Fluke 845AR User manual

845AR
High Impedence
Voltmeter
Null Detector
Instruction Manual

WARRANTY
Notwithstanding
any
provision
of
any
agreement
the
following
warranty
is
exclusive:
The
JOHN
FLUKE
MFG.
CO.,
INC.,
warrants
each
instrument
it
manufactures
to
be
free
from
defects
in
material
and
workmanship
under
normal
use
and
service
for
the
period
of
1-year
from
date
of
purchase.
This
warranty
extends
only
to
the
origina!
purchaser.
This
warranty
shall
not
apply
to
fuses,
disposable
batteries
(rechargeable
type
batteries
are
warranted
for
90-days),
or
any
product
or
parts
which
have been
subject
to
misuse,
neglect,
accident,
or
abnormal
conditions
of
operations.
In
the
event
of
failure
of
a
product
covered
by
this
warranty,
John
Fluke
Mfg.
Co.,
Inc.,
will
repair
and
calibrate
an
instrument
returned
to
an
authorized
Service
Facility
within
1
year
of
the
original
purchase;
provided
the
warrantor's
examination
discloses
to
its
satisfaction
that
the
product
was
defective.
The
warrantor
may,
atits
option,
replace
the
product
in
lieu
of
repair.
With
regard
to
any
instrument
returned
within
1
year
of
the
original
purchase,
said
repairs
or
replacement
will
be
made
without
charge.
If
the
failure
has
been
caused
by
misuse,
neglect,
accident,
or
abnormal
conditions
of
operations,
repairs
will
be
billed
at
a
nominal
cost.
In
such
case,
an
estimate
will
be
submitted
before
work
is
started,
if
requested.
THE
FOREGOING
WARRANTY
IS
IN
LIEU
OF
ALL
OTHER
WARRANTIES,
EXPRESS
OR
IMPLIED,
INCLUDING
BUT
NOT
LIMITED
TO
ANY
IMPLIED
WARRANTY
OF
MERCHANTABILITY,
FITNESS,
OR
ADEQUACY
FOR ANY
PARTICULAR
PURPOSE
OR
USE.
JOHN
FLUKE
MFG.
CO.,
INC.,
SHALL
NOT
BE
LIABLE
FOR
ANY
SPECIAL,
INCIDENTAL,
OR
CONSEQUENTIAL
DAMAGES,
WHETHER
IN
CONTRACT,
TORT,
OR
OTHERWISE.
li any failure occurs, the foilowing steps should be taken:
4.
Notify
the
JOHN
FLUKE
MFG.
CO.,
INC.,
or
nearest Service
facility,
giving
full
details
of
the
difficulty,
and
include
the
model
number,
type
number,
and
serial
number.
On
receipt
of
this
information,
service
data,
or
shipping
instructions
will
be
forwarded
to
you.
2.
On
receipt
of
the
shipping
instructions,
forward
the
instrument,
transportation
prepaid.
Repairs
will
be
made
at
the
Service
Facility
and
the
instrument
returned,
transportation
prepaid.
SHIPPING
TO
MANUFACTURER
FOR
REPAIR
OR
ADJUSTMENT
All
shipments
of
JOHN
FLUKE
MFG.
CO.,
INC.,
instruments
should
be
made
via
United
Parcel
Service
or
“Best
Way”
prepaid.
The
instrument
should
be
shipped
in
the
original
packing
carton;
or
if
it
is
not
available,
use
any
suitable
container
that
is
rigid
and
of
adequate
size.
If
a
substitute
container
is
used,
the
instrument
should
be
wrapped
in
paper
and
surrounded
with
at
least
four
inches
of
excelsior
or
similar
shock-absorbing
material.
CLAIM FOR DAMAGE IN SHIPMENT TO ORIGINAL PURCHASER
The
instrument
should
be
thoroughly
inspected
immediately
upon
original
delivery
to
purchaser.
All
material
in
the
container
should
be
checked
against
the
enclosed
packing
list.
The
manufacturer
will
not
be
responsible
for
shortages
against
the
packing
sheet
unless
notified
immediately.
If
the
instrument
is
damaged
in
any
way,
a
claim
should
be
filed
with
the
carrier
immediately.
(To
obtain
a
quotation
to
repair
shipment
damage,
contact
the
nearest
Fiuke
Technical
Center.)
Final
claim
and
negotiations
with
the
carrier
must
be
completed
by
the
customer.
The
JOHN
FLUKE
MFG.
CO.,
INC,
will
be
happy
to
answer
all
applications
or
use
questions,
which
will
enhance
your
use
of
this
instrument.
Please
address
your
requests
or
correspondence
to:
JOHN
FLUKE
MFG.
CO.,
INC.,
P.O.
BOX
43210,
MOUNTLAKE
TERRACE,
WASHINGTON
98043,
ATTN:
Sales
Dept.
For
European
Customers:
Fluke
(Holland)
B.V.,
P.O.
Box
5053,
5004
EB,
Tilburg,
The
Netherlands.
*For European customers, Air Freight prepaid.
John
Fluke
Mfg.
Co.,
Inc.,
P.O.
Box
43210,
Mountlake
Terrace,
Washington
98043
Rev. 4/80

