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AUDIO
IWO
Om
POT
N
PASSE
SHIFT
NET-
WORK
The frequency ratios
(just
as
the resistance ratio
previously mentioned) are far more
important than
the actual values of frequency
(or
resistance) used.
Fig.
9.
Audio
phase
-shift network
test
layout
sideband suppression ratio of
39
db
is
possible at the
worst
points within this range. The average suppres-
sion
ratio
will be
about
45
db.
Proper
phase -shift
network operation
is
necessary to obtain this class of
performance,
so
the adjustment procedures have
been
explained
in
great detail
as an aid
toward this
goal.
The phase shift network should never require read-
justment,
so
that
when you are satisfied with the
adjustment
you may seal
the trimmers with cement.
r
It
will be
noted
that
the frequency ratios are such
that
the
12th harmonic of 326.7 CPS, the 8th har-
monic of
490
CPS and the 3rd harmonic of
1306.7
CPS are
all
the
same as the 2nd harmonic of
1963
CPS, namely,
3920
CPS. Thus,
if
a
stable source
of
3920
CPS frequency (such
as
a
thoroughly warm
audio oscillator)
be
used as
a
reference, the frequency
of
the test oscillator can
be
set
very
closely
to
one-
half,
one-third, etc., of this reference frequency
if
both oscillators
feed
an oscilloscope and the resulting
Lissajous
figures observed.
Use of
a
calibrating frequency
in
this manner
assures
that
the frequency
ratios used are
correct,
even
though the exact frequencies used are unknown.
R12
AUD.
BAL.
+
¡t
/
DPDT
SWITCH
Iá
(SB-REV.)
A.F.
INPUT
JACK
Install the
phase -shift network
in
the chassis,
remove the
6AG7
output
tube,
plug
in a
crystal
(3850
to
4000
KC)
or
supply
a
signal to the crystal
socket
from
a
VFO
at
not
less
than
a
10
volt (RMS)
level,
set
L1
and
L2
for
minimum inductance (slug
out, counterclockwise) and apply
power.
The current
drain should
be
about
35
to
40
MA
at
300
volts
under this condition with the oscillator operating. If
the current drain
is
over
45
MA,
turn
off
the
B+
power,
adjust
LI,
reapply
power, etc.,
until the crystal
oscillates. This may
be
checked by means of
a
re-
ceiver
tuned
to
the crystal frequency. Continue to
advance the
slug
in LI
with the crystal operating until
oscillation ceases. Then back the slug out
a
few
turns
to assure stable crystal operation.
For
VFO
input
simply
adjust
L1
for
minimum total current.
Apply an audio signal of
1225
CPS to the input
jack
of
the
exciter and connect the horizontal
de-
flection of
the
oscilloscope to
a
cathode (pin
3)
of the
12AT7, and the
vertical deflection to the other cathode
(pin
8)
after making certain
that
the
oscilloscope
is
phase -compensated
at the frequency
of
1225
CPS.
Adjust
RS
to produce
a
circle
on
the screen. Adjust
R12
to about mid -range. This test should
be
made at
a
reasonably
low
audio signal level (in general, the
lower
the better).
Now plug
in
the
6AG7,
after
checking to
see
that
a
bias of
about
101..f,
volts
is
supplied. Connect the out-
put link
on L;
to the vertical plates
of
the
oscilloscope
(no amplifier used).
Deliberately unbalance
one of
the
/
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7
MOD.
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BAL.
1
3"
38
/
MOD.
/
LOAD
/
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1---i-
\L3
R17
7"
+
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-
116
MOD.II
3"\
BAL 5x7"X2"
4
,
CHASSIS
P.
A.
--+
/L4
2
8
1r12AU7
I
SHIELD
I
I
SOCKET
12"X2!"
L-+-
R.
F.
I1,7
2
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GAIN\\
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Í
OSC.
IPLATE
PHASE
+-IO\
2
i
l
I
+
1
4
L
8
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XTALOR
VFO
4
1
Am,/
SOCKET
I"
I"
I"
I"
--i
4
16
18--I8
I16
~
IÍ6
Fig. 10.
SSB
Jr. panel
layout (front view)
P.A.
PLATE
3
15"
12
1"
1
1
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