HP 400D Quick start guide

@ MODELS 400D, 400H, AND 400L
VACUUM TUBE VOLTMETERS
TEST, ADJUSTMENT, AND CERTIFICATTON METHODS
These Service Notes have been prepared as an aid in
testing and adjusting all
@ Model 400D, 400H, and
400L Vacuum Tube Voltmeters. The procedures that
follow supersede those in other ServiceNotes or Op-
erating and Servicing Manuals for these instruments.
All circuit references apply to one of the three ref-
erence schematic diagrams in these Service Notes.
No attempt should be made to make an instrument
agree completely
with any one of these schematics.
All recommended circuit changes are described in
Section
I of these Service Notes.
Voltages mentioned herein are measured between
the indicated points and the chassis
unless specified
otherwise. The voltmeter power supply should al-
ways be tested before undertaking repair and adjust-
ment of any other section of the instrument.
These Service Notes are divided into
five sections
as follows:
Section Page
I MODIFICATIONS 1
I1 TUBE & CRYSTAL DIODE REPLACEMENT 2
111 TEST EQUIPMENT REQUIRED 3
IV TEST & CALIBRATION PROCEDURE 3
A. Setting Mechanical in Models 400D
or 400H 3
B. Setting Mechanical Zero in Model 400L 3
C. Check Regulated Power Supply
4
D. Set dc Heater Voltage 4
E. Input Divider Circuit Identification 4
F. Input Circuit Preliminary Frequency
Response Adjustment
5
G. Basic Calibration (Setting Amplifier Gain) 5
H. Basic Calibration (Setting Input Divider) 5
I. Miscellaneous Checks 5
J. Recheck dc Heater Voltage 6
K. Check Calibration and Tracking 6
L. Check Effect of Line Voltage Change 6
M. Frequency Response Adjustment 6
V TESTS FOR INSTRUMENT CERTIFICATION 8
N. Final Check and Adjustment 7
SECTION I - MODIFICATIONS
Modifications in certain serials of ($9 Model 400D,
400H, and 400L Voltmeterscan simplifytesting.Speci-
fications have not been changed, Modernization of
instruments will simplify repair and testing and will
add to instrument reliability. The changes that follow
apply only to those 400D, 400H, and 400L instruments
having the specified serial numbers.
A. MODEL 400D SERIAL NO. 2221 AND BELOW
Change V2 plate load resistor R23 from an 8200
ohm,
*5%, 1 watt, composition resistor to a metallic
oxide film-type having thesameresistance, tolerance,
and wattage rating. This Corning Glass Works style
N25 resistor
is availableunder@StockNo. 0761-0001.
This change
is particularly recommended for those
instruments in which the residual noise reading is too
high and/or unstable. Mount the new resistor as
nearly
as
possible in the same location as the old
resistor.
‘b distribution abcfg
B. MODEL 400D SERIAL NO, 3944 AND BELOW
Add resistor R67 in
series with the control grid
of the
series regulator tube V7. Transfer the wire
(usually green) from V7 pin 1 to vacant pin 6 on the
sametube socket. Connect a 470 ohm
f lo%, 1/2 watt
composition resistor
(@ Stock No. 0687-4711)between
V7 socket pins 1 and 6.
This resistor prevents parasitic oscillation in the
power supply. Such oscillation can cause the meter
to be deflected
full scale against the right hand stop.
C. MODEL 400D SERIAL NO.
4344 AND BELOW I
Add 10 ohm *lo% 1 watt, composition resistor
(@ Stock No. 0690-1001) R68 in parallel with heater
voltage adjustment control R66. Thisresistor reduces
power dissipation in R66 and extends the
life of the
control. Readjust the dc heater voltage after
in-
stalling R68.
00665-4
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Page 2
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400D-2C
D. MODEL 400D SERIAL NO. 5130 AND BELOW
Transpose the connections to electrolytic capa-
citor C30
so that C30B is 20 pf and C3OC is 40 pf.
This transposition will reduce power supply ripple.
E. MODEL 400D SERIAL NO. 14770 AND BELOW
MODEL 400H
S ERIAL NO. 1137 AND BELOW
Replacement of control R29 or three-section resistor
R28 in the amplifier feedback
circuit will necessitate
revising and rewiring the feedback circuit.
The parts for this
circuit revision are supplied as a
kit with complete instructions under
@ Stock No.
400D-95A.
