HP 400D Owner's manual

OPERATING AND SE RVICING MANUAL
FOR
MODEL 40OD/H
VACUUM TUBE VOLTMETER
MODEL 4OOD
Serial 179?1 and above MODEL 4OOH
Serial 2238 and above
Copyright HEWLETT-PACKARD COMPANY 1955
275 PAGE MILL ROAD, PALO ALTO, CALIFORNIA, U.S.A.
400DH001-1

SPECIFICAT!ONS
MODEL 4OOD
VOLTAGE RANGE:
0.1 millivolt to 300 volts.Twelve ranges
selected with front Panel switch.
FulI scale readings of:
0.001
0.003
0.010
0.030
0.100
0. 300
1.000
3.000
10.00
30.00
100.0
300.0
DECIBEL RANGE:
-72 lo +52 db, in 12 ranges.
FREQUENCY RANGE:
10 cps lo 4 mc/s.
ACCURACY:
With line voltage of 115 volts, t1ffi
(103 to 127 volts), overall accuracy is
within:
x21o of full scale value, 20 cps to 1 mcl
x37o of. full scale value, 20 cps to 2 mcl
+\Voof. full scale value, 10 cps to 4 mc.
LONG TERM STABILITY:
Reduction in Gm of amplifier tubes to 7570
of nominal value results in error of less
tfian U.\Eo, 20 cps to 1 mc.
CALIBRATION:
Reads rms value of sine wave. Voltage
indication proportional to average value
of applied wave. Linear voltage scales,
0 to 3.0 and 0 to 1.0; db scale, -12 db to
+2 db, based on 0 dbm = 1 mw in 600
ohmsl 10-db intervals between ranges.
MODEL 4OOH
0. 1 millivolt to 300 volts. Twelve ranges
selected with front Panel switch.
Full scale readings of:
0.001
0.003
0.010
0.030
0.100
0. 300
1.000
3.000
10.00
30.00
100. 0
300.0
-72 to +52 db, in 12 ranges.
10 cps to 4 mc/s.
With line voltage of 115 volts, +10/6
(103 to 12? volts), overall accuracy is
witfiin:
tt%of. full scale value, 50 cps to 500 kc;
nflo d full scale value, 20 cPs to 1 mc;
+Wo of full scale value, 10 cps to 4 mc.
Reduction in Gm of amplifier tubes to 757c
of nominaL value results in error of less
than 0.\Va, 20 cps to 1 mc.
Reads rms value of sine wave. Voltage
indication proportional to average value
of applied wave. Linear voltage scales,
0 to 3.0 and 0 to 1.0; db scale, -12 db to
+2 db, based on 0 dbm = 1 mw in 600
ohmsl 10-db intervals between ranges.

SPECtFICATTONS (CONT'D.)
MODEL 4OOD
INPUT MPEDANCE:
10 megohms shunted by 15 p1"rf on ranges
1.0 to 300 volts; 25 pt on ranges 0.001
volt to 0.3 volt.
AMPLIFIER:
Output terminals are provided so volt-
meter can be used to arnplify srnall sig-
nals or to inonitor waveforms under
test with an oscilloscope. Output ap-
proximately 0. 15 volt rms on all ranges
with full-scale meter deflection. Aarpli-
fier frequency response sarne as that
of voltmeter. Internal impedance approx-
imately 50 ohms over entire frequency
range.
POWER SUFFLY:
tl5/230 volts, + l}Eo, 5Q/1000 cps, ap-
proximately 70 watts.
SIZE:
Cabinet Mount - t1-l/2 inches iligh,
- 7 -L/2 inches wide,
- 12 inches deep.
Rack Mount - 7 inches high,
- 19 inches wide,
- ll-3/4 inches deep.
WEIGHT:
Cabinet Mount - 18 lbs.; shipping .veight
approx. 25 lbs.
Rack Mount - 22Lbs.; shipping weight
approx. 35 lcs.
ALSO EVAIL.TIBLE:
@vloaet 400D-db, with a special meter
face having the db meter scale uppermost
to permit greater resolution in db read-
ings.
MODEL 4OOH
10 megohms shunted by L5 gfi. on ranges
1.0 to 300 volts; 25 ptrrf on ranges 0.001
volt to 0.3 volt.
Output terminals are provided so volt-
meter can be used to amplify small sig-
nals or to monitor waveforms under
test with an oscilloscope. Output ap-
proximately 0. 15 volt rms on all ranges
with full-scale meter deflection. Ampli-
fier frequency response same as that
of voltmeter. Internal impedance approx-
imately b0 ohms over entire frequency
range.
lL5/230 volts, r1ffi, 50/1000 cps, ap-
proximately 70 watts.
Cabinet Mount - ll-l/2 inches high,
- 7-l/2 inches wide,
- L2 inches deep.
Rack Mount - 7 inches high,
- 19 inches wide,
- lL-3/4 inches deep.
Cabinet Mount - 18 lbs.; shipping weight
approx. 25 lbs.
Rack Moturt - 22 Los.; shipping weight
approx. 35 lbs.
,iE tvioAet 400H-db, with a special meter
face having the db meter scale uppermost
to permit greater resolution in db read-
ings.
;
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(, CONTENTS
SECTION IGENERAL DESCRIPTION Page
-1
-1
-1
II-1
II-1
1-1
t-2
1-3
t-4
1-5
General Description
Damage in Transit
Power Line Voltage
Power Cable
Accessories -t
-2
SECTION II OPERATING INSTRUCTIONS
2-l
2-2 Controls and Terminals .
Operation
3-1
3-2
3-3
3-4
i* SECTION III THEORY OF OPERATION
SECTION IV MAINTENANCE
TABLE OF REPLACEABLE PARTS
General
Input Voltage Divider
Amplifier, Rectifier and Meter
Power Supply
Introductory .
Meter Zero Adjustment
Case Removal
Capacitor Replacement
Replacement of Crystal Diodes
Tube Replacement
Adjustments
Trouble Shooting
Power-Supply Localization Checks
Voltmeter Localization Checks
m
trI
III
UI
-1
-1
-2
-4
I
4-1
4-2
4-3
4-4
4-5
4-6
4-7
4-8
4-9
4-10
ry
ff
w
ry
ff
ry
w
ry
ff
ry
-1
-1
-1
q
-5
-7
-10
- 11
SECTION V
5-1 Table of Replaceable Parts v-1

