HP 400D Service manual
nlf
OPERATING
AND
SERVICE
MANUAL
(HP
PART
NO.
400D/H/L-903}
MODEL
400D
SERIALS
PREFIXED:
310-
MODEL
400H
SERIALS
PREFIXED:
313-
MODEL
400L
SERIALS
PREFIXED:
313-
AND
SPECIF.
HO2-400D
SERIALS
PREFIXED:
310-
VACUUM
TUBE
VOLTMETER
Appendix
C,
Manual
Backdating
Changes,
adapts
this
manual
to:
Models
400D/H02-400D,
Serial
Nos.
313-28977
and
below
Models
400H/L,
Serial
Nos.
310-52371
and
below
Models
400DR/HR/LR,
_All
Serial
Nos.
Copyright
Hewlett-Packard
Company
1961
P.O.
Box
301,
Loveland, Colorado,
80537
U.S.A.
00102-5
Printed:
APRIL
1966
T.
O.
33A1-12-349-1
Figure
1-1.
Vacuum
Tube
Voltmeters
Models
400D, 400H,
400L
1-0
00102-2
T.O.
33A1-12-349-1
Section
I
Paragraphs
1-1
to
1-5
SECTION
|
GENERAL
DESCRIPTION
1-1.
INTRODUCTION.
(Sce
figure
1-1.)
1-2.
This
manual
contains
operating
and
servicing
instructions,
anda
parts
breakdown,
for
the
Models
400D, 400H,
and
400L
Vacuum
Tube
Voltmeters
manu-
factured
by
the
Hewlett-PackardCompany.
The
Model
400D
Voltmeter
is
similar
toa
military
counterpart,
Electronic
Voltmeter
ME-30A/U,
in
appearance
and
operation,
but
contains
modified
electrical
circuits
to
obtain
improved
performance.
Applicable
Federal
Stock
Numbers
for
the
voltmeters
are
as
follows:
Model
400D:
6625-643-1670
Model
400H:
6625-557-8261
Model
400L:
6625-729-8360
1-3.
The
Models
400D,
400H,
and
400L
Voltmeters
are
the
same
except
forthe
differences
listed
in
Fig-
ure
1-2.
a.
Voltage
Range:
400D/H
-
0.1
millivolt
to
300
volts;
400L
-
0.3
millivolt
to
300
volts,
in
12
ranges
providing
full-scale
readings
of
the
following
voltages:
0.001 0.100
10.00
0.003
0.300 30.00
0.010
1.000
100.00
0.030 3.000
300.00
b.
Decibel
Range:
-72
to
+52
db,
in
12
ranges.
c,
Frequency
Range:
10
cps
to
4
mc.
d.
Input
Impedance:
10
megohms
shunted
by
15
pf
(15
uf)
on
ranges
1.0
volt
to
300
volts;
25
pf
on
ranges
0.001
volt
to
0.3
volt.
e.
Stability:
Line
voltage
variations
of
+10%
do
not
reduce
the
specified
accuracy,
and
line
voltage
transients
are
not
reflected
in
the
meter
reading.
Electron
tube
deterioration
to
75%
of
normal
transconductance
affects
accuracy
less
than
0.5%
from
20
cps
to
1
mc,
{.
Amplifier:
OUTPUT
terminals
are
provided
so
that
the
voltmeter
can
be
used
to
amplify
small
signals
or
to
enable
monitoring
of
waveforms
under
test
with
an
oscilloscope.
Output
voltage
is
approximately
0.15
volt
rms
on
all
ranges
with
full-scale
meter
deflection.
Amplifier
frequency
response
is
same
as
the
voltmeter.
Internal
impedance
is
approximately
50
ohms
over
entire
frequency
range,
a.
The
front
panel
meters
are
different
in
each
model,
as
described
in
paragraph
1-6.
b.
The
accuracy
specifications
are
different
for
each
model,
as
described
in
figure
1-2.
1-4.
DESCRIPTION.
1-5.
The
Hewlett-Packard
Models
400D,
400H,
and
400L
Vacuum
Tube
Voltmeters
are
general
purpose,
portable
electronic
a-c
voltmeters
of
high
sensitivity
and
stability.
They
are
suited
to
both
laboratory
and
field
use.
Models
400D/H
measure
a-c
voltages
from
0.
001
to
300
volts
and
Model
400L from
.003
to
300
volts
rms
fullscale,
witha
frequency
bandwidth
cover-
ing
10cps
to4
megacycles.
The
voltmeters
are
com-
pact,
accurate,
and
rugged
and
have
fast
meter
re-
sponse,
high
input
impedance,
stable
calibration
ac-
curacy,
and
freedom
from
the
effects
of
normal
line
voltage
variations.
The
voltmeters
are
designed
for
long
instrument
life
with
a
minimum
of
servicing.
g.
Accuracy:
Model
400D
-
+2%
of
full
scale,
20
cps
to
1
me;
+3%
of
full
scale,
20
cps
to
2
mc;
+5%
of
full
scale,
10
cps
to
4
mc.
Model
400H
-
+1%
of
full
scale,
50
cps
to
500
ke;
+2%
of
full
scale,
20
cpsto
1
me;
+3%
of
full
scale,
20
cpsto
2
me;
45%
of
full
scale,
10
cpsto
4me.
Model
400L
-
+2%
of
reading
or
+1%
of
full
scale,
whichever
is
more
accurate,
50
cps
to
500
ke.
+3%
of
reading
or
+2%
of
full
scale,
whichever
is
more
accurate,
20
cps
to
1
mec.
+4%
of
reading
or
+3%
of
full
scale,
whichever
is
more
accurate,
20
cps
to
2
mc,
+5%
of
reading
10
cps
to
4
mc.
h.
Power
Requirement:
115/230
volts
410%,
50
to
1000
cps,
approximately
100
watts.
i,
Size:
11-3/4
in.
high,
7-1/2
in,
wide,
12
in,
deep,
j.
Weight:
23
Ibs.
18
lbs;
shipping
weight
approximately
Figure
1-2.
Table
of
Specifications
00102-3
1-1
Section
I
Paragraphs
1-6
to
1-12
1-6,
Each
model
voltmeter
has
three
calibrated
scales
on
the
panel
meter.
The
Models
400D
and
400H
have
two
linear
VOLTS
scales,
0
to
1
and
0
to
3,
and
one
DECIBELS
scale,
-12
to
+2
db.
The
meters
used
in
the
Models
400H
and
400L
are
larger
and
include
a
mirror
to
eliminate
parallax
in
viewing
and
to
facilitate
use
of
the
higher
scale
calibration
accuracy
of
these
models.
The
Model
400L
VOLTS
scales
are
logarithmic
in
calibration,
from
0.3
to
1
and
0.8
to
3;
and
the
DECIBELS
scale
is
linear.
In
all
models,
the
VOLTS
scales
are
calibrated
to
indicate
the
root-mean-square
(rms)
value
of
an
applied
sine
wave.
Actual
meter
deflection
is
proportional
to
the
average
value
of
the
applied
signal,
thereby
minimizing
additional
meter
deflection
due
to
noise
and
harmonic
distortion.
1-7,
A
voltmeter
output signal
is
provided
at
the
front
panel
OUTPUT
terminals.
This
output
is
proportional
to
the
meter
reading
and has
a
waveshape
similar
to
the
applied
signal.
This
signal
level
is
about
0,15
volts
rms
for
a
full-scale
meter
reading,
regardless
of
the
input
signal
level.
The
internal
impedance
at
the
OUTPUT
terminal
is
50
ohms
over
the
full
frequency
range.
High-impedance
loads
(above
100K)
will
not
adversely
affect
the
accuracy
of
the
voltmeter,
This
output
is
valuable
for
increasing
the
sensitivity
of
bridges,
etc.,
where
distortion
added
to
the
waveform
is
not
a
factor,
1-8,
The
voltmeter
chassis
is
constructed
of
aluminum
alloy
throughout.
