HP 400D Operating And Service Manual

Section V
Paragraphs 5-9 to 5-16
c. Rotate mechanicalzero-adjustment screw clock-
wise until meter pointer is tothe left of zero and mov-
ing upscale toward zero.
d. Continue to rotate adjustment screw clockwise;
stop when pointer is exactly on zero. Ifpointer over-
shoots zero, repeat steps c and d.
e. When pointer is exactly on zero, rotate adjust-
ment screw approximately 15 degrees counterclock-
wise. This is enough to free the zero adjustment
screw from the meter suspension. If pointer moves
during this step, because the adjustment screw is
turned toofar counterclockwise, repeat the procedure
of steps c through e.
5-9. CABINET REMOVAL.
a. Remove the two cabinet retaining screws at the
rear of the instrument.
b. Push the instrument chassis forward out of the
cabinet. The bezel ring remains attached to the front
panel.
c. When replacing cabinet, pull power cable through
opening at rear of cabinet. Be sure power cable is
not caught between chassis and cabinet. Replace re-
taining screws.
5-10. TUBE REPLACEMENT.
Do not remove tubes from the voltmeter when
power is applied. To do so may damage the
voltmeter.
5-11. In many cases instrument malfunction can be
corrected by replacing a weak or defectivetube. Check
tubes by substitution while following the voltmeter
T.O. 33A1-12-349-1
performance check procedure in paragraph 5-22, Re-
sults obtained through the use of a "tube checker" can
be misleading. Before removing the tubes from the
instrument, mark the original tubes so they can be
returned to the same socket if they are not defective.
Replace only those tubes proven to be defective.
5-12. Figure 5-2 lists each tube in the voltmeter with
its function and the check or adjustment required if
the tube is replaced.
5-13. REPLACEMENT OF SPECIAL PARTS.
5-14. PRECISION RESISTORS AND INDUCTORS, Sev-
eralparts usedinthe voltmeter have closer tolerances
than those used in most test equipment. Resistors
R104, R105, R108, and R111 through R116 are pre-
cisioncomponents. If these resistors require replace-
ment, use the same value and type as the original, as
shown in the parts breakdown. If different values
are used or component positions are moved, the cali-
bration of the voltmeter may be inaccurate or the fre-
quency response may be altered. The inductance of
L10 and Lil affects the frequency response of the
voltmeter. Do notalter the shape or position of these
coils. Install replacement components in the same
positions the original components occupied, as nearly
as possible.
5-15. DIODE RECTIFIERS. Special high-performance
silicon diodes selected by the Hewlett-Packard Co.
are used for CR1 and CR2. When replacing the sili-
con diodes, be careful in soldering; heat can damage
them, Place a heat sink (such as a long-nose pliers)
on each diode lead close to the diode body to conduct
the heat away. If CRi and CR2 are replaced, the
voltmeter calibration and frequency response must
be checked as described in paragraph 5-22.
5-16. RANGE SWITCH. Because of the critical con-
struction and wiring of switch S1, it is not practical
to attempt a major repair onthe switch. When mech-
anical failure occurs in switchS1, replace the complete
CIRCUIT CHECK OR
Cathode Follower
1st Amplifier
2nd Amplifier
3rd Amplifier
4th Amplifier
High Voltage Rectifier
Series Regulator
Control Tube
Reference Tube
Calibration and frequency response (para, 5-22)
Test of the power supply (para. 5-20)
* Note that Vi must be replaced by a 6CB6, aged and selected for low noise and microphonics (4,
Part No. 5080-0621).
Figure 5-2. Adjustments Required When Tubes Are Replaced
5-2
00102-3

HP 400D Specifications

General

Brand	HP
Model	400D
Category	Measuring Instruments
Language	English