845AR
SECTION I
INTRODUCTION AND SPECIFICATIONS
1-1. INTRODUCTION
1-2. The Fluke Model 845AR High Impedance Volt-
meter-Null Detector allows measurement of de voltages
from one microvolt to 1000 volts de in 19 ranges. When
used as a null detector on the 100 millivolt range and
below, the input impedance is an excellent 19 megohms.™
A linear recorder output allows the instrument to be
used for production testing, and also as a de amplifier
with a maximum gain of 120 db.
1-3. The instrument may be wired to operate from
a line power source of 115 volts ac or 230 volts ac, as
desired. The instrument is designed to be mounted
directly in a standard EIA 19 inch relay rack. Resilient
feet are also provided for bench top use.
1-4. ELECTRICAL SPECIFICATIONS
INPUT VOLTAGE RANGE
1 microvolt to 1000 volts de end scale in nineteen ranges,
using X1 and X3 progression.
INPUT RESISTANCE”
100 megohms on"300 millivolt range and abovef 10 meg-
Wy
grounding. Better than 1010 ohms up to 80% relative
humidity and 35°C. With driven guard, isolation im-
proves by at least one order of magnitude up to 1013
ohms. Any input terminal may be floated 1100 volts
off chassis ground.
iy DC COMMON MODE REJECTION
‘¢Better than 160 db, input short-circuited, 80% relative
humidity; better than 140 db, open-circuited, 50% rela-
tive humidity; better than 120 db, open-circuited, 80%
relative humidity.
AC COMMON MODE REJECTION (below 100 kHz)
100 volts rms or 120 db greater than end scale, which-
ever is less, will effect reading less than 2% of end
scale, Input open-circuited.
AC NORMAL MODE REJECTION (60 Hz and above)
AC voltages 60 db above end scale will effect reading
less than 2% of end scale. Maximum voltage not to ex-
ceed 750 volts rms.
RECORDER OUTPUT
0-1 volt, one side at chassis ground; linear to 0. 5% of
end scale, Source impedance, 5k to 7, 5k.
ohms on 100-millivolt raage-and-below, Vesbuy RASE
JOO WAKO Fame | Mea ghwi ow VWeSTABILITY OF ZERO
ACCURACY =} mith voll anda, avb Yeko.
+(3% end scale + 0.1 microvolt),
MAXIMUM NOISE (input shorted)
Range Noise (peak-peak)
0. 20 microvolt
0.25 microvolt
0.30 microvolt
1 microvolt
3 microvolt
10 microvolt - 1000 volt
METER RESPONSE TIME (to 90% of reading)
Range Time
1 microvolt 5 seconds
3 microvolt 3 seconds
10 microvolt - 1000 volt 1-1/2 seconds
INPUT ISOLATION
Better than 1012 ohms at less than 50% relative humidity
and 25°C regardless of line, chassis, or recorder
Better than 0.15 microvolt/hr, better than 0.3 micro-
NOx hajg pYout/day.
TEMPERATURE COEFFICIENT OF ZERO
Less than 0.1 microvolt/°C from 15°C to 35°C. Less
than 0,2 microvolt/°C from 0°C to 50°C.
ZERO CONTROL RANGE
+5 microvolt minimum.
OVERLOAD PROTECTION
Up to 1100 volts de may be applied on any range. Typical
recovery time is 4 seconds.
INPUT POWER
115/230 volts ac 410%, 50 to 440 Hz, approximately
3 watts.
1-5. ENVIRONMENTAL SPECIFICATIONS
OPERATING TEMPERATURE RANGE
Within all specifications from 15°C to 35°C,
t-I