The revised feedback circuit is recommended for all
400D and 400H instruments whenever repair of the
feedback circuit
is necessary. The revised circuit
is superior to earlier circuitry but doesnot offer suf-
ficient advantages to warrant modification merely to
include the new circuit in an instrument. The problem
of adjusting frequency response following anychanges
in the feedback circuit would also make
it advisable
to avoid making unnecessary changes,
F. MODEL 400D SERIAL NO. 16371 AND ABOVE
The regulated power supply was changed. Series
regulator tube V7 was changed from
a 6AU5 toa
12B4A. Control tube V8 was changed from a 6CB6
to a 6U8 and reference tube V9 was changed from an
OB2 to a type 5651. The circuit differences can be
determined by referring to the schematic diagrams
included in these Service Notes.
Field modification to incorporate this change
is im-
practical and is not recommended.
SECTION II - TUBE &
Tubes with standard JEDEC characteristics can be
used for replacement.
In a great number of cases,
instrument trouble can betraced toadefectiveor weak
tube, Replace only those tubes proven to be defective
orweak. Check tubesby substitution. Results obtained
through the use of a “tube checker” canbe erroneous
and misleading. Mark original tubes to insure that
they are returned to the samesocket
if not replaced.
The power supply should be checked according to the
instructions in these Service Notes
after replacing
V6, V7, V8, orV9 in the power supply.
Heater voltage must be checked and adjustedafter
re-
placing V1, V2, V3, or V4. Factory selected tubes
are available for V1 and V2. These tubes aretested
for low noise and microphonics. Contactyour@ Sales
Office for the
@ Stock Numbers and prices on these
tubes.
G. MODEL 400H SERIAL NO. 1338 THROUGH 1537
Resistor R83 should be changed in value and
location. The
new value is 470,000 ohms *lo%, 1/2 ,‘-
watt instead of 100,000ohms. A second resistor with
a value of 470,000 ohms
&lo%, 1/2 watt and a sche-
matic
reference of R85 is connected in series with
R83. The
series combination of R83 and R85 is then
connected between the regulated B+ output atthecath-
ode of the regulator tube and chassis ground. The
transformer winding supplying heater voltage for V6,
V7, and V8
is connected to thejunction of R83 and R85.
These connections can be made by mounting both re-
sistors between terminals on the socket for V7.
4
H. MODEL 400D SERIAL NO. 17970 AND BELgW
MODEL 400H SERIAL NO. 2237 AND,BELaW
Residual noise as indicated by the meters in these
instruments can be reduced by a modification of the
input attenuator.
A higher degree of residual noise
reduction
is obtained on the upper voltmeter ranges
by this modification.
The input attenuator
is changedfroma capacitydivider
to a
resistive divider system by the addition of a kit
of parts available under
@ Stock No. 400D-95B. The
instructions supplied with thekit giveallnecessary de-
tails for changing the InputAttenuator ResistorBoard.
Basic calibration and frequency responseadjustments
will be required after this modification has been
completed.
IC
L/
I. MODEL 400D SERIAL NO. 25970 AND BELOW
MODEL 400H/L SERIAL NO. 5225 AND BELOW
The value of resistor R45 can bevaried to control low
frequency response
if cathode resistors R38, R47, and
R53 are changed from 120 ohms to 100ohms
&lo%,
1 watt.See SectionIV stepM1-d. Thevtvm performance
will be the same after changing the resistors but low
frequency response adjustments aresimplified.
CRYSTAL DIODE REPLACEMENT
Special high-performance junction type silicon diodes
areavailable for replacement of diodes CR1 andCR2.
These special diodes aremanufactured by
@ and are
available under
@ Stock No. G-29B-5. Orderdirectly
from the factory or from your
@ Sales Office.
The
@ Stock No, 212-1N82S diodes once specified for
CR1 and CR2 have been superseded by the superior
G-29B-5 diodes. The cathode end of the diode has
a
gold plated wire. The anode end of this diode has a
silver plated wire.
The G-29B-5 diodes have a junction that is less than
1/2 mil in diameter. If the diodes aredropped, a
mechanical failure may occur at the junction.
After
installation the diodes will withstand any shock that
the entire instrument can withstand.
Voltage calibration and frequency response should be
checked and adjusted after replacing
V1, V2, V3, V4,
V5, CR1, or CR2.
4
00665-3

*.
