Fluke 8600A - Page 37

8600A
c.
Short
the V-£2
INPUT
terminal
HI
to
LO.
Select
the
200V
RANGE and
VAC
FUNCTION.
d.
Connect
the return
lead
of
the test
equipment
voltmeter
to the
8600A
V/ft
LO input
terminal.
e. Connect
the high
input
lead
to
the
ac
converter
output
at
pin 9.
Note the
voltage
level.
f.
Move
the
high input
lead
to
pin
10.
The voltage
level
should be the
same
as
that noted
in step e.
4-54.
A
difference
in voltage
levels
between
the two
pins
would
be
displayed
as an
offset
on
the front
panel
readout.
The
level
at
either
pin
should
not
exceed +30
millivolts
nor
be less
than +20
millivolt.
If
the voltage
level
is
too
high or
there
is a difference
in voltage
level
between
the
two,
check
current
source
CL1
or
transistors
Ql,
Q7,
and
Q9,
or
capacitors
C12
and C13.
4-55.
The
operation
of
the ac
converter
with
an
ac in-
put
applied,
observed
at the circuit
locations
described
below,
may
indicate
the fault
area.
With
the
8600A in
the
2V
range
and
AC
V function
apply
a
IV rms
500 Hz
signal
to the
V/£2
INPUT
terminals
and
proceed
as
described
below.
a.
Connect
the
input return
lead
of an
oscilloscope
to the
8600A
MA
LO
terminal.
b.
Connect
the
scope input
probe
to
the
ac
convert-
er
input
at the wire
connection
on
S2A.
Note
the
amplitude
of
the signal;
it
should
be
2.8
volts
peak-to-peak.
c.
Move
the
scope
input
probe
to
U2
pin
6. The
ac
signal
at
this point
should
be about
one-quarter
of
the
amplitude
of the
input
signal,
or about
0.7
volts
peak-to-peak.
If correct
go to
step e.
d.
If
the signal
is not
present
at
this
point or
the
dc voltage
level
is close
to either
the
-1
5 volt
or
+
15
volt
supply,
U2,
Q7,
or CL1
are
probably
defective.
If the
signal
is
distorted
check
the
operation
of
Ql.
e.
Connect
the
scope
input
probe
to the
base of
Q7,
the
signal
here
should
be
one-quarter
of
the
amp-
litude
of
the
input
signal.
f.
The
ac
converter
output
at pin 9
should
be
one
volt
dc
plus the
dc
voltage
offset
level
at pin
10.
4-56.
A/D
CONVERTER
OR
CONTROL
AND DISPLAY
4-57.
An
A/D converter
or
control
and
display
failure
will
generally
affect
the operation
of the
8600A
in all
functions.
The
exceptions
to this would
be
a malfunc-
tion
in an
individual
function’s
control
signal or
range re-
lay
signal.
The
following
information
is provided
to aid
in
isolating
problems
within
the
A/D converter
and control
and
display
circuitry.
4-58.
The
A/D
Converter
can be
checked
by using
the
following
procedure.
a.
Select
the
DCV
function
and the
2V range.
b.
Apply
+1
V dc to
8600A
Vf2
terminals.
c.
Connect
the
oscilloscope
probe to TP2.
The
probe
ground
return
may be
connected
to TP3
or
the
mA LO
terminal.
d.
The signal
at
TP2
should
be
as shown
in figure
4-3.
4-59.
The
stable
+5V
section
of the
waveform
shown
in
figure 4-3
should
be
200msec
long
for a
+1
V dc input. If
the
dc input
voltage
is
increased
to 2V, the
stable
portion
of
the
waveform
should be
300msec
long. If
the waveform
is
correct,
the
problem
is in the
display
section.
If
the wave-
form
is incorrect,
refer
to
figure 44
for further
test points
and
waveforms
in the
A/D
Converter.
Check the
wave-
forms
at the
buffer,
U4, for
the
correct
voltage
from
Q14
and the
appropriate
reference
(U17,
Q17,
and
Q21).
U4
has a gain
of 5 so the
waveform
at
its output
should
be 5
times
its input
(not
inverted).
The output
of
U5
should be
triangular
as
shown
and
opposite
in polarity
to the buffer
input.
Figure
4-3.
A/D CONVERTER
OUTPUT
SIGNAL
4-60.
The control
waveforms
and timing are
important
to the
correct
operation
of
the
A/D Converter.
The wave-
forms
and
test
points
are
shown
in figure
44. Continue
with the
test
setup
in 4-58.
First
check the
signal at the
gate of
the FET.
If
absent
or wrong,
check
that output
from
U8, keeping
in mind that
the
driver transistors
act
as
inverters.
The
operation
of
U8
with respect to
the A/D
Converter
may be
checked
as follows.
a.
Select
DCV
function
and
TV range.
1/77
4-9