOW TO SELECT POWER.
or rechargeable nickel-cadmium bafteries (when the Model 1688 is installed).
The Model 168 has a built-in line-voltage power supply and line cord. If
the accessory Model 1688 rechargeable Battery Set is ordered and installed,
then Lhe user has the option of selecting line or battery operation via the
front panel pusbbuteons labeled LINE and BAT.
The accessory Model 1688 Rechargeable Battery Set my be ordered at
the fime of purchase of the Model 168 or may be purchased and field
installed at a later time if so desired.
plug-in wiring.
the Model 168 chassis.
a. HO” to operate From Line Power.
ranges of line voltage from a minimum of 90 volts to a maximum of 250 volts,
ms, 50-60 Hz.
ZlO-25OV.
sear Panel
Line S"fGxhf?S 90-110 105-125
This section provides infarmation needed to operate the
me line voltage ranges are 90-llov, 105-125V, 195-235V, and
summary of Line Voltage Switch settinga
OPERATING INSTRUCTIONS
The Model 168 may be powered from line voltage
NOTE
The Model 1688 features
As a result, no modifications need to be made to
The Model 168 is operable over four
TABLE 3-l.
Line Voltage Ranges - Volts, nns, 50-60Hz
195-235 210-250
117/234V Switch (5102)
LOW/NORM Switch (SlOl) LOW
b. HO" to operate From Battery Power. The Model 168 my be used with
the optional accessory Model 1688 Rechargeable Battery Set to provide
portable operation in addition to line operation.
power, depress the BAT pushbutton. Check the battery vaJtage at test
points A and B co ensure that barreries are charged sufficiently.
12 0374
117 117 234
NORM LO"
To operate from battery
234
NORM
MODEL 168
Refer to Section 2-3b for instructions concerning installation of the Bat-
eery Pack, battery voltage checks, and recharging.
fully charged NI-CAD batteries is 6 hours minimum. No fuses are required
for operaeion in battery mode.
OPERATING INSTRUCTIONS
Battery operation from
Summary of flattery Voltage Levels (BAT mode)
Test Point Acceptable Battery Levels Recharge Bdtteries
A
8
Summary of Operation in LINE, CHG, and BAT Modes.
PUShbUttOn Line connected
Depressed
OFF
LINE ON
CHG OFF
BAT OFF
The insfrument will be turned off if all power pushbuttons are
released (all non-depressed). A lock-out feature prohibits
selection of two or more puehbuttons at the same time.
3-3. HOW TO MAKE INPUT CONNECTIONS. The Model 168 has two front panel
terminals identified aa “HI” (red) and “LO” (black). These terminals
accomodate banana plugs, alligator clips, spade lugs, bare wires, and
other similar input connections. Leads may be fabricated using a good
quality copper wire terminated by single banana plugs such as Keithley
Part No. 80-S or dual banana plug such as Keitbley Part No. W-7.
Rraay-made tese lead8 are also available from Keifhley. accessory Model
1681 Clip-on Test Lead Set includes two 40 inch long leads terminated by
a banana plug and spring-loaded clip which easily attaches to wires and
terminals on pc boards, etc.
interchangeable probe tips for various applications. The Kit includes
regular probes, alligator clipe, banana plugs. spade lugs, and phone tips.
1688 not installed 1688 installed
RtS”,P$
2.5V - 9V 4.m 2.5V M-29
7v - 10.5v 8.4”
OFF OFF OFF
TABLE 3-2.
Normal
TABLE 3-3.
Condition of Inserument:
Line connected Line not-connected
ON
ON OFF
ON ON
NOTE
Model 1683 Universal Test Lead Kit features
if below Tested
7V
1680 installed
b-30
OFF
0374
13
OPERATING INSTR”CTIONS
MODEL 168
14
0374
3-4. HOW TO SELECT FUNCTION. The front panel pushbuttons are arranged
to permit selection of five functions including DC voltage, AC voltage,
DC curcent, AC current, and resistance.
a. DC Voltage. To select DC voltage operation, depress the “DC”
pushbutton. Voltage function is implied by the “DC” function indicator.
b. AC Voltage. To aelect AC voltage operation, depress the “AC” push-
button. Voltage function is implied by the “AC” function indicator.