4OOD-2C Page 3
SECTION 111 - TEST EQUIPMENT REQUIRED
The following equipment is required to test and cali-
brate
@ Vacuum Tube Voltmeter Models 4WD, 400H,
and 400L.
A. A dc voltmeter capable of measuring 250, 6.3,
12.6 volts. This voltmeter musthave a
sensitivity
of 1000 ohms/volt or better. A vtvm such as the @
Model 410B is satisfactory,
B. An @ Model 738AR (formerly @ Spec. 23678)
Voltmeter Calibration Generator or equivalent.
Model 738AR generator will deliver a 400-cps output
voltage from 0.3 millivolt rms to 300 volts
rms with
an accuracy of &0.25%and not more than 0.2% hum
and distortion. This generator will also supply
+ dc
or -dc voltages from 0.3 millivolt to 300 volts for
calibration of dc voltmeters.
C. An
@ Model 739AR (formerly @ Spec. 23679)
VTVM Frequency Response Generator. Thisgen-
erator has an internal signal source to cover
the fre-
quency range from 350 kc to 11 mc.
Up to a 3-volt
output signal can be obtained. The internalattenuator
and reference voltmeter are designed for frequencies
from 10 cps to
11 mc. Internal impedance is 50 ohms.
D. An @ Model 200s (formerly @ Spec. 23680) Wide
Range Oscillator. This oscillator has aninternal
impedance of
50 ohms and will deliver a 3-volt signal
from 5 cps to 600 kc. Use
with Model 739AR VTVM
Frequency Response Generator.
b E. A battery operated oscillator covering 60 and 120
cps with an output of at least
1 volt and not more
than
1% distortion.
F.
A square wave generator capable of delivering a
2-volt peak-to-peak
1 kc signal, $The@Model
211A Square Wave Generator ortheinternal calibrator
in the @ Model 150A High Frequency Oscilloscopecan
be used.
G. A high frequency oscilloscope such as the @
Model 150A.
H.
The @ Model 739AR VTVM Frequency Response Gen-
erator and the @ Model 200s Wide Range Oscillator
aredesigned to
be operated together when ma
quency response measurements. These
tw
ments greatly increase the speed and accuracy of
frequency response measurement.
The frequency response adjustment procedure given
in Section
IV is particularly suited for use with
Model 739AR and Model 2005 instruments. Theinstru-
ments in the following list are suitable alternates.
-- A low distortion oscillator covering the frequency
range from 10 cps to at least 6 or 7 mc and an output
voltage of at least 3.0 volts rms. The
($9 Model 650A
Test Oscillator will meet these requirements,
-- An rms reading, average responding, acvoltmeter
with a known frequency response from 10 cps to at
least 4.0 mc. The reference voltmeter can be an
@ Model 400H or Model 400L. A thermocouple type of
reference meter can also be used. Absolute voltage
values arenot neededwhen makingfrequency response
measurements.
An adjustable line voltage source.
SECTION IV - TEST & CALIBRATION PROCEDURE
When all the following procedures aretobecompleted,
they can most easily be done in the order listed. In
many instances, only one or two tests will be needed
and they can be done without completing all other
tests. For example, basic calibration can be checked
and adjusted at any time without making any other
adjustments.
A 10 to 15-minute warmup and a check of the
power
supply output voltages is always recommended before
making any other tests or adjustments.
The specifications for your
@ Model 400D, 400H, or
400L Vacuum Tube Voltmeter are given
in the front
of the manual supplied
with the vtvm. The following
test procedure contains extra checks to help you
analyze a particular instrument. These extra checks
and the data they contain cannot be considered as
specifications.
NOTE: The test frequencies and the voltages specified
in many of the steps in this procedure are based upon
the use of
the test equipmentlisted in Section 111. Other
frequencies and voltages canbe used,
Any vtvm can be
adjusted for optimum performance on the range you
most commonly
use.
L
00665-2
A.
SETTING MECHANICAL ZERO IN MODELS400D
QQ
1) Allow 400D 20 minute warm up. Turn instrument
off.
2) Rotate mechanical zero-adjustment screw clock-
wise
until meter pointer is to left of zero and moving
upscale toward zero.
Continue to rotate adjustment
screw clockwise;
stop when pointer is right on zero, If pointer
overshoots zero, repeat
this adjustment.
3) When pointer is exactly on zero, rotateadjustment
screw approximately 15 degrees counterclockwise.
