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Section
VHI
TROUBLESHOOTING
Model 34S5A
controller
or
from
the
main
controller to the
HP-IB.
Ex-
ample:
the
main
controller receives a
message from
the
Bus
to
read
DC.
After a reading
is
taken, the
main
con-
troller
sends the reading
to the
Bus.
It
should be
noted,
as with the front panel, the
bus
circuitry
can
interrupt
the
main
controller
whenever
necessary
(to clear
inter-
face,
etc.).
B-185. Inguard Section.
8-186.
The
Inguard
Section
consists
of
(he
measuring
circuitry,
a
controller,
and power
supplies.
The
main
function
of
these
circuits
is
to
perform Auto-Cal,
DC,
AC,
and
Ohms
measurements. These
circuits
arc
con-
trolled
by an
inguard
controller,
which
in
turn are par-
tially
controlled
by
the
outguard
controller.
8-187.
The
circuits
used
for
Auto-Cal
and
DC
measurements
are
basically
the
same.
The
Auto-Cal
measurements
consists
mostly
of
gain
and
offset
measurements of
various
op-amps and FETs. The
Auto-
Cal
function
can be
turned
on
or
off,
as desired
by
the
operator.
8-188.
The
following procedure
outlines a typical
DC
measurement.
a.
A DC
signal
is
applied to the input
of
the
3455A.
This
signal
may
or
may
not
be
attenuated
by
the input
attenuator
circuits.
b.
The
signal
is
next applied
to
the
Main
Amplifier
through
the
Auto-Cal and
Measurement
Switching
cir-
cuits.
After pre-amplifications
by
the
Main
Amplifier,
the signal
is
applied
to the
A/D
convertor
(10
V
DC
for
full
scale).
c.
The
A/D
convertor changes
the
analog
signal
to a
digital
signal
and
sends
the
digital
signal
to the
inguard
controller.
The
inguard
controller
(hen
transfers
this in-
formation
to the
outguard
controller.
d.
The
outguard
controller processes the
information
and
displays the
reading
on
the front panel.
8-189.
Auto-Cal measurements
are
taken
in
the
form of
Auto-Cal
constants
and
are
used
to
compensate
for
in-
ternal
measurement
errors.
To
help generate the
cal
con-
stants (gain
and
offset),
stable
reference voltages
(±
10
V) and
stable
resistive
divider
(1
kO, 100 kR, 900 kR,
and
1
MR)
are used.
These
circuits
are located
on
the
reference
module.
The
reference voltages are
also
used
for
the
operation of
the
A/D
convertor.
8-190.
The
ohms
convertor
is
used
to
supply
the current
for
an
ohms
measurement and
in
turn causes
a
voltage
drop
across the
unknown
resistor.
The
voltage
drop
depends
on
the value
of
the
unknown
resistor
and
the
range of
(he
instrument.
This
voltage
is
measured
along
with
a
voltage
drop
across a reference
resistor,
by
the
DC
circuits
of
the
3455A.
The
DC
readings are then
converted
to
digital
readings
and
passed
on
to the
main
controller.
The
reading
is
then
calculated
by
the
main
controller to
an
ohms
reading to
be
displayed
on
(he
front panel.
8-191
.
The 3455A
offers a
choice of
either
a
True
RMS
or
an Average Responding
AC
Convertor.
Both
conver-
tors
changes an
AC
voltage to a
DC
voltage with
an
amplitude of approximately
6.7
V
for
a
full
scale
in-
put.
This
resultant
DC
voltage
is
(hen processed
by
the
DC
circuits,
as
explained
in
paragraph
8-188, with the
exception
of
the
DC
attentuator
circuits.
The
attenua-
tion
is
done on
the
AC
convertor board.
The
main
con-
troller
receives
the
digital
information
from
the
inguard
controller
and
is
then processed
to
be
displayed
as
an
AC
reading
on
the front panel.
The
following
is
an
ex-
planation
of
the differences
between
the convertors.
a.
True
RMS
Convertor;
The
True
RMS
Convertor
can
cither
be
AC
or
DC
coupled.
Using
operational
cir-
cuitry,
the input voltage
to (he
convertor
is
changed
to
a
DC
level
proportional
to the
RMS
value
of
the input
voltage.
b.
Average
Responding
Convertor:
The
Average
Responding Convertor
is
only
AC
coupled.
An
average
responding
circuit
calibrated to the
RMS
value
of
a
sinasoidal input voltage,
is
used
in this
convertor.
The
resultant
DC
output
of
the
convertor
is
a voltage pro-
portional
to the
average value
of
the input voltages ab-
solute value.
8-192.
The
inguard
controller controls the
operation
of
the
inguard
section
after
receiving instructions
from
the
outguard
controller.
The
inguard
circuits
being
control-
led
are
used
to
perform
the various
measurements.
8-193.
For
a
more
detailed
explanation
of
the
34SSA's
circuitry,
refer to
the
Theory
of Operation
Section
in
this
manual
(paragraph
8-10).
8 194.
PRELIMINARY
TROUBLESHOOTING
CHECK.
8 195.
INSTRUMENT
HALF
SPLITTING
TECHNIQUES.
8-196.
Before proceeding
to
a particular
service
group
for
troubleshooting the
34S5A
should be
half-split.
This
is
done
to
determine
if
the
failure
is
in
the
inguard or
outguard
section
of
the instrument.
The
following pro-
cedure can be
used.
a.
Haif-splitting
can
easily
be accomplished
with
an
Inguard/Outguard
Service
Cable
(part
number
03455-61609)
and
a
working
3455A
(a
second
in-
strument)
as follows:
I.
With
each
34S5A
turned
off,
disconnect the
AlOWl
Inguard/Outguard
cable
assembly
from
the
outguard connector (A1J7)
on
each 3455A.
8-36
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