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Section VIII
SERVICE
GROUP
E
Model 3455A
b. If
the
0
detect signal
of
older
instruments
is
unstable,
modify
the
instrument
in
the following
way:
1.
Change
A
I4R44 from
a
10
M
ohm
resistor
toa
2
M
ohm
resistor (part
number
0683-20S5).
2.
Change A12R7,
R8. R16,
and R17 from
4.99
K
to
10
K ohm
(part
number
0757-0442).
c.
Check
for the correct
operation
of
AI4US
and
its
associated
circuitry.
Since
U5-and
its
associated
circuitry
is
an
absolute
amplitifer,
the
output
of
the amplifier (emitter
of
U6)
should be
the
same
as
the
signal
at
TPl.
Therefore, pins 14
and
15
of
the
A
14
board should
also
show
the absolute
value
of
the
signal at
TPl.
The
correct signals for
a
-10
V
input to the
3455A
are
shown
in
Figure
8-E-l.
If
the
signals
at
A14P1
pins
14, IS,
and
S
are
incorrect,
troubleshoot the
detect
circuits.
d.
Make
sure
any
oxidation
layers
have
not
formed on
the pins
of
the
AI4
board.
The
pins
can be
cleaned with
a
soft
lead
eraser.
B-E-8.
AID Noise (Schematic
6).
8-E-9.
A/D
noise
will
usually
show
up
in
all
ranges
and
ail
functions
of
the
34S5A.
Two
circuits
to
check
for
noise
on
the
A/D
board
are the integrator
and
the input
circuits.
a.
Check
for
a
noisy
A14Q5
or U3.
Make
sure there are not
oscillations
present
at
TPl.
b.
Check
for a stable
zero
detect signal
at
A14U6
pin
1.
If
the
signal
is
unstable,
U4
or
U6 may
be
noisy.
c.
Check
for
a noisy
AI4Q3.
B-E-10.
Inguard Logic Troubleshooting (Schematic
7).
8-E-l
1.
Before troubleshooting
the
inguard
logic
make
sure the
outguard
logic
is
operating
correctly.
Use
the
half-splitting
technique of paragraph
8-195.
a.
Check
the
back
gate
bias voltage
(B.G.)
of
the processor
(A10U26
pin
387).
The
voltage
should
be within
±
.25
V
of
the voltage
marked on
the processor.
If
the voltage
is
incorrect,
check
for the
correct
value
of
pad
resistor
A10R105
(pad
list
in
parts
list).
If
the
pad
resistor
is
the correct value
and
the
bias
voltage
is
incorrect,
replace the processor.
8-E-12.
Inguard
Logic
Troubleshooting with no
AfO
Waveform.
a.
The
signals
at
A10U26
pins
34
to
37 should be
the
same
as
those
on
A3TP4
to
TPl.
The
only
ex-
ception
is
the
signal at
A10U28
pin
9.
If
the
signals
do
not
agree,
check
for
malfunctions
in
the
in-
guard
light
isolators.
A10U34
and U35,
plus
their
associated
circuits.
Line
F0 and
FI
transfer
data
from outguard
to
inguard (F0
is
the
data
transfer
line
and
FI
is
the
data
transfer rate
line).
b.
If
pin
9 of
AIOU28
is
different
than
pin
37 of
U26
and
the
HAZ
line
(pin
1
of U27)
is
low,
the
pulse
transformer
and/or
associated
circuits
may
be
defective.
This
interrupt
circuit
can
be
checked
by manually
clocking
A
IOTP
10.
and
can be
achieved
by
pulling
TP
10
low
and
then
releasing
it.
If
no
toggling
is
taking
place,
troubleshoot the
interrupt
circuit
consisting
of
A10U32A
and U27.
If
there
is
toggling,
check
T2
or
the
outguard
section (Al).
c.
The
interrupt request
lines at
pin
29 of
A10U26
must
be
high.
Troubleshoot
the interrupt
cir-
cuitry
if
necessary.
d.
Data
lines
D0
to
D7
(pin 18 to
35
of U26) and program
address
lines
(PA0
to
PA7
pins
1
to 8
of
U26)
should have
voltage
levels
from
approximately
zero to
approximately
+
4.5
V.
Check
for
any
circuits
causing
these
lines
to
be loaded
down.
It
is
possible
and normal
to
observe sharp peaks of
3
V
to 3.5
V
on some
lines
of U26. These peaks
are present
when
the
line
is
in
a
tri-state
mode.
This
is
a
possible
and normal
operation.
8-70
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