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Model 3455A
SERVICE
GROUP
G
Section VIII
nearest -hp- Service Office,
if
inoperative.
The
only checks
that
can
be
made
are the following.
1.
If
the reading
on
the display
jumps
10
or 100 counts
when
adjusting
any
adjustments
on
the reference assembly, the
wiper of
the
pot
may
be
dirty.
Work
the
adjustment screw of
pot
back
and
forth to clean the wiper.
2.
Make
sure
an
oxidation
layer
has not
formed on any
pins
of
the
assembly
printed
circuit
board.
The
pins
can be
cleaned with
a
soft
lead
eraser.
3.
Typically, the
maximum
noise
allowed
on
the reference voltages (use
a
DVM
with input
Z
>
1010)
are
20#tV
for the
+
10
V
reference
and
30
/tV for
the
-10
V
reference.
Replace
the
assembly
if
excessive noise
is
present.
4.
The
+
10
V
reference voltage
at
A10TP8
should be
adjustable
to
-f
10
V
±
100
fiV
and
the -10
V
to
-10
V +20 mV.
Replace
the
assembly
if
both
the
+
10
V
and
-10
V
are
not
adjustable.
Replace
A10U7
if
only
the
-10
V
reference
is
incorrect.
8-G-4.
Turn-Over
Errors
(Schematic
1,
S,
and
B).
8-G-S.
Turn-over
errors are present
when,
for
example, a
positive
input reading
is
good and
the
negative input reading
is
out of
tolerance.
This can be checked by
taking a
positive
reading
and
then
reversing the input
leads.
The
following are a
few
turn-over checks
and
hints.
a.
When
checking
for
turn-over
errors,
the
10
V
range
and
zero
offsets
should be
the
first
things to
check.
b.
Check
the
A/D
converter (A14)
if
turn-over differences arc observed. Replace,
if
necessary.
c.
Turn-over
errors
on
all
ranges:
Unsolder
CRl
and
CR2
from
the
AlO
board.
If
the error disappears,
CRl
and/or
CR2 may
be
leaky.
Make
sure the
+
10
V
and
-10
V
references are
good (A10TP8
should be
+
10
V
±
100
/»V
and
A10TP7
should be -10
V
±
20
mV).
d.
Turn-over
differences
on
the 10
V
range:
Check AlOUl,
U2, Q7,
or QI8.
Other
possible
causes
may
be Kl,
Al, Q2, Q4, Q19, and
their
associated
circuits.
e.
Turn-over
errors
on
the
top
three ranges:
Check
A10K6
and QIS.
f.
Turn-over
differences
on
the
l(X)
V
and
KXX)
V
ranges:
Lift
A10Q9
and CR29.
If
the error
disappears,
CR31
and/or
CR29 may
be
leaky.
Do
the
same
with
CR16
and
CR17.
Q8 may
also
be
defective.
g.
The FETs
connected
to
A10TP2 may
cause turn-over
errors,
if
leaky.
(^40
may
also
be
leaky.
h.
If
the negative readings are
good and
all
positive
readings
above 20
V
are unstable
on
the 100
V
range,
check
AI0Q36.
B-G-6.
Othar Troubleshooting (Schematic B
and
111.
a.
If
the
instrument
fails
to
sample
in
the
dc
volt,
high resolution
mode
and
the
ac
volt
normal
resolution
mode,
check
A1C29
or
U48.
b.
If
the
HP-IB
operation
is
intermittent
with the instrument's
LED's
dim,
make
sure the
50/60
Hz
switch
is
in
the correct position.
c.
If
the fan refuses to spin after
repeated turn-ons,
change
AIRIS,
19,
and
24 from
11.8
kll
to
13.3
kll
(0757-0289).
This change should not be
made
on
operating fan
circuits.
d.
A
good
fan
measures approximately 30
(1
between
the
brown
and
yellow
wires
of
the
fan.
A
defective
fan
will
usually
measure between
10
(1
and
IS Q.
8-77
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