Western Electric 407A-4, Инструкция по эксплуатации

Страница: 56 / 136

The s l ider adjustment on resistances R6 0A, R61A, R6 2A
and R63A, with the approximate res is tances and voltages which
shou ld b e obtaine d are given in Table 4. Measurements of
these resis tances can be made after removing the interconnect­
ing and ground leads from R61A and R6 2A.
Adjus tment of Feedback and As s ociate d Circuits
After the entire transmitter has b een tuned, adjust­
ments of the feedback circuit and associated circuits should
be made . Approximately 28 db of feedback is requ ired to ob ­
tain the des ired low nois e level of 60 to 6 5 d� b el ow 100
per cent single-frequency modu lati on.
To measure the feedback, remove the feedback recti fier
tube V9A of the oscillator amplifier and modulate the trans­
mitter w ith a s ingle f�equency of 400 cycles . The audio in­
put level should b e approximately -32 db at 100 per cent
modulati on. When the tube is r eplace d the audio input level
for 100 per cent modulation should be -4 db .. The difference
of the two input levels, -32 db and -4 db, or 28 db is then
the value of feedback .
The fo llowing procedure should b e fol lowed to obtain
the above value of stable feedb ack. For initial tests the
capacity value of condens er C37A should be 50
:mm.f'.
However,
after the adjustment of the transmitter is approximately
co.ro.plete, and b efore d istortion measurements are made, con­
denser C 37A should b e s lightly readjus te d for minimum dis­
tortion at the h igher audio frequencies . The "Feedb ack
Adj . 11 control should be set at ab out 50. With the "RF Out­
put" control set at z ero, operate the "High-Voltage Recti ­
fier" switch on the control unit. Slowly increase the
transmitter output and ob s erve the carrier on an os ci l lo­
scope . If s purious os cillati on appears on the carrier, it
will be necessary to adjust either one or b oth elements of
49