nections to
the plate side of interstage audio transformers,
resistance or
impedance coupling
networks or to any
other
circuits
wherein the potential is either above or below
ground.
Before aligning
a
radio receiver, it
is
always good practice
to test the
audio section first,
by
applying
a
stage-by-stage
signal.
In
this manner one may readily locate the
cause
of
non-functioning in a
receiver
as being due solely to circuit
troubles in the
audio section.
Note: When it is desired to
feed the Audio Test Signal
into
a low
impedance circuit, such as a
15, 50,
200,
500,
or 600
ohm,
etc.,
input, the
audio output
jacks should not be connected directly
to such loads or
else-
the
audio
oscillator
may
be
thrown
out
of
oscillation due
to the reflected
low impedance into the high
impedance
oscillator grid and
plate networks. In such instances,
an
appropriate impedance matching
transformer
or resistive
pad should be
employed to transform the high
impedance audio
"T" output
to the
desired
low impedance. A pad
effect may be
fflt
roughly approximated
by utilizing a 50.000 to 100,000
ohm re-
o-°
sistor
in series
with
the test
lead from the "high" pin jack and
a
resistor across
the
low impedance amplifier
terminals. This
latter resistor
should
approximate the input impedance in
order
to
properly load the
amplifier input circuit.
For
example, if the input impedance of an
amplifier were
500
ohms and
an impedance matching transformer was
not
available to
match the audio oscillator
output, nor a properly
designed pad,
merely place
a 50,000
to
100,000
ohm re-
sistor in
series with the lead from the "high" pin jack to
the
high side of
the amplifier 500
ohm input, and then a 500
ohm
resistor directly
across the 500
ohm input terminals;
with
the
"ground" pin jack, of course, connected to
the
low or ground side of
amplifier
input terminals.
When
doing this,
the
maximum voltage
actually
supplied
to the
amplifier input terminals is
materially reduced,
since
the two resistors
act (in addition to an
impedance matching
device),
as a
voltage divider. In this
particular case, only
about
500/100,000
or l/200th of the oscillator output, or
about .5 volt
maximum is available; but this is still more
than
sufficient
to
feed the input stage of an
amplifier.
Many
other applications
will
be
found
for using
your
Signal
Generator.
As an
example,
the
voltage available
through the A.V.C.
Substitution
System is pure D.C., and is
completely variable from zero to
approximately
JjfJ
volts._
The
D.C.
voltage output is obtained from the
polarized
"A.V.C. Voltage" tip
jacks. This voltage may
be
employed
in place of batteries in tests or experiments as long as the
current drain is not above
approximately
Wjniljiarripergs,
A few other
applications of this A.V.C. Voltage
Substitu-
tion System are:
1.
Ohmmeter circuits
2. "B" Supply for low current drain
45 volt battery
operated radios and hearing aids
3. Grid bias for audio as well as R.F. amplifier
stages
4.
Polarizing voltag
es
for testing leakage in
low workin
g
voltage
electrolytic condensers, etc.
5.
^
<W-i>
I
S^SLS
g
GENERALIZED
R.F. ALIGNMENT
AND
ADJUSTMENT
and
use of
THE A.V.C. SUBSTITUTION METHOD
It is almost impossible to give exact alignment
procedures
for the many varieties of circuits that will
be
met with in the
course of one's servicing
experience.
It is advised that
AT
ALL TIMES direct reference
should
be made to either
service
manuals
or
manufacturers' instructions for specific details.
The
manufacturer
is
always best
qualified
to state the special
steps
necessary for proper
alignment of his particular
set.
The
information which
follows
is
therefore presented
in
an
all-inclusive nature, being
fully
aware of the fact that
there
is
no
rule
so
definite,
to
which there is no exception.
However,
the general approach is in all cases more or less
similar.
Before beginning
alignment, all
tube
shields and
chassis
bottom
plate (if any)
, if
possible,
should be in
position.
The
line-up
screw driver or socket-type trimmer adjusting tool
should have
little or no metal, to prevent capacity effects
upon the tuned circuits. A "tuning
wand" is sometimes
desirable,
but
not
necessary,
to
determine
the direction of
the required alignment correction
before touching
the in-
ductive
or
capacitative adjusting
screws
or
nuts.
As
with
the Signal Generator, it is advisable
to allow
ap-
proximately a 10 minute warm-up
period
of
the receiver,
to
permit all circuits and
components
to come to normal operat-
ing temperatures.
Adjustments
may then commence
with the
Signal Generator
set to the same frequency
as
the
R.F. or
I.F. stages being
aligned.
USUAL ALIGNMENT
PRACTICE
and A.V.C.
It has been commonly accepted
practice to automatically
align
all radio
receivers
incorporating
A.V.C, with
the
volume control
set
at
maximum
and the
Signal
Generator
adjusted to deliver
very low signal output. This
has been
done
in an attempt
to
avoid the
broad resonance curve
effect
introduced
whenever
the signal was of sufficient
intensity
to
cause
action of the A.V.C. system
as
outlined
on page
(5).
But as
can
be
readily
proven and demonstrated,
a receiver
so
aligned has
not been
properly
adjusted for
NORMAL condi-
tions of broadcast
reception, because the volume
control, in
regular usage,
is
never
set to
maximum
position
and very
definite
A.V.C. voltage
(5
developed.
This
can be understood
when
one realizes that the received signal
is of much
greater
intensity than
the small Signal Generator
output, necessarily
employed to operate BELOW
the A.V.C. threshold.
Therefore, when the receiver is aligned
under
conditions of
maximum sensitivity
(as it
would
be
when
the A.V.C. system
is not
working), there
is a
very
great
difference between
the
grid
bias conditions
on the
R.F.
and I.F.
stages than when
the receiver is
installed
in the home
or elsewhere
for
normal
conditions of reception.
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