C. DC current. To select DC current operation, depress the “DC” push-
button.
sired range. The %A” and “A” pushbuttons are “push-push” type with lock-out,
such that one pushbutton muet he released before the other can be selected.
d. AC Current. To select AC current operation, depress the “AC” push-
button.
desired currenf range. The “mA” and ‘A” pushbuetons are “push-push”
type with lock-out, such that one pushbutton must be releaeed before the
other can be selected.
e. Re8iStance.
“LO OHMS” or “HI OHMS” pushbutton depending an the desired resistance
range. Either “kR” or “Mn” are displayed depending on range.
DC Voltage
DC Milliamperes
DC Amperes
AC “oltage x
AC Milliamperes x
AC Amperes x
Then select either the “ti” or “A” pushbutton depending on the de-
Then select either the “mA” or
To select resistance operation, depress either the
If all function pushbuttons are released, the inpue to the Model
168 will be disconnected. A lock-out feature prohibits selection
of inconsistent funcrlon pushburtons at the same time.
TABLE 3-4.
Pushbuttons Depressed
DC AC
x
x x
x
Function
Des lred
Summary of Function Settings
Voltage & Current
‘A” pushbutton depending on the
NOTE
CUrrent
d A
x
x
x
Resistance
LO OHMS
“I OHM
Resistance (LO)
Resistance (HI)
x
x
OPERATING INSTRUCTIONS MODEL 168
3-5. HOW TO MEASURE VOLTAGE. The Model 168 measures ac and dc voltage
in five ranges: O.lV, lv, IO”, IOOV, and IOOOV dc orlOOOVac (both ac
and dc volts have a maximum inpur of 1200 volts peak ac + dc). The
displayed voltage is direct-reading with decimal point located autonlatica11y.
a. DC Voltage.
f.OOO1 valts dc to $999 voles dc. Above 999 volts, the display reads
properly and flashes to indicate an overrange condition.
at the HI eerminal is negative,
minus sign.
lighted “DC” and decimal point are also displayed for all dc measurements.
1. How to Measure DC Voltage. Select the dc function by depressing
the “DC” pushbutton.
I.0 terminals. Observe the displayed digits, polarity sign, and decimal
point lacacion. The lighted “DC” indicates that the dc voltage function has been selected.
2. How to Select Range. The Model 168 autamaeically selects the
appropriare range for all valtage measurements.
3. How to Determin+ Accuracy.
reading ~1 digit.
will have an uncertainry of ~0.1% 51 digit or +.002 volts. The input
resistance in rhe dc mode is 10 megohms. Measurements from relatively
high source resistances could cause an additional reading error. The
amount of error due to loading can be determined by ehe following
relatianship:
For example, a scurce resistance of 10,000 ohms will result in a
loading error of approximaeely 0.1% of reading.
4. How fca Determine Maximum Allowable Input. The maximum allowable
voltage input is 1200 volts dc + peak ac. The Model 168 displays dc
voltages to i999 volts. Beyond 999 volts fhe display reads properly
and blinks to indicafe an overrange condition.
The Model 168 automatically displays voltage from
If the polarity
the Model 168 display indicates a (-)
If the minus sign is off, a positive voltage is implied. A
Connect the signal to be measured between I+1 and
The Model 168 accuracy is ~0.1% of
For example, a display reading of 1.000 volts dc
% error = 100 x Ks i (Es i 107)
where R, = source resistance in ohms.
IMPORTANT
The Model 168 provides ac rejection of greater than 75 dB (NMRx).
However, a large ac signal superimposed on a dc level could case
damage if the inpue exceeds 1200 volts dc + peak ac.
16
0374
/
MODEL 168
“PERATINC TNSTRUCTTONS
5. HOW to ZerO the Display. The Model 168 has a front panel zero adjuscment which may he used to zero the display to compensate for zero
offset. Apply a &xc connection bctwee” the input terminals or select
“A” function and adjust the zero control (screwdriver required) to obtain a .OOOO display with the minus (-) polarity flashing on and off.
b. AC Voltage. The Model
168 automatically
displays voltage fror .OOO1
volts ‘co 499 volts ac rms. Above 499 volts, the display reads properly
and flashes to indicate an overrange condition. In the ac function, the
Model
168 operates as an
average-reading voltmeter, calibrated in terms of
the root-mean-square (rms) of a sine wave. ‘The ac-to-dc converter is a fullwa”e rectifier type, and as such the calibration is exact for sinusoidal
waveforms.
lifier so that dc is blocked.
