Advertisement
Model 3455A
SERVICE
GROUP
E
Section Vlll
f.
For no
A/D
waveform
at
TPl, go
to
paragraph
8-E-4
for
troubleshooting.
For an
incorrect
waveform go
to
paragraph
8-E-6.
8 E4.
No
A/D Wavoform.
8-E-S. Since the
A/D
waveform
is
dependent
on
various
circuits in
the
3455
A
(input,
main
amplifier,
etc.),
isolation
of
these
circuits
is
necessary.
The method
used
is
simply
a
signal tracing
method
with
limited
operational checks.
a.
Set
up
the
3455A
using the
procedure of paragraph
8-E'3a,
b,
and
c.
b.
Measure
the voltage
at
the
multiplex
node
(sources
of
AlOQI,
Q2, Q3, and
Q4).
If
the voltage
is
not -10
V
dc, the input
circuit
may
be
inoperative.
The
multiplex
node
may
also
be loaded
down
by
one
or
more
FETs.
c.
Measure
for
a >10
V
dc
voltage
at
A10TP4.
If
the voltage
is
incorrect,
troubleshoot
the
main
amplifier
circuit.
Make
sure
A10Q19
is
turned on.
d.
Measure
the instrument's reference voltages.
A10TP8
should be
-f
10
V
±
100
and
TP7
should be
•
10
V
±
20
mV.
If
the reference voltages are incorrect,
troubleshoot the reference
assembly
(A
1
1
or
A20) and/or
U7.
The
reference voltages are
used
on
the
A/D
board and should be
correct for
proper
A/D
operation.
e.
Short across capacitor
A14C2
and measure
the voltage
at
A14TP1. The
voltage
should be
ap-
proximately
zero.
If
there are
any
great
offsets,
troubleshoot
A14U3
and
associated
circuits. If
the
voltage
at
TPl
is
good, remove
the short
from
C2
and
continue with
this
procedure.
f.
Short
AI4TP1
to
ground and measure
the
0
detect,
10
V
detect,
and
.2
V
detect
levels.
The
table
below
gives the correct detect
levels.
Remove
the short
from
TPl
and
apply -10
V
to
TPl. Measure
the
levels
of
the
0
detect,
10
V
detect,
and
.2
V
detect.
See
the table
below
for the correct
levels.
A14TP1
Shorted
A14TP1
at
-10
V
0
Detect
Level
s
5
V
s
0
V
1
0
V
Detect
Level
s
0
V
s
5
V
.2
V
Detect
Level
s 0 V
s
0
V
If
the
levels in
the table are
incorrect,
troubleshoot
A14U4,
5,
6
and
their
associated
circuits.
g.
Other
circuits
on
the
A/D
board
may
affect
A/D
operation.
These
circuits
are the input
circuits
and
diode
array
U\
and #2 and
their
associated
circuits.
Also
make
sure
A14Q3,
Q4, and
their
associated
circuits
are operating
correctly.
If
these
circuits
appear
to
be working
correctly,
the
in-
guard
logic
may
be
at fault.
Go
to
paragraph 8-E-lO
for further
troubleshooting.
8-E-6.
Incorrect
A/D Waveform.
a.
Check
for a
leaky
AI4C2, Q5,
U3,
or
U5
and
associated
circuits.
Circuits past
U4 may
also
be
defective.
b.
Circuitry
preceeding
the integrator
may
also
cause
an
incorrect
A/D
waveform.
Make
sure
A14Q2,
Q4, and
their
associated
circuits
are
operating
correctly.
c.
Check
for correct
operation
of
the detect
circuitry.
Paragraph
8-E-5f
may
be
helpful
in
troubleshooting
these
circuits.
B
E-7.
Correct
A/D
Waveform.
a.
If
the
A/D
waveform
is
correct
and
the
A/D
board
is still
inoperative,
check
the zero
detect
cir-
cuit.
Make
sure the zero
detect signal
is
stable
with
the correct
voltage
levels
(approximately 0
V
or
5
V).
8-69
Advertisement