CAUTION
The ground terminal (G) of the INPUT and OUTPUT binding posts, the instru-
ment chassis and cabinet, and the third (green) grounding conductor in the
three -conductor power cable are electrically connected together at aII times.
When the NEMA connector is used in the proper manner, a ground path is
established between the 400D/H and equipment which is also grounded. Do not
connect the ground terminal of the instrument to any point which is not at
ground potential, or a short circuit wiII be created. If such a measurement
is necessary, disconnect the NEMA ground in the power cable by using an
adapter with the grounding pig -tail removed, and the cabinet insulated from
ground. The 400D/n cabinet wiII be at the same potential as the ground Iead.
Caution must be used if the clip lead is connected to a point which is more
than a few volts off ground. Because of the potential hazard to personnel,
this method is not recommended.
One side of almost all power distribution systems is grounded. Extreme cau-
tion must be used if direct measurement of power system voltages is attempted.
If the ground Iead is accidentally connected to the ungrounded side of the Iine,
severe damage to the 40 OD/H is possible because of the short circuit created.
Fower line voltages can be safely measured by using the ungrounded (upper)
terminal only. Contacting the grounded power conductor will give a reading
of 0 volts, while contacting the ungrounded lead wiII give full line voltage
reading.
When very low level voltages are being measured, there is a possibility of
error caused by current flowing in the equipment grounding conductor and the
ground signal input conductor. This current flow will cause a voltage to be
developed in the connecting leads which is in series with the signal under
measurement and wilt be read by the meter. This source of error can be re-
moved by eliminating the ground toop between the meter and the circuit under
test.
I'
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Sect. I Page 1
stcTr0lr r
o
I.I GENERAL DESCRIPTION
Throughout this manual, where reference is made
to the 400D the information applies equally to the
400H. The major differences between the two in-
struments are tolerance refinements and a more
accurate meter movement in the Model 400H.
The Model 400D Vacuum Tube Voltmeter is an accu-
rate ard sensitive average-req)onding rms calibrated
voltmeter which will measure a-c voltages from 0.001
volt full scale to 300 volts full scale over a frequency
range of 10 cycles to 4 megacycles. It has an input
impedance of 10 megohms, effectively preventing
disturbance to circuits under test. The distinctive
features of this voltmeter make it valuable for mea-
suring.gain, network response, and ou@ut level witl
speed and accuracy. The wide frequency range makes
it suitable for audio, r-f, and video measurements.
The sensitivity of t}re voltmeter is sufficient in many
instances to measure hum and noise level directly.
The 400D voltmeter also may be used as an audio
level (VU) meterl as a high-gain broadband ampli-
fier to give increased sensitivity to oscilloscopes,
bridges, and other equipment requiring additional
o
Table 1-1. Ranges
Switch
Designator Voltage
Range Decibel.
Range
-60 .0001
-50 .003
-40 .01
-30 .03
-20 .1
-10 .3
10
3 +10
10 +20
30 +30
100 +40
300 +50
0 to 0.001
0 to 0.003
o to 0.01
0 to 0.03
0to0.1
0to0.3
0to1
0to3
0to10
0to30
0 to 100
0 to 300
-72 to -58
-62 to -48
-52 to -38
-42 to -28
-32 to -18
-22to - I
-l2to+ 2
- 2to+12
+ 8to+22
+18 to +32
+28 to +42
+38 to +52
GEl{TRAT DTSCRIPTIO]{
sensitivity; and to detect nulls. In conjunction with
an oscilloscope, the voltmeter may be used to monitor
waveforms, and in conjunction with an oscillator,
may be used to measure wide ranges of L and C as
well as moderate ranges of R and Z. The 400D also
may be used as an indicating device to measure
coil Q.
T.2 DAMAGE IN TRANSIT
If upon initial inspection this instrument is found
to be damaged in any way, refer to CLAIM FOR
DAMAGE IN SHIPMENT for the necessaly instruc-
tions. (See last page in instruction manual. )
I.3 POWER LINE YOTTAGE
When the instrument leaves the factory normally
it is wired for 115-volt operation. ff it is desired to
operate the instrument from a 230-volt source, refer
to the transformer conversion data on the schematic
diagram for the correct procedure.
As shown on the schematic diagram, the alteration
changes the primary windings of the transformer
from a parallel (for 115-volt input) to a series (for
230-volt input) arrangement.
I.4 POWER CABTE
The three conductor power cable supplied with this
instrument is terminated in a polarized three-prong
male connector recommended by the National elec-
trical Manufacturersr Association. The third con-
tact is an offset round pin, added to a standard two-
blade connector, which grounds the instrument chas-
sis when used with the appropriate receptacle. To use
this NEMA connector in a standard two-contact re-
ceptacle, a three-prong adapter should be used.
The ground connection emerges from the adapter
as a short lead which should be connected to a
grounded receptacle box for the protection of oper-
ating personnel.

