MAINTENANCE
I. CALIBRATION: After
construction
of
the instrument is
completed,
it
is
necessary
to
carry
out
the calibration
procedure
described below.
If
a
change
occurs in the accuracy
of
the instrument
aftera long
period
of
use, it is
probably
due to aging
of
the components.
The accuracy
of
the
instrument
may readily
be restored by repeating this
calibration
procedure.
Recalibration
wil
I also
be necessary
,
whenever parts
(tubes
,
etc
.)
are
replaced.
A.
I N
I
Tl
AL
STEPS: Follow
the
procedure
described
in
"INITIAL
STEPS"
in
the
OPERATING
INSTRUCTIONS
section.
In addition,
check to
see
that
the isolation resistor,
R—
3
1 ,
is
properly connected within the
D-C test probe.
B.
D~C
VOLTMETER CALIBRATION;
(See
Fig.
2)
To
calibrate the
d-c
voltage ranges,
use
two flashlight batteries connected
in
series.
The
terminal
voltage
will
be 3. 10 volts when fresh
batteries
are
used. Set
the
FUNCTION
switch to "-DC"
and the
RANGE
switch
to
"5V".
Short
the
D-C
(red)
probe
to
the
COMMON
lead
(ground)
and
turn
the
ZERO ADJ.
control until
the
meter
pointer
is at zero (ignore
any
change after the test leads
are
disconnected).
Connect
the batteries between
the D-C
test leads
with the COMMON (ground)
lead
touching
the positive
side
of
the batteries. Adjust the "-DC"
calibration
potentiometer R-29
until
a 3.
10 volt reading
is
obtained
on
the meter
(3.10
on
the
0
to
5
D-C
scale).
To
calibrate
the
"+DC
n
voltage ranges, repeat
the
above
steps
with
the
FUNCTION switch set
at
"+DC" and
the
positive end of
the cells connected
to the D-C probe. Adjust the "+DC"
calibration
potentio-
meter
R~27.
NOTE:
The electrostatic
pickup
which
appears
on the lowa-c
and d-c
voltage
ranges, when either the
AC“OHMS
or
DC
probe is held or
touched is normal
in
a
sensitive vacuum
tube voltmeter,
due
to
the extreme
sensitivity
of
the
instrument. However,
if
the AC'OHMS or DC probe (depending upon
the
function)
is
shorted to the
COMMON (ground)
lead
when
the zero adjustment
is
made,
the zero
obtained
will
result
in correct meter
readings
and no
error
will
be introduced
because
of
electrostatic pickup.
C.
A-C
VOLTMETER CALIBRATION: (See
Fig.
2)
To calibrate the a-c
voltage ranges, set the FUNCTION
switch
at
"AC" and the
RANGE
switch at
"1000V".
Short
the
AC”OHMS
(black)
test lead to the COMMON
lead
(ground) and turn the ZERO ADJ.
control until the
meter pointer is
at
zero.
Turn
the
RANGE
switch to the
"5V"
position and
adjust
the A“C
shift balance
potentiometer,
R-5, until the
meter pointer returns
to
zero.
Then turn
the
RANGE switch to the
"500V"
position; the
meter pointer
should move very
little, usually
not
at
all. Connect
the COMMON and AC
-
OHMS test
leads
to
the
1
15
volt
A-C supplyand adjust theA-C
calibration potentiometer,
R30,
unti I the meter reads
115 volts
.
Cal
ibration
with
the
1
1 5
vol t A~C
supply
wi
1
1
result in
the instrument being accurate
within
5%.
If
greater
accuracy
is
desired, the
instrument should be cal ibrated against a
known
,
standard
A-C
voltage.
-
9
“
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