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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