The
panel
is
finished
in
non-reflecting,
light-grey
baked
enamel;
the
cabinet
is
finished
in
dark-blue,
baked
wrinkle
paint,
The
cabinet
is
equipped
with
rubber
feet
and
a
leather
carrying
handle.
Control
markings
on
the
front
panel
are
engraved
and
black
filled.
INPUT
and
OUTPUT
terminals
are
special
binding
posts
which
accept
either
bare
wire
or
banana
plugs;
the
3/4-inch
spacing
between
binding
posts
accepts
standard
dual-banana
plugs.
The
"ground"
side
of
the
INPUT
and
OUTPUT
terminals
is
connected
to
the
instrument
chassis
which
is
in
turn
connected
to
the
power
line
ground
through
the
third
(round)
prong
of
the
plug
on
the
power
cable.
1-2
T.O,
33A1-12-349-1
1-9.
The
voltmeter
is
equipped
with
a
non-detachable
power
cord.
Test
leads,
which
may
be
plain
wire
leads
or
coaxial
cable,
and
test
probes
must
be
supplied
by
the
user.
1-10.
Instruments
designated
Models
400DR,
400HR,
and
400LR
are
rack
mount
configurations
of
the
400D,
400H,
and
400L,
respectively.
They
are
identical
to
their
cabinet
model
counterparts
in
every
other
re-
spect.
They
are
designed
to
be
mounted
ina
stan-
dard
19inchwide
x
7inch
high
relay
rack
space.
Re-
fer
to
Appendix
C
for
Replacement
Parts
information.
1-11.
ACCESSORIES.
1-12.
Accessory
instruments
for
the
voltmeter
are
available
(not
supplied)
to
increase
its
range
of
opera-
tion
and
application,
such
as
increasing
voltage
mea-
surement
range
and
input
impedance,
converting
to
current
measurement,
providing
line
matching,
ete.,
as
follows:
a.
H-P
11004A
Line
Matching
Transformer.
Pro-
vides
balanced
135-ohm
or
600-ohm
input,
5
ke
to
600
ke.
b.
H-P11005A
Bridging
Transformer.
Allows
volt-
age
measurement
on
balanced
lines.
20
cps
to
45
kc.
c,
H-P
11039A
Capacitive
Voltage
Divider.
Safely
measures
power-frequency
voltages
to
25
kilovolts.
Division
ratio,
1000:1.
Input
capacity,
15
pf
+1
pf.
d.
H-P
11041A
Capacitive
Voltage
Divider.
Accu-
racy
+3%.
Division
ratio,
100:1.
Input
impedance,
50
megohms,
resistive,
shunted
with
2.75
pf
capa-
city.
Maximum
voltage,
1500
volts.
e.
H-P
456A
AC
Current
Probe.
Allows
current
measurements
without
breaking
the
circuit,
Sensitivity
1
mv/ma
+2%
at
1
kc.
Maximum
input
1
amp
rms;
2
amp
peak,
Output
noise
less
than
50
uv
rms,
f.
H-P
11029A-11034A
Shunt
Resistors,
For
mca-
suring
currents
as
small
as
1
microamp
full
scale,
Accuracy
+1%
to
100
ke,
+5%
to
4
me
(470A,
45%
to
1
mc),
Maximum
power
dissipation,
1
watt.
00102-3
T.O.
33A1-12-349-1
Section
IL
Paragraphs
2-1
to
2-11
SECTION
II
INSTALLATION
2-1.
UNPACKING
AND
INSPECTION.
2-2.
There
are
no
special
precautions
for
unpacking
the
voltmeter.
Save
the
shipping
carton
and
packing
materials
for
possible
storage
or
reshipment.
When
unpacking,
inspect
instrument
and
packing
materials
for
signs
of
damage
in
shipment.
Make
an
operation
check
as
directed
in
paragraph
2-10
to
determine
if
performance
is
satisfactory.
If
there
is
any
indication
of
damage,
immediately
file
a
claim
with
the
transport
service
used
or
other
cognizant
authority.
2-3.
LINE
VOLTAGE
REQUIREMENT.
2-4.
The
voltmeter
is
wired
at
the
factory
for
use
on
115-volt
a-c
power.
This
voltage
may
vary
+10%
without
adverse
effect
upon
voltmeter
performance.
The
volt-
meter
can
be
wired
for
use
on
230-volt
a-c
power
by
reconnecting
the
dual
primary
windings
on
the
power
transformer
as
shown
in
the
schematic
diagram
in
Section
V.
When
using
230-volt
power,
change
from
a
l-amp
to
a
1/2-amp
slow-blow
fuse.
If
necessary,
provide
an
adapter
for
attaching
the
standard
115-volt
plug
on
the
voltmeter
to
the
230-volt
outlet.
2-5.
POWER
LINE
CONNECTION.
2-6.
The
three-conductor
power
cable
on
the
voltmeter
is
terminated
in
a
polarized
three-prong
male
connector.
The
third
contact
is
an
offset
round
pin
added
to
a
stand-
ard
two-blade
connector,
which
grounds
the
instrument
chassis
when
used
with
the
appropriate
receptacle.
To
connect
this
plug
in
a
standard
two-contact
receptacle,
use
an
adapter.
The
chassis
ground
connection
is
brought
out
of
the
adapter
in
a
green
pigtail
lead
for
connection
to
a
suitable
ground.
2-7.
The
power
plug
normally
supplied
with
the
volt-
meter
is
made
of
molded
rubber
and
is
an
integral
part
of
the
power
cable.
On
certain
military
contracts,
a
modification
of
the
Model
400D,
termed
the
H02-400D,
is
equiped
with
a
removable
plug
having
the
same
pin
configuration
but
constructed
of
corrosion-resistant
material.
In
all
other
respects
the
H02-400D
is
the
same
as
the
Model
400D
and
carries
the
same
Federal
Stock
Number.
00102-2
The
lower
INPUT
and
OUTPUT
signal
terminals
on
the
panel
of
the
voltmeter
are
connected
directly
to
the
chassis
of
the
voltmeter.
Any
voltage
applied
to
the
lower
terminal
will
be
shorted
directly
to
ground.
If
the
ground
con-
nection
in
the
power
cord
is
disconnected
by
use
of
an
adapter,
the
entire
voltmeter
cabinet
will
carry
whatever
potential
is
applied
to
the
lower
terminal
and
may
be
a
hazard
to
the
operator.
2-8.
INSTALLATION.
2-9.
The
voltmeter
is
a
portable
instrument
requiring
no
permanent
installation.
The
voltmeter
is
for
bench-
top
operation,
standing
on
its
rubber
feet
with
its
front
panel
near
the
vertical
plane.
A
bail
is
provided
for
raising
the
front
of
the
cabinet
to
obtain
a
better
viewing
angle.
2-10.
OPERATION
CHECK.
2-11.
The
voltmeter
is
ready
for
use
as
received
from
the
factory.
The
simple
check
described
below
can
be
made
by
incoming
inspectors
to
determine
if
electrical
damage
was
incurred
in
shipment.
If
more
complete
proof
of
instrument
performance
is
required,
the
over-all
performance
check
described
in
paragraph
5-22
must
be
used.
Make
a
simple
performance
check
as
follows:
a.
Connect
voltmeter
to
the
power
line
through
a
variable
transformer.
Set
transformer
for
115
volts,
turn
on
and
allow
a
five-minute
warmup.
b.
Measure
any
sine
wave
voltage,
excepting
the
power
line,
from
0.01
to
300
volts
whose
exact
voltage
is
known.
Note
that
the
lower
INPUT
terminal
is
connected
to
the
power
line
ground.
c.
While
making
the
above
measurement,
adjust
the
line
voltage
from
103
to
127
volts,
The
reading
on
the
meter
must
not
change
by
more
than
the
width
of
the
pointer.
