Frequency Response Adjustment - HP 400D Service Note

Page 6
Replace V 1 and/or V2 to remove excessive micro-
phonics. Pre-selected tubes a r e available from@ for
V1 and V2. See Section I1 of these Service Notes for
details,
3) Check Output vs Shock
a. Connect a 1.0 volt rms, ac signal to the INPUT
and set the vtvm to the 1-volt range. Connect
the vtvm OUTPUT to an oscilloscope and adjust
oscilloscope to display several cycles.
b. Strike either top front corner of the instrument
with
the palm of your hand, There should be no
sudden change in amplitude nor a momentary
collapse of the pattern. This test will detect
another type of microphonic tube or poor elec-
trical connections within the vtvm.
4) Check Beat Notes at 60 and 120 Cycles
a. Connect a battery operated oscillator to the IN-
PUT terminals of the vtvm. Set the range switch
to the 1-volt range, Adjust the oscillator output
to provide a vtvm reading of 0.9 volt at about
58 cps. Adjust
the
oscillator frequency toobtain
maximum meter swing which will normally not
be more than about 1/8 in. along the meter scale.
b. Repeat step a at 120 cps. The maximum meter
swing will usually not be more than about 1/8
in.
along
the
meter scale.
High beat notes a r e usually caused by defective tubes
anywhere in the instrument.
J. RECHECKDCHEATERVOLTAGE
Check the dc heater voltage by the method given
in paragraph D.
If either electrolytic capacitor in the heater circuit
has been replaced, a heat-run of several hours should
be allowed. After
the
heat-run to form
the
capacitor
plates, reset
the
heater voltage.
K. CHECK CALIBRATION AND TRACKING
1). Check full scale accuracy
of
each voltmeter range.
Use
the Voltmeter Calibration Generator. The cali-
bration should be within *2% of full scale for a 400D
Voltmeter or
i l g
of full scale for a 400H or 400L
Voltmeter, The test voltage should be between 50 cps
and 500 kc. Avoid using the power line frequency or
a harmonic of
the
power line. Refer to
the
accuracy
specifications
if
you use a test frequency outside of
this range.
2) Check meter tracking on any mid-range. The 1-volt
range is usually convenient. A sticky meter can cause
the vtvm to be outside of
the
calibration specifications.
Defective diodes for CR1 and CR2 can also introduce
excessive tracking error. Refer to Section I1 of these
Service Notes for details on suggested replacement
diodes.
400D-2C
L. CHECK EFFECT OF LINE VOLTAGE CHANGE
1) Connect a sine wave signal to the INPUT of
the
vtvm under
test,
U s e a convenient mid-range and
adjust the input signal amplitude for a convenient ref-
erence level such as 0.9. Use any frequency within
the
frequency range of the voltmeter. The line volt-
age should
be
at 115 volts for at least 2 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 &2% of full scale after a 2-min-
U t e
period.
These effects a r e more noticeable at the upper and
lower limits of the vtvm frequency range. If the change
is excessive, check tubes V5 through V 1 in that order,
M. FREQUENCY RESPONSE ADJUSTMENT
In most applications, frequency response adjust-
ments will not be necessary; particularly
if
the vtvm
is used at frequencies below approximately 500 kc.
The results obtained will depend entirely upon the
"reference voltmeter" used to monitor input signal
level to the vtvm under test. You a r e urged to avoid
making frequency response adjustments unless the fre-
quency response characteristics of your reference
voltmeter a r e accurately known. The following fre-
quency response adjustment data is given for use in
those cases where frequency response is critical and
the required
test
equipment is available.
The voltage at the input terminals of the vtvm under
test
must be monitored by the reference voltmeter,
The reference voltmeter indication at 400 cps is
called "reference level" in these instructions, The
adjustments a r e listed in a sequence that is most con-
venient when using the
@
Model 739AR and 200s in-
struments described in Section 1 1 1 . If you use other
equipment, you may have to change the sequence to
suit the instruments.
1) For Models 400D serial 001-34370 and below and
Models 400H/L serial 001-11226 and below:
Note: For Models 400D/H/L with serials prefixed
017-, proceed to step 2.
a. Adjust C21 at
4
mc on the .001 volt range after
establishing a 400 cps ''reference level" on the
same range.
b. Adjust response at 1 mc on the ,001-volt range.
Use the same ''reference level" as step a.
Capacitor C26 in the plate circuit of V4 can be
changed in value between limits of 15 and 82 pf to
control 1 mc response. If you change C26,
repeat step a.
c.
Frequency response above
4
mc can be roughly
adjusted by adding or adjusting the values of
capacitor C40 and resistor R82 in the cathode
circuit of V2.
00665-2