This
is enough to free adjustment screw from the
meter suspension,
If pointer moves during this
step repeat entire adjustment.
B. SETTING MECHANICAL ZERO IN MODEL 400L
The special meter
in the 400L is adjusted with
the vtvm turned on since the meter pointer rests
against the stop at the left end of the scale
when the
instrument
is turned off.

Page 4
I
In most 400L instruments the mechanical zero is pre-
set during manufacture of the meter movement, The
adjustment screw
in such meters is inoperative. Most
400L instruments will have no adjusting screw on the
front of the meter case.
The following procedure
is for use only with those
instruments
in which themechanical adjustment screw
is effective.
1) Connect a 1.0 volt rms, 400 cps signal from
the
Voltage Calibration Generator to the 400L INPUT
terminals. Set the 400L to the 1-voltrange and adjust
the feedback control (R29) for a
full scale meter
reading of 1.0 volt. Any convenient range could also
be used along
with a corresponding change in input
signal level. The input signal frequency
is also not
critical.
2) Switch
the Calibration Generatortodeliver a signal
of 0.3 volt, or 0.3 of the signal level used in step 1.
Set the meter mechanical zero adjusting screw to ob-
tain a reading that
is 0.3 of the reference reading in
step 1.
Use the same vtvm rangefor both steps 1 and
2. If you overshoot, continue to rotate the screw
clockwise and again approach the 0.3 point from the
high side of the scale, Thescrew should not be turned
counterclockwise during
this procedure.
3) Repeat steps
1 and 2 until no further improvement
can be noted. Compromiseadjustment of themechani-
cal zero can be made for optimum tracking.
After
final adjustment, cover the screw with a small piece
of black tape.
1
I
C. CHECK REGULATED POWER SUPPLY
1) The B+ voltage at the outputof the regulated power
supply should be 250 volts i5 volts. Measure this
voltage between the chassis and the cathode of the
series regulator tube
with the line voltage set to
115volts.
2) Vary the line voltage between 103 and 127 volts.
The regulated
B+ will usually change no more than
2 volts from one extreme to the other, Ripple volt-
age
in the regulated B+ is usually 3millivolts or less
under these same test conditions.
The dc voltage at the plate of the
series regulator
tube or at the cathode of the rectifier tube should be
between 400 and 420 volts with a 115-volt line, Low
voltage at this point may be from adefective rectifier
tube,
filter capacitor, or power transformer.
To check the rectifier tube,connectadcvoltmeter be-
tween chassis and the rectifier cathode. Reduce
line
voltage from 115 to 103volts. The voltmeter reading
should drop as line voltage
is reducedandthen remain
steady. If the dc voltage continues to drop at a slow
rate, the rectifier tube
is probably weak and should
be replaced.
If the regulated B+ voltage is not correct, replace-
ment of any or
all four tubes in the power supply may
be necessary.
If tube replacement does not correct
the dc voltage, precision resistors R62 and/or
R64
may have changed value and should be replaced. Ab-
normal current drain caused by component failure
elsewhere in the vtvm can result in poor power sup-
k
\ ply regulation,
400D-2C
Excessive ac ripple may be from a defective tube in
the power supply other than
the rectifier tube, Ca-
pacitor C36 may be open,
D. SET DC HEATER VOLTAGE
1) Adjust control R66 to
set the dc heater voltage for
tubes
V1 and V2 to 6.3 volts rt0.2 volt. The dc volt-
ages across
the heaters of tubes V3 and V4 must also
be 6.3 volts rt0.2 volt.
DO NOT USE A VOLTMETER
THAT IS CONNECTED TO
A COMMON GROUND WITH
THE VTVM UNDER TEST WHEN MEASURING THESE
HEATER VOLTAGES.
A SHORT CIRCUIT WILL
RESULT. Slight readjustment of control R66 will
often bring these two voltages
within tolerance.
2)
If both voltages cannotbe set to6.3volts, f 0.2 volt,
adjust R66 to bring the lowest of the two voltages
within tolerance. Connect a resistor
in arallel with
the two heaters with the highest voltage. )?he correct
resistor value
will enable you to bring bo* heater
voltages within tolerance by readjusting control R66.
Use only one resistor
in parallel with one pair of
tubes at a time.
A resistor value below 220 ohms
1 watt should not be used.
