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Model
3484A
GAIN• RI +R2
R2
3482A- B-3035
4
DC
AMPLIFIER
Figure
5-9.
DC
Amplifier
Diagram.
mately half the
correct
value,due to the
altenuation of
Rl
,
R2C,
and
RS.
However attenuation
will
be correct on the
two
higher ranges.
5-39.
If
readings are
correct
with the
filter
OUT, but low
on all
ranges with the filter
switch set
to A
or B,
attenuation
is occurring
in the filter
circuit.
This problem
could be caused
by
leakage
through
a
filter
capacitor or
from a
reed to
relay
shield.
5-40.
The
gain of
the DC
Amplifier
is
adjusted by
varying
the
feedback
resistance
R
I
in
Figure 5-9.
Leakage
across a
relay reed when the reed
should
be
open
will
cause
Rl to
be
low and the gain
will be low.
If
a reed does not close,
or
the contact resistance becomes
excessive,
RI will be
high
and the
gain
will be
low.
5-41
.
DCV REED RELAY CHECKS.
In
many
cases,
a malfunction
can
be traced
to a faulty reed
relay by interpreting the
symptoms evident in
the
front
panel display.
Table 5-5
lists
a
number
of such symptoms
and
the probable
causes.
Symptoms
are listed with the
ranges
on
which
t hey
appear.
Check for each symptom on
each
range to determine which ranges are
affected.
Any
problem which
appears on
only
one range
may be due to
a
defective relay drive
circuit.
A
known
voltage
source, such
as a
de
standard,
is required
for
these
checks.
An
ohmmeter
should
be used to determine
whether a
recd
switch
is
open
or
closed.
Be
careful
not
to apply stress to the
leads
of the
reed
switch.
5-42. RANGING CHECKS.
5-43. Selection of any range causes the
Enable
signal for
that
range to
be
LOW, enabling
the associated Range
Relay
Driver which
energizes
the
appropriate reed
relays.
If
selection
is manual,
the Enable
line
is grounded
through
the
RANGE
switch
.
In
remote
range
selection, a ground
connection of the Remote Program
line is
applied to a
series
pair
of
inverters,
causing
the Enable
line to
be LOW.
In
autorange operation,
outputs
of
the
Range
Register
are
decoded by
a series of
NAND
gates
to
cause the
proper
Enable line
to
be LOW. The ranging logic
circuits are shown
in
Figure
7-4.
Section
V
5-44. Some troubles can
be
traced
by
analyzing
the
symptoms evident in
the
front
panel
display.
For
example,
if a certain range cannot
be
selected
by
any of the t hree
modes of selection,
the trouble
could
be
in the Range Relay
Driver c
ircuit.
However, if the problem is
limited
to only
one function,
the
trouble is more likely
in
t he
range relays
for that function, or the Function Selection
Circuits.
(V
AC(AC)
and
V AC(DC)
functions
use
the same
range
relay c
ircuits.) If
the
decimal
and fu
nction
indications are
correct
but
the
numerical
display is off
by
a factor of
10
or
100,
the
trouble
could
be
either
in
the range
relays
or driver
circuit.
On
the other
hand, if
the numerical
display is
correct
but
either
the decimal
or
function indication
is
incorrect, the trouble is probably in
the
Decimal and
Annunciator
Lamp
Drivers (Figure
7-5).
5-45. Remote Range Check.
Two
general
types of
malfunction
may occur
in
the Remote
Range
Selection circuits.
If
may be
impossible
to
select
a
certain
range remotely,
or
a
range
may
remain
"selected" at
all
times.
In
either
case,
the trouble is
probably in
t he
remote selection
inverters,
A31Cl and 5.
lf
none
of the
remote
range
program
lines is
grounded
, t he 3484A
automatically selects
the
highest range. Paragraph
4-29 and
Figure 4-5
explain
the method
used
to select
t
he highest
range
in
both
vo ltage
and
o
hms
functions.
5-46.
Autorang
e
Check.
If
the instrument
does
not
autorange
properly but manual
selection is correct,
refer
to
Autorange
Troubleshooting
Tree, Figure
5-14.
5-47.
DELAY GENERATOR
CHECKS.
5-48.
The purpose
of the
Delay
Generator
is to
prevent
the
3480A/B
from sampling, following an autorange signal,
until the
instrument
has had
time to change
ranges
and the
amplifier
has
settled
.
At the
end of
the delay time,
the
Delay Generator resets
and causes the
3480A/B t
o
take a
measurement.
5-49. Each
time an
autorange command
is present
at the
Range
R egist er
input, this
same signal changes
t
he state of
the Delay
Flip-Flop,
initiating
a delay period. The length of
the
delay period depends
u pon
the function and
degree of
fil tering
selected
.
The Delay Generator
consists of
three
main parts; the
Delay
Flip-Flop, the delay
t iming
capacitor
circuits, and
the
reset circuit.
5-50.
Delay
Flip-Flop.
The
Delayed
T
rigger
Initiate
signal
at
A31C6 pin
2
must
be
HIGH and
either t he Auto
Enable
or
Delay Enable
input to
the Delay Gate
must be
LOW, causing the
level
at
pin
3 also
to
be
HIGH
in order for
the
Delay Flip-Flop
to
be t riggered
by an a utorange command.
If
the flip-flop does not
trigger,
check
voltages
at these
points.
An
emitter-to-collector
short
in
A3Q8 would
also cause
pin
3
to be
LOW
and
prevent
triggering.
If
the flip-flop fails
to
trigger,
no delay
will
5-1
J
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Table of Contents
Troubleshooting
