HP 400D Operating And Servicing Manual page 25

2) Switch the vtvm to the
the
capacitor C4 for best square wave pattern on the
oscilloscope.
BASIC CALffiRATION (SETTING AMPLIFIER
E.
GAIN)
Connect a O. 3 volts rms, 400 cps signal from the
Calibration Generator to the vtvm INPUT. Switch
vtvm the
the to 0. 3 volt range and adjust the feedback
attain a reading of O. 30 volts.
control (R29) to
F. BASIC CALmRATlON ( SETTING INPITr DIVIDEIl
1) Connect a 1. 0 volt rms, 400 cps, signal from
the Voltmeter Calibration Generator to the INPUT
of the vtvm under te st. Switch the vtvm to the 1. 0
volt range.
2) The voltmeter indication will normally be 1. 0
If
volts. the reading is high, decrease the value
of the pad in parallel with R3.
increase the value of the pad. Resistor R3 and pad
are on the input circuit board.
G. RECHECK DC HEATER VOLTAGE
the de
Check heater voltage by the method given
in paragraph 4-7C .
1f
either electrolytic capacitor in the healer Circuit
has been replaced, a heal-run of several hours
should be allowed. Afler the heat-run to form the
capacitor plates, reset the heater voltage.
H. CHECK CALffiRATION
scale
1) Check full accuracy
Use the Voltmeter Calibration Generator. The
calibration should be within :1:2% of full scale for a
400D voltmeter or ±l% of full scale for a 400H or
400L voltmeter. The test voltage should be between
kc.
50 cps and SOO
Avoid using the power line fre­
quency or a harmonic of the power line frequency.
to the
Refer accuracy specifications if you
frequency outside of this range.
2) Check meter tracking on any mid-range. The
I volt range is usually convenient. A sticky meter
can cause the vtvm to be outside of the calibration
specifications. Defective diodes for CRI and CR2
can also introduce excessive tracking error. Refer
to paragraph 4-4 for information On replacement
diodes.
1.
CHECK EFFECT OF LINE VOLTAGE CHANGE
1) Connect a sine wave Signal to the INPUT of the
vtvm under test. Use a convenient mid-range and
adjust the input Signal amplitude for a convenient
0 volt range and adjust
1 .
H the
reading is low,
&
TRACKING
of each voltmeter range.
use
a test
reference level such as 0 . 9 . Use any frequency
within the frequency range of the voltmeter. The
line voltage should be at 115 volts for at least two
minutes before setting your reference level.
2} When the line voltage is changed from 115 to
103 or from 115 to 127 volts, the indication on the
vtvm
under test will normally not change from the
reference level by more than :1:2% of full scale after
a two minute period.
These effects are more noticeable at the upper and
lower limits of the vtvm frequency range.
change is excessive, check tubes V5 through VI in
that order.
J. RESPONSE
SPEC. 23679
&
The following frequency response adjustment pro­
cedure is based upon the use of the dj}Spec. 23679
VTVM Frequency Response Generatorand
Range
23680 Wide Oscillator.
the vtvm.
The input voltage to
monitored by the reference meter in the Frequency
Response Generator. The imUcatiOn on this monitor
meter at 400 cps will
be
steps that follow.
Follow procedure
in
the
23679 and Spec. 23680 equipment is not available.
The frequency response of
ably not need adjustment. These steps have been
included to verify the instrument response and to
restore proper operation in case the original ad­
justment is altered. Failure to meet any of the
can
following tests usually be traced to one or more
weak tubes.
Connect a 400 cps sine wave signal to the INPUT
1)
of the vtvm under test. Adjust signal level for a
reading of 0. 90 on the . 001 volt range. Adjust mon­
itor meter "SET LEVEL".
to
2) Change input signal to 4 me and set amplitude
to " SET LEVEL". Adjust capacitor C21
under test for a reading of 0.90.
3) Change input signal to I mc and set amplitude
to "SET LEVEL". Capacitor C26
cuit of V4 can be adjusted in value between limits
of 15 �f and 82 /.ij.tf to control 1 mc response.
you change C26, repeat steps
IV
Sect, Page 7
H
ADJUSTMENT WITH
the
$Spec.
is
under test essentJall y
in the
called "SET LEVEL"
paragraph e
if th
4-7K Spec.
NOTE
the
instrwnent will prob­
in
the vtvm
in
the plate cir­
through 3.
1
the
H