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RCA Crystal. Calibrated
Marker Generator
WR·99A
SCOPE
M~RKER
GENER~TOR
WR-b4A
LOW-C~P~CITANcE
PROBE HERE
INTERMEDIATE
__.-o'IJ\
rREQUENCY
MARKER
o
o 0
SOUND
TR~NS
FM
DETECTOR
Figure 9. Test-equipment setup for alignment of sound-if
ClIlpJilier.
The test-equipment setup for alignment of traps in
the if amplifier is the same as that used in conventional
if alignment described above. A sweep·response curve
is obtained on the oscilloscope screen, and a marker
from the WR·99A, which is set to the frequency of the
trap, is fed into the mixer stage.
Because the response of the amplifier is very low at
the trap frequencies, the marker may often be difficult
to see on the response curve. The use of a marker-
adder unit, such as the RCA WR-70A RF /IF /VF
Marker Adder is recommended for trap alignment. The
unit is especially valuable in trap-alignment
applica-
tions because the marker is added to the response
curve after the sweep signal is taken out of the receiver,
thus eliminating all suckout. If a marker adder is not
used, the WR·99A should be set for maximum output
and the scope gain set to maximum to increase the size of
the marker. If difficulty is experienced, more precise
adjustment may be achieved by connecting a Volt-
Ohmyst, set for de-voltage measurements, across the
second-detector load resistor, and tuning the trap for
minimum voltage reading on the meter.
The general procedure in aligning picture-if ampli-
fiers is first to set the traps and then to align the am-
plifier circuits. Since any adjustment of the amplifier
circuits usually will slightly detune the traps, the traps
may have to be "touched up" during the picture-if am-
plifier alignment. The manufacturer's
alignment in-
structions will again determine the exact procedure to
follow.
Alignment of Sound-If' Amplifiers and
FM
Detectors
A typical test set-up for aligning an intercarrier-type
sound-if amplifier is shown in Figure 9. This system
employs a ratio detector which reeei ..es its signal di-
rectly from the last sound-if amplifier stage. Circuits
which use a discriminator detector will employ a lim-
iter stage ahead of the detector circuit.
An over-all response curve of the sound-if amplifier
and detector is ohtained by connecting the sweep gen·
erator and the "\\R·99A at point "A", the grid of the
first sound-if amplifier. Set the CAL/MOD control on
the WR·99:\ to "4.5 :\-IC". The oscilloscope is con-
nected at point "C", where a demodulated signal ap·
pears. The sweep width control on the sweep generator
should
be
set to ~ve a sweep width of approximately
1 Me. An "S'vshaped curve, similar to the curve shown
in Figure 10. will appear on the oscilloscope screen.
To check alignment of the first if-amplifier stage
only, the oscilloscope probe should be moved to point
"B" where a sweep response curve similar to that
shown in Figure 11 should be obtained. The marker
indicates the 4.5·Mc center frequency of the curve. If
the marker does not appear exactly at the center of the
curve, the amplifier should be adjusted
as recom-
mended by the manufacturer
until the marker
is
exactly at the center of the curve. This assures that re-
sponse is symmetrical and adequate audio bandwidth
is obtained.
• 18 •
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