Fluke 8600A Instruction Manual page 36

8600A
verter to the input
of
the
A/D
Converter.
By
inserting a
voltage
at
TP1
1
,
all
input
signal
conditioners
will
be by-
passed.
The
operation of
the
A/D
Converter and
the
Control
and
Display sections (with the
exception of
the
range controls)
can be checked with
the following
test.
If
the instrument
passes this test,
the indication
is
that
the
fault
is
in the
Input Divider or
in
the range
selection.
a.
Select the
DCV
function and
the
200m
V
range.
b.
With no
input to the
Vf2
terminals, check
the
voltage
at
TP1
1
,
and
at
buffer input
(U4
pin
3).
If
it is
OV
proceed to the next
step.
A
voltage
on TP1
1
or
at
the buffer input
indicates that
one
of
the control
FETs
(Q14, Q15, Q16, Q17,
Q21)
or
Q13
or
Q1
1
may
be shorted or
leaky.
c.
Connect
+190mV
to
TP1
1
and
the return lead to
TP3.
Refer
to table
4-10 and
observe the front
panel
display
as
the front panel range switches
are
changed.
RANGE
SETTING DISPLAY
200
mV
+190.00
2V
+
.19
20V
+
1.90
200V +
19.00
1200V
+190.00
AUTO
DECIMAL
MAY
BE
ANYWHERE
REPEAT FOR NEGATIVE
INPUT
Table 4-10
INPUT DIVIDER TEST
446.
OHMS CONVERTER
447.
An
instrument malfunction
affecting
only the
ohms mode
of
operation
is
generally the
result
of the
ohms
converter producing an improper
level
of
current
for the particular resistance range being
used.
The
follow-
ing
procedure should be used
to evaluate the operation
of
the
ohms
converter.
a.
Select the
Kfi function and
20
K
range.
b.
Place
a
good
quality short across the V-S2 input
terminals.
c.
Measure
the voltage
level at
the cathode of
CR2.
It
should be
+10 ±0.1V
dc.
d. If
the
voltage
is
incorrect
measure
the voltage
at
pin
3 of
U
1
.
This point should be 0.00
volts
+ 100
microvolts.
e.
A
voltage
level greater
than ten
volts
at
CR2
and
zero
volts
at
pin 3 of
U1 would
indicate that
the current source
(Q3,
U3
and
associated
com-
ponents)
is
supplying
more
current
than normal.
If
the
voltage
at
CR2
were
less
than ten
volts
the current
from Q3 would
probably be
less
than
normal.
448. The
operation of the current source can be
checked by measuring the
voltage
at
pins 2
and
3
of U3.
The
voltage
on
both
pins
should be approximately
—8.6
volts.
Uneven
voltages
may
be caused by improper
re-
sistance value
of
R35
or
R9
1
,
or a
failure in
CR1
0.
The
voltages
at
pin 2
and
3
of
U3 may
be proper and the
cur-
rent supply
from
Q3
improper
if
the gate
of
Q3
were
leaking current
back to pin 6 of
U3.
449.
CURRENT
SHUNTS
4-50.
Failures
in
the current
mode
of
operation
affect-
ing
all
current ranges
would
probably be
attributed to a
failure in
one of
the following
areas:
1)
the input fuse
(FI) open,
2)
one
or
more
of diodes
CR1, CR6, CR7,
or
CR8
shorted, or 3) improper connection
in
the con-
tacts
of
function switch
S2
(ACI), or
S4 (DCI)
or range
switches
S6-S10.
4-51.
AC
CONVERTER
4-52. Generally a
failure in
the ac converter
will
do
one of
two
things:
1)
create a dc
voltage
output without
an ac input signal applied to the
8600A
input terminals
or
2)
not produce
the
proper dc
voltage output
when
an
ac
input
is
applied to the
unit.
A
dc offset voltage
created
within the ac converter
may
cause the
multimeter
display
to indicate
some
substantial value
of
ac voltage
when
a
short
is
placed
across
the input
terminals.
When
the
ac
converter
failure
causes the display to
remain
at
zero
when
an
ac
signal
is
applied to the input, the converter
is
not
producing the proper dc voltage output
for
the
A/D
con-
verter.
4-53.
The
operation
of
the
ac
converter
can be checked
for
the dc voltage
offset
by
performing the following pro-
cedure.
a.
Remove
the
molded
plastic
outer
case
from
the
8600A
(see par.
4-7 Access/Dissassembly).
b.
Remove
the
ac
converter pcb, take the
metal
shield
off the
side
and
reinstall
the
board
into
mainframe.
NOTE!
Take
care that
all
the connector pins
on
the
board
are
properly aligned before
seating
the
pcb.
4-8
1/77