2-1
Section
HI
T.O.
33A1-12-349-1
REFERENCE
NUMBER
DESIGNATION
FUNCTION
Panel
meter
Indicates
rms
volts
and
decibels
of
sine
wave
signals.
Indicator
light
Indicates
that
voltmeter
is
turned
on.
ON
Power
switch
Applies
line
power
to
voltmeter.
INPUT
terminals
Receive
voltage
to
be
measured
or
signal
to
be
amplified.
RANGE
(DB-VOLTS)
switch
|
Selects
full-scale deflection
sensitivity.
OUTPUT
terminais
Supply
signal
level
proportional
to
meter
reading,with
same
waveform
as
applied
to
INPUT
terminals.
Zero
adjust
screw
Meter
zero
adjust
screw
(for
400D
and
400H
only).
Figure
3-1.
Voltmeter
Front
Panel,
Showing
Controls
and
Connectors
3-0
00102-2
T.O.
33A1-12-349-1
Section
III
Paragraphs
3-1
to
3-9
SECTION
Ill
OPERATING
INSTRUCTIONS
3-1.
INSTRUMENT
TURN-ON.
3-2.
The
voltmeter
is
ready
for
use
as
received
from
the
factory
and
will
give
specified
performance
after
a
few
minutes
warmup.
See
Section
II
for
information
regarding
connection
to
the
power
source
and
to
the
voltage
to
be
measured.
Controls
are
shown
in
figure
3-1.
3-3.
GENERAL
OPERATING
INFORMATION.
3-4.
METER
ZERO
CHARACTERISTIC.
When
the
Model
400D
and
400H
Voltmeters
are
turned
off,
the
meter
pointer
should
rest
exactly
on
the
zero
calibration
mark
on
the
meter
scale.
If it
does
not,
zero-set
the
meter
as
instructed
in
paragraph
5-7.
The
meter
supplied
in
the
Model
400L
Voltmeter
is
not
provided
with
a
mechanical
meter
zero
adjustment.
When
the
voltmeter
is
turned
on
with
the
INPUT
terminals
shorted,
the
meter
pointer
may
deflect
upscale
slightly;
this
deflection
does
not
affect
the
accuracy
of
a
reading.
NOTE
When
the
voltmeter
RANGE
switch
is
set
to
the
lowest
ranges
and
the
INPUT
terminals
are
not
terminated
or
shielded,
noise
pickup
can
be
enough
to
produce
up
to
full-scale
meter
deflec-
tion.
This
condition
is
normal
and
is
caused
by
stray
voltages
in
the
vicinity
of
the
instru-
ment.
For
maximumaccuracy
on
the
-001-volt
range,
the
voltage
under
measurement
should
be
applied
to
the
voltmeter
through
a
shielded
test
lead.
3-5.
METER
SCALES.
The
two
voltage
scales
on
each
of
the
voltmeter
models
are
related
to
each
other
by
a
factor
of
1710
(10
db).
In
conjunction
with
the
calib-
rated
RANGE
switch
steps,
this
provides
an
intermediate
range
step
spaced
10
db
between
"power
of
ten"
ranges,
which
are
20
db
apart.
The
relationship
of
the
DECIBELS
scale
to
the
0
to
1
VOLT
scale
is
determined
by
making
0
db
on
the
DECIBELS
scale
equal
to
the
voltage
required
to
produce
1
milliwatt
in
600
ohms
(0.775
volts).
Thus,
the
DECIBELS
scale
reads
directly
in
dbm
(decibels
referred
to
one
milliwatt)
across
a
600-ohm
circuit,
and
can
be
used
to
measure
absolute
level
of
sine
wave
signals.
It
can
also
be
used
to
measure
relative
levels
of
any
group
of
signals
which
have
the
same
waveform,
across
any
constant
circuit
impedance.
The
RANGE
switch
changes
voltmeter
sensitivity
in
10-db
steps
accurate
to
within
+1/8
db.
The
RANGE
switch
position
indicates
the
value
of
a
full-scale
meter
reading.
3-6.
CONNECTIONS.
Voltmeter
test
leads
must
be
provided
by
the
user.
The
type
of
leads
and
probes
used
will
depend
upon
the
application,
as
listed
below:
a
For
connection
to
low-impedance
signal
sources,
plain
wire
leads
often
are
sufficient.
00102-2
b.
For
high-impedance
sources,
or
where
noise
pickup
is
a
problem,
low-capacity
shielded
wire
must
be
used
with
a
shielded,
dual
banana
plug
for
connection
to
the
voltmeter
terminals.
c.
If
a
probe
is
used,
it
should
also
be
shielded
to
prevent
pickup
from
the
hand.
d.
For
signals
above
a
few
hundred
kilocycles,
the
capacity
of
the
test
leads
must
be
kept
to
a
minimum
by
using
very
short
leads,
preferably
unshielded.
An
alligator
clip
should
be
used
at
the
test
end
so
that
connection
can
be
made
without
adding
the
capacity
of
the
user's
hands.
3-7.
MAXIMUM
INPUT
VOLTAGE,
Do
not
apply
more
than
600
volts
dc
to
the
INPUT
terminals.
To
do
so
ex~-
ceeds
the
voltage
rating
of
the
input
capacitor.
3-8.
If
an
applied
voltage
momentarily
exceeds
the
selected
full-scale
voltmeter
sensitivity,
a
few
seconds
may
be
required
for
circuit
recovery,
but
no
damage
will
result.
3-9.
INPUT
VOLTAGE
WAVEFORM.
The
voltmeter
is
calibrated
to
indicate
the
root-mean-square
value
of
a
sine
wave;
however,
meter
pointer
deflection
is
proportional
to
the
average
value
of
whatever
waveform
is
applied
to
the
input.
If
the
input
signal
waveform
is
not
a
sine
wave,
the
reading
will
be
in
error
by
an
amount
dependent
upon
the
amount
and
phase
of
the
harmonics
present,
as
shown
in
figure
3-2
below.
When
harmonic
distortion
is
less
than
about
10%,
the
error
which
results
is
negligible.
INPUT
VOLTAGE
CHARACTERISTICS
METER
INDICATION
Fundamental
=
100
100
Fundamental
+10%
100
2nd
harmonic
Fundamental
+20%
2nd
harmonic
100-102
Fundamental
+50%
100-110
2nd
harmonic
Fundamental
+10%
3rd
harmonic
Fundamental
+20%
3rd
harmonic
Fundamental
+50%
3rd
harmonic
Note:
This
chart
is
universal
in
application
since
these
errors
are
inherent
in
all
average-respond-
ing
type
voltage-measuring
instruments.
96-104
94-108
90-116
Figure
3-2.
Effect
of
Harmonics
on
Voltage
Measurements
3-1
Section
TI
Paragraphs
3-10
to
3-16
=—
POSSIBLE
GROUND
LOOP
+
T.O.
33A1-12-349-1
INSERT
UNGROUNDED
ADAPTER
TO
BREAK
o
T
USE
GROUND
LOOP
eae
ONE
SIDE
OF
POWER
LINE
IS
ALSO
GROUNDED
Figure
3-3.
Test
Setup
for
Avoiding
Ground
Loop
3-10. Since
the
voltmeter
meter
deflection
is
propor-
tional
to
the
average
value
of
the
input
waveform,
it
is
not
adversely
affected
by
moderate
levels
of
random
noise.
The
effect
that
noise
has
on
the
accuracy
of
the
meter
reading
depends
upon
the
waveform
of
the
noise
and
upon
the
signal-to-noise
ratio.
A
square
wave
has
the
greatest
effect,
a
sine
wave
intermediate
effect,
and
"white"
noise
has
the
least
effect
on
the
meter
reading.
3-11.
