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RCA Crystal-Calibrated Marker Generator WR-99A
with the initial one for sensitivity, switching noise,
and general performance.
If the tuner is satisfactory· in these respects, it is
advisable to check the alignment by observing the
response curves for each channel. Curves for the in-
dividual channels should be examined and compared
with those shown in the manufacturer's
service notes.
If a response-curve check indicates that alignment is
required, the technician should refer to the alignment
curves given in the service notes as guides and follow
closely the recommended alignment procedure.
Alignment should not be attempted until these pre-
liminary tests have been completed. Futhermore, the
technician should be aware that most tuners, unless
tampered with, are correctly aligned. This knowledge
can often prevent misalignment of a good tuner.
The primary purpose of alignment is to obtain a
response curve of proper shape, frequency coverage,
and gain.
Most tuners merely require
"touch-up"
alignment in which relatively few of the adjustments
are used.
Generally, complete over-all alignment is
required only when a person with inadequate knowl-
edge or equipment has worked on the tuner.
For a
complete alignment job, it is desirable to follow a
specific sequence of adjustments, the sequence depend-
ing upon the type of tuner.
However, where only
touch-up alignment is required, the sequence of adjust-
ment is usually unimportant.
In principle, complete front-end alignment includes
alignment of the antenna input circuits and adjustment
of the amplifier and rf oscillator circuits. The antenna
input circuits are usually aligned to give a response
curve which has a sharp drop-off slightly below channel
2 and which is flat up through channel 13.
In effect, the input circuits, which consist of two or
more traps and high-frequency peaking circuits, act as
a high-pass filter. Their correct alignment is important
in keeping low-frequency interference from entering
the receiver through the tuner circuits. Adjustment of
P
5
Figur.
6. Typical tuner curve for channel 8.
the input circuits is usually critical but they seldom
require service.
Alignment of the rf, amplifier and oscillator stages,
however, is a more familiar job. Adjustments include
setting the oscillator. frequencies for channels 2 through
13, setting one or more traps to their correct frequen-
cies, and adjustment of tracking with the rf amplifier.
The converter transformer
may also require adjust-
ment along with the tuner.
Al! these adjustments require that a sweep signal
from the sweep generator and a marker signal from
the WR-99A be fed into the tuner so that a response
curve with markers will be reproduced on the oscillo-
scope screen,
Alignment is accomplished by setting
adjustments
so the waveshape on the oscilloscope
screen resembles the waveshape shown by the manu-
facturer in his service notes. The notes show separate
curves for each of the 12 channels. Each channel is
aligned separately to obtain the desired curve shape.
The marker signals from the WR-99A are USEdto pro-
vide frequency reference points to aid in shaping the
curve.
With the sweep generator set to deliver output on
channel 8, for example, and 4.5-Mc interval markers
injected from the WR-99A at 181.25 and 185.75 Mc,
a typical tuner curve for channel 8 will resemble that
shown in Figure 6. The markers on the curve show
the separation between the picture and sound carriers.
Since the rf sections of the TV receiver must pass both
sound and picture signals, a bandpass of approximately
6 Me is required. The locations of the sound and pic-
ture carrier frequencies for all vhf channels, which are
shown in Table I
l
page] 7' ,are individually marked
on the dial scales.
A necessary preliminary to alignment is a check of
!he test setup. For example, rf bias should be checked
and proper connection points for test equipment deter-
mined. All equipment should be given a 20-minute
warm-up time to enable circuits to stabilize operation.
If the tuner is to be aligned in the receiver, the tuner
curves should be observed with the first if-amplifier
stage out of operation. Removal of the first if-amplifier
tube is generally sufficient to avoid any curve distortion
caused by the if amplifier. In some tuners, resonance
in the mixer plate circuit may also produce undesirable
reflections.
Generally, to remedy this situation, the
picture-if amplifier input must be loaded or the
if-
transformer primary must be detuned.
The tuner oscillator should be in operation during
alignment. If it is not, the lack of oscillator injection
voltage at the mixer grid will alter the mixer bias,
.
"
.
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