The input signal is ac coupled (capacitive) to the input amp-
me 1np”t bLocking capacitor effectively
reduces the Model 168 low-frequency response to approximately 20 HZ.
1. HOW to Measure AC Voltage.
Select the ac function by depressing
the “AC” pushbutton. Connect the signal to be measured between HI and
LO terminals.
Observe the displayed digits, and decimal point location.
The lighted “AC” indicates chat the ac voltage function has been selected.
2. mw to se1ecr Range.
‘The Model 168 auramarically selects tire
appropriate range for a volksge Ineas”reme”t.
3. How to Determine Accuracy.
The Model 168 accuracy is 1(0.5X of
reading ?0.3% of range). For example, a display reading of 1.000 volts
ac “ill have an uncertainty L.008 volts over a frequency range from
20 HZ to 10kHz. An additional reading error may result if the source
resistance is relatively high.
is frequency dependent.
For example, with an input resistance of 9
The input impedance of the Model 168
megohms and shunt capacitange of less than 90 picofarads, the effoc-
tive input impedance at 60 HZ is approximately 8.23 megohms. The impedance at other frequencies may be determined by the following relationship.
Zin =
Rin
1 + (2nf R&)Z
I
0374
where 2.1~ = effective input impedance
R
sin < 90
in =
9 x
lob
ohms
x lo-6 farads
= frequency in HZ
17
MODEL 168
Source loading can be determined by the following relationship:
% error = 100 x z,
% + Zi”
4. How to Determine Maximum Allowable Injxt. The maximum allowable
voltage input is 1200 volts dc + ac peak. The Model 168 displays ac
voltages to 499 volts rms.
Beyond 499 “olfs the display reads properly
and blinks to indicare an overrange condifion.
NOTE
The Model 168 blocks dc signals at the input as a result of the
capacitive coupled input. Howe”er, a large dc si@,al superimposed
on an ac level could cause damage if the inpuf exceeds 1200 “olcs
dc + peak ac.
C. “oltaae Measurements using Model
1682. The Model
168 may be
used
with the accessory Model 1682 RF Probe to permit ac voltage measurements
from 100 kHz to 100 EMz. The transfer accuracy of the 1682 is t5%, cal-
ibrated in rms of a sine wave.
Input impedance is 4 megohms shunted by
2pF. Maximum allowable input is 30” rms AC, 200” DC. To use the 1682
with rhe Model 168, connect the probe to HI and LO. Select DC voltage
function. Voltage is direct reading in “olts ac rrxs.
The Model 1682 is designed for use with a voltmeter having an
input resistance of 10 megohms ilO%.
I
18
0374
MODEL 168
OPERATING INSTR"CTIONS
3-6. HO" TO MEASURE CURRENT. The Model 168 me=="ree ac and dc current
in four ranges; O.lmA, ImA, O.lA, and 1A.
The displayed current is direct
reading in terms of milliamperes ("!A) or amperes (A) (depending on mode
selected) with decimal point located ="tam=tic=lly.
DC current.
a.
The Model 168 me=="ree dc current from .OOOl mA to 2 mA
end frbm .OOOlA to 1A. If the Polarity at the "I terminal is negative,
the Model 168 display indicate= a (-) minus sign. If the minus sign is
off, a positive current is implied. A lighted "DC" is displayed when the
dc function is selected. Current may be selected for direct reading in
terme of milliamperes (mA) or amperes
current measured.
When the ampere=
(A)
depending on the magnitude of
(A)
mode is selected, ehe Model 168
measures current from 0.0001 ampere to 1 ampere. (The display will Permit a reading up to 1.999 amperes or until the 2A fuse blows.) When the
milliampere (mA) mode is selected,
0.001 milli=mpere to 2 mi11*=nlperes.
the Model 168 measures current from
(The display will permit a reed-
ing up to 9.99 IDA or until the lOmA fuse blows.)
1. How to Measure DC Current.
the "DC Pushbutton.
First select "A" mode.
