Fluke 8600A Instruction Manual 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