Sect.I Page 2
r.5 ACCESSOR.IES
Accessories available for use with the Model 400D
Vacuum Tube Voltmeter are listed below. These
accessories are not supptied with the instrument
but may be purchased from the Hewlett-Packard
Company.
Model AC-60A Line Matching Transformer -
t to 600
otrm unUatanced output, for measurements on bal-
anced lines.
Terminating Resistance:
600 ohms or 10,000 ohms
FrequencY Range:
5 kc to 600 kc
Power Handling CaPacitY:
+22 dbm (10 volts at 600 ohms)
Balance:
Better than 40 db, entire frequency range
Model AC-60B Line Bridging Transformer -
unbal-
anced output, for measurements on balanced lines'
Terminating Resistance:
600 ohms or 10,000 ohms
Frequency Range:
20 cps to 45 kc
Power Handling CaPacitY:
+15 dbm (4.5 volts at 600 ohms)
MoCel 452A Capacitive Voltage Divider -
Division ratio: 1000:1
Model 4StCapacitive Voltage Divider -
Division ratio: 100:1
Accuracy: +370
Frequency range: 20 cps to 4 mc
Input Impedance: 50 megohms shunted with 2' 75 u4u.f'
Maximum Voltage: 1500 volts
Model 4?0A thru 470F Shunt Resistors -
These shunt resistors adapt the Model 400D for
measuring currents as small as 1 microamp full
scale.
Accuracy: +l\o lo 100 kc, 470A-F
+SVo to 1 mc, 470A
+}Vo to 4 mc, 4708-F
Maximum Power
dissipation: 1 watt
Accuracy:
Input caPacitY:
Maximum volt-
age rating:
xflo
15 gi., +1 grf
60 cps 25 kv
1mc 20kv
20 mc ?kv
100 kc 221r.t
10 mc 15 kv
Model
a70r
470B
470C
470D
4?08
470F
Shunt Resistance
--T'.I-oIms -
1. 0 ohms
10.0 ohms
100.0 ohms
600.0 ohms
1000.0 ohms

