Fluke 893A - 1 Of 2)

CIRCUITRY
1
ADJUSTMENTS
AND THEIR
EFFECTS
ZENER REFERENCE
SUPPLY
A2R19 Used
to adjust for
17i0. 01 volts do
at A2TP9 and has
a minor effect
on
instrument
calibration.
Adjustment of this control
establishes
a specific voltage
for the zener
diodes which minimizes
the zeners
temperature
coefficient
and increases their
voltage stability.
NULL DETECTOR
A4R27 Adjusts
the gate
signal drive level of
Ql for optimum chopping
efficiency.
Inadequate drive to
the gate of
Ql
will
result in a lower
than specified
null detector
input resistance
and a lower
gain.
Excessive
drive to
the gate of
Ql
(clockwise
setting)
will reduce the
;
null
detector stability.
;
A4R31
Adjusts the level
of the signal applied
to the drain of
Ql.
This signal
is used to
cancel out the
null detector open-circuit
offset effects
produced
by the
gate to drain capacitance
of
Ql. Misadjustment of
this control
will produce
null detector
open
circuit
offsets particularly
notieable on the
.001 volt sensitivity.
A4R5
Adjusts the
null detector gain
to compensate
for variations
in the
carrier
amplifier gain,
full-scale
meter sensitivity, and
feedback
resistor
tolerances.
A4R17
Adjusts the
carrier amplifier gain which
determines the
input
resistance
of
the null
detector and to
a minor degree
affects null
detector sensitivity.
A3R8 Adjusted
to provide
uniform null detector
voltages of ±6.
8 volts dc on
all voltage ranges.
This adjustment
does not affect instrument
calibration.
A3R5
Adjusted to reduce the
3 kHz common
mode signal from
A3T1
that
is present
at the common terminal
of the null detector.
This adjust-
ment
does not affect instrument
calibration.
KELVIN-VARLEY
:
DIVIDER
A5R36
Provides
a means of trimming
the full-scale
vernier dial range to
compensate for
potentiometer resistance
tolerance,
1000 VOLT
RANGE A2R26
Adjusts
the value of the
reference voltage
for
the
+18.5
volt dc
regulator to
a value necessary to calibrate
the 1000
volt range.
Adjustment
of this resistor
will also effect calibration
of tr»e
lOO
volt
range but does not
effect the
1
and
10 volt range calibration.
100 VOLT RANGE
A3R14
Adjusts the ilO
volts dc applied to the
Kelvin-Varley Divider on the
lOO volt range.
Adjustment of
this resistor will not
effect the
calibration
of any other
voltage range.
10
VOLT RANGE
A2R33
Adjusts the 11 volts
dc applied to
the Kelvin-Varley
Divider
on
the
10 volt
range. Adjustment
of this resistor
will not effect the
calibration
of any other
range.
1
VOLT RANGE
A2R35
Adjusts
the
1. 1
volts
dc applied to the Kelvin-Varley
Divider on the
1
volt
range. Adjustment
of this resistor
will not
effect
the cali-
bration of
any other range.
1000
VOLT AC RANGE
(400
Hz)
A7R29
Adjusts the AC Converter
gain at low frequencies
on the
1000 volt
ac range. Adjustment
of this resistor will
not affect calibration
of any other voltage
range.
100 VOLT AC
RANGE
(400
Hz)
A7R31 Adjusts the AC
converter gain
at
low
frequencies on the
100 volt ac
range. Adjustment
of this resistor
will not affect calibration
of any
other
voltage range.
Figure 4-n.
SUM/Vu4RY OF CALlBRATiON
ADJUSTMENTS (Sheet 1
of
2)
4-13
8 93
A