sary to set the A.C.
meter to ranges
any
higher than
about 3
to 12 volts AND
a blocking
condenser is
not required when
the
voice coil
winding is
isolated from the tube
D.C.
volt-
ages, except
in
some special circuts
involving
inverse
feed-
back via
series
connection
with
the
speaker
output
winding.
Inasmuch as the output
meter
is
connected to
the
audio
amplifier, a
modulated
R
V,
signal is
required so that
the
detector
can deliver
an auuio
signal voltage to
the
amplifier
circuit
proportional to the
R.F. circuit
adjustments.
There-
fore, all
trimmers and
padders are
adjusted until
a peak
indication is
obtained on the
output
meter.
One note of caution :
always keep the R.F.
input signal at
a
point whereat none
of the stages of the
receiver is ever
overloaded.
When using an
A.C.
output meter with receivers employ-
ing
A.V.C.,
it is necessary
(if not using the
A.V.C. substitu-
tion
method) to operate with
small signal generator
output
to
keep below the
point where A.V.C. action
levels off
the
receiver
response.
Otherwise a broad
selectivity curve
results,
with
consequent
difficulty
in obtaining correct peak adjust-
ment of
the
trimmers, as described on
page
(5)
.
In
this
connection it may
have
been
the reader's
problem
to
have noted that
when using other types of
Signal
Gen-
erators, with the
output set below the
A.V.C. threshold, there
was
not enough audio
voltage
appearing at
the amplifier out-
put stage to
properly swing the output
meter. Such
difficulty
is
usually a
matter of insufficient audio component,
or too
little
modulation of the carrier, which generally
had
been
fixed at
approximately
30%.
It is
in cases
such
as
these that
the
high modulation
per-
centage capability of
the
Series E-200 and
E-200-C
is of
particular
advantage. Simply raise the modulation
to
the
level
required
for good output meter indication
on the
low-
est output
meter range employable. In this manner, although
the R.F.
signal
intensity can
be readily maintained below the
point at
which A.V.C. action takes place, the audio com-
ponent OF
THE CARRIER CAN BE INCREASED BY
OVER
300%
and
naturally
the audibility
or
utility
of the
R.F.
test signal
is
proportionately
increased.
See page
(20).
Because of
variable modulation percentage,
as well as the
A.V.C.
substitution
system, incorporated
in the E-200
Series,
the simple
A.C. output
meter need
be the
ONLY
one
used for
all
cases of
A.M. receivers, making for
standardized align-
ment
procedures.
Other types of
resonance indicators
may, of course,
be
employed, but the
previous means is
by
far
the simplest.
A
few of the
other methods
include
the following,
and
as will
be
noted,
make use of an unmodulated
R.F. test signal.
2.
—The
reader
is,
by
this
time,
familiar
with
the basic
theory
of
automatic volume control. The A.V.C.
voltage
developed
in the
second detector circuit, is directly propor-
tional
to
the
intensity
of the R.F. signal voltage
to
which
the receiver
is tuned.
Hence, the relative magnitude of the
A.V.C.
voltage should
be a good means
of determining
when
the various receiver
circuits
are properly
aligned.
Any de-
vice
that will indicate the
relative value of
the A.V.C. volt-
age,
WITHOUT
SERIOUSLY DISRUPTING
THE A.V.C. CIRCUIT,
can
be
employed.
The A.V.C.
diode load
resistor
may be anywhere from
100,000 ohms to
2 megohms.
The measuring device
for
a
resonance
indicator
should be either
a vacuum
tube volt-
meter or high sensitivity
(20,000
ohms
per volt) multi-range
meter.
In this
connection there is
another valuable point
to
consider.
If the receiver
incorporates
a TUNING EYE, this
tube is actually
a form of
vacuum tube voltmeter and is
already
bridged
directly across
the A.V.C. circuits,
for reson-
ance indications.
In this
case
the eye may be adjusted for
minimum
shadow, whereas
the V.T.V.M. or multi-range
meter is adjusted for
MAXIMUM
indication.
3.—If
a receiver employs
a
power
detector in the form of
a triode or pentode,
a V.T.V.M.
or
20,000
ohm
per volt
multi-range
meter may be used
to
measure the D.C. voltage
developed
across the
cathode
load resistor. This voltage
is
also proportional
to the
intensity
of the R.F.
input to
the
detector.
4.
—
Inasmuch
as the R.F. and
I.F. amplifier
tubes in
A.V.C.
controlled
receivers
have their grid bias directly
con-
trolled
by the voltage
developed in the
diode detector circuit,
the plate
current, or for that
matter,
the cathode or screen
current
of the R.F.
or I.F. stages
also varies in direct
pro-
portion
to the degree
of alignment.
A D.C.
milliammeter
in
the plate, screen
or cathode
circuit
of one
of
the A.V.C.
con-
trolled
tubes may
be employed
as the
resonance indicator.
In such
case
adjustments are
made for
MINIMUM
meter
swing.
The reason for this
is quite
obvious when
it is under-
stood that the stronger
the
R.F. signal,
the greater
will be
the A.V.C. voltage.
Consequently
the
greater will
be the
negative bias
on the controlled
stages,
and along
with
a
greater negative bias
comes
a decreased
amount of
current
in the
various tube circuits.
The
D.C. milliammeter
can be
one
of the
low
current
ranges
of your A.C.-D.C.
multi-range
meter.
5.—If the A.V.C. controlled
tubes employ
a fixed mini-
mum
bias
resistor
in the
cathode returns,
a low range,
high
resistance voltmeter
can
be
connected
across
any one of these
fixed bias resistors.
Since
the cathode
current decreases
with increased R.F. signal
(
due
to A.V.C.
action)
,
the
volt-
age
developed
across the
cathode bias
resis
tor
will
also
de-
crease, and we would
adjust
trimmers and
padders
for
minimum voltmeter
swing.
6.—There are some receivers
which
employ tuning
meters
rather than tuning
eyes. These
tuning
meters in
most cases
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