Sect.II Page 1
sEcTl0il il
i
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2.1 CONTROLS AND TERMINALS
oN-
T-his toggle switch closes the Iine voltage to the
power-supply transformer. With the switch at ON,
the red indicator lamp will glow as soon as the power
transformer is energized.
DB VOLTS
TE'G wmFtype rotary switch connects the proper
multiplier resistors and capacitors into the circuit
for the desired voltage range. The position of the
switch indicates 1) the full scale voltage of ttre range
in use and 2) the db level, when the m d.er pointer
is at zero on the DECIBELS scale. Lirnits of each
range are shown in Table 1-1.
INPUT AND OUTPUT TERMINALSPUT are con-
nected to the input circuit of the instrument. The
two binding posts designated OUTPUT are the output
terminals for the amplifier. The lower binding post
in each pair, designated G is connected to tfie Chas-
sis. The binding posts will accommodate either a
banana plug or wire, and are so arranged that any
double banana plug with a 3/4 Lnch spacing may be
used.
CAUTION
The maximum voltage (the sum of the d-c voltage
and a-c peak voltage) applied to the input terminals
of the Model 400D Vacuum Tube voltmeter must not
erSggd_qlqJglts. Higher voltages will break down
the capacitors in the input system of the instru-
ment.
METER
mA ,i00-D meter is a d-c milliammeter calibrated to
indicate the rms value of a sine wave.
FUSE
if[E-fuseholder, Iocated on the back of the instru-
ment, contains a l-ampere cartridge fuse which is
OPERATII{G I}ISTRUGTIO}IS
in the power supply input circuit. Replacement fuses
must be of the rtSlo-Blot' type as specifiedin the
Table of Replaceable Parts.
NOTE
When the power transformer is connected for 230-
volt operation, use a l/Z-ampere " Slo-Blo, car-
tridge fuse.
2.2 OPERATION
When the Model 400D is received from the factory,
the meter pointer should indicate zero before the
instrument is turned ol. If it does not, adjust the
pointer to zero as explained under Section IV,
paragraph 4-2. After the instrument is turned on,
the meter pointer may show an indication of as much
as two scale divisions, principally on the one-milli-
volt range. This effect is normal and does not im-
pair the accuracy of the instrument.
On the lowest three ranges of the instrument the
high input impedance coupled with the gain of the
amplifi.er causes the meter needle to be forced
against the right-hand stop of the meter when the
input terminals are unshielded. This condition is
normal and is caused by stray voltages in the vicinity
of the instrument.
If measurements are made from a high-impedance
source, hum pick-up can affect the meter indication
because of the high impedance of both the source and
the Model 400D. Shielded leads wiil reduce pick-up,
although they wiil cause an increase in the capacity
shunted across the source, with the possibility of
excessive circuit loading.
a. VoltageMeasurements
rct ptugged intoapower
source of specified voltage and frequency, and the
toggle switch at ON, allow the instrument about
five minutes to reach a state of stable operation.
o

Sect.II Page 2
=
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oo
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r -E
LrJ
rooo loK
I M PE DA NCE - OHMS
-25 t
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Figure 2-1. Model 400D/H Impedance Correction Graph

2) Set tlle DB VOLTS (range) switch to the desired
voltage range.
3) Connect the voltage being measured to the INPUT
binding posts.
4) The value of voltage being meaeured will be in-
dicated on one of two ecaleg: 0-to-1 volt or 0-to-3
volts. Wh6;-the-?tnge used contains the integer
tr ltt , th€ value of the voltage being measured will
be indicated on the l-volt scale: when the range used
contains the integer ,3tt , th€ value of the voltage witl
be lndicated on the 3-volt scale. Each range swltch
posltion shows the maximum voltage that can be
measured on the appropriate meter scale.
PRECAUTION
In order to maintain accuracy of measurement, it
must be kept in mind that the instrument is an aver-
age-responding device but that the meter is cali-
brated in terms of the rms voltage of a pure sine
wave. If the waveform of the voltage being mea-
sured contains appreciable harmonic voltages or
other spurious voltages, the meter indication will
deviate from the true rms value on the order in-
dicated by Table 2-1.
b. Decibel Measurements -
ffis of decibels are made
in the same way as voltage measurements except
that the indication is read on the db scale (-12 to
+2 decibels). The level in decibels is the algebraic
sum of the m eter db-scale indieation and the DB
VOLTS (range) switch position.
Sect.II Page 3
1) To read power directly in dbm (0 dbm = 1 milli-
watt into 600 ohms), the m@surement must be made
across 600 ohms.
2) Comparative db measurements (without respect
to the reference level) may be obtained by direct
reading provided each measurement is made across
the same value of impedance. Made in this manner,
the dilference in decibels between two or more mea-
surements may be obtained directly from the db-
scale indications.
3) To obtain tfie level in dbm with respect to im-
pedances other than 600-ohms, tfie meter correction
graph shown in Figure 2-1 may be used. The level
in dbm will be the algebraic sum of the level as in-
dicated on the meter and the correction shown on the
graph. For orample, if the range switch is at the +30
db position, the measurement made across g0 ohms,
and tfie indication on the DECIBELS scale +1, the
level in dbm is obtained as follows:
+ 1 (db scale indication)
+30 (range switch position)
;gT (level in db as indicated by meter)
+ 8 (correction for 90 ohms impedance)
;59 dbm
For tfie same conditions, with the measurement
made across 60,000 ohms:
+ 1 (db scale indication)
+30 (range switch position)
;51 (levet in db as indicated by meter)
-20 (correction for 60,000 ohms impedance)
;TI dbm
Table 2-1. Effect of Harmonics on Model 400D/H Voltage Measurements
Input Voltage Characteristics True
RI\II'S Value Value Indicated by Model400D
Fundamental = 100
Fundamental +LlEo 2nd harmonic
Fundamental +20V0 2nd harmonic
Fundamental +50Vo 2nd harmonic
Fundamental +llflo 3rd harmonic
Fundamental +20Vo 3rd harmonic
Fundamental +5070 3rd harmonic
100
100. 5
102
112
100. 5
102
tt2
100
100
100-102
100-110
96- 104
94-108
90- 116

