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Page 4
technique
requires
current
through
the probe,
procedures
given in the Model 428A/B manuals will
require
some
modification.
Such modifications are
discussed in the following paragraphs.
30,
CALIBRATION,
Instead
of the accuracy
check
procedures given in the manuals
(Model 428A, par.
4-6A, and/or step 23, table 4-2; Model 428B, par.
9-39, and/or step 24, table 5-2), check instrument
calibration
by re-measuring
the
earth's
magnetic
field strength,
and comparing the reading obtained
with
the
reference
reading
made
as instructed
in
paragraph
6.
If the readings
are
not the
same,
follow the adjustment procedure given in the instru-
ment manual.
31.
SIGNAL
SOURCE.
Some
of the techniques
des-
cribed
in the paragraphs listed below require that
there be an input signal. A suitable ac input may be
obtained by placing the magnetometer
probe near a
power-supply
transformer.
With such an input, the
following procedures can be used.
a. Troubleshooting.
Waveforms
can
be used
as
describe in
(1) Model 428A, par. 4-8A
(2) Model 428B, par. 5-29, 5-30
b. Adjustments.
(1) Model
428A,
table
4-2,
steps 9 through 13
(2 Model
428B,
table 5-2, steps 8 throught 12
32.
LOOP
GAIN
CHECK.
To
measure
the gain of
the amplifier, proceed as follows:
a. Model 428B. See step 18, table 5-2.
Model 3529A
(1) Obtain a signal as follows: winda5- to 50-turn
coil around the magnetometer probe head, and
couple the coil to an oscillator which furnishes
Signals in the 20 eps to 8--cps range, such as
the @ Model 200CD. Set the audio oscillator
for 20 eps.
(2) Proceed
as described in step 18 of table 5-2
except
for
setting
the
0 db point on 20 cps
(instead of 50).
b. Model
428A.
Since
the
Model
428A
does not
have
an ac output,
the procedure is somewhat more
complieated than that specified for the Model 428B;
measuring
the true
gain of the amplifier involves
disconnecting the feedback loop. Proceed as follows:
(1) Follow
the instructions given in step 14, table
4-2, of the Model 428A manual through zero-
setting the meter.
(2) Wind a 5- to 10-turn coil around the magneto-
meter probe head,
(3 Feed
enough
direct
current
of the proper
polarity through the coil togivea 3 ma. reading
on the Model 428A meter.
Then turn off this
current until step 5.
(4) Disconnect
the jumper across the Test Dis-
connect terminals on terminal board 428A
- 75A
(board nearest front panel); this jumper will
be
found
across
the terminals
nearest
the
front panel.
Removing the jumper disconnects
the feedback loop.
(5) Turn
instrument
on, and rezero.
Turn
on
Same
current
as
applied
in step
3; meter
Should now read approximatley 60 to 90 ma. If
meter
reads less, amplifier may have deteri-
orated.
Proceed
as
described
in step
14,
table 4-2.
01364-2
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