845AR
Within all specifications from 0° to 50°C except:
Derate by a factor of two — :
Maximum Noise and Meter Response Time.
DC Common Mode Rejection —
Derate by 20 db.
STORAGE TEMPERATURE RANGE
-40°C to +70°C.
RELATIVE HUMIDITY RANGE
0 to 80%.
SHOCK
VIBRATION
Meets 10 Hz to 55 Hz tests of MIL-T-945A.
1-6. MECHANICAL SPECIFICATIONS
MOUNTING
Standard EIA relay rack. Resilient feet provided for
bench use.
WEIGHT
9 pounds,
Meets hammer blow requirements of MIL-T-945A and SIZE
MIL-~S-901B.
3.47 inches high x 19 inches wide x 8.26 inches deep.
19,00"
14,875"
Figure 1-1. MODEL 845AR OUTLINE DRAWING

static
awareness
A Message From
John Fluke Mig. Co., Inc.
Some semiconductors and custom IC's can be
damaged by electrostatic discharge during
handling. This notice explains how you can
minimize the chances of destroying such devices
by:
1. Knowing that there is a problem
2. Learning the guidelines for handling them.
3. Using the procedures, and packaging and
bench techniques that are recommended.
The
Static
Sensitive
(S.S.)
devices
are
identifiedin
the
Fluke
technical
manual
parts
list
with
the
symbol
The
following
practices
should
be
followed
to
minimize
damage
to
S.S.
devices.
3. DISCHARGE PERSONAL STATIC
BEFORE HANDLING DEVICES
1. MINIMIZE HANDLING
2. KEEP PARTS IN ORIGINAL CONTAINERS
UNTIL
READY
FOR
USE.
4.
HANDLE
5.5.
DEVICES
BY
THE
BODY
Page 1 of 2

5. USE ANTI-STATIC CONTAINERS FOR
HANDLING AND TRANSPORT
7. AVOID PLASTIC, VINYL AND STYROFOAM®
IN WORK AREA
PORTIONS REPRINTED
WITH PERMISSION FROM TEKTRONIX, INC.
AND GENERAL DYNAMICS, POMONA DIV.
@ Dow Chemical
Page 2 of 2
WHEN REMOVING PLUG-IN ASSEMBLIES,
HANDLE ONLY BY NON-CONDUCTIVE
EDGES AND NEVER TOUCH OPEN EDGE
CONNECTOR EXCEPT AT STATIC-FREE
WORK STATION. PLACING SHORTING
STRIPS ON EDGE CONNECTOR USUALLY
PROVIDES COMPLETE PROTECTION TO
INSTALLED SS DEVICES
HANDL S. DEVICES ONLY AT A
STATIC-FREE WORK STATION
. ONLY ANTI-STATIC TYPE SOLDER-
SUCKERS SHOULD BE USED.
ONLY GROUNDED TIP SOLDERING
IRONS SHOULD BE USED.
Anti-static bags, for storing S.S. devices or pcbs
with these devices on them, can be ordered from the
John Fluke Mfg. Co., Ine.. See section 5in any Fluke
technical manual for ordering instructions. Use the
following pari numbers when ordering these special
bags.
John Fluke Part No. Description
463622 6” X 8" Bag
453530 8" X 12” Bag
453548 16” X 24” Bag
454025 12” X 16" Bag
Pink Poly Sheet Wrist Strap
30”x60"x60 Mii P/N TL6-60
P/N RC-AS-1200 $7.00
$20.00
J0089B-07U781G0/SE EN Litho in U.S.A.

