5 .
Rese
t the RANGEsel
ector
to the
po
si
tion
wh
ich gives a reading nearest
to
full
scale
and read
the
ac
voltage.
NOTE: All RMSscales
are
inblack
with full-sca le values
of
1.5,5,15,50
,
500,
and 1500 volts. All
peak-to-peakscales
are
in
red with
full-scale
values
4,
14,42,
140,420,
1400,
and
4200 vol
ts
. When the instrument
is
set
at
the
1.5
volt
range, the
RMS
or P-P scales
on
the lowest arc (designated as
LOW
-
AC)
are
read; on higher ranges, the scales on the two
center
arcs
are
read.
It shou
Id
be noted
that
the
fi
xed
ratio
of
2 .
83
to 1 between corresponding
peak-
to-peak
and
rms
scales
is
derived
from
the r
elation
sh
ip between the
peak-
to-peak
and
rms
values
of
a sine wave. There fore, while peak-
to-peak
read-
ings
are
valid
regardless of whether the wavefo
rm
is complexor
sine,
rms
read-
ings
are
valid
only for sine waves.
Note
a l
so
that the
ti
me
delay between the
instant
the
leads
are
removed
from
the
source being measured and the instant
the meter pointer returns to
zero
is
normal and
is
the
result of
circuit
constants
selected
to permit
accurate
measurement
of
recurrent pulses with
low
repeti-
tion rates.
applications
This
instrument may be used
fo
maintain and service television receivers,
fm-am and communication
receivers,
transmitters, audio equipment, and
pul-
sed
electronic
and
electro-mechanical
eqpipment. Indicative
of
itsversatility
are
some of the special
applications
described below.
OSCILLATOR GRID-BIAS
MEASUREMENT:
The
negative
dc
voltage
devel-
oped on
the
oscillator
grid
is
always
directly
proportional to
the
strength'
of
oscillation.
This
voltage
can
be measured very
readily
at
the
oscillator
grid
while the band switch
is
turned to the various bands, and
In
each
of
its posi-
tions the main tuning condenser
Is
rotated
from
minimum
capacity.
This
will
give
an
Indication
of
the strength of oscillation
at
all frequencies within the
oscillator's
range.
AVC-VOl
TAGE
MEASUREMENTS:
The
automatic volume control
voltage
de-
veloped
by the incoming signal
can
be measured
at
a number
of
places in the
receiver.
This
negative
voltage
first appears across the diode load resistor. It
may also be measured along
the
avc
bus and
at
the grids of
therf
tubes being
controlled.
The
dc
voltage
measured
at
the
diode load resistor
Is
a very
con-
venient
output indication during
receiver
alignment.
Owing to
the
high Input resistance of this Instrument,
it
is
possible to meas-
ure bias
(avc)voltage
on the grid
of
rf and
If
amplifier tubes without disrupting
the
signal.
DC
SUPPLY
VOLTAGE
MEASUREMENTS:
Power supply dc voltages can
be
I
J
I
I
measured
at
the
rectifier
filaments and
in
the
filter
circuits.
Plate,
screen,
and
ca
thode dc voltages
can
be
measured
at
the corresponding pins of the tube
sockets.
BIASCELL
VO
LTA
GE
MEA
SU
REMENTS:
Th
is
instrument will
accur
a
telym
ea
s-
ure the
voltage
of a bias
cell
. Current drawing voltmeters
are
not
ca
pa
bl
e
of maki
ng
th
is
measurement and in many cases wi II damage the
cell.
DETECTION OF GA
SSY
TU
BES:
One effect of a ga
ss
y tube
is
to reduce the
normal
negati
ve
grid bias, or even make the grid
po
sit
ive.
Th
is
instrument
is
ideal for mea
su
ri
ng the
voltage
directly
at
the control grid of any tube in
order to
dete
rmine whether or not this e
ff
ect
is
presen
t.
Excessive gas
wi
II
cause the tube to c
ea
se operati
ng
norm
ally
, and in an audio amplifier will
usually cause
the
vol
ume
co
ntr
ol
to become noisy.
Th
is amount of gas will
not always produce a
no
tic
eab
l e change in the operation of the radio
receiver
.
Consequently if r
epeated
difficulty
is
experienced with volume
cont
rols
be-
coming noisy in this type of circui
t,
this instrument should be used to check
for incorrect bias.
OUTPUT
INDICATION:
To
measure output in
the
alignment of
am
and
TV
receivers, the instrument
is
prepared for de
voltage
measurement and
II
sua Ily
connected to the load resistor
of
the second
detector
while
the
circuit com-
ponents
are
adjusted for optimum output.
In
an
fm
receiver,
the
instrument
is
connected across the limiter load resistor. The
zero-center
feature
is
very
useful for
the
alignmf'nt
of
fm
discriminators.
DB
MEASUREMENTS:
In
order to avoid crowding
of
frequently used
scales,
there
is
no
db
scale
on the meter. Another reason for the absence of this
scale
is
that
there
are
many different reference levels in use and each reference level
resu
I
ts
in a
differentsca
Ie. Fig. 1
is
a graph for
one
accepted
reference
level,
namely
0.775
volts across 600
ohms
resistive load
(1
milliwatt), with which
rms
ac
voltage
readings
can
be
converted to db readings. However, the db
value
read
from
the
chart
is
correct only when the
voltage
reading has been
taken across a
600
ohm
resistive load.
If
the reading has not been taken across a
600
ohm
load, the db
value
read
from
the
chart
must be corrected by adding
a Igebraically to
it
the correction increment spec ifJed in the
chart
of Fig. 3
for the
particular
resistive load.
If
the resistive load
is
not included
in
the
chart,
the
cor
r
ection
increment
maybe
calculated
from
the followi
ng
formula.
Correction Increment
;0
10 log 600 (where R
is
the
resistive load)
R
It should
be
noted
that
decibel measurements must be made with a sine wove
form
to ovoid waveform error and
that
the
correlation between decibels and
ear
response
is
greatest
at
1000 cyc les .
· 7 ·
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