
Model 428B
SECTION V
MAINTENANCE
5-1. INTRODUCTION.
5- 2. This section contains the service information which is
required to properly maintain the 428B Clip-On
Milliammeter.
Included
are
performance
checks,
adjustment and calibration procedures, and servicing and
troubleshooting information.
5-3. TEST EQUIPMENT REQUIRED.
5-4. The test equipment required to perform the operations
outlined in this section is listed in Table 5-1. This table
includes the type of equipment, critical specifications,
purpose, and recommended model. If the model
recommended is not available, equipment which meets or
exceeds the critical specifications listed may be
substituted.
5-5. IN-CABINET PERFORMANCE CHECKS.
5-6. The performance checks described in Paragraphs 5-7
through 5-24 are front panel procedures designed to
compare the Model 428B with its published specifications.
These
tests
may
be
incorporated
into
periodic
maintenance, post repair, and incoming quality control
checks. The performance checks should be conducted
before any attempt is made to adjust or calibrate the
instrument. During the in-cabinet performance checks, the
Model 428B should be connected to the ac line through a
variable voltage device so that line voltage may be varied
± 10% from 115 or 230 Vac to assure that the instrument
operates correctly at various ac line voltages.
5-7. CLEANING OF PROBE JAWS.
5-8. When the probe shows excessive sensitivity to the
magnetic fields around it, the probe jaws should be
cleaned
(to
determine
excessive
sensitivity,
see
Paragraph 5-24 Step a). Excess ive sensitivity to external
fields can be caused by foreign material between the
probe jaws or by overheating (see Paragraph 3-3 Step d).
The probe jaws should always be cleaned prior to
calibration, adjustment or repair of the 428B.
5-9. To clean the probe jaws, open the probe and brush off
the mating surfaces with the brush which is supplied with
the instrument. If the brush will not remove the
contaminant, disassemble the probe and clean it with a
pencil eraser as illustrated. When reassembling the probe,
be sure that the arrow on the body of the probe points to
the heavy red wire as shown in Figure 6 -1 .
Figure 5-1. Cleaning Probe Jaws.
5-10. ELECTRICAL ZERO SET.
5-11. In the following procedure the numbers in
parentheses refer to Figure 5-2.
a.
Connect the 428B to the power line through a
variable transformer .
b.
Turn on power (1) and allow about two minutes of
warmup time.
c.
Place probe (2) away from any stray fields such as
meter magnets, or open core transformers.
d.
Insure that the probe jaws (3) are completely
closed (Paragraph 5-24 Step a, outlines a way to
do so).
e.
Set RANGE switch (4) to 1 mA.
f.
Zero-set the 428B with the ZERO (5) control. If the
meter cannot be zeroed, degauss the probe
(Paragraph 3-29) or clean the probe jaws
(Paragraph 5-8).
g.
Change the line voltage from 103 to 127 volts (±
10%) and read zero drift on the 428B meter. Zero
drift should not exceed ± .03 mA .
Figure 5-2. Electrical Zero Set.
14
Содержание 428B
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