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Model 3455A
Section Vlll
SERVICE
GROUP
D
8
0-1.
OHM
TRQUBlESHQOTtNG
(SCHEMATIC
1. 4).
6-0-2.
Ohms
Circuit Isolation.
8-D-3.
Before troubleshooting
the
ohms
convertor, the
3455A
should be operating
correctly
in
thedc
mode.
Because
some
of
the
dc
and Auto*Cal
circuits
are
used
in
ohms,
there
circuits
should be check-
ed before
working
on
any
ohms
circuit.
The
procedure below
may
be
helpful
in
isolating
the
ohms
section
of
the instrument.
a.
With
the
instrument
set
to
the
dc
function,
check
the zero
and
full
scale
reading
on
the
display.
These checks
should be
made
on
all
ranges (100
mV,
I
V,
10
V, 100 V,
and
1000
V
ranges).
If
any
malfunctions
occur,
go
to
Service
Croup B
for further
troubleshooting.
b.
Using
the
self-test
mode
of
the
instrument
(see
paragraph
8-B-3
for
an
explanation of
the
self-
test),
check
for
any Auto-Cal
constant
failure.
Co
to Service
Croup B
if
any
constants
fail.
c.
If
the
dc
readings
on
the instrument are
good and
the
self-test
passes,
continue with
this
service
group
for
ohms
troubleshooting.
8-D4.
Ohms
Servicing.
8-D-S.
The
following checks
may
be
useful
if
the
ohms
function
is
completely
inoperative.
a.
Set the
3455A
to the
2
wire
ohms
function,
1
K ohms
range,
and Auto-Cal
off.
b.
With no
load applied to the terminals
of
the
3455A,
check
the voltage across the input
ter-
minals.
The
voltage
should be approximately
-4.7
V
dc
(typically -4.5
V
to
4.8 V).
If
the voltage
is
in-
correct,
the
ohms
convertor
board (A12)
or
the input
relays
may
be
defective.
Co
to
paragraph 8-D-6
for further
troubleshooting.
c.
When
approximately
-4.7
V
is
observed
at
the input terminals, the
ohms
convertor
is
in
voltage
limit.
This
is
a
correct indication
with
an open
circuit
input.
Connect a
1
K ohm
resistor
across
the
in-
put terminals of
the
34SSA. Measure
the voltage
drop
across the
resistor.
The
voltage
should be
ap-
proximately
-.7
V
dc
and
indicates correct
constant current operation
of
the
ohms
convertor.
Co
to
paragraph 8-D-6
if
the voltage
is
incorrect.
d.
The
above
steps
should
isolate
malfunctions
in
the current
source
circuitry
of
the
ohms
conver-
tor. If
all
the steps indicate correct
ohms
operations, the miscellaneous troubleshooting section
of
this
service
group
may
be
helpful
(paragraph
8-D-8).
8-D-6.
Ohms
Convertor Troubleshooting (Schematic
1, 4).
8-D-7.
Ohms
convertor malfunctions
may
be caused by
the
ohms
convertor
board
itself,
or
by
the
AlO
mother
board.
It
is
important
to
remember
that the
mother board and
ohms
convertor
have
in-
terconnecting
ohms
circuitry.
Try
the
following
procedure
to
troubleshoot
ohms
malfunctions.
a.
With
a
dc
voltmeter,
measure
the voltage across
A12C1. The
low
side
of
the
meter should be
connected
to
TP
•
V
and
the high
side
connected
to
the plus
side
of Cl
.
The
meter should
read
-f
19
V
dc.
If
the
reading
is
low by
.5
V
or
more,
A12TI
or
AlOTI
may
be
at fault.
b.
With
an
oscilloscope,
measure
the
ripple
across
A12C1. The
ripple
should not be
more
than
.1
V
peak
to
peak.
If
the
ripple
is
too
high,
check
AlOTl, A12TI,
C3,
CR4,
or
Cl.
c.
If
the voltage across
A12C!
is
very
low
or
zero,
look
for
an approximately 30
V
peak
to
peak
square
wave
at
the
anodes of
CR3
and
CR4.
This
signal
should be
around
31
MHz
with the
345SA
set
8-65
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