Section
VIll SERVICE GROUP B
Model
3455A
strument set to the 100
V
range and
with
Auto-Cal on, measure for any readings at AtOTPl. If this
voltage
is incorrect, check the bias
of
the input FET’s by shorting the drains of
Q38
to the sources of
Q38.
TPl
should
now
read
zero.
The
drains of
Q37
should
be
approximately
+
10 V and
the sources
of
Q3
should read
between + 1 V and
+ 2
V. Troubleshoot
the
active attenuator, if
the
readings
are
bad.
b. For the 100
V
range zero check, set the 34S5A to the 100
V
range
with
Auto-Cal off. Adjust R66
for a zero reading at TPl. If unable to adjust for a zero voltage, check for a leaky C21, C22, C26,
CR32, or Q37. If the zero reading is good, the 34S5A should display 0
V
±
1 count, a short time after
Auto-Cal is turned on. If an offset remains on the display,
Q36
may be
defective.
c. For a 100
V
range full scale check,
observe
for a cal constant 5 failure,
when
the
34S5A
is
in
the
self-test mode. If cal constant 5 fails, troubleshoot its circuitry by using the procedure of
paragraph
8-B-l
1.
If
cal constant
5
passes, step to
cal
constant 5 by
using
the
procedure of paragraph 8-B-I I. If
cal constant
5
passes,
step to
cal constant
5 by
using the procedure of paragraph 8-B-l I. Measure
TP2 for approximately
-1
V
and measure for approximately
-.1 V
at the 10:1
divider
(pin 9 of J3).
Check for exactly
-i-
10
V
at the Junction of R47 and
Q33,
and for
+
9.9
V
at the
junction
of R47 and
K6.
If 10
V
is
measured
at
R47 and K6 instead of + 9.9
V
or R63 may be open.
8 B-27. 1000 V
Range
or Constant 6
Fails.
a. For the 1000 V range zero check,
remember
that some of the same
circuits are used
in the
100 V
range. The 100
V range zero and
gain
should
be
working before troubleshooting the 1000 V range.
Check for the proper switch closures used in cal constant
6
(use the procedure of paragraph 3-B-1 1 to
locate constant
6).
b. For a 1000
V gain
check, use
the procedure of paragraph 8-B-24b.
c. Set the
34SSA
to
the
1000
V range with Auto-Cal on, and apply 1(XX) V to the input terminals. If
the reading
on the display changes intermittently from 100 counts to 200 counts, A10K5 or K6 may be
breaking down.
KS
and K6 can be checked by connecting channel A of an oscilloscope (set to 20
V/div)
at the Junction of R47 and K6.
WARNING
For
safely,
connect the scope
probe
lo the 100 K
resistor
R47.
Connect channel B of the scope
(2
V/div) to pin 10 of U24. Set the scope to the chop mode and trig-
ger on channel B. If channel B indicates a 5
V
spike
when arcing
occurs, as
seen on channel
A, then
K6 may be breaking
down.
If the
indication on channel B appears
to
be good, KS may be defective.
C21, C23, or the input node of
Q37,
may also be
defective.
8 B-2B. Various Other Malfunctions.
8-B-29. Shorted FET’s.
a. Occasionally FET’s on the multiplex node may short.
Two ways
can be used to isolate shorted
FET’s.
1. Measure the gate to on resistance with an ohmmeter.
2. Short the input of the 34SSA and turn the high
resolution
and
Auto-Cal functions
off.
While
monitoring A10TP3 step through all the dc
ranges
(1
V, 10 V,
etc.)
and
ac functions.
Make sure Auto-Cal is turned off after
every range and
function
change. If TPS
dips to
-24
V,
a
FET may be shorted. Usually, the
defective FET
is normally
turned off
in
that
particular
range
or function.
b. If AlOUl gets
very
hot Ul,
Q2, Q13,
or
Q14
could possibly
have their gates shorted to their
can.
8-56
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