EXAMPLES:
Range
Switch Meter
Scale Meter Reads Actual
Volts
VOLTAGE tdEASUREMENT
1.8
0.44
2.3
.27
+ 2db
- ?db
- 6db
0db
- 8db
-11 db
- 1db
180
44
0023
0002?
+42 db
+33 db
+ 4db
-30 db
-38 db
-61 db
-61 db
300
10 .003
.001
DB MEASU]
3
1
3
1
TEMENT
*
+40 db
+40 db
+10 db
-30 db
-30 db
-50 db
-60 db
,t
db
db
db
db
db
db
db
* NOTE:ln case where a meter scale reading below
-8 db is obtained, it is best to switch to the
nel* tronrer range on tfie instmment so a reading will
be obtained in the upper portion of t}te scale where
highest accuracy may be obtained.
Sect. II Page 4
The same situation exists for voltage measurements.
Wtren a reading is obtained in the lower 1/3 scale, the
range switch should be switched to the next lower
range to obtain a reading in the upper 2/3 scale.
c. Amplifier
The Model 400D rnay be used to amplify small signals.
ilith full-scale meter deflection, the amplifier open-
circuit output is approximately .15 volt rms on all
ranges. The response of the amplifier is flat across
the band from 10 cycles to 4 mc. The impedance,
looking into the OUIPUT terminals, is approximately
50 ohms over the entire frequency range. To obtain
maximum gain, proceed as follows:
1) Turn the toggle switch to ON, and allow a warm-
up period of approximately five minutes.
2) Set the DB VOLTS (range) switch at the .001-volt
position.
3) Cormect to the OIITPUT btuding posts the equipment
which is to receive tlte amplified signal. When the
impedance of the load across t}te amplifier output is
approximately 50 ohms, the output voltage will be
approximately half t}re open-circuit voltage, or ap-
proximately .0?5 volt.
4) Connect the voltage to be amplified to tfie INPUT
binding posts. Up to .002 volt may be applied to the
amplifier with the range switch at the .001-volt
position.
NOTE
Amplification also may be obtained on the .003,
.01, . 03, and . I volt ranges, but maximum gain
is available only on the .001 volt range because of
the 10 db loss per step inserted by the DB VOLTS
switch as the switch is turned in a clockwise di-
rection.

Sect.III Page 1
$ECTI0]{ ilt
3.I GENERAL
The circuit of the Model 400D includes a two-step
voltage divider in the input, a stabilized broadband
amplifier, a rectifier and meter circuit, and a
regulated power supply. Arrangement of the circuit
is shown in block diagram form in Figure 3-1, and
in detail on the schematic diagram.
The voltage under measurement is applied to the
voltmeter at the input terminals. On the lower ranges
the voltage is corpled direcily to the grid of a catlrode
follower, the cathode of which is connected as a volt-
age divider tapped for six outputs. On the higher
ranges the voltage is reduced to a thousandth of its
value at INPUT before it is applied to the grid of
the cathode follower. Out of the six-tap divider t}te
voltage is coupled to a four-stage amplifier. The
output of the amplifier feeds into a full-wave recti-
fier bridge with a d-c milliammeter across its mid-
points, and a current proportional to the input volt-
age flows through the meter movement. The meter
is so calibrated that the resulting deflection indicates
NPUT
o
IOUTPUT
?
iI
VOLTAGE
REGULATOR
v7
T}IEORY OT OPERATIOI{
the rms value of a sine-wave voltage applied at
INPUT.
The circuit of the 400D voltmeter is discussed in
greater detail in tfie following paragraphs.
3.2 INPUT YOTTAGE DIVIDER
The input voltage divider consists of a 1000:1 re-
sistive voltage divider feeding into the grid of tube
V1, and a six step resistance voltage divider feeding
into the first stage (V2) of ttte amplifier. The 6 step
voltage divider is connected into the cathode circuit
of V1, which is arranged as a cathode follower. Con-
nections to both resistive dividers are set up by the
DB VOLTS switch, by means of which the circuit
may be arranged to operate on any one of twelve
ranges.
a, 1000:1 Voltage Divider -
@nge) switch on any one
of the six lower ranges (.001 volt to . 3 volt), section
VOLTAGE
DIVI DER
vt
AMPLIFIER
v2J5,V4,V5
RECTIFIER
AND
METER
POIYER
SUPPLY
V6
50 -lOOOar
Figure 3-1. Model 400D/H Block Diagram

