Fluke 8600A Instruction Manual page 40

8600A
4-62. Errors
in the display
concerning decimal point
location
and upper
or
lower annunciators can be
the
result
of range control problems.
A
+5V
is
required
from U8
to
turn
on
the annunciators. Decimal point
logic
is
a positive
pulse
output from
U8
occuring
at
the
same time
as
the
appropriate strobe
signal.
Either
STO
or
ST7
is
selected
by
U16
before being applied
to the display board.
The
digit
information
to be presented
on
the front panel
is
delivered
by
U8
in
BCD
format on
lines
W,
X,
Y, and
Z.
4-63.
A
malfunction caused
by
the display section of
the circuitry will generally cause the
8600A
display
to
indicate the failure in
one of
five
ways;
1)
all
LED's
are
dark, 2)
one
segment of any
one
or
all
LED
displays are
dark, 3)
any
single
LED
display
is
dark, 4) the
numbers
containing
a particular binary
code
(1, 2,
4
or 8)
will
not
display or 5)
one
digit is
brighter
than normal and
all
others are
off.
The
probable cause
for
each
possible
fail-
ure indication
is
given
below.
4-64.
When
all
LED's
are
dark,
check
for
+5
volts
at
the emitter
of
Q30. Using an
oscilloscope
check
U10
pins
9 through
15,
LED
segment
drive
signals,
for a
squarewave
signal
alternating
between
+3.5
volts
and
+0.4
volts.
4-65.
When
a
segment of only one
LED
is
dark
the
LED
is
the probable cause of
the
failure.
If,
however,
the
same segment
in
all
LED'
s is
out
the particular segment
drive
signal,
U10
pins 13,
12, 11, 10,9,
15,
or 14
corre-
sponding to segments A,
B, C, D,
E, F,
and
G
respectively,
can be checked
for the required
voltage
change from +3.5
volts
to +0.4
volts as the segment
lights.
4-66.
When
one
digit in
the display remains dark
the
strobe
signal for that
digit
should be checked.
The
base
of Q37 (MSD), Q29
(2SD),
Q3
1
(3SD),
Q33 (4SD) and
Q35 (LSD)
should go
to
+5
volts as
each
is
strobed
on
for 300qs.
4-67.
When
the display
indicates that
one
of the binary
codes
(
1
,
2,
4,
or 8)
is
missing; the bed output from
U8
can be observed
at
pin 3
1
(1),
32
(2),
33
(4)
and 35
(8).
The
output
at
each pin should drop from +5
volts to
about
zero
volts
when
that
code
is
used
to
produce
the
digit
being
displayed.
If
the
bed
information
at
U8
is
cor-
rect
the probable cause of the
failure
is
U10.
4-68.
When
one
display
digit
is
bright
and
all
others are
off, the clock
oscillator
is
the probable
cause.
Check
the
operation of
U7, U8, and
the
1
MHz
crystal
Yl.
4-12
1/77