Model 345SA SERVICE GROUP B
Section VIII
c. If the
reading changes faster than .25
V
per second, there is leakage on the multiplex node. If
the reading on the display changes
positively,
either AlOQS or Q17 may be leaky. If the change is
negative, A10Q2,
Q3,
Q4, Q13,
QIS,
or
QI6 may be leaky.
8-B-22. The test in the
above paragraph, paragraph 8-B-21, is a quick leakage test and should find
most leakage
failures. A more thorough test
involves
checking zero and full scale
voltages
on all dc
ranges.
Start with the 10
V
range and the other ranges in the
following
order: 1
V,
100
mV,
100
V,
and 1000
V
ranges. The
following paragraphs contain the procedures which should be used for
leakage
failures.
B B-23. 10 V
Ranga or Constant 10 and 11 Fail.
a. Set the 34S5A to the 10
V
range.
Auto-Cal on and short the input terminals. If the reading
on
the display is
positive
(more than 5 counts),
AlOQl may
be
leaky. If the reading
is
negative (more
than 5 counts),
Q2
may
be leaky. To doublecheck for a
defective
Q1
and
Q2,
note the reading on the
I
V
and 100
mV ranges. The bad reading should also be present on those ranges.
b. Apply +
10 V
or
-10
V
to the
input terminals of the 345SA. Make sure the readings are within
specification. Check the reference
voltages
and adjust them, if necessary. A10TP8 should be -f 10
V
±
100
pV and TP7 should be
•
10
V
±20
mV.
If the reference
voltages
are good and the instruments
readings is
low,
A10Q4 may be leaky.
B-B-24.
1 V Range or Constant
0, 1,
and 2 Falls.
a. For
the 1 V range check, do the procedure as explained in paragraph 8-B-23a.
b. For the 1 V range full
scale check, do the
following:
1 . With the 34S5A set the 1
V
range and Auto-Cal off, apply -f 1
V
to the input terminals. A
voltage of -f 1
V
should appear on the
multiplex mode
and
-t-
10
V should
be at
A10TP4.
If the
multiplex mode reading is bad,'
troubleshoot the
input circuit. If
the reading at TP4
is bad,
make sure A10Q21 is
turned
on.
Check for leaky
Q22,
CR12, CR13, or a defective U3. If TP4
reads good, set up the 34S5A for the self-test mode by pressing the TEST button. Check for cal
constant 0 failing and if it does, troubleshoot the failure by using the procedure of paragraph
8-B-lO.
Continue with
the next step
if constant 0 does
not
fail.
2. By using the procedure of paragraph 8-B-lO, step to
cal constant
0.
Adjust the active at-
tenuator for a zero reading, as read at AlOTPl (adjust R66). Measure the
voltage
at J3 pin 9 for
exactly -t- 1
V.
If the reading is
low,
Q39 or Q18 may be
leaky. If the reading
is good,
check the
operation of Q36.
This can
be done by
changing the high
voltage
amp offset. The 1 V reading at
J3
pin 9 should change, because the gain of the I
V
range is changed.
B-B-25. 100 mV Range or Constant 3
Fails.
a. For the 100
mV
range zero check, do the
procedure of paragraph 8-B-22a.
b. For the 100 mV range full scale
check
do
the following:
1. Apply + 100
mV
to the input of the
3455A.
The
instrument should
be
set
to the
100 mV
range with Auto-Cal. Measure for approximately -t- iO V at AI0TP4 and 1/10 of this
voltage
at
TPS. TP2 and TP6 should read approximately the same as TP5. If the reading at TPS and TP6
are incorrect, check the
power
supplies of U3 (pins
4
and 7). The supplies
should have
approx-
imately the
voltage
at
TP6
± 5
V. Troubleshoot U3 and associated circuitry if necessary.
2. Adjust the high
voltage
amp
(A10U18)
to zero, as
read at TPl (adjust R66). Measure the
voltage at the 10:1
divider
(J3 pin
9)
for exactly
1/10
the
voltage
at
TP2. If
this
voltage is incor-
rect, QI6 or Q2I may be
leakly.
B B-2B. 100 V Range or
Constants
5,
4,
and
7
Fail.
a.
The active attenuator can be checked by applying -i- 10
V
to (he input of the 34SSA.
With
the in-
8-55
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