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Sect. II Page 2
provides a maximum attenuation of 120 db in 10 db
ranges.
The output level may be varied between
ranges
by changing the input to the attenuator with
the ATTENUATOR
VERNIER
control.
Input is
monitored
by the rf level meter.
The 3 VOLT rf
output is obtained by reducing the dc component of
the demodulated rf feedback.
F.
CRYSTAL CALIBRATOR
A small
signal,
coupled
from
the attenuator,
is fed to the control grid (pin 2) of a mixer ampli-
fier
(triode
section
of V10).
The
output
of the
crystal oscillator (pentode section of V10) is also
fed to this grid.
The mixed
signal from V10 is
fed into an output triode amplifier, V9B. The beat-
frequency output of V9B is brought out to the front
panel
CRYSTAL
CALIBRATOR
OUTPUT
phone
jack,
The crystal oscillator consists of the pentode sec-
tion of V10 operating as an electron-coupled oscil-
lator,
Positive feedback to the control grid is ob-
tained from the screen
grid through the crystals.
Two crystals provide oscillation at either 1 mc or
100 kc, as selected by the XTAL
CALIBRATOR
switch.
The plate of the oscillator provides the
signal
necessary
to drive the mixer grid of the
triode section,
G.
AUDIO OSCILLATOR
The audio oscillator is a modified Wien Bridge
oscillator with amplitude stabilization.
The feed-
back signal is taken from the secondary
winding
of the output
transformer.
Two oscillator fre-
quencies
(400 and
1000 cps) can be selected by
Switching different resistors
in the Wien Bridge.
The level of oscillation is set by the MODULATION
OSCILLATOR
ADJUSTMENT
(R51) which controls
the feedback tothe lamp. Increased feedback causes
the lamp to heat up, increasing its resistance.
The
increased
resistance
causes
more
degeneration
in the cathode of V5, limiting the gain.
In the INTERNAL positions the modulating voltage
is available at the front panel MODULATION IN-
PUT-OUTPUT
jack.
This
voltage is supplied for
synchronization
purposes.
It has a source im-
pedance of 82K ohms,
H.
PERCENT
MODULATION
METER
CIRCUIT
The modulating signal goes to pin 2 of V9A, a
cathode-follower,
This tube feeds shunt diode CR5
which rectifies the signal.
A dc voltage corres-
ponding to the modulation is fed to the PERCENT-
Model 606A
AGE
MODULATION
meter.
Clamp
diode
CR4
prevents
the cathode of V9 from going negative to
protect
capacitor
C56,
whose
voltage
rating
is
20 volts.
This point would otherwise go towards
-200
volts
if V9
was
removed
or
was
weak,
The
modulating
signal also goes to pin 2 of V8.
The
signal
level
to V8 can be controlled by the
ATTENUATOR
VERNIER,
When
the signal level
is varied the output modulation
is varied, as ex-
plained in the following paragraph.
This circuit
is also used to reduce the voltage onthe RF Ampli-
fier during
switching.
As
soon as the RANGE
switch is rotated out of the detent position, S7 dis-~
connects
the +300
volt power
supply,
hence
grid
voltage of V8 drops to ground potential.
The B+
is not
reconnected
until after the turret has made
contact in its new position.
This action keeps the
amplifier
tubes
from
drawing
excessive
screen
current
when
the turret is disengaged from
the
plate circuit.
When
the +300 vdc supply is dis-
connected
R64 is substituted as a load to keep the
power supplies in regulation.
I,
DIFFERENTIAL AMPLIFIER
The
external
modulation
signal fed into the
MODULATION jack is combined with the de refer-
ence
level in the same
manner
as the
internal
modulation. Either of these signals is applied to a
resistive network consisting of R57, R62 and R68
and
is added
to the de reference voltage.
The
combined
voltages
appear
at the ATTENUATOR
VERNIER (R63),
This control varies the dc, which
controls
the carrier,
and the modulation signal at
the same rate.
Thus the percentage of modulation
remains
constant
regardless
of carrier
level.
The dc (carrier level) and ac (modulation) signals
are
fed
through
an
rf filter to pin 2 grid of the
triode
section of V8 as a reference signal to be
compared
to a signal from the output which is fed
to the pentode grid (pin 7).
This triode plus the
pentode section of the same tube forma differential
amplifier,
The ac level of both of these signals
is proportional to the modulation.
The modulating
signal is the reference voltage and the actual modlu-
lation of the output is compared to this reference.
The de level of the output (proportional to the rf)
is compared to a reference dc level that is pro-
portional
to the desired
rf level, as set by the
ATTENUATOR VERNIER control.
Since
the cathodes
of V8 are
connected togetler,
the reference signal applied to the triode section
will also appear
on
the cathode
of the pentode
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