Model
3455A SERVICE GROUP
B Section
VIll
c. Shorted FET's and U! may
show
up as on "OL” indication on the display of the 3455A. This
condition can
be
checked
by
measuring
the
voltages
at
TP4.
If the
voltage
reads
approximately +
16
V or
-16
V, then measure TP3. If TP3 appears
to be
floating
or is at a
-24
V level,
short the multiplex
node
to
ground. If the “OL" condition disappears, a FET on the multiplex node is shorted.
Use the
procedure of paragraph 8-B-29a, b to rind the shorted lET. Some possible.FET failures may
be
Q3,
Q4. Q15.
Q19,
or
Q21.
6-B-30. Other Troubleshooting Hints.
a. If either A10K5 or K6 slicks closed, it may damage the other relay. Both should
be
replaced.
b. A
sticking KS
could also damage R47,
when
K6 closes.
c.
If 9.9009 V
is
displayed
on
the 100 V and 1000 V ranges
of the 3435A
with
the input open,
A10K6 is probably shorted.
d. AlOLl should not be too close to R63. Arcing could occur for 1000
V
inputs.
e.
With Auto-Cal on and A10R66 adjusted, the instrument
should
temporarily indicate
an offset
on either the I V, 100 V, or 1000 V ranges. If the offset remains,
Q36
may be
open.
f. If all tests pass and the 100
mV
range is out of tolerance, then AI0Q28 may be open.
g.
If all tests pass the 34SSA reads zero
volts
on the 1000
V
range
with
an input voltage,
Q39
may
be
open.
h. If all tests pass and then 100
mV,
1
V,
100
V,
and 1000 V ranges are out of tolerance, then
AI0Q29 or
Q31
may be open.
i.
If all tests pass and the 100 V and 1000 V ranges are
out
of tolerance, then
A10Q4 may be
open
or
K6 may
be
shorted.
j.
If Auto-Cal constant S fails and the 100
V
and 1000
V
ranges are
way
out of tolerance, then
A10R46 or
R63
could
have
changed
value.
k. If
17
V appears on the multiplex node, check for a defective A10Q15,
Q18,
CR12, or
U12.
8 B-31.
General Noise.
8-B-32. Noise in the 34S3A may
show
up in one or more functions. If more than one function is
noisy
it usually
indicates
dc noise. The dc
noise source
should be found first,
before
troubleshooting
any ac or
ohms noise.
Go to Table
8-3,
to
find the correct service group for
ac
and
ohm noise.
8-B-33. DC Noise
(Schematic
1, 5,
and
B).
a. Equal amount of noise on all ranges:
Noise
of this nature is usually caused by the output of the
dc amplifier (A10U2), the reference assembly (All or A20), or the A/D
convertor
(A 14). The follow-
ing
two methods can
be used to
find noise causing
circuits.
1. Try replacing the A/D
convertor
board (AI4)
with
a
known
good one. If the
noise
disap-
pears, go to
Service
Group E paragraph 8-E-14 for further troubleshooting. If the noise is
still
present or a good A/D board is not
available,
use the next procedure.
2.
Set
the 3435A to the 10 V with Auto-Cal off. Using a high impedance DVM
(10
V
input
impedance >
1010
ohms), measure the 10
V
reference at A10TP8. If the reference
voltage
is
noisy, replace the
reference
assembly
(All
or A20). If TPS is good, unsolder R38 at the
multiplex node. With a clip lead, connect TPS to the unsoldered end
of R38. Measure the
voltage at TP4. If TP4 is noisy, U2 and its output circuit may be noisy. If the voltage
at
TP4 is
quit, the A/D
convertor
is most likely noisy. Go to
Service
Group E paragraph 8-E-14 for fur-
ther
troubleshooting.
8-57
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