845AR
SECTION Ii
OPERATING INSTRUCTIONS
2-1, RECEIVING INSPECTION 2-5, PRELIMINARY OPERATION
2-2. This instrument has been thoroughly tested and 2-6, Connect the Model 845AR line plug to a 115 volt
inspected before being shipped from the factory. Im- ac power outlet or to 230 volts ac if the instrument is so
mediately upon receiving the instrument, carefully in- wired,
spect for damage which may have occurred during ship-
ment, If any damage is noted, follow the instructions | WARNING! |
tlined in th he back of thi A
outlined in the warranty page at the back of this manual The round pin on the polarized three-prong
2-3. CONTROLS, TERMINALS, AND INDICATOR plug connects the instrument case to power
system ground. Use a three-to-two pin
2-4, The location and function of the front-panel adapter when connecting to a two-contact
controls are described in Figure 2-1. Detailed oper- outlet. For personnel safety, connect the
ating descriptions are given in the following para- short lead from the adapter to a high-quality
graphs. earth ground.
METER ; PR ZERO CONTROLS
Indicates the voltage applied ERO ~ Opens input terminals ISOLATED OUTPUT
i
nd
shorts
amplifier
input
allow-
LEVEL
CONTROL
Sax ng zeroing of the instrument === For full-scale meter
INPUT AND ith the lower zero control. POWER LIGHT deflection, this control
COMMON TERMINALS PR = Instrument is ready for Indicates line allows adjustment of th
Provide connection to vol- peration as a voltmeter or power is applied — isolated output voltage
tage being measured. ull detector. to the instrument from 0 to 1 volt de,
HH Saree CALAS
GUARD TERMINAL S MECHANICAL ZERO
This terminal is usually strapped to the common terminal Screwdriver adjustment used
The terminal is connected directly to an inner chassis o set the meter to zero. ovide connection to
shield. By removing the strap, a guard potential equal an external recorder
to the common terminal potential may be applied to the
guard terminal, shunting the leakage current from the ANGE CONTROL
common terminal to ground, Provides selection of the desired full-scale sensitivity.
Figure 2-1, CONTROLS, TERMINALS, AND INDICATOR
2-1

845AR
a. Place the Model 845AR controls as follows:
POWER ON
RANGE 10 MICROVOLTS
OPR/ZERO ZERO
b. Adjust thezero control for an initial zero meter
deflection. Place the RANGE switch to the 1
MICROVOLT RANGE and re-zero with the zero
control,
2-7, MECHANICAL ZEROING
2-8, It may become necéssary to adjust the mechani-
cal zero control of the Model 845AR at more frequent
intervals than complete calibration. To mechanically
zero the instrument proceed as follows:
a. Place the RANGE switch to 1000 VOLTS and the
POWER switch to ON.
b. Adjust the mechanical zero adjustment screw for
zero meter deflection.
c. Place the RANGE switch to 10 MICROVOLTS and
electrically zero the instrument as outlined in
paragraph 2-5.
d. Repeat steps a and b.
2-9. OPERATION AS A HIGH IMPEDANCE
VOLTMETER
2-10. To operate the Model 845AR as a High Impedance
Voltmeter perform the preliminary operations according
to paragraph 2-5 and proceed as follows:
a. Place the controls as follows:
POWER ON
OPR/ZERO OPR
RANGE 1000 VOLTS
Mote!
When measuring voltages in the microvolt
ranges, use copper wire having low thermal
EMF's.
b. Connect the voltage to be measured to the Model
845AR INPUT terminal and connect the common
point of the voltage being measured to the COMMON
terminal.
c. Deflection of the meter indicates the polarity and
magnitude of the measured voltage. Increase the
sensitivity of the Model 845AR for maximum on-
scale deflection.
2-11, OPERATION AS A NULL DETECTOR
2-12. The Model 845AR may be used to monitor small
voltage differences in bridge circuits, potentiometers,
and other measuring apparatus. In most of these appli-
cations the circuits are adjusted for zero deflection or
a null on the Model 845AR. Equipment connections for
2-2
various types of null detector configurations are illus-
trated by Figure 2-2 through 2-4. To operate the Model
845AR as a Null Detector, perform the preliminary
operations according to paragraph 2-5 and proceed as
follows:
a. Select the desired equipment application as illus-
trated by Figure 2-2 through 2-4 and make the
appropriate equipment connections.
b. ' Place the Model 845AR controls as follows:
POWER ON
OPR/ZERO OPR
RANGE as desired
c. Adjust the circuit being measured for zero or a null
deflection on the Model 845AR meter.
Figure 2-2. BRIDGE DETECTOR - FLOATING SUPPLY
Figure 2-3. BRIDGE DETECTOR - HIGH RESISTANCE