Sect.I[ Page 2
S1A of the range switch sets up connections in such a
manner that the input voltage is applied directly to
the grid of cathode follower V1 without being reduced
by the 1000:1 divider. With the range switch on any
one of the six higher positions (1 volt to 300 volts),
section S1A establishes the connection between the
grid of V1 and tfie resistive divider at the junction
of resistor R4 and R3' artd the input voltage is re-
duced a thousand-to-one before it is applied to tfie
grid of V1.
b. Six-Step Voltage Divider -
ffirewound resistance
(R10A, B and R11A, B, C, D) in the catiode circuit
of cathode follower V1 constitute a resistance di-
vider tapped for six output voltages.
The six taps are brought out to contacts on section
S1B of the DB VOLTS switctr. The movable member
of S1B is fashioned with two contacting arms, spa.ced
at 180 " . Thus as t}te switch is moved through the
full range of its rotary travel, contact is made twice
with each of the six taps, once on the travel through
the six lower ranges and once on t}te travel through
the six higher ra.nges. Since on the six higher ranges
the input voltage is divided by a thousand before it is
applied to t}te 6 step divider, each tap in the re-
sistive divider serves two ranges, t}tus making a-
vailable a total of twelve ranges.
The output from Vl is aplied to the grid of V2, the
first stage of t}te amplifier. For full scale deflection
of the meter, the maximum voltage that can be ap-
plied to the grid of V2 is .001 volt. The resistance
divider in the cathode circuit of V1 provides such
reduction on each range that for maximum voltage
at the INPUT terminals, the voltage applied to the
grid of V2 will not exceed .001 volt.
The r-c networks in the cathode circuit of V1 mini-
mize d-c switching transients while the riurges are
being changed. The variable capacitors switched
into the circuit on tie .0L/10 volt and .003/3 volt
ranges are provided for adjustment of the high-fre-
quency response. The trimmer capacitor connected
across t}te 1000:1 divider compensates for stray
capacity to keep the division ratio constant over the
full frequency range.
The V1 input circuit is stabilized with 35 db of feed-
back over t}te entire frequency range of the instru-
ment.
3.3 AMPLIFIER. RECTIFIER AND METER
The four-stage amplifier provides high gain over
a wide frequency range. The amplifier output is
applied to a full-wave rectifier actuating a l-milli-
ampere meter movement. The amplifier-rectifier
system is stabilized with an overall feedback loop
which has a level of 60 db at the middle of the fre-
quency range. At the edges of the frequency range,
the amount of negative feedback is so proportioned
as to provide the marimum stability consistent with
the gain there available.
a. Amplifier -
Beffieen -the grids of V2 and V5 the amplifier
yields a net gain of approximately 55 to 60 db over a
lO-cycle to 4-megacycle band. A high level of nega-
tive feedbac\ frequency compensating networks in the
plate circuit of each stage, and catfiode degeneration
at low frequencies (the cathode resistors are not
bypassed at low frequencies) provide an amplifier
of high stability which can operate over an extremely
wide (10-cycle to 4-mc) band. The feedback of the
amplifier is returned from the plate of the last stage,
V5 through tfie rectifier-meter circuit, to the cathode
of first stage V2 in such phase as to be degenerative
in effect. The gain is adjusted by means of variable
resistor R29 in the feedback loop. Another adjustment
in the feedback loop, variable capacitor C21, is used
for adjusting tlre frequency response of the amplifier
at high frequencies.
The stages of the amplifier are resistance-capacitamce
coupled. The coupling circuitry between each stage is
frequency-compensating, and provides separate
coupling for low and high frequencies. This feature
of the circuit design contributes to the flatness of the
frequency response across the wide band over which
t}te voltmeter is rated to operate.
When the 400D is used as an amplifier, pentode V5
is operated as a cathode follower and supplies volt-
age at the OIJTPIJ'I terminals. The impedance, look-
ing into the OUTPUT terminals, is approximately
50 ohms.
The output from the plate of V5 is delivered to a full-
wave rectifier.
b. Rectifier and Meter -
ffi circuit is arranged in a
bridge-type configuration, with a crystal diode and a
capa.citor in each branch ard a d-c milliammeter con-
nected across its midpoints. The diode connection
provides fullwave rectification of the input current.
The design of the bridge is sueh that 1) a pulsating
direct current is delivered to the meter circuit and
2) an alternating current of the same frequency as the
current at tle rectifier input is delivered to the out-
put of t}te bridge. From t}re rectifier-bridge output,
t}te ac flows through the feedback loop to the cathode
of V2.
The current through tlre meter is proportional to the
average value of the waveform of the voltage applied

