Western Electric 407A-4, Instruction Manual

Page: 73 / 136

emis sion of the tubes in the power ampl ifi er is suffic ient
to furnish the increas ed peak power .
T o faci litate the modulating adjustment, the operator
should calculate in advance what his antenna current or
transmiss i on-l ine current is to be at complete modulation,
in acc ordance with the ris e he wishes to obtain with respect
to the carr ier current .
Adjust the "Feedback Adjustment" control on the oscil­
lator-ampl ifier to about one-fifth full scale . With the
filament voltages adjusted to 2 7 volts on the power amplifier
and 18 volts on the modulating amplifie r, and with 50 kilo­
watts carrier output from the transmitter, apply to the audio
input terminals the output of a 400-cycle os cillator of goo d
wave form. The modulated output wave of the transmitter may
be ob served by means of a cathode-ray os ci llograph connecte d
at jack JlD "V2 Plate . " Increas e the audi o input unti l on
the downward swing of modulation the radi o-frequency ampli­
tude comes just to z ero. Inspect the wave form to make cer­
tain that there is no s inging at any part of the wave. I f
the antenna current r i s e is not as great as i s desired with
2 7 volts on the filaments, the load impedance to the modu­
lating amplifier should be lowere d by changing the tap at
the high-potential end of L30C or by changing the pos ition
of the tap connecting terminal 63 to this co il. For each
tap setting the tuning condenser C5C "Plate C ircuit Tuning"
should be adjusted for minimum plate current on the modulat­
ing amplifier. The total plate current of the modu lating
amplifier should not b e increased to more than
o.
8 ampere at
the c arr ier output.
To re duce the antenna-current ris e, the filament volt­
ages on the power-amplifier tubes should be reduced alter­
nately in 0. 5-volt steps until a furthe r reducti on for
either tube would bring the antenna-current rise below the
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