Select the dc function by depressing
The "mA" and "A" pushbuteona
are "push-push" type, such that one pushbutton m"se be released before
the other can be selected.
terminals.
Observe the displayed reading,
tive inputs), and decimal point position.
that the dc function ha= bee" selected.
Connect the input signal between HI and LO
the polarity sign (for negaThe lighted "DC" indicates
The lighted ",,!A" or "A" indi-
cates that the current mode has been selected.
2. HO" to Select Ranges.
sensitivity for either %A" or
is between 0.0001 mA and 2 mA, select the %A" mode.
The Model 168 automatically defermine= the
“A”
modes.
If the current to be measured
If the current to
be measured is between 0.0001 A and lA, select the "A" mode. For either
%A" or "A" modes, the Model 168 has two sensitivities, 0.1000 and 1.000
full range.
while the maximum reading ie .1999.
On the most sensitive range, the minimum reading is .OOOO
When the input exceeds .1999, the
Model 168 a"tom=tically "p-ranges to the higher range and decimal Point
location is changed appropriately.
Down-ranging occurs when the input
level is reduced, causing a dieplayed reading to be less than 1-9-O on
any range.
0.189, the display will change from .190 to .1890.
For example, if =n input signal is reduced from 0.190 to
However, if the sig-
nal is increased from .1899 to .1900, the Model 168 will not "prange,
but remain on the 0.1000 range.
0374
19
OPmATING 1NSTR”CTIONS
FRONT PANEL
MODEL 168
CURRENT RANGE FUSES
20
FIGURE 11. Locafion of
Current Range Fuses.
0374
3. HOW to Determine Accuracy.
of reading + 0.1% of range).
have an uncertainty of +.004 ampere.
uses a 1000 ohm shunt resistor.
The accuracy of the Model lb8 is t(O.3%
For example, a display of 1.000 ampere will
I" rhe ?,,A" mode, the Model 168
In the "A" mode, the Model 168 uses a
1 ohm shunt resistor. An additional reading error shauid be considered
if the so"rce resistance is not greater then 1000 times the shunt re-
sistor. For example, in the "mA" mode,
a source resistance of 100 kilohm would result in a loading error of approximately 0.1% of reading.
Loading error for other eource resistances can be determined by the
following relationship:
% error - 100 X R, (mA mode)
R, + 1000
where R, = eource resistance
or % error =loo X %( A mode)
R
6
4. Ho" to Determine Maximum Allowable Input. The Model 168 uses
separate fuses for each current mode.
a 10 milliampere fuse.
The "A" function ie protected by a 2 ampere
The ?,,A" mode ie protected by
fuse.
a) HOW to Check the Current Range Fusee.
To determine the condition of fuses for "IDA" and "A" modes, select the "Lo OHMS" resistance mode. Since the shunt resistors are connected between HI end
LO terminals, the resistance including fuse can be meaeured directly
an the Model 168.
observe the display.
kn to indicate a "good fuse".
fuse.
display.
For the "A" mode, select the "A" pushbutton and observe the
The display should read approximaeely .0012 kn to indicate
For rhe "mA" mode, select the ?,,A" pushbutton end
The display should reed approximately 1.180
A flashing display indicates a "blown"
a "good" fuse. A flashing display indicates a "blown" fuse.
b) How to Replace the Current Range Fuaee. The current range fuses
F103 and F104 are located on the printed circuit board. The fuses are
accessible by removing the fop cover and internal shield. TO replace
the fuses, turn the instrument over 80 that the bottom cover faces up.
Use a screwdriver to remove the four slotted screwe on the bottom
cover as shown in Figure 4. After the ecre"s ere removed taking cere
CO hold the top and bottom covers together, turn o"er the inetrumen~
so that the top cover faces up. Remove the top cover to gain access
to rhe printed circuit board. If the Model 1688 Rechargeable Battery
OPeRATING INSTRUCTIONS
MODEL 168
Pack is installed, lift off the battery pack from the stand-offs cod
set to the left of the instrument.
The fuses for current ranges are
located on the prineed circuit board near ehe front right side under
the shield.
rhe fuses.
a single Phillips head screw.