to the input of the rectifier. Since calibration of the
meter in rms volts is based on the ratio tlnt exists
behveen the average and effective values of a voltage
that is a true sine wave, deviation in a waveform
from that of a true sine wave may cause meter mea-
surements to be in error. Table 2-1 gives an indi-
cation of the limits of possible error due to the
presence of harmonics in the waveform of a voltage
under measurement.
NOTE
This table is universal in its aplication since these
errors are inherent in all voltage measuring equip-
ment of the average responding type.
The cunent flow through the rectifier-meter circuit
is shown in Figure 3-2. For purposes of e:<planation,
Sect.I[ Page 3
the amplifier (tubes V2-V5) may be considered as a
signal generator with the output from the plate of V5
as one side, and the cathode of V2 as the other side.
The rectifier-meter circuit may be considered as
a bridge across t}re generator. On the positive half
of the cycle, current flows into the bridge from the
top. On the negative half of the cycle, current flows
into the bridge from the bottom.
On the positive half of the cycle, diode CR1 conducts,
and cument flows through CR1 to the juncture between
the meter and capacitot C32, where it divides, a por-
tion placing a charge on capacitor C32 and t}re rest
flowing through the meter. Since diode CR2 is non-
conducting, the current from the meter output flows
to capacitor C33, placing a charge thereon. As
capacitor C32 and C33 discharge, current flows to
the generator.
+250 LEGEND.
DIRECTION OF CURRENT FLOW
POSITIVE HALF OF CYCLE
NEGATIVE HALF OF CYCLE
POSITIVE HALF OF CYCLE
NEGATIVE HALF OF CYCLE
cRr
0'n--,,''-
I
------------+
----)
,l
I
I
v3 -v4 -v5
FROM VI
,l/'t ,2
I
l_.J
\
ec32 c33
I
Figure 3-2. Model 4O0D/H
Meter Bridge
R28 C
Direction of Current Flow through Rectifier-
Partial Schematic
i \_, I
I
t

Sect.I[ Page 4
On the negative half of the cycle, diode CR2 is
biased to conduct, and diode CR1 is nonconducting.
The current flowing into the bridge at the juncture
between capacitors C32 and C33 divides, half plac-
ing a charge on C32 and the other half on C33. Since
dioCe CR1 is nonconducting, as capacitor C32 dis-
charges, current flows through the meter and diode
CR2 to the generator. As capacitor C33 discharges,
current flows tfirough CR2 to the generator. The
action of capacitors C32 and C33 results in the flow
of an alternating current in the feedback loop, so
phased t}tat it is negative in effect with respect to
ttte signal on the grid of V2. Capacitor C34 across
the meter provides a bypass path for any a-c com- section.
ponent that may be present.
3.4 POWER SUPPLY
The power supply circuits provide a high-voltage
regulated dc for t}re plate circuits of tubes Vl to V5
and a low-voltage unregulated dc for the filament
circuits of tubes Vl to V4. The filament circuits of
tubes V5 to V8 are supplied directly with 6. 3-volt
ac from windings in the secondary of power trans-
former T1.
a. Input Circuit -
may be connected for operation from either a 115 volt
(xljl] or a 230 volt (+ 10Eo\ 50/1000 cps source.
The primary circuit is fused. When switch 52 is
in the ON position, power from the line is applied
to the primary winding of transformer T1.
b. High-Voltage Supply -
eurrenflorEengn-voltage supply is rectified
by dual diode V6, and regulated by triode V7. Am-
plified control voltage is supplied to grid of V7 by
the pentode section of V8. The triode section of V8
acts as a d-c cathode follower driving the pentode
Noise from reference tube V9 is eliminated by low-
pass filter R63 and C35.
D-c drift due to changes in V8 filament voltage is
canceled by differential action between tfie two
sections of V8.
c. Filament Supply -
Current for the low-voltage d-c filament supply
is rectified by selenium rectiJier SR1 and is filtered
by capacitors C39A and C398. Variable resistors
R66 provides a means for adjusting the level of the
rectified voltage.
The primary windings of power transformer T1
I
I
lalI
:

4.I INTRODUCTORY
This section contains instructions for maintaining
and trouble shooting the Model 400D as well as
procedures for replacing tubes and making internal
adjustments. A tube replacement chart lists the
checks and adjustments required aJter tube replace-
ment, and a trouble shooting chart assists in local-
izing most types or troubles which might occur.
Photographs showing tlre physical location of compo-
nents, a voltage and resistance diagram, and a
schematic diagram are also provided for conveni-
ence in trouble shooting.
The more intricate adjustment procedures described
in tlis section are provided for those who have t}te
necessary skill and test equipment. When qualified
personnel and test facilities are not available, it is
suggested that adjustments not be made in the field.
Instructions are given on the Warranty page at tle
back of t}is instruction manual for the procedure to
be followed should repair service be required.
The following information appears in this section:
4-2 Meter Zero Adjustment
4-3 Case Removal
4-4 Capacitor Replacement
4-5 Replacement of Crystal Diodes
4-6 Tube Replacement
4-7 Adjustments
4-8 Trouble Shooting
4-9 Power-SupplyLocalizationChecks
4-10 Voltmeter Localization Checks
4.2 METER ZERO ADJUSTMENT
Whenever the mder pointer does not indicate exactly
on zero, the pointer should be reset to zero. For
the most accurate positioning of tfie meter pointer,
the instrument should be allowed to reach operating
temperature before the adjustment is made. With
the instrument in its case, the instrument should
be aLlowed to heat for at least 15 minutes. Shut off
Sect.fV Page 1
sEcTl0l{ lll
1llAI1{TEI{41{CE
the power and wait at least 2 minutes for all capaci-
tors to completely discharge, then proceed as follows:
The adjust screw is in the meter frame at a midpoint
immediately below the meter face. The adjustment
is made properly only when the pointer is traveling
in the opposite direction to the turn of t}te adjust
screw. Although the adjust screw may be turned in
either direction, a practical procedure is to turn
the screw in a clockwise direction until the pointer
starts to swing back toward zero. Then, still turn-
ing the scre'tr clockwise, bring the pointer (now
traveling counterclockwise) back to zero.
4.3 CASE REMOVAL
The instrument case is fastened to t}le rear of the
chassis with two screws. To remove ttre case, re-
move the screws, and slide the case to the rear and
off the instmment.
When replacrng the case, pr.ll tlre powercablethrough
the opening at the rear of the case. Be sure the cable
is free of the case along the entire length of the cable
so tfiat it carurot get caught between chassis and case
as tie case is slid onto the instrtment.
4.4 CAPACITOR, REPLACEMENT
Electrolytic capacitors in the Model 400D are high-
quahf capacitors that have a useful life of from five
to ten years. Do not replace these capacitors unless
proved defective by accurate tests.
4.5 REPLACEMENT OF CRYSTAL DIODES
Meter tracking is affeeted by the characteristics of
the diodes in the rectifier-meter circuit. When a
diode is replaced, therefore, it is important 1) that
the replacement diodes have t}te proper character-
istics and 2) that the meter calibration be checked
and, if necessary, readjusted.

