Western Electric 407A-4, Instruction Manual

Page: 57 / 136

the series circuit, composed of R44A and Cl3A, until th e
undesired oscillation disappears. When the carrier is ad­
j usted for normal output modulate the tra..�smitter with a
single frequency of 400 cycles, and observe the wave for
different values of modulation. I f a spurious oscillator
appears it will be necessary to adj ust one or both elements
of the series circuit until the oscillation is eliminated.
Tne setting of the "Feedback Adj.
11
control should be in­
creased and the above procedure followed for each setting
until a total feedback of 31 or 32 db is obtained. This
would represent an input level of approximately 0 db at 100
per cent, 400 cycle modulation. Observations should also be
made with carrier output from zero to normal and when modu­
lating at different levels with an audio frequency of 50
cycles. The final value of R44A should not be less than 500
ohms when all adjustments are completed. When the test s in­
dicate no spurious oscillations for this value of feedback
the "Feedback Adj . " control should be reduced until the in­
put level for 100 per cent, 400-cycle modulation is -4 db or
the value of feedback is 28 db. This gives an operating
margin of 3 to 4 db .
Whenever maj or adj ustments of the radio-frequency cir­
cuits of the transmitter are to be made, th e feedback should
be removed by opening link Dl7A. For minor radio-frequency
adjustments the feedback can be eliminated either by remov­
ing tube V9A or reducing the setting of the "Feedback Adj . 11
control to zero.
Audio Monitoring Rectifier
A radio-frequency rectifier tube VlOA and its associated
circuit furnish a source of audio monitoring output from the
transmitter. A 500 ohm termination should always be provided
for the output of TllA . This can be either in the form of a
resistor or a matching transformer. The output level with
50