3)
If you are unable to obtain the correct heater
voltage aftercompleting steps 1 and 2, checkelectro-
lytic capacitors
in the heater circuit. If both heater
voltages are low, check selenium rectifier
SR1.
E. INPUT DIVIDER CIRCUIT IDENTIFICATION
1) An identification of the type of input circuit will
enable you to select the proper test required for your
instrument. Instruments are classified by type of
input circuit into two general groups:
a. Models 400D, serial001-34370and below;Models
400H/L, serial 001-11226 and below
, . , these
instruments may have either a “capacitive input
divider” or a
“resistive input divider”. Com-
plete steps
2 through 6 todeterminewhich one is
present.
b. Models 400D/H/L with serials prefixed 017- and
048-
. . , all these instruments employ a “re-
sistive divider”. Omit steps 2 through 6 and
proceed with Section IV-F.
2) Set the vtvm to the 1.0 volt range and connect a
1.0 volt rms, 400 cps signal from the Calibration
Generator to the INPUT terminals.
Any convenient
range higher than the 1-volt range can
be used. The
signal frequency should be approximately 400 cps.
3) Rotate capacitor C4 (on input circuit board) one
full turn while watching the vtvm meter reading. The
meter reading will vary only about
547, in some instru-
ments and approximately 5v0 in others.
4)
If the meter indication varies approximately 5vo,
capacitor C4 is a Voltage Calibration Adjustment,
This will be referred to as a “CAPACITIVE INPUT
DIVIDER” circuit in the balance of the procedures
in these Service Notes.
00665-3

400D-2C Page 5
5) If the meter indication varies approximately 547,,
capacitor C4 is a Frequency Response Adjustment.
This
will be referred to as a “RESISTIVE INPUT
DIVIDER” circuit in the balance of the procedures
in these ServiceNotes.
i/
6) Mark the type of input circuit on the input circuit
shield, This reference will be used in later steps
of this Test Procedure.
F. INPUT CIRCUIT PRELIMINARY FREQUENCY
’RESPONSE ADJ USTMENT
2) For Models 400D/H/L with serials prefixed 017-:
a. Connect a
1.0 volt rms, 400 cps, signal from
Voltmeter Calibration Generator to INPUT of
vtvm. Switch vtvm to
1.0 volt range.
b. Adjust
RlOl for a vtvm reading of 1.0 volt.
I. MISCELLANEOUS CHECKS
NOTE:
All checks in this section aremade with
the vtvm cabinet in place.
This adjustment
is for “RESISTIVE INPUTDIVIDER”
circuits only:
1) Connect a 1 kc, 2 volt peak-to-peak, square wave
signal to the INPUT
of the vtvm under test. Connect
the OUTPUT terminalsof the samevtvm tothe vertical
input of a high frequency oscilloscope such as the
@ Model 150A.
2) Switch the vtvm to the
1.0 volt range and adjust
capacitor C4 for the best square wave pattern on the
oscilloscope.
G. BASIC CALIBRATION (SETTING AMPLIFIER
GAIN)
1) For Models 400D serial 001-34370 and below and
Models 400H/L serial 001-11226 and below:
Connect a 0.3 volt rms, 400 cps signal from the Cali-
bration Generator to vtvm INPUT. Switch the vtvm
to 0.3 volt range and adjust the feedback control,
R29, to obtain a reading of
0.3 volt.
2) For Models 400D/H/L with serialsprefixed 017-:
Connect a 0.001 volt rms, 400 cps signal from the
Calibration Generator to vtvm INPUT. Switch the
vtvm to the 0.001 volt range and adjust the feedback
control, R107, to obtain a reading of 0.001 volt.
i/
H. BASICCALIBRATION (SETTING INPUT DIVIDER)
1) For Models 400D serial 001-34370 and below and
Models 400H/L serial 001-11226 and below:
a. Connect a 1.0 volt rms, 400 cps, signal from
Voltmeter Calibration Generator to INPUT of
vtvm. Switch vtvm to
1.0 volt range.
b. For “CAPACITIVE INPUT DIVIDER” circuits,
adjust C4 for a reading of 1.0 volt.
c. For “RESISTIVE INPUT DIVIDER” circuits, the
vtvm indication
will normally be 1.0 volt. If
reading is high, decrease value of pad (R87) in
L/ parallel with R3. If reading is low, increase
value of pad (R87). Resistor R3andpad R87 are
on input circuit board.