If
the
noise
signal
is
a
50%
duty
cycle
square
wave
and
the
signal-to-noise
ratio
is
10:1
(between
peak
voltages),
the
error
will
be
about
1%
of
the
meter
reading.
If
the
noise
signal
is
"white"
noise
and
the
signal-to-noise
ratio
10:1,
the
error
is
negligible.
3-12.
LOW-LEVEL
MEASUREMENTS
AND
GROUND
CURRENTS.
3-13.
When
the
voltmeter
is
used
to
measure
signal
levels
below
a
few
millivolts,
ground
currents
in
the
meter
test
leads
can
cause
an
error
in
meter
reading.
Such
currents
are
created
when
two
or
more
ground
connections
are
made
between
the
instruments
of
a
test
setup
and/or
between
the
instruments
and
the
power
line
ground.
Two
ground
connections
complete
an
electrical
circuit
(ground
loop)
for the
voltages
which
are
generated
across
all
instrument
chassis
by
stray
fields,
particularly
the
fields
of
transformers.
These
ground
currents
can
be
minimized
by
disconnecting
the
ground
lead
in
the
power
cord
from
either
the
voltmeter
or
the
signal
source
being
measured,
at
the
power
outlet
as
shown
in
figure
3-3,
and
by
making
sure
that
in
the
test
setup
no
other
ground
loop
is
formed
that
can
cause
a
ground
current
to
flow
in
the
voltmeter
test
leads.
Although
the
resultant
voltage
developed
across
a
test
lead
is
in
the
order
of
micro-
volis,
it
is
enough
to
cause
noticeable
errors
in
measurements
of
a
few
millivolts.
The
presence
of
ground
currents
can
sometimes
be
determined
by
simply
changing
the
grounds
for
the
instruments
in
the
3-2
setup
and
watching
for
a
change
in
meter
reading.
If
changing
the
ground
system
causes
a
change
in
meter
reading,
ground
currents
are
present.
3-14.
MEASUREMENT
OF
VOLTAGE.
3-15.
The
meter
has
two
VOLTS
scales,
0
to
1
and
0
to
3.
When
the
RANGE
switch
is
set
to
.001,
.01,
-1,
1,
10,
or
100
VOLTS,
read
the
0
to
1
scale.
When
the
RANGE
switch
is
set
to
.003,
.03,
.3,
3,
30,
or
300
VOLTS,
read
the
0
to
3
scale.
The
lower
(black)
signal
INPUT
and
OUT-
PUT
terminals
and
the
instrument
case
are
connected
tothe
power
system
ground
when
the
instrument
is
used
witha
standard
three-
terminal
(grounding)
receptacle.
Connect
only
ground-potential
circuits
to
the
black
INPUT
and
OUTPUT
terminals.
3-16.
Operate
the
instrument
as
follows:
a.
Connect
the
voltmeter
to
the
a-c
power
source.
b.
Turn
the
Power
switch
ON
and
allow
a
warmup
period
of
approximately
five
minutes.
c.
Disconnect
any
external
equipment
from
the
OUT-
PUT
terminals.
d.
Set
the
RANGE
switch
to
the
VOLTS
range
which
will
read
the
voltage
to
be
measured
at
mid-scale
or
above.
If
in
doubt,
select
a
higher
VOLTS
range.
e.
Connect
the
voltage
to
be
measured
to
the
INPUT
terminals.
00102-2
T.O,
33A1-12-349-1
AVOID
A
SHORT
CIRCUIT
ACROSS
THE
POW-
ER
LINE!
To
measure
power
line
voltage,
first
connect
only
the
upper
(red)
INPUT
terminal
to
each
side
of
the
power
line,
in
turn,
leaving
it
connected
to
the
side
that
causes
meter
indi-
cation.
Then
connect
the
lower
(black)
INPUT
terminal
(grounded
internally)
to
the
other
side
of
the
line.
If
this
procedure
is
not
followed,
the
power
line
may
be
short-circuited
through
the
grounded
INPUT
terminal
of
the
voltmeter.
f.
Read
the
meter
indication
on
the
appropriate
VOLTS
scale,
in
accordance
with
the
full-scale
value
indicated
on
the
RANGE
switch.
Evaluate
the
reading
in
terms
of
the
full-scale
value
indicated
on
the
RANGE
switch.
Study
the
following
examples:
Example
1
When
the
RANGE
switch
is
in
the
.1
VOLTS
range,
read
the
0
to
1
VOLTS
scale.
If
the
meter
indicates
.64
on
that
scale,
the
voltage
being
measured
is:
.64
(meter
indication)
x
[eritetiseslerted|
1
[voltage
range
T
(full-scale
value)
~
*°64
volt
Example
2
When
the
RANGE
switch
is in
the
30
VOLTS
range,
read
the
0
to
3
VOLTS
scale.
If
the
meter
indicates
1.6
on
that
scale,
the
voltage
being
measured
is:
1.6
(meter
indication)
x
30
Fascseaueces|
voltage
range
3
(full-scale
value)
—
ne
volts
3-17.
MEASUREMENT
OF
DECIBELS.
3-18.
The
DECIBELS
meter
scale
is
provided
for
measuring
dbm
directly
across
600
ohms
and
for
measuring
db
ratio
for
comparison
purposes
when
each
measurement
is
made
across
the
same
circuit
impedance.
To
measure
signal
level
directly
in
dbm
(0
dbm
equals
1
milliwatt
into
600
ohms)
proceed
as
follows:
a.
Connect
the
voltmeter
to
the
a~c
power
source,
b.
Turn
the
Power
switch
ON
and
allow
a
warmup
period
of
approximately
five
minutes.
c.
Disconnect
any
external
equipment
from
the
OUT-
PUT
terminals.
d.
Set
the
RANGE
switch
to
the
DB
range
which
will
give
an
upscale reading
of
the
signal
to
be
measured.
If
in
doubt,
select
a
higher-level
scale.
e.
Connect
the
voltage
to
be
measured
to
the
INPUT
terminals,
00102-2
Section
TI
Paragraphs
3-17
to
3-21
f.
Note
the
meter
indication
on
the
DECIBELS
scale
{-12
to
+2
db).
The
signal
level
is
the
algebraic
sum
of
the
meter
indication
and
the
db
value
indicated
by
the
RANGE
selector.
Study
the
following
examples:
Example
1
If
the
indication
on
the
DECIBELS
scale
is
+2
and
the
RANGE
switch
is
in
the
+20
DB
position,
the
level
is
+22
dbm.
Example
2
If
the
indication
on
the
DECIBELS
scale
is
+1.5
and
the
RANGE
switch
is
in
the
-40
DB
position,
the
level
is
-38.5
dbm.
3-19.
To
measure
db
across
impedances
other
than
600
ohms,
follow
the
above
procedure
and
evaluate
the
results
as
follows:
NOTE
Since
the
measurement
is
made
across
other
than
600
ohms,
the
level
obtained
in
step
f
is
in
db,
but
not
in
dbm.
a.
To
obtain
the
difference
in
db
between
measure-
ments
made
across
equal
impedances,
algebraically
subtract
the
levels
being
compared.
b.
To
obtain
the
reading
of
a
single
measurement
in
dbm,
note
the
impedance
across
which
the
measure-
ment
is
made
and
refer
to
the
Impedance
Correction
Graph,
described
in
paragraph
3-20.
c.
To
obtain
the
difference
in
dbm
between
measure-
ments
made
across
different
impedances,
convert
each
measurement
to
dbm
using
the
Impedance
Correction
Graph
described
in
paragraph
3-20.
Then
algebraically
subtract
the
dbm
leveis
being
compared,
3-20.
IMPEDANCE
CORRECTION
GRAPH.
3-21.
As
the
voltmeter
DECIBELS
scale
is
calibrated
to
indicate
dbm
for
measurements
made
across
600-ohm
circuits,
a
correction
factor
must
be
used
when
meas-
urements
are
made
across
circuit
impedances
other
than
600
ohms,
if
absolute
dbm
levels
are
desired.
