B&K 415 - Sweep Alignment Methods

2.3.2 SWEEP ALIGNMENT METHODS
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The best method of checking alignment and
determining which stages require alignment
is to inject an r-f sweep frequency signal at
t?e tuner antenna terminals. The age bias
lme must be clamped by application of bias
or grounding the age line. The outputs of the
1-f and chroma circuits are then observed on
an oscilloscope and compared to the manu-
facturer's recommended response curves.
~he technician can then decide which por-
tions of the receiver require alignment. For
example, if the i-f response is satisfactory but
!he chroma response is not, then the problem
1s
~etween the video detector output of the i-f
stnp and the output of the bandpass ampli-
fier. If the i-f response and the chroma
re-
~ponse ?re
poor then it is most likely that the
1-f
requires touch-up, particularly if the re-
sponse is poor on the slope aflecting chroma
response.
The r-f portion of the tuner seldom creates an
alignment problem because the passband is
so much greater than that of the i-f section;
however, the mixer output circuit, which is
loc_at~d on the tuner, may require attention.
This 1s part of the tuned matching network
between the tuner and the first i-f stage. A
separate prealignment procedure is given for
the link circuits by some manufacturers.
Once the deficient portion of the receiver is
determined, an alignment check of that sec·
tion can be performed. The alignment proce-
dures vary with manufacturers. Some sug-
gest signal combinations at the tuner antenna
terminals which can generate i-f and video
sweep frequencies in the receiver so that
over~ll alignment can be done by selecting
the nght combination of input signals. One
way of doing this is to first connect an r-f
sweep generator
for i-f alignment. After this
1s complete, the picture carrier frequency for
the channel being used is selected and this
is modulated by a video sweep signal (This
1s the
VSM, or video sweep modulation,
method). This video sweep modulation is
demodulated at the video detector of the TV
n:_icei:7er and applied to the chroma bandpass
circuits for the alignment of these stages.
Other
marn1f?cturers recommend an i-f sweep
~reque~cy miected at the mixer grid (or base,
if tr';lns1storized) for i-f alignment. The i-f pix
earner frequency (45.75 MHz) is then mod-
ulated with a video sweep voltage
(VSM
again). As before this is detected at the video
detector of the TV receiver and the recovered
sweep voltage is used for the chroma circuit
alignment.
Another method is to first video-sweep align
the chroma circuits directly. The i-f is then
aligned and then video sweep modulation of
the i-f pix frequency (45.75 MHz) is used to
check the combined effect on the chroma re-
sponse of i-f alignment and chroma align-
ment. Usually a touch-up of the chroma cir-
cuits is done to obtain the desired final overall
chroma response.
In conjunction with i-f alignment practically
all manufacturers recommend pretuning
i-f
traps by injecting spot frequencies into the i-f
(usually at a specified tuner test point). Other
procedures outline a prealignment of all
tuned circuits in the i-f before sweep align·
ment procedures.
In all cases the manufacturer's method is the
best for his particular receiver and the manu-
facturer's service manual is preferred for
alignment. SAMS Photofacl procedures are
also reliable and in most cases repeat the
~anufacturer's procedure. If complete rea-
lignment of an apparently deficient receiver
does
_n<Jt restore the required response, the
techmcian must then consider that a com-
ponent failure has occurred and must employ
standard trouble shooting procedures.
The quantity of test equipment required to
align any set by any method as well as the
work involved for setting up a series of align-
ment tasks on a given receiver are a serious
consideration for the serviceman and the
technician who must perform efficiently. With
this in mind we have developed the
Mode}·
415 SWEEP/MARKER GENERATOR which
enables you to do
any set with no additional
equipment other than an oscilloscope and a
VTVM. All "inlercabling changes and gen-
erator selections" are done internally for you
by a master function switch and front panel
controls. Because a single output jack is
used, several different outputs can be
switched to the same injection point on the
receiver
v<:7it~out disconnecting or changing
leads.
This 1s done by selecting the desired
output with the function switch. All outputs
are properly terminated and stray radiation
from the unit is negligible so that there are
no_ co_nfusing "hot" leads in the test set-ups.
This mstrument is unequalled in flexibility
and functional design.
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