Fluke 8506A Instruction Manual page 159

8506A
4-131.
The
values of
some components
in
the
Thermal
True-RMS
Converter
are selected
at the factory.
These
components do
not normally need
reselection unless
there
is
a
failure
and subsequent replacement
of
some
component
in the circuit.
For
those cases the
selection
procedure
is
given
in
the following
paragraphs. After
selecting
the appropriate
value,
refer to
paragraph 4-143
for ordering information.
4-132.
AMPLIFIER
OFFSET SELECTION
4-133.
If
certain
components
are replaced, the
Amplifier
Offset
Adjustment R15
may
not bring the
reading within the accepted
tolerance.
These components
include the amplifier
U
I
,
components
associated with
U
(especially in the
U
1
input
circuit),
and components
in
the
attenuator
circuit. If
R15
cannot
be properly
adjusted,
use the following procedure
to
reselect
RIO
or R13,
1.
Set
R15
to
approximately
the
mechanical
center of
its
range.
2.
Connect
the
test
DMM
between
TP3
(HI) and
TP1
(LO). Insert
a
10
kilohm,
5%
resistor in line
with the
test
DMM
connection
to
TP3
(HI). This
resistor
must be connected within
one
foot of
TP3.
3.
On
the
UUT,
select
the 100
mV
range
and
apply
a high
quality,
low-thermal
short across the
SENSE
HI
and
LO
inputs.
4.
Note
the
test
DMM
reading.
Match
this
reading
(positive
or
negative) to the appropriate
voltage range
shown
in
Table
4-29.
5.
For
a
positive
test
DMM'
reading,
compute
a
new
value
for
RIO
by adding
the respective
resistance
shown
in
Table
4-29 to the
original
RIO
value.
Then
replace
RIO
with
a
1%
resistor
nearest
this
new
value.
6.
For
a negative
test
DMM
reading, compute
a
new
value
for
R13
by adding
the respective
resistance
shown
in
Table
4-29 to the
original
R13
value.
Then
replace
R13
with a
1%
resistor nearest
this
new
value.
7.
Adjust
R15
for a
test
DMM
reading of
0.000000 ±.000002.
8.
If
the
R15
adjustment does
not
yield the
required
test
DMM
reading,
select
a 1%
resistor
next
closest to the
newly computed
value
for
RIOor
R13.
9.
Replace
PviO (or
R13) and
repeat
step
7.
10.
Once R15
is
satisfactorily adjusted,
remove
the
test
DMM
leads
and
the
UUT
short.
Table
4-29.
Amplifier Offset Resistor Selection
VOLTAGE
(±»
onys
VOLTAGE
{+)
OHMS
0.050982 0.018962
0.047762
25.3
0.017606
9.4
0.045763
24.2 0.016250
8.7
0.043855
23.2
0.041944
22.2 0.014991
8.0
0.040031
21.2
0.013727
7.4
0.038116
20.2 0.012467
6.7
0.036298
19.2
0.011301
6.1
0.034572
18.3
0.010134
5,5
0.032843
17.4 0.008966
4.9
0.031213
16.5
0.007798
4.3
0.029574
15.7
0.006728
3.7
0.027940
14.8 0.005654
3.2
0.026398
14.0 0.004583
2.6
0.024855
13.2
0.003507
2.1
0.023311 12.4 0.002435
1.5
0.021864
11.6
0.001458
1.0
0.020411
10.9 0.000480
0.5
0.018962
10.1
0.000000
0.0
4-134.
RMS SENSOR
GAIN SELECTION
4-135.
If the
RMS
Sensor
U6
or
any
of
its
associated
components
are replaced, the
RMS
Sensor Gain
Adjustment
R26
may
not be able to bring
the
reading
within
the
accepted
tolerance. If this occurs, use the
following procedure
to reselect
R24
or
R28
so that
R26
can
adjust the
RMS
Sensor Gain.
1.
Remove R24
or
R28,
if
either
is
installed,
and
replace the resistor in the
circuit
with a bus
wire.
2.
Perform
the
Sensor Adjustment
portion of the
Calibration Procedure.
3.
If
the
Sensor Adjustment procedure cannot
be
successfully
completed, check
the direction of
rotation of
R26.
If
R26
runs out of adjustment
in
the
clockwise
direction replace the bus wire
in the
R24
position
on
the Amplifier
PCB
with a 20
kilohm, 1%, metal
film
resistor. If
R26
runs out of
adjustment
in the
counterclockwise
direction,
replace the bus wire
in
the
R28
position
on
the
Amplifier
PCB
with a 20 kilohm, 1%, metal
film
resistor.
4.
Perform
the Sensor Adjustment portion of
the
Calibration
Procedure
if
a
resistor
was
inserted in
either the
R24
or R.28 position.
4-136.
LOW VOLTAGE
50
kHz
CAPACITOR
SELECTION
4-137. Capacitor
07
is
selected
depending upon
the
value of the
components
installed in
C5
and C6. Use
the
following procedure
to select the value of C7.
4-45