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Model 3455A
SERVICE
GROUP
C
Section VIII
lifting
RS2
and
R64,
the other
circuits
are causing the
malfunction
(absolute
amp,
squaring
amp,
etc.).
8-C-6
Absoivta
AmplHiar
Circuitry.
8-C-7.
If
the
waveshape
at
TPS
appears
to
be
incorrect, try
the following
checks
(except
where
noted, the input
signal
should be
a
10
V,
100
Hz
sinewave).
a.
To
check
if
other
circuits
on
the
A14
board
causes
failures in
the absolute amplifier, the ab-
solute amplifier
can be
isolated.
This can be accomplished
by
lifting
RS2 and
R53.
The
amplifier
should
now
be
operating
correctly.
Troubleshoot
the amplifier
circuit,
if
defective.
b.
Check
the
power
supply
voltages
at
pins
4 and
7
of U7. Pin 4 should be approximately
-IS
V
and
pin 7
approximately
+
15
V.
c.
Check
for
an
approximately
2.8
V
peak
to
peak sinewave
at
U4
pin
6.
Troubleshoot
U6
and
associated
circuitry,
if
necessary.
d.
If
the
sinewave
at
U4
pin
6 has
oscillations,
reduce
C22
to 10
pF
(part
number
in
parts
list).
Do
not reduce
C22
below
10
pF,
as the
frequency response
of
the ac
convertor
may
be
affected.
e.
The
cathode/anode
junction
of
CR5
and
CR4
should
have
a
sinewave with
slight
distortion
at
the zero crossover point.
Replace
CR4
or
CRS,
if
necessary.
f.
If
the
signal
at
TPS
is
distorted,
CR4
may
have
leakage.
CR4
and
CRS
can be
interchanged.
g. If
QI4
or
QIS
appear
defective,
check
with
an
ohmmeter
and
replace,
if
necessary.
8-C-8.
Squaring
Amplifiar, Integrator,
and
Antilog
Circuitry.
8-C-9.
The
squaring
amplifier, integrator,
and
antilog
circuits
are
connected by feedback
paths.
Isolation
of
these
circuits
may
be
difficult.
There
are,
however,
some
checks used
to help
troubleshoot
these
circuits
(except
where
noted, the input
signal
should be a 10 V,
100
Hz
sinewave).
a.
In
some
cases
it
is
possible to
isolate
the integrator
from
the other
circuits
on
the
A15
board.
This
can be accomplished by
shorting
TP6
to the
cathode of
CR3. The
reading
at
TPl
should be ap-
proximately
zero.
If
there are great offsets
at
TPl.
troubleshoot the
integrator.
b.
Apply
a 100
mV,
100
Hz
sinewave
to
the input terminals
of
the
345SA.
The
instrument should
be
in
the 10
V
range
and
display
approximately
.10(X)
V.
Check
for
dc
readings
of
+
.82
V
at
TP4,
-9
V
at
TP3,
-.48
V
at
TP2, and
+
.067
V
at
TPl.
These
readings
may
be
helpful
in isolating
the squar-
ing
amp,
integrator,
and
antilog
circuits.
c.
Reapply
a
10 V, 100
Hz
sinewave
to
the input
of
the
34SSA.
The
instrument should be
on
the 10
V
range.
Check
for
dc
readings
of approximately
+
.66
V
at
TP4,
-1.15
V
at
TP3,
-.6
V
at
TP2, and
-I-
6.7
V
at
TPl.
Again,
these readings
may
be
helpful
in
isolating
the
squaring
amp,
integrator,
and
antilog
circuits.
d.
Check
the
power
supply
voltages
of Ul, U2,
and
U3.
Pins
7 of
the
op-amps
should be approx-
imately
-I-
15
V
and
pins
4
should be approximately
-
IS
V.
e.
Check
for a
voltage
drop of
approximately
+
3.3
V
across
R36.
If this
voltage
drop
is
incorrect,
Q12
or
Q13
and
their
associated
circuitry
may
be
defective.
f.
For
parasitic oscillations at
TP3
change
R36
to
649
0
(part
number:
0698-4460).
g.
If
the display
of
the
3455A
indicates
an overload
condition with a
good
waveshape
at
TPS,
8-61
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