Figure 2-4, BRIDGE DETECTOR ~- FLOATING
NULL DETECTOR
2-13. MEASURING VOLTAGES WITH A
STANDARD CELL
2-14. The Model 845AR may be used with a voltage
divider and a standard cell to calculate unknown volt-
ages with a high degree of accuracy. Connect the equip-
ment as illustrated in Figure 2-5. Perform the pre-
liminary operation as outlined in paragraph 2-5 and
proceed as follows:
a. Place the Model 845AR controls as follows:
POWER ON
OPR/ZERO OPR
RANGE as desired
b. Adjust the voltage divider for zero or null deflection
on the Model 845AR meter while placing the RANGE
switch to successively more sensitive ranges.
Figure 2-5. STANDARD CELL VOLTAGE MEASUREMENTS
845AR
c. Calculate the unknown voltage by dividing the
standard cell voltage by the final division ratio of
the divider.
2-15. USE OF ISOLATED OUTPUT
2-16. DC ISOLATION AMPLIFIER
2-17. The Model 845AR may be used as a dc isolation
amplifier having a voltage gain of up to 120 db, de~-
pending on the settings of the RANGE switch and the
OUTPUT LEVEL control. To compute the maximum
voltage gain on any range of the Model 845AR, use the
following formula:
1 volt (maximum
isolated output)
Voltage gain in db = 20 log i
Range (in volts)
2-18. RECORDER OUTPUT
2-19.. The Model 845AR ISOLATED OUTPUT may be
used to provide an output voltage, adjustable from zero
to one volt for a full-scale meter deflection for use with
a recorder. Since the output is isolated from the input,
floating measurements can be made without the use of a
floating recorder. To use the adjustable recorder out-
put, proceed as follows:
a, Connect the recorder to the ISOLATED OUTPUT
terminals.
Note!
The lower ISOLATED OUTPUT terminal is
connected to chassis ground. If a ground
reference is undesirable, remove the
jumper wire above R202 on the power supply
circuit board. Refer to Figure 2-6 for
jumper wire location.
b. Turn the recorder on.
c. Proceed as outlined in paragraph 2-9 or 2-11, as
desired.
d. Adjust the ISOLATED OUTPUT LEVEL control
for the desired output to the recorder. This control
has a log taper so that smooth control is possible
at both high and low settings.
Note!
The ISOLATED OUTPUT current capability
is 100 microamperes with a 5 kilohm source
impedance.
2-20. OPERATING NOTES
2-21. SPURIOUS VOLTAGES AND CURRENTS
2-22, Voltage measurements at the microvolt level
involve the persistant problems of thermoelectric ef-
fects. These effects may be compensated for by tempo-
rarily disconnecting the voltage from the circuit under
measurement and noting the meter deflection of the
2-3

B45AR
se tA eRe NARADA near
Model 845AR on the desired r
then be subtracted from all sub:
ments. A thorough underst:
lead to reducing or eliminating
2-23, THERMOELECTRIC VOLTA
2-24. Wa circuit is composed of twa d
a net voltage will result if iwo dissi
are maintained at different temperaiur
moelectric voltages, also known
couple voltages, or Seebeck ve
by using metals having iow thermoele
and keeping all junctions af the same tem
terminals of the Model 84548 are made of
gold-flashed to prevent tarnish.
voltages, all connections te the Modsi $
made with pure copper wires. Silver
solder coated copper also produce sath
Timed copper is less satisfactory than si
copper coated copper. Nickel and ni
are not suitable for connections t
Excellent results can be cbtaine
twin lead, or even lamp cord if }
sistance is not required. Hf shiciding
a length of flat braid over the cable.
For
2-25. HIGH SOURCE IMPEDANCE
2-26. Due to the very high input
sensitive. Thus, a perso
static voltage, can caus
input to the instrument and r
flection as a hand app ches the input t
Careful shielding will
due to charges that may be
nals when the OPR-ZE
appreciable transi
to OPR if nothing is “connected t t
Turning the switch back and
charge, eliminating the prob
impedances, the response of the
ably slow due to the low pass
superimposed noise. Howe
pass filter is such that commo:
tremely high while the response t
encountered low source impedance:
2-4
» Also,
input rI
IRO, an
GROUND « JUMPER
R WIRE LOCATION
OVERLOAD VOLTAGES
The instrument is designed to withstand up to
1100 volts peak ac continuously applied
the three ingut terminals or between
gr ound and any of the three input terminals
tting of the RANGE or OPR-ZERO
er, repeated or continuous overloads
nthe ranges below 3 millivolts will
ion in protective, low-pass-filter
is will result in thermal voltages
several minutes to subside after the
oved,
ent hag an imer chassis connected
inal on the front panel. Ordinarily,
ninal is strapped to the COMMON termi-
nnected in this way the inner chassis
id, This greatly improves the leakage
ground and the common mode rejection.
the inner chassis is available at the
t may be driven at the same voltage
ION terminal, This further increases the
nce and common mode rejection by about
he voltage used to drive the GUARD termi-
should be obtained from a separate source or by
fa voltage divider connected directly across
» drops across impedances in the circuit under
meagurement.
2-31, INCREASING INPUT RESISTANCE
2-88, Inthe 1 microvolt to 1 millivolt ranges, a 40
megohim resietor is connected directly across the input
of the instrument. The input resistance may Hin in-
creased oa these ranges by disconnecting the 46 meg-
ohm resistor where it attaches to the RANGE switch.
However, the input resistance will no longer be well
defined. Typical input resistances with the 1) imegohm
noved are as follows: `
Input Resistance
300 megohms
1,000 megohms
3,000 megohms
10,000 megohms
PAY
Riles
f
|
j
7
j
Í

