Model
3455A
SERVICE GROUP D Section
VIII
6-D-11. Ohms Noise (Schemitic
1,
4).
8-D-I2. To
prevent
possible
damage
to
sensitive components being measured, the ohms current
source of the
3455 A
is
limited
to
5 V.
Lower
currents are used to keep this
voltage
low. Because of the
small currents, the ohms section of the instrument may be susceptible to noise.
8-D-13. Before troubleshooting for any ohms noise, make sure the dc noise level is good. Check for
noise on
all
dc
ranges of the 3455A. If the dc operation is good, troubleshoot for ohms noise. A few
troubleshooting hints for servicing ohms noise are
given below:
a.
Excessive
noise on all ranges: Check for a 19
V
dc
voltages
across
A12CI.
If the
voltage
is
low
by .5
V,
A12TI or AlOTl may be at fault.
b. Readings decrease on
each successive sample and then suddenly jump back, with the procedure
repeating.
The
case
of AIOR63 may
be
touching the case of
Q37.
c. Noise on the
.1, 1,
and 10 ranges: A10K9 may be
defective.
d. Possible noise on the
100,
I K, and 10 K ranges
with very
high readings at 1/iO scale: A12CR7
may be
defective.
e. 1/10 scale reading on the 100 K range is noisy and
low:
Check for oscillation at AI0TP4. This
can be accomplished by setting the 3455A to the ohms function. The
instrument should
be on the 100
K
range, with Auto-Cal off, and placed in Hold/Manual. TP4 should be monitored with an
oscilloscope. Press the HOLD/MANUAL button and
observe
for any oscillation at TP4. If any
oscillation is noted, try
changing A10C4
to
.0068 (part number 0160-0159) and AlORl 1 to 1.3 K
(part number
0757-0426).
f. Noisy at I M and/or 10 M ranges: Push all
wiring away
from the ohms board and all input
wiring away from the top guard
cover,
place the red
wire,
connected
between
the front/rear
switch
and the AlO board, next to the guard sheet metal. All
wires
should be kept
away
from
transformers
and transistors.
g.
10 M range very noisy: Make sure the 50/60 Hz
switch
is in the correct position.
h. Noise on the 1 K range: If noise
shows
up
on older instruments, make sure on
18
guage teflon
coated
wire
is
installed
in the
instrument. The
wire
should be connected
between
pin E of A10J3 and
the
cardinal
ground
terminal located between K7 and K9. The wire may reduce noise on the 1 K ohms
range
as
well as the I V ac and dc ranges.
i.
Noise
in
ohms
function: To
reduce external noise in ohms function, shielded cables are very
useful.
When measuring resistance
in
the 2 wire and 4 wire ohms function, connect the resistor to the
3455A
with
one or more shielded cables. The shields should be connected to each
low
terminal.
Most
noise, associated
with
external body
capacitance, should
be
shunted
to
ground rather than through
the
measuring instrumentation. The cables should not be
reversed
(the shields connected to the high
terminals
and
the
center conductor connected
to
the low terminals), or no shielded cables should be
used.
No shielded cables or reversed cables may cause
excessive
noise in ohms.
8-67
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