The
correction
factor
is
not
necessary
in
measuring
relative
db
levels
(not
dbm)
across
the
same
impedance,
but
it
is
required
for
comparison
of
db
levels
measured
across
different
impedances.
The
Impedance
Correction
Graph
in
figure
3-4
gives
the
correction
factor
for
conversion
of
the
meter
reading
to
dbm
when
the
impedance
of
the
circuit
under
test
is
known.
To
use
the
graph,
read
the
conversion
factor
corresponding
to
the
test
circuit
impedance
and add
it
to
the
meter
reading
determined
by
the
method
of
paragraph
3-17.
Observe
the
algebraic
sign
of
the
correction
factor
in
making
the
algebraic
addition.
Use
the
following
examples:
Example
1
If
the
measurement
is
made
across
90
ohms,
the
indication
on
the
DECIBELS
scale
is
+2,
and
the
RANGE
switch
is
atthe
+30
DB
position,
the
level
in
dbm
is
obtained
as
follows:
3-3
Section
II
Paragraphs
3-22
to
3-25
+
2
(meter
indication)
+30
(RANGE
switch
position)
+32
(sum)
+
8
(correction
factor
from
the
Impedance
+40
dbm
Correction
Graph)
Example
2
For
the
same
conditions
as
given
above,
except
that
the
measurement
is
made
across
an
impedance
of
60,000
ohms,
the
level
in
dbm
is
obtained
as
follows:
+
2
(meter
indication)
+30
(RANGE
switch
position)
+32
(sum)
~-20
(Correction
factor
from
the
Impedance
+12
dbm
Correction
Graph)
3-22.
USE
OF
VOLTMETER
AMPLIFIER.
3-23.
The
amplifier
in
the
voltmeter
may
be
used
for
amplifying
weak
signals.
With
full~scale
meter
deflec-
tion,
the
open-circuit
output
of
the
amplifier
is
approxi-
mately
0.15
volt
rms
regardless
of
the
RANGE
switch
position.
The
impedance
looking
into
the
OUTPUT
terminals
is
approximately
50
ohms.
The
frequency
3-4
T.O.
33A1-12-349-1
response
and
calibration
of
the
voltmeter
may
be
affected
by
the
impedance
of
a
load
applied
to
the
OUTPUT
terminals.
To
check
the
effect
of
the
applied
load:
observe
the
meter
reading
obtained
with
no
load
connected
to
the
OUTPUT
terminals
and
then
note
any
shift
of
reading
when
the
external
circuit
is
connected
to
the
OUTPUT
terminals.
If
the
shift
is
negligible,
the
measurement
is
not
being
affected
appreciably
by
the
load.
Whenever
the
input
signal
is
changed,
i.e.,
a
different
frequency
or
band
of
frequencies
is
applied,
repeat
the
quick
check
described
above.
3-24.
Maximum
gain
from
the
amplifier
is
obtainable
only
on
the
lowest
(.001
volts)
range,
since
output
level
is
the
same
for
all
bands.
This
is
due
to
the
10-db
amplification
loss
per
step
inserted
by
the
RANGE
switch
as
it
is
turned
clockwise.
Amplification
may
also
be
obtained
on
the
.003,
.01,
.03,
and
1
volt
ranges.
3-25.
When
the
voltmeter
is
used
as
an
amplifier,
select
a
range
which
gives
a
meter
deflection
near
full
scale.
Off-scale
signals
more
than
twice
the
value
of
the
position
of
the
RANGE
switch
will
cause
severe
distortion.
00102
-2
T.O,
33A1-12-349-1
Section
III
DBM
METER
CORRECTION
1000
IMPEDANCE
-
OHMS
6-8-7
Figure
3-4.
Impedance
Correction
Graph
00102-2
3-5
T.O.
33A1-12-349-1
Section
IV
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43M071104
SQOHLYD
LndNi
00102-2
4-0
T.O.
33A1-12-349-1
Section
IV
Paragraphs
4-1
to
4-11
SECTION
IV
CIRCUIT
DESCRIPTION
4-1.
BLOCK
DIAGRAM.
4-2.
The
electrical
circuits
of
the
voltmeter
are
shown
in
the
block
diagram
in
figure
4-1;
they
consist
of
an
input
voltage
divider
controlled
by
the
RANGE
switch,
a
cathode
follower
input
tube,
a
precision
step
attenuator
controlled
by
the
RANGE
switch,
a
broadband
amplifier,
an
indicat-
ing
meter,
and
a
regulated
power
supply.
The
voltage
applied
to
the
INPUT
terminals
for
measurement
is
divided
by
1000
before
application
to
the
input
cathode
follower
when
the
RANGE
switch
is
set
to
the
1-volt
range
and
higher;
the
input
voltage
is
applied
directly
to
the
cathode
follower
on
the
lower
ranges.
The
voltage
from
the
cathode
follower
is
divided
in
the
precision
attenuator
to
be
less
than
1
millivolt
for
application
to
the
voltmeter
amplifier.
The
output
of
the
amplifier
is
rectified
in
a
full-wave
bridge
rectifier
with
a
d-c
milliammeter
across
its
midpoints.
The
resultant
direct
current
through
the
meter
is
directly
proportional
to
the
input
voltage.
4-3.
INPUT
VOLTAGE
DIVIDER
AND
STEP
ATTENUATOR.
4-4,
The
input
voltage
divider
limits
the
signal
level
applied
to
the
input
cathode
follower
to
less
than
0.3
volt
rms
when
voltages
above
this
level
are
measured
with
the
RANGE
switch
set
at
the
1-volt
range
or
above.
The
divider
consists
of
a
resistive
branch
with
one
element
made
adjustable
to
obtain
exact
1000:1
division
at
middle
frequencies
and
a
parallel
capacitive
branch
with
one
element
made
adjustable
to
maintain
exact
1000:1
division
to
beyond
4
megacycles.
The
input
impedance
of
the
voltmeter
is
established
by
this
divider
and
is
the
same
for
all
positions
of
the
RANGE
switch.
On
the
six
low-voltage
positions
of
the
RANGE
switch,
the
input
divider
provides
no
attenuation
of
the
input
voltage.
(See
figure
5-10
for
the
complete
schematic.)
4-5.
The
step
attenuator
in
the
cathode
circuit
of
the
input
cathode
follower
reduces
the
voltage
to
be
measured
to
1
millivolt
or
less
for
application
to
the
voltmeter
amplifier.
Each
step
of
the
attenuator
lowers
the
signal
level
by
exactly
10
db
(1:¥10).
The
attenuator
consists
of
six
precision
wirewound
resistors
which
are
selected
to
very
high
accuracy
and
carefully
mounted
on
a
12-
position
rotary
switch.
The
RANGE
switch
rotor
has
two
contactors
(see
figures
5-9 and
5-10);
the
first
contacts
each
resistor
in
turn
while
the
input
divider
is
in
the
non-attenuating
position;
the
second
rotor
finger
repeats
these
contacts
while
the
input
attenuator
is
in
the
attenu-
ating
position.
On
the
.001-volt
range
a
fixed
capacitor
(C15)
is
automatically
connected
to
provide
flat
frequency
response
beyond
4
megacycles,
In
the
.003-
and
the
.01-
volt
ranges,
separate
adjustable
capacitors
(C14,
C16)
are
automatically
connected
to
the
attenuator
to
permit
setting
the
frequency
response
at
4
megacycles.
C14
and
C16
are
also
connected
to
the
attenuator
on
the
3-
and
10-volt
ranges.
Fixed
capacitor
C106
(permanently
connected)
flattens
frequency
response
on
the
.03-
and
30-volt
ranges.
00102-2
4-6.