845AR
SECTION IH
THEORY OF OPERATION
3-1. INTRODUCTION
3-2. The Model 845AR High Impedance Voltmeter~
Null Detector theory of operation is contained in this
section of the manual. A block diagram is illustrated
in Figure 3-1, and a functional schematic diagram is
located at the end of Section V. The block diagram and
functional schematic diagram are to be used as an aid
in understanding circuit theory, and in troubleshooting.
3-3. BLOCK DIAGRAM ANALYSIS
3-4. The Model 845AR is a photo-chopper stabilized
amplifier with the overall gain of the amplifier being
RECTIFIER
LINE POWER AND
FILTER
84 HERTZ
precisely controlled by negative feedback. The instru-
ment's main circuits are an input range divider, a
photocell modulator, an ac amplifier, a synchronous
demodulator, a dc amplifier, a meter, an isolation
converter, a neon drive, an 84 Hz multivibrator, a
supply rectifier, and a rectifier filter.
3-5, The input range divider provides a fixed input
impedance to signals of less than 1 millivolt while al-
lowing reduction of input signals above i millivolt.
Photochoppers modulate the input signal to the ac am~
plifier at 84 Hz. The drive signal for the photo modu-
lator is provided by the neon drive which is composed
of neon lamps driven alternately at 84 Hz by the 84 Hz
MULTIVIBRATOR]
3
r
NEON } SUPPLY
DRIVE | RECTIFIERS
SYNCHRONOUS
DEMODULATOR
£ £ afv -ísv
INPUT RANGE
DIVIDER
NEGATIVE
FEEDBACK
RECORDER
OUTPUT
GUARD SHIELD
Figure 3-1. MODEL 845AR BLOCK DIAGRAM
3-1

