
Model 428B
5-21. AC OVERLOAD.
5-22. An oscillator or function generator, a 50 ohm
resistor, an ac voltmeter or oscilloscope, and the meter
calibrator are required to complete this check (a
filament transformer and suitable current limiting
resistors can be substituted for the oscillator).
a.
Set the meter calibrator for 10 mA. With the
428B on the 10 mA range clip the 428B probe
onto the lead which connects the calibrator
terminals.
b.
Note the 428B reading.
c.
Set the function generator to 60 Hz, its output
to generate a 10 mA peak sine wave into a 50
ohm resistor (1 volt peak-to-peak or 0.35 volt
rms across a 50 ohm resistor).
d.
Clip the 428B probe onto BOTH the wire from
the meter calibrator and the resistor lead from
the function generator (see Figure 5-4).
e.
The 428B should read within 2% of the
reading noted in Step b.
Figure 5-4. AC Overload.
5-23. NOISE CHECK.
5-24. A battery operated ac voltmeter and a 1 kilohm
resistor are required to complete this check.
a.
Point probe East and West and rotate about its
axis.
b.
Note peak-to-peak change in meter reading, if it
doesn't exceed 0.1 mA on the 1 m A range,
proceed. If the change is excessive, perform
Paragraph 5-8.
c.
Zero the Model 428B on the 1 mA range.
d.
Connect the output of the 428B to the input of a
battery operated 403 B with a 1 kilohm resistor in
parallel with the 403B input.
e.
Section V
f.
With the 428B on the 1 mA Range check for a
maximum of 15 m V reading on the AC Voltmeter.
g.
Switch the 428B to the 3 mA range. Check for a
maximum of 5 m V reading on the AC Voltmeter.
h.
Switch the 428B to the 10 mA range. Check for a
maximum of 2 mV reading on the AC Voltmeter.
i.
Check each range from 10 mA to 10 amps using
the procedure of Step f. No ac output reading
should exceed 2 mV.
5-25. ADJUSTMENT PROCEDURE.
5-26. When the instrument shows signs of defective
components, use the troubleshooting procedure to find
and correct the problem. It is quite easy to compound
a 428B trouble by misaligning the instrument in an
attempt to adjust out the effects of a defective
component.
5-27. POWER SUPPLY.
5-28. Connect an electronic dc voltmeter to test point 5
(Pin 1, V9) of the 428B. The voltage at this point
should be 272 ± 6 V if not, adjust R109 for 272 V.
5-29. MECHANICAL ZERO SET.
5-30. When meter is properly zero set, pointer rests
over the zero calibration mark on the meter scale when
instrument is (1) at normal operating temperature, (2)
in its normal operating position, and (3) turned off.
Zero set as follows to obtain best accuracy and
mechanical stability:
a.
Allow instrument to operate for at least 20 minutes;
this allows meter movement to reach normal
operating temperature.
b.
Turn instrument off and allow 30 seconds for all
capacitors to discharge.
c.
Rotate mechanical zero adjustment screw
clockwise until meter pointer is to left of zero and
moving upscale toward zero..
d.
Continue to rotate adjustment screw clockwise;
stop when pointer is right on zero. If pointer
overshoots zero, repeat Steps c and d.
e.
When pointer is exactly on zero, rotate adjustment
screw slightly counterclockwise to free adjustment
screw from the meter suspension. If pointer moves
during this step, repeat Steps c through e.
5-31. DC AMPLIFIER BALANCE.
5-32. Open the 428B and short across the brass pins
on the circuit board which are connected to the ends of
C1 0 (these pins are located on either side of C14).
Connect a 1500 ohm resistor between Pin 2 of V6 and
the 6.3 V filament supply. Disconnect the feedback
disconnect (see Figure 7-3). Zero the 428B meter with
R46.
16
Содержание 428B
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