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RCA Crystal-Calibrated Marker Generator WR-99A
Figure 7. Sweep-response
curve for picture-if amplifier and de'-
tector. Marker shows picture-carrier frequency.
(2) the sound carrier frequency must be at 1.0% or
less of maximum
response.
The picture
carrier
is
placed at approximately
50% of maximum response
because of the nature of single' sideband transmission,
the system used in transmitting
television signals.
If
the circuit is adjusted to put the picture carrier too
high on the response curve, the effect will be a general
decrease in picture quality caused by the resulting low-
frequency accentuation;
placing the picture carrier too
low on the curve will cause loss of the low-frequency
video response and result in poor definitio~.
Loss of
blanking and proper synchronization
will also occur.
The skirt selectivity of the picture-if curve is made
sharp enough to reject the sound component
of the
composite signal.
The sound carrier is kept at a low
level to prevent interference with the video signal. To
achieve this selectivity
in split-sound
receivers,
an
absorption
circuit, consisting of a trap tuned to the
sound intermediate
frequency, is used. Some receivers
include additional traps tuned to the higher frequency
of the adj acent channel sound carrier. These traps have
a marked effect on the shape of the response curve.
Alignment
of traps is described
under the headinz
"T
~
rap Alignment".
Checking Response of Individual Stages
The response of individual 'if-amplifier stages or of
two or more stages together may be checked by setting
up the sweep generator
and WR-99A as shown in
Figure 8, The sweep signal is fed into the stage im-
mediately preceding the stage being checked,
The re-
sponse curve is checked on the oscilloscope, which is
connected across the second-detector
load resistor.
Be-
fore attempting sweep alignment of the amplifier sec-
tion, the age circuit should be rendered inoperative by
either disconnecting the age bus or by using a bias box,
as described in the section, "Eliminating
AGC Action".
The rf cable from the sweep generator is connected
to the grid of the if-amplifier stage ahead of the stage
CRO,
WR-8tA
o
TO
MIXER
o
0
TI
1
OtT
LOAD
RES
Figure 8. Method for checking response of third if-amplifier stage
and detector.
to be checked to isolate the test equipment
from the
stage being checked. The rf output cable should not be
connected to the grid of the stage being checked be-
cause even slight loading of the high-impedance
grid
circuit may cause a change in circuit impedance and
result in distortion
of the normal response character-
istic, The marker signal from the WR-99A should be
fed to the grid of the mixer tube.
With equipment set up as shown in Figure 8, a re-
sist-or of small value, such as 470 ohms, should be
connected across the primary of the following if trans-
former. The resistor acts to swamp the primary winding
and prevents inductive reactance of the winding from
affecting the bandpass characteristics
of the amplifier
being checked. The shape of the response curve on the
oscilloscope will be determined by the bandpass char-
acteristics
of the amplifier
stage and the detector
circuits.
If it is desired to check the bandpass characteristics
of the detector circuit only, the sweep generator output
cable should be moved from point "B" to point "C"
and the swamping resistor, R., placed across the pri-
mary of T4. Response of the second and third if ampli-
fier stages and the detector
stage together
may be
checked by moving the rf output cable to point "A"
and connecting the swamping resistor across the pri-
mary of T2.
Trap Alignment
One or more traps may be contained in the rf unit
and picture and sound if amplifiers, depending
upon
the type of receiver.
Traps are included to attenuate
specific frequencies, such as adjacent picture and sound
carriers,
or picture.
and sound-if signals in v.ari~us
parts of the receiver.
(Continued on page 18)
. 13·
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