845AR
a
a
multivibrator. Eighty four Hz is used to provide the
Model 845 with an operating frequency asynchronous
with the power line frequency and its harmonics. The
84 Hz multivibrator also drives the following circuits;
(1) the supply rectifiers which provide operating volt-
ages for the amplifiers, (2) the synchronous demodu-
lator which demodulates the amplified dé signal, (3) the
isolation converter which produces thé Ineter and iso-
lated recorder output. The entire amplifier and the
secondaries of both transformers are surrounded by a
guard shield which permits the use of external guard
voltages.
3-6. The ac amplifier is a high impedance amplifier
whose gain is. controlled by the resistance selected by
the RANGE control. The amplified v signal is then
detected by the synchronous demodulator.
3-7, The synchronous demodulator is driven by the
84 Hz reference signal and detects the amplified dee.
signal. The detected aé'Signal is then amplified by a
de amplifier whose gain is controlled by fixed feed-
back. The output signal of the dc amplifier is applied
to the isolation converter which drives the isolated re-
corder output, and the meter which indicate the polarity
and magnitude of the measured voltage. This same de 4c
signal is also fed back to the input of the ac amplifier
to control overall amplifier gain. The feedback ratio
is determined by the setting of the RANGE control and
allows overall amplifier gain to be precisely controlled.
3-8. CIRCUIT DESCRIPTION
3-9. POWER SUPPLY
3-10. Input power transformer T201 receives 115 volts
ac, or 230 volts ac if the instrument is so wired, through
the power switch, S1. The primary winding of T201
is constructed in such a manner as to utilize either 115
volts ac input, windings parallel, or 230 volts ac, wind-
ings in series. Fuse, F1, protects the Model 845AR
circuitry from overloads.
3-11. The secondary voltage of T201 is rectified by
bridge rectifier CR201 through CR204, The bridge
rectifier output voltage is filtered by C201 and regu-
lated by zener CR207. This regulated output voltage is
used as the operating voltage for the 84 Hz multivibrator.
3-12, The 84 Hz multivibrator is used to provide syn-
chronous drive voltages and de operating voltages for
the Model 845AR amplifier circuits free from any power
line frequency variations and harmonics. The multi-
vibrator is a transformer-coupled free running multi-
vibrator composed of transistors Q201 and Q202, trans-
former T202, and frequency determining components
C203 and R206 through R208. Variable resistor R206
is used to adjust the frequency of the multivibrator to
84 Hz. The voltage at the secondary of T202 is recti-
fied by CR104 and CR105 to produce the positive and
negative 15 volt dc operating voltages for the amplifier
circuits, The same winding furnishes the synchronous
demodulator and isolation converter drive signals and
is tapped at a higher voltage level to drive the neon
lamps DS101 and DS102. These neon lamps provide the
drive signal for the photocell modulators V101 and
V102.
3-2
D Aeg
3-13. INPUT DIVIDER
3-14, The basic full-scale sensitivity of the Model
845AR is limited to a maximum of 1 millivolt, There-
fore, input signals above this value must be reduced.
The input divider consists of R101 through R109 and
RANGE switch S101A. On ranges being a multiple of 1,
input voltages above 1 millivolt are divided down to 1
millivolt or less, upon selection of the proper range.
On ranges being a multiple of 3, input voltages above 1
millivolt are divided down to 300 microvolts or less,
upon selection of the proper range. On ranges of 1
millivolt and below, a 10 megohm resistor, R104, is
connected across the input to provide a fixed value of
input impedance,
3-15. AC AMPLIFIER
3-16. The input signal from the input divider is filtered
by a three stage, low-pass filter composed of R110,
C101, R111, C102, R112, and C103. This filter re-
duces any ac voltage having a frequency above 1 Hz,
The filtered dc voltage is then square-wave modulated
by photocell modulators V101 and V102, which are
driven by DS101 and DS102. The resulting square-wave
signal is coupled through C104 and amplified by Q101,
Q102, and Q103 which form a three stage amplifier
having a high input impedance. The gain of the ac am-
plifier is controlled by the common emitter resistance
selected by the RANGE switch S101B. Maximum gain
is used on the 1, 3, 10, and 30 microvolt ranges and is
gradually reduced by the selection of R124 through R126
as the range is increased. The output of Q103 is ca-
patitively coupled to a two stage current amplifier
composed of Q104 and Q105. The current amplifiers
have a constant gain controlled by fixed negative feed-
back through R130 and C111.
3-18. SYNCHRONOUS DEMODULATOR
3-19. The synchronous demodulator detects the magni-
tude and phase of the amplified signal. The 84 Hz drive
signal is applied to the base of transistor Q106 which
references the synchronous demodulator to the same
phase as the photocell modulator. The demodulated
signal is filtered by R134 and C114 before being applied
to the de amplifier.
3-20, DC AMPLIFIER
3-21, The dc amplifier amplifies the detected de signal
from the synchronous demodulator. Transistors Q107
through Q112 comprise a two-stage differential amplifier
with a complementary emitter-follower output. Negative
feedback through R149 and C116 is applied to the base of
Q108 and controls the de amplifier gain, The output
from the common emitter of Q111 and Q112 is one volt
de for a full range input on any range, which drives the
isolation converter, Overall negative feedback through
the resistive network of R138 through R142 and R114 is
controlled by the position of the RANGE switch 8101C.
This negative feedback allows precise control of the
overall gain of the Model 845AR amplifiers.

3-22, ISOLATION CONVERTER
3-23. The isolation converter drives the recorder cut-
put and meter while providing isolation from the Model
845 amplifier circuitry. The output signal from the am-
plifier is applied to the transistors Q113 and Q114. An
84 Hz reference drive signal is applied to the bases of
transistors Q113 and Q114 which causes modulation of
the de input signal to occur. The resulting modulated
845AR
signal is coupled to the secondary of T203 where tran-
sistors Q203 and Q204 demodulate secondary signals
occuring at their 84 Hz base signal rate. Capacitor
C204 charges to the peak of the demodulated signal and
discharges through the OUTPUT LEVEL control R1,
R211 through R213, and the meter M1. The meter M1
indicates the polarity and magnitude of the input voltage.
Capacitor C3 and resistor R2 filter the resulting dc out-
put voltage for the recorder output.
3-3