equal, the audio
balance control
arm
should
be in the middle
third of
its range or a
bad 12AT7'
or wiring
error, is indicated.
Another source of
possible
difficulty
in obtaining
unwanted sideband
suppression
is the audio
phase
shift network:
The
AC
voltages
atpins
5-1 and 3-7
of the audio
phase
shift
network
socket
b should be in the
ratio of 2 to
? for a steady
1000 cps audio
signal input. The lower
voltage
will
appear
on
pins
3-7
and the
ratio
should
be adjustable
by rotation of
the
phase
balance
control.
A morepreclse checkfor
proper
phase
shift
would betofeedthe
signal
from D2tothe
horizontal
input
and the
signal from
D? to the
vertical
input of an oscilloscope
and adjusting
both vertical
and horizontal oscilloscope
gains
for
equal
deflection,
Adjusting the
phase
balance
control'
a
perfect
circle
pattern
on
thJ scope
should be
obtained indicating
exact
90o
audio
phase
shift,
providing
the
phase
shifts
in the
horizontal
and
vertical oscilloscope
circuits are
equal.
The interstage
and modulation
transformers
can
be easily checked
as a
possible
source
of
dif-
ficulty by
me-asuring
the
DC
resistance
of the
windings.
The DC resistance
of the
primary
of
T4
is
65b
A
and the
secondary
is
40
Q. Both T3 and T1
exhibit a
primary
DC
resistance
of
625
5?
and a secondary
DC
resistance
of
850
O.
If no audio signalcan
apparentlybe
passed
through
the audio stages, useaVTVM,
signal
tracer,
or headset
(cipacitively
coupled
for safety)
to apply
normal audio trouble
-shooting
techniques.
The audio circuitry
is straight
forward.
If
the
actual
generated
sideband
is opposite
to that
indicated on
the
panel,
after
proper
operation
is obtained,
reverse
the
transformer
leads
to
lugs ? and
8 of the sideband
selector switch'.
RS'
Poor
linearity
in anyadapter
stagewill
be
indicated by
flattening or distorting
of the
transmitter
RF
output wave
form obtained
on an
oscilloscope
with the selector
switch
in AM
position
and a
1000
cps audio
input,
(This
is the
familiar
"T\tro-Tone"
test.
)
No
indication of
non-linearity
shouldbe apparent
before
rated
output
is obtained. Check
the operating
voltages,
especially
bias voltages,
on all stages
against
the voltage
chart.
AIso
check the
operating voltages
on the
final
ampllfier
of the companion
transmitter
per
the modification instructions
given
on
page 50'
Adjusting
the loading
of the
final amplifier
will
also affect
linearity and should be
checked.
If the
VOX
and/or
the anti-trip
circuits
appear
to be inoperative.
check the
negative
voltages
produced
at
pin
2 of the 6AL5
socket
B as
follows: Turn
the receiver sensitivity
full CCW and
the transmitter sensitivity
full CW,
and speak
into the microphone.
Note the maximum
peak
negative voltage which should
be
approximately
-40
volts, depending
on the microphone
output
voltage. Turn
the transmitter
sensitivity
full
CCW
and the
receiver sensitivity
fuli
CW.
Con-
nect the audio output
of a receiver
to
terminals
1
and 2 on the
rear of the SB-10
and
adiust the
gain
for
normal Iistening
voiume.
Note
the maximum
peak
negative voitage at 82,
which should
be
approximately
-90
volts,
depending
on
the
receiver volume. If either
of these voltages
is low
o. nii
present,
check the
c6rresponding
triode
amplifier
connections and the
6AL5
connections.
AIso
check for a bad tube at
either
of these
locations. If
the relay action
is
erratic,
check the
voltages at
F8 against the
voltage
chart
for the
conditions listed. If the
voltage is
improper,
check for an error
in function switch
wiring,
a short
in the voltage divider
network,
or a
bad
resistor.
Page
60
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