Sect. W Page 2
cc6 R5? CR2
V7
c36
V6
R6e
R64
V3
V8
t?z
R8t
FI
4tt#-
R23
clT R$n
R54
Re9
c2l
il8
ca4
c40
JUNCTIONI
Rl, R2, Rl c lA
RA8 A. B"C
R82
+450v tRrG,]
c4
Kb
Figure 4-1. Model 400D/H View of Left Side Instrument Case Removed

The diodes used in the Model 400D rectifier-meter
circuit are special high performance junction type
silicon diodes manufactured by the Hewlett-Packard
Company, and must be obtained from @. The part
number given this silicon diode is G111A and re-
placements should be ordered by this number.
Sect.IV Page 3
When checking tubes by replacing with new tubes, if
no improvement in operation is noticed, return
original tube to its socket. Do not transpose tubes
since this may mask any m@al operation that
may exist.
CAUTION
These diodes are of the junction type. Because tJre
junction is less than l/2 mil in diameter, extra care
must be taken not to subjectthemtoexcessive me-
chanical shock. Dropping them on a table or on the
floor, etc. may cause a mechanical failureatthe
junction. When installed in the instrument, how-
ever, the diodes will withstand any shock which can
be witfistood by the instrument as a whole.
NOTE
If the gain of the replacement tube differs from that
of the original tube, resistor R29 is adjusted to re-
store calibration. If the frequency response of the
400D is altered by a replacement tube, it is recom-
mended that another replacement tube be sought that
has capacitance characteristics similar to those of
the original tube.
4.6 TUBE REPLACEMENT
Any tube in the Model 400D may be replaced with a
tube having conesponding RETMA standard charac-
teristics. However, for the utmost in accuracy frorn
the Model 400D, it is recommended that whenever a
tube is replaced, Table 4-1 be consulted and the
specified adjustment checked. The procedure for
performing these adjustments may be found in
paragraph 4-6.
The 6CB6 tubes in positions V1 and V2 have been
replaced by type 6DE6 tubes. This change has been
made.because of an appa.rent superiority of type 6DE6
tubes with respect to micmphonics.
Because of the decreased filament drain of the 6DE6
tubes, a resistor (R84, 2?0 ohms, xlfio, 1 W) has
been added to the instrument across C39B to balance
6CB6 tubes
Table 4-1. Tube Replacement
TUBE FIJNCTION CIRCUTT ADJUSTMETiTT
vl - 6C86 (6DE6)
v2 - 6CB6 (6D86)
v3 - 6CB6
v4 - 6CB6
v5 - 6CB6
v6 - 6AX5
v7 - L2B4A
v8 - 6u8
v9 - 5651
Cathode Follower
1st Amplifier
2nd Amplifier
3rd Amplifier
4th Amplifier
High Voltage Rectifier
Power-Supply Series
Regulator
Power-Supply Control
Tube
Reference Tube
Calibration,
Calibration,
Calibration,
Calibration,
Calibration,
None
Check +250 vdc regulated
Check +250 vdc regulated
Check +250 vdc regulated
steps 1, 2,3,4,5. Filament Adjustment
steps 1, 2,3. Filament Adjustment
steps 1, 2, 3. Filament Adjustment
steps 1,2,3. Filament Adjustment
steps 1,2,3.

Sect. IV Page 4
ct
cr4
cr6
tsrA-D)
{C? THRU C16}
{NI? THRU RI9}
{ftr0A-8, Rl lA- D}
k*
\
sJ
Figure 4-2. Model 4A0D/H View of Right Side Instrument Case Removed