The amplifier shield must be removed to gain access to
The shield.&? fastened to rhe circuit board by means of
Before the ecrew can be removed, the
front panel most be lifted out of, the way. Gently lift the front
panel and pull forward until panel is clear of puehbuttoos.
and remove the Phillips head screw which holds the shield.
Locate
The metal
shield is held by clips to fhe printed circuit board and must be pried
Replace fuses as necessary.
UP.
FlO3:
O.OlA, 25OV,
type 8AC, fast acting (%A" mode)
Fuse types are as follows:
~104: 2.4, 250", type 8AG, fast acting ("A" mode)
After fuses are replaced, re-assemble shield, spring, washer, ana
Phillips Screw as shown in Figure 11.
Model 1688 battery peck on stand-offs.
Replace front panel. Replace
Replace top cover. Take care
fo hold top and bortom covers together and turn over instrument so
that bottom cover faces up.
Replace Screw to complete re-assembly
of chassis.
b. AC Current.
The Model 168 measures ac c"rrent from .OOOlmA to 2mA
and from .OOOlA to 1A. A lighted "AC" is displayed when the ac function
is selected.
milliamperes (nut) or amperes
Current may be selected for direct reading in cerme of
(A)
depending oo the magnitude of current
?a be measured. When the amperes (A) made is selected, the Model 168
measures currenf from .OOOl ampere to 1 ampere.
(The display will permit
a reading up to 1.999 amperes or until the 2 ampere fuse blows.) When
the milliampere (mA) mode is selected, the Model
from .OOOl milliampere to 2 milliamperes.
168 neasures current
(The display will permit a
reading up to 9.99 mA or until the 10 m.4 fuse bl.ows.)
1. How to Measure AC Current. Select the ac function by depressing
the "AC" pushbutton. First select "A" mode. The "mA" and "A" pushbuttons are "push-push"
before the other can be selected.
type, such that one pushbutton must be released
Connect the input signal between HI
and LO terminals. Observe the display4 reading, and decimal point
position.
selected. The li.ghrcd "ti" or
The lighted "AC" indicates that the ac fuoctioo has been
“A”
indicates that the currant mode has
been selected.
22
0374
MODEL 168
ROW to Select Ranges.
2.
sensitivity for either “IDA” or “A” modes.
is between O.OOOlmA and 2mA, select the “m,” mode.
be measured is between O.OOOlA and lA, select the “A” mode. For either
““A” or “A” modes, the Model 168 has two sensitiviries, 0.1000 and 1.000
full range. on the most SenSitiVe range,
while the maximum reading is
Model 168 automatically up-ranges to the higher range and decimal point
location is changed appropriately.
3. How to Determine Accuracy. The accui-acy of the Model 168 is k(l%
of reading + 0.3% of range). For example, a display of 1.000 ampere
will have an uncertainty of t.013 amperes (over a frequency range from
30 RZ to 5lcHz).
sister. An additional reading error should be considered if the source
resistance is not greater than 1000 times the shunt resistor.
in the “ti.” mode, a source resistance of 100 kiloh would result in a
loading error of approximately 0.1% of reading.
source resistances can be determined by the followinS relationship:
In the YnA” mode, the Model 168 uses a 1000 ohm shunr re-
% error = 100 X R, (“,A mode)
The Model 168 automatically determines the
If the current to be measured
the minimum reading is .OOOO
.1999. When the input exceeds .1999, rhe
R, + 1000
OPERATING INSTRUCTIONS
If the current to
For example,
Loading error for other
4. Uaw to Determine Maximum Allowable Input. The Model 168 uses
separate fuses for each current mode. The “mA” mode is protected by
a 10 milliampere fuse.
a) HOW to Check the Current Range Fuses. TO determine the condition of fuses for %A” and “A” modes, select the “LO OHMS” resistance mode.
LO terminals, the resistance including fuse can be measured directly
on the Model 168. For the “mA” mode, select the “,“A” pushbutton and
observe the display.
kQ to indicate a “good. fuse”.
fuse.
display.
a “good” iuse.
0374
Since the shunt resistors are connected between HI and
For the “A” mode, select the “A” pushbutton and observe the
The display should read approximately .0012 kn to indicate
A flashing display indicates a “blown” fuse.
The “A” function is protected by a 2 ampere fuse.
The display should read approximately 1.180
A flashing display indicates a “blown”
23