Cathode
follower
V1
provides
a
constant,
high
input
impedance
to
the
input
voltage
divider
and
INPUT
ter-
minals
of
the
voltmeter
and
provides
a
relatively
low
impedance
in
its
cathode
circuit
to
drive
the
step
at-
tenuator.
The
voltage
gain
factor
across
V1
is
0.95.
4-7.
BROADBAND
VOLTMETER
AMPLIFIER.
4-8.
Amplification
of
the
signal
voltage
is
provided
by
a
four-stage
stabilized
amplifier
consisting
of
tubes
V2
through
V5
and
associated
circuits,
The
amplifier
provides
between
55-
and
60-db
gain
with
about
55
db
of
negative
feedback
at
mid-frequencies.
The
feedback
signal
is
taken
from
the
plate
of
the
output
amplifier
(V5)
through
the
meter
rectifiers
and
gain-adjusting
circuit
to
the
cathode
of
the
input
amplifier
(V2).
Variable
resistor
R107
in
the
feedback
network
adjusts
the
negative
feedback
level
to
set
the
basic
gain
of
the
amplifier
at
mid-frequencies,
while
adjustable
capacitor
C102
permits
setting
amplifier
gain
at
4
megacycles.
Variable
resistor
R118
in
the
coupling
circuit
between
V4
and
V5
permits
adjusting
the
gain
of
the
amplifier
at
10
cycles
per
second
by
controlling
the
phase
shift
of
low-frequency
signals
between
these
two
stages
(increasing
phase
shift
decreases
degeneration
and
increases
gain).
4-9.
Variable
resistor
R119
in
the
grid
return
path
for
V3,
V4,
and
V5
adjusts
the
total
transconductance
of
these
tubes
in
order
to
restrict
the
maximum
gain-
bandwidth
product
of
the
amplifier.
The
gain-bandwidth
product
must
be
restricted
to
give
a
smooth
frequency
response
rolloff
above
4
megacycles
and
to
prevent
possible
unstable
operation
at
frequencies
far
above
4
megacycles
when
tubes
having
unusually
high
trans-
conductance
are
used
(tube
transconductance
tolerances
during
manufacture
permit
wide
variations
in
new
tubes;
the
adjustment
permits
the
use
of
such
tubes).
The
plate
voltage
from
V5
is
rectified
by
the
meter
rec-
tifiers
and
drives
the
feedback
network.
The
cathode
voltage
of
V5
is
fed
to
the
meter
OUTPUT
terminals
for
monitoring
purposes.
The
current
through
V5,
and
thus
the
signal
voltage
at
the
cathode,
is
affected
by
the
loading
of
the
meter
rectifiers.
For
signal
levels
causing
third-scale
or
more
meter
deflection,
this
dis-
tortion
consists
of
a
very
small
irregularity
near
0
volts
on
the
waveform
as
each
diode
begins
conduction.
4-10.
INDICATING
METER
CIRCUIT.
4-11,
The
meter
rectifier
circuit
consists
of
two
silicon
diodes
and
two
capacitors
connected
as
a
bridge
with
the
indicating
meter
across
the
mid-points
as
shown
in
figure
4-2.
The
diodes
provide
full-wave
rectification
of
the
signal
current
for
operating
the
meter.
Electron
flow
through
the
meter
is
supplied
in
the
following
manner
(see
figure
4-2).
During
the
positive-going
half
cycle
of
plate
voltage
on
V5,
rectifier
CR1
conducts
electrons
from
both
C32
and
C33
back
to
the
B+
buss.
The
portion
of
electrons
from
C33
flows
through
the
meter
on
the
way
to
B+.
At
this
point
in
the
cycle,
both
C32
and
C33
are
charged
to
the
potential
of
B+
less
some
small
drop
in
R51
and
R52.
4-1
Section
IV
Paragraphs
4-12
to
4-16
4-12.
During
the
negative-going
half
cycle
of
the
plate
voltage
of
V5,
rectifier
CR2
conducts
electrons
back
to
both
C32
and
C33
from
the
plate
of
V5.
That
portion
of
electrons
going
back
to
C32
flows
through
the
meter
on
the
way
(in
the
same
direction
that
the
electrons
flowed
in
the
first,
positive,
half
cycle).
At
this
point
in
the
cycle,
both
C32
and
C33
are
discharged.
The
pulsating
current
through
the
meter
is
smoothed
by
C34
to
prevent
meter
pointer
vibration
when
measuring
low-frequency
signals.
The
current
is
proportional
to
the
arithmetic
average
value
of
the
waveform
ampli-
tude
of
the
signal.
Meter
calibration
in
rms
volts
is
based
on
the
mathematical
ratio
between
the
average
and
rms
values
of
true
sine
wave
current.
4-13.
In
addition,
the
bridge
serves
as
a
segment
of
a
voltage
divider
(in
series
with
L11
and
R108)
connected
across
the
output
of
the
amplifier.
The
negative
feedback
voltage
fed
to
the
input
of
the
amplifier
is
obtained
across
L11
and
R108,
The
alternating
charge
and
discharge
of
C32
and
C33
produce
at
their
junction
with
L11
an
al-
ternating
current
of
the
same
phase
and
waveform
as
that
at
the
plate
of
V5.
This
phase
is
negative
with
respect
to
the
input
signal
applied
to
the
first
stage
of
the
amplifier
(V2),
and
drives
the
negative
feedback
network.
4-14.
POWER
SUPPLY.
4-15,
The
power
supply
consists
of
tubes
V6
through
V9
and
the
associated
circuits,
as
shown
in
the
complete
T.O.
33A1-12-349-1
schematic
diagram,
figure
5-10.
The
power
supply
furnishes
regulated
+250V
d-c
voltage
for
the
grid
and
plate
bias
circuits
of
tubes
V1
through
V5,
unregulated
12.6V
d-c
voltage
for
the
heater
supply
of
tubes
V1
through
V4,
and
6.3V
a-c
voltage
for
the
heater
supply
of
tubes
V5
through
V8.
The
power
supply
is
designed
to
operate
from
either
a
115-volt
(410%)
or
a
230-volt
(£10%)
a-c
power
source
of
50
to
1000
cps.
The
primary
winding
of
power
transformer
T1
is
arranged
in
two
sections,
which
can
be
strapped
either
in
parallel
or
in
series,
to
permit
operation
on
115V
or
230V,
respectively.
4-16.
The
output
of
rectifier
V6
is
applied
to
the
voltage
regulator
circuit
consisting
of
V7
through
V9
which
supplies
a
constant,
+250
volts
de
to
the
stabilized
ampli-
fier
circuit
of
the
voltmeter.
Tube
V7
is
the
series
regulator
tube,
and
V9
provides
a
fixed
reference
voltage
drop,
with
which
the
output
voltage
is
compared
in
ampli-
fier
V8B.
V8A
is
a
cathode
follower
which
couples
the
reference
voltage
from
V9
to
V8B
without
loading
V9.
The
regulated
output
voltage
is
applied
to
the
control
grid
of
V8B,
while
the
reference
voltage
is
applied
to
its
cathode.
The
difference
between
the
control
grid
and
cathode
voltages
controls
the
operating
point
of
V8B
and
thus
its
plate
voltage,
which
in
turn
supplies
the
grid
voltage
for
regulator
V7.
Any
change
in
the
regu-
lated
output
of
V7
produces
a
correcting
change
in
the
grid
bias
of
V7
through
the
action
of
V8B,
thus
maintaining
an
essentially
constant
output
voltage
despite
changes
in
line
voltage
or
load
on
the
supply.
The
gain
of
V8B
is
high
enough
to
keep
the
output
at
the
V7
cathode
regulated
LEGEND:
DIRECTION
OF
CURRENT
FLOW
POSITIVE
HALF
OF
CYCLE
———»
NEGATIVE
HALF
OF
CYCLE
———=
posimve
HALF
oF
crete
/_\
NEGATIVE
HALF
OF
cycLE
/
\
Figure
4-2.
