
indicated.
The
troublesho
oting
pro cedures
later
in
this
c
hapter
detail
the
steps
to
trace
the
cause
of
the
problem.
Shutdo
wn
Circuit
The
sh
utdo
wn
circuit
is
triggered
b
y
o
v
ercurren
t,
o
v
erv
oltage,
underv
oltage,
or
o
v
ertemp erature.
It
protects
the
instrumen
t
b
y
causing
the
regulated
v
oltage
supplies
to
b e
sh
ut
do
wn.
It
also
sends
status
messages
to
the
A9
CPU
to
trigger
w
arning
messages
on
the
analyzer
display
.
The
v
oltages
that
are
not
sh
ut
do
wn
are
the
+5VD
and
+5V
CPU
digital
supplies
from
the
preregulator,
the
fan
supplies,
the
prob e
p o
w
er
supplies,
and
the
display
supplies.
The
sh
utdo
wn
circuit
can
b e
disabled
momen
tarily
for
troublesho
oting
purp oses
b
y
using
a
jump er
to
connect
the
SDIS
line
(A8TP4)
to
ground.
V
ariable
F
an
Circuit
and
Air
Flo
w
Detector
The
fan
p o
w
er
is
deriv
ed
directly
from
the
+18
V
and
018
V
supplies
from
the
A15
preregulator.
The
fan
is
not
fused,
so
that
it
will
con
tinue
to
pro
vide
airow
and
co oling
when
the
instrumen
t
is
otherwise
disabled.
If
o
v
erheating
o ccurs,
the
main
instrumen
t
supplies
are
sh
ut
do
wn
and
the
fan
runs
at
full
sp eed.
An
o
v
ertemp erature
status
message
is
sen
t
to
the
A9
CPU
to
initiate
a
w
arning
message
on
the
analyzer
display
.
The
fan
also
runs
at
full
sp eed
if
the
air
o
w
detector
senses
a
low
output
of
air
from
the
fan.
(F
ull
sp eed
is
normal
at
initial
p o
w
er
on.)
Display
P
o
w
er
The
A8
assem
bly
supplies
v
oltages
to
the
display
through
a
wire
cable.
The
A8
supplies
+5V
CPU
and
+65
V
to
the
A19
GSP
,
then
the
+65
V
is
routed
to
the
display
.
They
are
not
connected
to
the
protective
sh
utdo
wn
circuitry
so
that
the
A18
display
assem
blies
can
op erate
during
troublesho
oting
when
other
supplies
do
not
w
ork.
Probe
P
o
w
er
The
+18
V
and
018
V
supplies
are
p ost
regulated
to
+15
V
and
012.6
V
to
pro
vide
a
p o
w
er
source
at
the
fron
t
panel
for
an
external
RF
prob e
or
milli-meter
mo
dules.
Digital
Control
Theory
The
digital
con
trol
functional
group
consists
of
the
following
assem
blies:
A1
fron
t
panel
A2
fron
t
panel
pro cessor
A9
CPU
A10
digital
IF
A16
rear
panel
A18
display
A19
GSP
These
assem
blies
com
bine
to
pro
vide
digital
con
trol
for
the
en
tire
analyzer
and
HP
85047A
and
85046A/B
S-parameter
test
set.
They
pro
vide
math
pro cessing
functions,
as
w
ell
12-6
Theory
of
Operation
DRAFT
3/21/106
15:14
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