00665-2
1) Noise and Switching Transients
- This check will
vary with the voltmeter model. Models 400Dand 400H
have one method while the400Lhasa different method.
a. For 400D and 400H VOLTMETERS
- Plug a
shielded connector ((8 Stock No. 1251-0019)over
the INPUT terhinals of the vtvm under
test,
A lO0,OQO ohm resistor should be connected be-
tween the terminals inside this shielded plug.
The residual reading on the 0.001 volt scale
will
normally not be more than 0.00005 volt (5% of
full scale).
b. For 400L VOLTMETER
- Connect a 0.0003 volt
rrns (0.3 millivolt) signal of about 400 cps from
the Calibration Generator
to the INPUT termi-
nals, The voltmeter reading should be steady
with the range switch inthe0.001position. Noise
in
V1 or V2 will cause this reading to vary
erratically.
c. Connect the 100,000 ohm shielded resistor (see
step a) between the INPUT termirials. Rotate
the range switch through all 12 positions in both
directions. The meter needle may be momen-
tarily deflew as high as
full scale between
some switch positions. Themeterpointershould
always drop back to zero at a uniform rate with-
out pause or a sudden change in the rate of fall.
d. Remove the shielded 100,000 ohm resistor and
connect a
0.001 volt rms,400cps signalfrom the
Calibration Generator to the INPUT terminals,
The voltmeter reading should be steady with the
range switch in the 0.001 position.
If the results obtained in this
test arenot satisfactory,
check tubes
V1 and V2, all coupling capacitors, elec-
trolytic capacitors in screen and plate circuits, V2
plate load resistor, and cathode bypass capacitors.
a. Connect a 0.001 volt rms, acsignaltothe INPUT
terminals and set the vtvm to the 0,001 volt
range. Hold your hand palm-upward and sharply
rap the front of themetercasewithyour knuckle.
bJ
The pointer will usually not move more than,.
about 1/8 inch along the meter scale.
b. Repeat step a at 300 volts on the300-volt range.
The results obtained should be the same.
2) Check for Microphonics

Page 6
Replace V1 and/or V2 to remove excessive micro-
phonics. Pre-selected tubes are 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 120Cycles
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 frequencytoobtain
maximum meter swing which will normally not
be more than about 1/8
in. alongthemeter scale.
b. Repeat
step a at 120 cps. The maximum meter
swing will usually not be more thanabout 1/8
in.
along the meter scale.
High beat notes areusually 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 eachvoltmeter range.
Use the Voltmeter Calibration Generator. The cali-
bration should be
within *2% of full scale for a 400D
Voltmeter or ilg 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 trackingon anymid-range. The 1-volt
range
is usually convenient. A stickymetercan 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 I1of 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, Use 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 115volts foratleast 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 changefrom the reference
level by more than &2%of full scale after a 2-min-
Ute period.
These
effects are more noticeable at the upper and
lower limits of thevtvm frequencyrange.
If the change
is excessive, check tubes V5 through V1 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 are urged to avoid
making frequency response adjustmentsunless the
fre-
quency response characteristics of your reference
voltmeter are 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 arelisted in a sequence that is most con-
venient when using the @ Model 739AR and 200s in-
struments described in Section 111. 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 15and 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

400D-2C
If you change or add these parts, repeat steps a and b.
Use same range and reference level as step a. A re-
sponse that “falls-off’’ above 4 mc is more desirable
than one that rises.
d. The 10 to 20 cps.responseadjustment varies with
voltmeter circuitry.
If cathode resistors R38, R47, and R53 for tubes V3,
V4, and V5 respectively all have values of 100 ohms,
the value of R45
in the plate circuit of V4 can be ad-
justed between limits of 0.75 and
1.0 megohms to
adjust the 10 to 20 cps response.
In any instrument, to increase the 10cps indication,
remove R80 if present in V2 grid circuit. Add R81 in
V3 grid circuit
if additional increase is needed after
R80 removal. Conversely, to decrease the indication,
remove R81 and add R80
if necessary. Either but not
both resistors can be added. Both resistors can be
omitted.