Simplified
Schematic
of
Meter
Bridge
Circuit
4-2
00102-2
T.O.
33A1-12-349-1
to
within
+1
volt
de
as
the
V7
plate
voltage
is
varied
+10%,
with
about
60
ma
of
load
current.
The
response
of
the
regulating
circuits
is
fast
enough
to
reduce
ripple
in
the
output
voltage
to
less
than
1
millivolt,
supplementing
the
filtering
action
of
C30.
C36
couples
the
ripple
com-
ponent
in
the
regulated
output
directly
to
V8B
to,avoid
attenuation
in
R62.
R57
shunts
a
small
portion
of
the
load
current
around
V7
to
prevent
excessive
V7
plate
dissipation
at
high
line
voltages.
R63
and
C35
constitute
a
low-pass
filter
which
prevents
noise
generated
in
V9
from
reaching
V8B.
4-17.
The
heater
supply
for
the
voltmeter
tubes
is
divided
into
two
sections.
One
section
supplies
d-c
voltage
for the
tubes
in
the
input
cathode
follower
and
00102-2
Section
IV
Paragraph
4-17
the
amplifier.
The
other
section
supplies
a-c
voltage
for
the
tubes
in
the
power
supply.
The
voltage
required
for
the
heaters
of
tubes
V1
through
V4
is
obtained
from
6.3V
and
7.3V
secondary
windings
of
transformer
T1,
which
are
series
connected.
The
voltage
developed
across
the
two
series-connected
windings
is
rectified
by
full-wave
rectifier
CR3,
reduced
to
12.6
volts
by
R66
and
R68
in
parallel,
and
applied
to
the
series-
parallel-connected
heaters
of
V1
through
V4,
as
shown
in
figure
5-10.
The
series-parallel
connection
of
the
four
heaters
establishes
a
voltage
of
6.3V
for
each.
The
heater
of
V5
receives
6.3V
ac
from
one
of
the
wind-
ings
which
drives
CR3.
The
heaters
of
V6, V7,
and
V8
receive
6.3V
ac
from
a
separate
6.3V
secondary
winding
on
T1.
4-3/4-4
T.O.
33A1-12-349-1
Section
V
Paragraphs
5-1
to
5-8
SECTION
V
MAINTENANCE
procedures.
If
an
adjustment
or
replacement
of
parts
is
made
without
following
instructions
or
understanding
the
effects,
further
trouble
shooting
may
be
complicated.
5-1.
SCOPE.
5-2.
This
section
contains
complete
instructions
for
repairing
and
calibrating
the
voltmeter.
This
material
is
covered
in
the
following
groups
of
paragraphs:
b.
Do
not
remove
tubes
when
the
voltmeter
is
turned
on.
Before
replacing
tubes
refer
to
paragraph
5-10.
Lead
F
Paragraph
tals
5-5.
TEST
EQUIPMENT
REQUIRED.
5-3.
Precautions
5-6.
The
test
equipment
required
for
complete
testing
5-5.
Test
Equipment
Required
of
the
voltmeter
is
listed
in
figure
5-1.
Equivalent
5-7.
Meter
Zero
Adjustment
instruments
may
be
substituted
for
those
listed.
5-9.
Cabinet
Removal
5-10.
Tube
Replacement
5-7.
METER
ZERO
ADJUSTMENT.
5-13.
Replacement
of
Special
Parts
5-17.
Trouble
Shooting
5-8.
The
meter
is
properly
zero-set
when
its
pointer
5-20.
Testing
the
Power
Supply
rests
over
the
zero
calibration
mark
on
the
meter
scale
5-22.
Testing
Voltmeter
Performance
when
the
instrument
is
1)
at
normal
operating
tempera-
5-24
Calibration
and
Frequency
Response
ture,
2)
in
its
normal
operating
position,
and
3)
turned
Adjustments
off.
Adjust
the
zero-set
if
necessary,
as
follows:
5-3.
PRECAUTIONS.
a.
Allow
the
voltmeter
to
operate
for
20
minutes
so
that
the
meter
movement
will
reach
normal
operating
5-4.
Observe
the
following
precautions:
temperature.
b.
Turn
the
voltmeter
off
and
allow
one
minute
for
all
capacitors
to
discharge.
a.
Make
no
adjustments
and
replace
no
parts
in
the
voltmeter
except
as
described
in
one
of
the
following
Electronic
Multimeter
0
to
300
a-c
and
d-c
volts;
accuracy
of
+3%
or
better;
input
impedance
100
megohms.
Voltage
and
resistance
ME-26B/U
or
measurement.
H-P
410B
Oscillator
10
cps
to
300
kc;
3
volts
Signal
source
for
Voltmeter
Calibrator
(Precision
Voltage
Source)
Frequency
Response
Test
Set
Oscilloscope
or
AC
Voltmeter
Variable
Transformer
D-C
Current
Test
Set
(Milliammeter)
00102-3
output
into
50-ohm
load,
400-cps
output
voltage;
0.001
to
300
volts
in
10-db
steps
+0.2%;
0.1
to
1.0
volt
in
0.1
volt
steps
+0.2%.
300-ke
to
4-mc
range;
3
volts
output
into
50-ohm
load;
10-db
steps,
0
to
70
db.
10-cps
to
4-mc
range.
Adjust
line
voltage
between
103
and
127V
ac
with
1-amp
load.
Clip-on
type
measurement;
current
range
up
to
100
ma.
testing
and
calibration
Calibrating
voltmeter
at
mid-frequencies.
Calibrating
voltmeter
frequency
response.
Trouble
shooting
by
signal
tracing.
Checking
voltmeter
operation
with
varying
line
voltage.
Checking
load
on
power
supply.
Figure
5-1.
Test
Equipment
Required
160B
or
400D
CN-16/U
or
Ohmite
VT2
5-1
;
1
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
T.
O.
33A1-12-349-1
switch
assembly.
Use
the
following
procedure,
(Locate
parts
by
referring
to
figures
5-3
and
5-4;
RANGE
switch
connections
are
shown
in
figure
5-9.)
a.
Remove
voltmeter
cabinet.
(See
paragraph
5-9.)
b,
Loosen
setscrews
in
RANGE
switch
knob
and
remove
knob.
ce.
Disconnect
capacitor
C104
from
switch
S1,
d.
Disconnect
white
leads
from
capacitors
C14
and
C16.
Label
each
lead
with
a
tag.
e.
Remove
the
two
screws
and
one
nut
which
retain
the
switch
shield
plate.
f.
Disconnect
white
leads
from
switch
contacts,
Tag
each
lead
to
permit
easy
connection
to
the
new
switch.
g.
Disconnect
the
heavy
dark-green
switch
lead,
the
heavy
light-green
switch
lead,
and
the
heavy
black
switch
lead
at
terminal
strips.
Tag
each
lead.
NOTE
The
input
shield
must
be
removed
for
access
to
the
terminal
board
connection
of
the
dark-
green
lead,
h.
Remove
the
nut
which
holds
the
switch
bushing
to
the
front
panel,
i,
Remove
RANGE
switch
assembly.
j.
The
sequence
for
installing
the
replacement
RANGE
switch
assembly
is
the
reverse
of
the
removal
procedure.
k,
After
replacement
of
switch
S1,
check
the
calibra-
tion
and
frequency
response
of
the
voltmeter
and
make
necessary
adjustments.
5-17.
TROUBLE
SHOOTING.
5-18.
The
first
step
in
trouble
shooting
is
to
learn
the
nature
of
the
symptoms
of
the
malfunction
with
as
much
detail
as
possible.
Inspect
the
test
setup
being
used
when
symptoms
of
malfunction
were
observed,
to
be
sure
that
the
source
of
trouble
is
not
external
to
the
voltmeter.
