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Model 3455A
SERVICE
GROUP
C
Section VIII
8
C
l.
AC CONVERTOR TROUBLESHOOTING.
8-C-2.
True
RMS
Convtrtor
Sarvieing
(Schamatic
3).
8-C-3.
Before troubleshooting
the
345SA's True
RMS
Convertor,
the
instrument should operate
properly
in
the
dc
mode.
Verify
for the correct
operation of
the
dc
section,
before
servicing
the
ac
convertor.
The
following
procedure should be followed before troubleshooting
or repairing the
ac
convertor.
a.
Check
the
dc operation of
the
3455A.
Verify
for correct
full
scale
and
zero
scale
readings
on
all
ranges.
b.
Set the
34SSA
to the 10
V
range, ac function,
and
short the input.
c.
Check
for
approximate
zero
levels at
A1STP8
and TPS,
with
the
low
input
of
the
meter con-
nected
to
TP6
(go
to
paragraph
8-C-4 or 8-C-6
if
bad).
d.
Short
TP3
to
TP6
and measure
the voltage
at
TPI.
TPl
should read approximately
zero.
Remove
the short (go
to
paragraph
8-C-8
if
bad).
e.
Check
for
proper
biasing
of
A1SU2. The
voltage
at
U2
pin 2
should be between
-2
mV
and
-3
mV.
Repad
R2I
if
necessary
(R2I padding
list
is
in
the parts
list).
f.
Remove
the short
form
the input
of
the
34SSA. Apply
a 10
V, 100
Hz
sinewave
at
the input
ter-
minals.
Check
for the
following
voltages.
1.
With
an
oscilloscope,
check
for
a sinewave
at
A15TP8. The
amplitude
of
the
sinewave
should be approximately
2.8
V
peak
to
peak
with
no
shift
in
the
dc
level
(go
to
paragraph
8-C-4
if
bad).
2.
A
halfwave
rectified
sinewave should be observed
at
TPS.
The
amplitude of
the
waveshape
should be approximately
1.4
V
peak
to
peak, with
no
shift in
the
dc
level
(go
to
paragraph
8-C-6
if
bad).
3.
The
waveshape
shown
below
with
an approximate +
.7S
V
dc
level,
should be observed
at
TP4
(go
to
paragraph 8-C-8
if
bad).
0
V
4.
The
waveshape
shown
below
with
an approximate
-1
V
dc
level,
should be observed
at
TP3
(go
to
paragraph
3-C-8
if
bad).
0
V
5.
Check
for
approximately
-f
.S
V
dc
at
TP2
(go
to
paragraph 8-C-8
if
bad).
6.
Check
for
approximately
+6.67
V
dc
at
TPl
(go
to
paragraph 8-C-8
if
bad).
7.
Check
for
approximately
zero
volts at
TP7
with the
34SSA
in
the 10
V
or
1000
V
ranges.
This voltage
level
should
change
to
approximately
-IS
V when
the
instrument
is
switched
to the
1
V
or 100
V
ranges (troubleshoot gain switching
circuit,
if
bad).
8-S9
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