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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