HP 400D Operating And Service Manual

T. O. 33A1-12-349-1
switch assembly. Use the following procedure, (Locate
parts by referring to figures 5-3 and 5-4; RANGE switch
connections are shown in figure 5-9.)
a. Remove voltmeter cabinet. (See paragraph 5-9.)
b, Loosen setscrews in RANGE switch knob and
remove knob.
ce. Disconnect capacitor C104 from switch S1,
d. Disconnect white leads from capacitors C14 and
C16. Label each lead with a tag.
e. Remove the two screws and one nut which retain
the switch shield plate.
f. Disconnect white leads from switch contacts, Tag
each lead to permit easy connection to the new switch.
g. Disconnect the heavy dark-green switch lead, the
heavy light-green switch lead, and the heavy black switch
lead at terminal strips. Tag each lead.
NOTE
The input shield must be removed for access
to the terminal board connection of the dark-
green lead,
h. Remove the nut which holds the switch bushing to
the front panel,
i, Remove RANGE switch assembly.
j. The sequence for installing the replacement RANGE
switch assembly is the reverse of the removal procedure.
k, After replacement of switch S1, check the calibra-
tion and frequency response of the voltmeter and make
necessary adjustments.
5-17. TROUBLE SHOOTING.
5-18. The first step in trouble shooting is to learn
the nature of the symptoms of the malfunction with as
much detail as possible. Inspect the test setup being
used when symptoms of malfunction were observed, to
be sure that the source of trouble is not external to the
voltmeter. Then remove the voltmeter cabinet as
directed in paragraph 5-9 and inspect the circuits of
the voltmeter, looking for signs of overheating, deteri-
oration, and physical damage or tampering. Check the
fuse. If the fuse is blown, try another fuse to see if it
blows; if it does, measure the d-c resistance of filter
capacitors C1, C17, C30, C39, rectifier CR3, and the
windings of transformer T1 to locate the short circuit
without applying power to the voltmeter.
5-19. If the voltmeter can be turned on safely (without
the fuse blowing), measure the line voltage applied to T1
and the voltmeter power supply output voltages (see
paragraph 5-20). Check the tubes of the power supply
if the regulated voltage is not the proper value or is
unstable. Use the procedures of figure 5-5 and the
tests described in paragraph 5-22 to learn the full
nature of the trouble symptom. Watch for marginal
00102-2
Section V
Paragraphs 5-17 to 5-21
operation by operating the voltmeter at 103 and 127
line volts while making tests. Check the tubes in the
voltmeter amplifier. Measure the tube element voltages
at the tube sockets and compare readings with the values
shown in the voltage and resistance diagram in figure
5-8. Apply a test signal to the input and measure the
voltage of the test signal while tracing it through each
coupling network and each stage of amplification.
Compare readings with those shown in the block diagram,
figure 4-1. In figure 4-1, an a-c current probe, H-P
Model 456A, is recommended for the measurement of
a-c current in the meter circuit without breaking any
leads. If this current probe is not available, avoid
measurement of the a-c current, Check meter indica-
tions as directed in paragraph 5-22 instead. An
oscilloscope may be used for observing test signal
waveshape and measuring amplitude, if desired,
5-20. TESTING THE POWER SUPPLY.
5-21. The regulated power supply produces a constant
+250 vdc to operate all the tubes in the amplifier section,
The stability of the voltmeter depends directly upon the
stability of the +250 volts from the supply. When the
supply is operating satisfactorily, the +250 volt output
remains constant and the ripple level on it remains less
than about 1 millivolt for line voltages between 103 and
127 volts. Weak tubes (V6, V7, and V8) are the usual
causes of instability. An unstable regulator tube is
indicated by excessive line frequency ripple and varying
output voltage as the line voltage is changed. Marginal
operation is indicated if a trouble symptom appears
only when a low or high line voltage is applied. To test
the complete power supply proceed as follows:
a. Connect the voltmeter to an adjustable line trans-
former so the applied line voltage can be varied between
103 and 127 volts. Set line voltage to 115 wlts, turn on
the voltmeter, and allow a five-minute warmup period.
b. Measure the d-c voltage between V6 (pin 8) and
ground. Normal value is 410 + 10 volts with exactly
115 volt power line input. Lower line voltage 10% to
103 volts for 2 minutes. If the d-c voltage slowly
drops below 360 volts, replace V6.
c. Measure the d-c voltage between V7 (pin 1) and
ground with line voltage adjusted to 115 volts. Cor-~-
rect value is 250 + 5 volts.
d. Vary line voltage from 103to127 volts. The d-c
voltage observed in step c must not change more than
+1 volt. For wrong voltage and/or poor regulation,
replace V7, V8 or V9.
e€. Measure the a-c voltage between V7 (pin 1) and
ground. Ripple voltage must be less than 3 mv for any
line voltage (103 to 127 volts). High ripple voltage is
caused by defective V8, V7, V6 or V9. Replace in
this order.
f, Measure the direct current in the lead from
V7 (pin 1) which must be less than 60 milliamperes.
If the current is much too high, the regulator circuit
will not function properly. Excessive current indicates
5-3

HP 400D Specifications

General

Brand	HP
Model	400D
Category	Measuring Instruments
Language	English