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ELECTRICAL CIRCUIT
R2
M
12AT1
HIGH
IMPEDANCE
MIKE
2
S. B.
CARBON
MIKE
B
4.5T06V.
DC
T
5
AUD0
OUTPUT
300
TC5
6.3V.
,(C4
AUDIO
OUTPUT
Fig.
8.
Suggested
microphone circuits
for
use
with
the
SSB
Jr.
Circuit Constants
(All
resistors and capacitors
*20%
tolerance
unless
specified otherwise)
Cs, Cz, Cs
0.05
mf
400
volt paper
R,
470 ohm,
3
watt
Ca, Cs
8
mf
450
volt electrolytic
R7
47,000 ohm,
M
watt
Rs
2
megohm,
3
watt
Re
33,000 ohm,
M
watt
R2
0
1
megohm,
1
watt
T
Microphone to grid transformer
R3
0
5
megohm,
Mj
watt
Adjust the horizontal and vertical
gains on the
oscilloscope to produce
a
line
about
WI inches long
slanted
at
45
degrees when the oscillator
is
set to
a
frequency
of 490 CPS (an exact method of
setting
frequency
will
be described
later).
If
the
oscilloscope
has negligible
internal phase shift the display
will be
a
straight
line
instead
of
a
narrow slanting
ellipse.
If the latter display appears
it
is
necessary to correct
the
oscilloscope
phase shift externally by using an
adjustable
series resistance (a 50,000 ohm poten-
tiometer) mounted at either the vertical
or
horizontal
input terminal, depending
on
what correction
is
necessary.
At any rate, the objective here
is
to get
a
single
straight
line
at
490
CPS. In some cases
a
series
capacitor may
be needed to provide
the necessary
correction. Try values from
0.05
to
0.0005 mf. Now
shift lead
1
from
terminal
A
to terminal
B on
the phase
shifter. Adjust the trimmer
of
C7
to obtain
a
circle
on
the
oscilloscope.
It
will
be
noted
that
as
this
ad-
justment
is
made the display
will
shift
from an ellipse
"leaning" to
one side
through
a
circle or ellipse
(with
axes parallel to
the deflection axes) to an ellipse
which leans the
other
way.
If
desired or necessary,
the appropriate
gain control
on
the
oscilloscope
may
be
changed
so
that
a
circle
instead
of
a
"right"
ellipse
is
obtained at the point
of
correct
adjustment.
After changing the
gain control on
the
oscilloscope,
check
(and correct,
if
necessary) the phase shift
in
the
oscilloscope by moving lead
1
back to terminal
A,
and then repeat the setting
of C,
with lead
1
back
on
terminal
B.
In general, always make certain
that
the
oscillo-
scope
is
used
in a
phase -corrected manner.
As a
double-check (if the deflection plates
in
the
oscillo-
scope
are skewed,
for
instance) connect lead
2
to
terminal A'.
If
the
circle changes
to
a
slanting
ellipse,
readjust
C7
to produce an ellipse "half -way" between
the
ellipse
(obtained by switching
lead
2)
and
a
circle. Changing lead
2
from
A' to
A
and back again
should
give equal and opposite skew to the display
when
C7
is
set correctly. Failure to get symmetrical
ellipses (egg
-shaped,
or
other display)
is
due to
dis-
tortion, either
in
the
oscilloscope,
the oscillator, the
transformer,
or
the cathode
follower.
Conduct the
test at
as low
a
signal level as possible
to avoid
dis-
tortion.
Next connect
leads M and N to terminals E and
E',
respectively. Connect leads
1
and
2
to E, set the
oscil-
lator frequency to
1960
CPS, correct oscilloscope phase
shift
as before, and move lead
1
to terminal
G.
Adjust
C10
for a
circle as was done for
C7,
using
the precautions
outlined
for
that
case.
Now connect lead M to
terminal
D,
and lead N
to
terminal F. Connect
leads
1
and
2
to terminal D,
set the oscillator frequency
at
1307
CPS, correct
oscilloscope phase shift as before,
and move lead
1
to
the junction
of
R9
and C,. Adjust
C9
for
a
circle
on
the
oscilloscope, as before.
Repeat the
above procedure for
the remaining
R
-C
pair,
Rs
and C,. Use terminals D and
C
this time
and set the oscillator
for 326.7
CPS. This completes
except
for
a
final check
the adjustment
of
the phase
-
shift network. Connect
A
to A', E to
E',
B
to
C,
F to
G,
and
A
to E.
Be
certain to remove the temporary
short circuiting connections between the
12AT7
plates and
T2, T3.
If
the
oscilloscope did
not require changes
in ex-
ternal compensation
over the four frequencies used
an over-all frequency check can now be made easily
on
the
phase -shift network. To do this, connect lead
1
to point
B, C, lead
2
to point F,
G, lead M
to point
A,
A', E,
E',
and lead N to point
D. Now
shift
the arm
of
the potentiometer toward
M
until
a
circle
appears
on
the
oscilloscope screen
at
a
frequency
of
250
CPS. Then,
as
the oscillator frequency
is
varied
from
250
CPS to
2500
CPS, this circle
will
wobble
a
little
from one side to the other, passing
through
a
perfect circular display at
440, 1225
and
2500
CPS.
The audio band
over which
the
wobble indicates
a
plus
or
minus
1.3
degree
deviation
from
90
degrees
is
225
to
2750
CPS,
or
12
to
1
in
range. This means
that
when
other circuits are properly adjusted,
a
7
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