Then
remove
the
voltmeter
cabinet
as
directed
in
paragraph
5-9
and
inspect
the
circuits
of
the
voltmeter,
looking
for
signs
of
overheating,
deteri-
oration,
and
physical
damage
or
tampering.
Check
the
fuse.
If
the
fuse
is
blown,
try
another
fuse
to
see
if
it
blows;
if
it
does,
measure
the
d-c
resistance
of
filter
capacitors
C1,
C17, C30, C39,
rectifier
CR3,
and
the
windings
of
transformer
T1
to
locate
the
short
circuit
without
applying
power
to
the
voltmeter.
5-19.
If
the
voltmeter
can
be
turned
on
safely
(without
the
fuse
blowing),
measure
the
line
voltage
applied
to
T1
and
the
voltmeter
power
supply
output
voltages
(see
paragraph
5-20).
Check
the
tubes
of
the
power
supply
if
the
regulated
voltage
is
not
the
proper
value
or
is
unstable.
Use
the
procedures
of
figure
5-5
and
the
tests
described
in
paragraph
5-22
to
learn
the
full
nature
of
the
trouble
symptom.
Watch
for
marginal
00102-2
Section
V
Paragraphs
5-17
to
5-21
operation
by
operating
the
voltmeter
at
103
and
127
line
volts
while
making
tests.
Check
the
tubes
in
the
voltmeter
amplifier.
Measure
the
tube
element
voltages
at
the
tube
sockets
and
compare
readings
with
the
values
shown
in
the
voltage
and
resistance
diagram
in
figure
5-8.
Apply
a
test
signal
to
the
input
and
measure
the
voltage
of
the
test
signal
while
tracing
it
through
each
coupling
network
and
each
stage
of
amplification.
Compare
readings
with
those
shown
in
the
block
diagram,
figure
4-1.
In
figure
4-1,
an
a-c
current
probe,
H-P
Model
456A,
is
recommended
for the
measurement
of
a-c
current
in
the
meter
circuit
without
breaking
any
leads.
If
this
current
probe
is
not
available,
avoid
measurement
of
the
a-c
current,
Check
meter
indica-
tions
as
directed
in
paragraph
5-22
instead.
An
oscilloscope
may
be
used
for
observing
test
signal
waveshape
and
measuring
amplitude,
if
desired,
5-20.
TESTING
THE
POWER
SUPPLY.
5-21.
The
regulated
power
supply
produces
a
constant
+250
vdc
to
operate
all
the
tubes
in
the
amplifier
section,
The
stability
of
the
voltmeter
depends
directly
upon
the
stability
of
the
+250
volts
from
the
supply.
When
the
supply
is
operating
satisfactorily,
the
+250
volt
output
remains
constant
and
the
ripple
level
on
it
remains
less
than
about
1
millivolt
for
line
voltages
between
103
and
127
volts.
Weak
tubes
(V6,
V7,
and
V8)
are
the
usual
causes
of
instability.
An
unstable
regulator
tube
is
indicated
by
excessive
line
frequency
ripple
and
varying
output
voltage
as
the
line
voltage
is
changed.
Marginal
operation
is
indicated
if
a
trouble
symptom
appears
only
when
a
low
or
high
line
voltage
is
applied.
To
test
the
complete
power
supply
proceed
as
follows:
a.
Connect
the
voltmeter
to
an
adjustable
line
trans-
former
so
the
applied
line
voltage
can
be
varied
between
103
and
127
volts.
Set
line
voltage
to
115
wlts,
turn
on
the
voltmeter,
and
allow
a
five-minute
warmup
period.
b.
Measure
the
d-c
voltage
between
V6
(pin
8)
and
ground.
Normal
value
is
410
+
10
volts
with
exactly
115
volt
power
line
input.
Lower
line
voltage
10%
to
103
volts
for
2
minutes.
If
the
d-c
voltage
slowly
drops
below
360
volts,
replace
V6.
c.
Measure
the
d-c
voltage
between
V7
(pin
1)
and
ground
with
line
voltage
adjusted
to
115
volts.
Cor-~-
rect
value
is
250
+
5
volts.
d.
Vary
line
voltage
from
103to127
volts.
The
d-c
voltage
observed
in
step
c
must
not
change
more
than
+1
volt.
For
wrong
voltage
and/or
poor
regulation,
replace
V7,
V8
or
V9.
e€.
Measure
the
a-c
voltage
between
V7
(pin
1)
and
ground.
Ripple
voltage
must
be
less
than
3
mv
for
any
line
voltage
(103
to
127
volts).
High
ripple
voltage
is
caused
by
defective
V8, V7,
V6
or
V9.
Replace
in
this
order.
f,
Measure
the
direct
current
in
the
lead
from
V7
(pin
1)
which
must
be
less
than
60
milliamperes.
If
the
current
is
much
too
high,
the
regulator
circuit
will
not
function
properly.
Excessive
current
indicates
5-3
Section
V
T.
O,
33A1-12-349-1
v7.
c
coz
OOIV
ADJ-4mc
(HIGH
FREG
comp)
ra
.
.
;
nor
“oolv
ads
400cPs
v+—~
ve
—
SH
ae
lik
2
a2
(26V
eee
HEATER
SUPPLY
INPUT
SHIELD
REMOVED
R101
Iv
ADS
sf
MOORS
"S
c4#
tv
ADS
20Kc
Figure
5-3.
Left
Side
View
of
Voltmeter
Chassis
5-4
T.O.
33A1-12-349-1
Section
V
Paragraphs
5-22
to
5-23
Figure
5-4.
Right
Side
View
of
Voltmeter
Chassis
a
short
circuit
or
partial
short
in
the
circuits
of
the
voltmeter
amplifier
section.
A
clip-on
type
milliam-
meter
should
be
used
for
this
measurement.
g.
If
the
output
voltage
is
stable
but
is
incorrect,
measure
the
resistance
of
R62
and
R64.
The
ratio
of
these
two
resistors
determines
what
the
output
voltage
will
be.
If
the
value
of
one
of
these
resistors
is
in-
correct
and
produces
the
wrong
output
voltage,
replace
it
with
a
resistor
which
provides
the
correct
output
voltage.
h.
Measure
the
d-c
voltage
across
C39A
which
must
be
12.6
volts
with
a
line
voltage
of
115
volts.
If
nec-
essary,
adjust
R66
to
obtain
12.6
volts.
If
the
voltage
cannot
be
set
to
12.6
volts,
check
the
a-c
voltage
from
the
associated
transformer
windings;
also
check
CR3
and
C39.
5-22.
TESTING
VOLTMETER
PERFORMANCE.
5-23.
The
following
test
procedure
checks
the
accuracy
and
stability
of
the
voltmeter
at
low
and
high
frequencies
00102-3
and
with
low and
high
line
voltages.
It
can
be
used
for
comprehensive
incoming
inspection,
for
proof
of
per-
formance,
and
for
trouble
shooting.
If
the
readings
are
within
specifications
during
these
tests,
the
voltmeter
is
operating
properly.
This
test
is
made
without
removing
the
cabinet.
Instruments
used
to
test
the
accuracy
of
the
voltmeter
(see
paragraph
5-5)
must
be
known
to
have
sufficient
accuracy
to
make
valid
measurements.
Proceed
as
follows:
a.
Connect
the
voltmeter
as
shown
in
figure
5-6.
(This
setup
measures
calibration
accuracy
at
mid-
frequencies.)
b,
Set
the
line
voltage
to
115
volts,
turn
the
voltmeter
on
and
allow
a
30-minute
warmup
period,
c.
Check
the
instrument
meter
zero
setting
as
in-
structed
in
paragraph
5-7.
‘&
Connect
the
voltmeter
tothe
voltmeter
calibrator;
set
voltmeter
RANGE
switchto.
001,
and
set
voltmeter
calibrator
VOLTAGE
SELECTOR
switch
to
provide
0
volts
output.
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