A value of 2.2 megohms or higher should be
used for R80 or R81. Check tubes, cathode capacitor
C20, and coupling capacitors
if you findthat a resistor
value below 2.2 megohms is required.
e. Adjust C14 at
4 mc on the .003-volt range after
establishing a 400 cps “reference level” on
the
same range.
f. Adjust C16 at 4 mc on the .Ol-volt range after
establishing a 400 cps “reference level” on
the
same range.
g. Check frequency response on the .03, .l, and .3
volt ranges. Establish a 400 cps “reference
level” for each range and then checkresponse at
4 mc. If an error greater than f 5% of full scale
is noted, make a compromise adjustment of
C21.
If you change C21, repeat steps a, b, c,
d, e, and
f. The compromise setting of C21
must be used when repeating
step a.
h. For “CAPACITIVE INPUT DIVIDER” Circuits
See step
E in Section IV to de-
Skip
step i.
termine input circuit type.
i. For “RESISTIVE INPUT DIVIDER” Circuits
Adiust C4 at 20 kc on the
1.0 volt range after
esiablishing a 400 cps “reference level” on
the
same range.
j. Adjust R6 in the input divider to bring the 4 mc
response
within specifications on the 1 .O-volt
range after establishing a 400 cps “reference
level” on the same range.
This resistor consists of several parallel resistors.
Increasing the resistance of R6 will raisethe response
at
4 mc. Lowering the resistance will decrease the
4 mc response.
00665-2
Page 7
k. The response of the 3-volt range can now be
checked.
If other ranges have been properlyad-
justed, this range
will be within specifications.
The response of any range can now be checked
at any frequency between 10 cps and
4 mc.
2) For Models 400D/H/L with serials prefixed 017-:
a. Adjust C102
“HIGH FREQUENCY COMP” at
4 mc on the 0,001 volt range afterestablishing a
400 cps “reference level” on the same range.
b. The oscillator should now be tuned through the
band of
4 mc to 10 mc. Note the voltmeter indi-
cation during this time;
If the readingincreases
more than
2% above the 4 mc level, adjust AM-
PLIFIER BIAS control R119 counterclockwise,
and repeat
4 mc ad‘ust (step a above). Repeat
the
4 mc response. Note: Be sure reference
meter
is set at SET LEVEL.
this procedure unti II peaking is less than 2% of
c. Set oscillatoroutput to“reference level”. Adjust
LOW FREQUENCY RESPONSE control R118 to
bring the 10 to
20 cps response within specified
limits. If R118 will not bring both the 10 and 20
cycle response into specifications, the AMPLI-
FIER BIAS control R119 must be readjusted
so
that R118 can bring the 10 and20cycle response
within limits.
d. If R119
is adjusted during the low frequency re-
sponse calibration (step c),
it will be necessary
to repeat the high frequency response adjust-
ment in step b. The adjustment of R119 will be
a compromise to meet both low and high
fre-
quency specifications.
e. Adjust C14 at 4 mc on the 0.003 volt range after
establishing a 400 cps reference level
on the
same range.
f. Adjust C16 at 4 mc on the .01 volt range after
establishing a
400 cps reference level on the
same range.
g. Adjust C4 at 20 kc on the 1.0 volt range after
establishing a 400
cps reference level on the
same range.
h. Adjust R6 in the
input divider to bring the 4 mc
response
within specifications on the 1.0 volt
range after establishing a 400 cps reference
level on the same range.
R6 consists of several
resistors connected
in parallel. Increasing the
sponse at
4 mc. Lowering the resistance will
decrease the
4 mc response. Note: Be sure
input shield
is in place before taking reading.
resistance of R6
will raise the frequency re-
\
N. FINAL CHECK AND ADJUSTMENT
in steps
G and H.
Repeat the Basic Calibration procedures given

Page 8 400D-2C ' ' .
SECTION V - TEST FOR INSTRUMENT CERTIFICATION
To certify a 400D,400H, or 400L Vacuum Tube Volt-
meter
it is only necessary to check the accuracy
specifications, These specifications aregiven in the
Operating and Servicing Manual supplied with your
vtvm.
Certification
is accomplished without removing the
instrument cabinet and without making anyinstrument
adjustments, Suggested procedures for checking the
accuracy specifications are given in Section
IV of
these Service Notes in steps K, L, and M.
The procedures in steps K, L, and M contain adjust-
ment methods, These adjustments must
be omitted
when certifying an instrument. Actually, if aninstru-
ment meets specifications no adjustments will be
necessary.
d
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Page 9
W
la

Page 10
2 . 'i
400D-2C
"
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400D-2C
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I I 1 ..-
4
No no e0 nO aY
00665 - 2