T EVIS i - AmericanRadioHistory.Com

quency definition probably wasn't
noticeable in the first place anyway,
due to the excessive "snow" present).
Similarly, the load resistor of the
video detector may be considerably increased
in value, to improve the overall
picture quality. A reduction in the
bias of the video amplifier may also
produce quite beneficial results.
It is possible that the changes just
described may impair the receiver's
synchronization due to the sync clipping
they may promote in the video
amplifier. In such a case, the sync
take -off point may be moved to the
video detector (if the original take -off
point is in the video amplifier circuit).
The change in take -off point may
not produce happy results in noisy
fringe locations, since a smaller sync
signal will be fed to the sync stages
when the sync take -off point is
changed; also, the noise accompanying
the sync signal will be worse, i.e.,
greater, due to the elimination of the
noise -clipping action generally provided
in the video amplifier. If changes
of the kind to be described improve
the sync performance, the video stage
changes may be left "as is." Other -
wise, some gain -promoting changes
will have to be sacrificed to restore
sync performance to normal.
If the receiver is being operated in a
very noisy fringe area, and a sync preamplifier
with a negative -going signal
input is present, reducing the plate
voltage of the stage may provide better
sync performance. Lowering the
plate voltage will reduce the cut -off
bias, that is, the bias necessary to produce
cut -off, and thus more effective
clipping of noise pulses that exceed
weak negative signal peaks in amplitude
will result. In other words, the
tube will act more like a limiter. The
desired reduction in plate voltage is
achieved by adding a bypassed dropping
resistor of suitable value in series
with the plate load resistor (see Fig.
2). The bypass condenser may be .05
or .1 pfd. The resistor's value will
have to be determined experimentally.
A 100,000 ohm potentiometer may be
used in place of the resistor. The pot
is adjusted to a point where optimum
results are noted in picture synchronization.
Then its resistance at this setting
is measured and a suitable resistor
is added in place of the pot.
Vertical sync stability is frequently
unsatisfactory in fringe areas. When
incoming signals are extremely weak,
the undesired symptom may be a continual
roll of the picture. If the received
channels are weak, but not as
weak as in the preceding case, vertical
slipping may affect a frame now and
then, particularly when a strong noise
pulse is getting through the integrator
(Fig. 6).
A number of changes may be made
to remedy this instability. One such
change is to chop off one resistor in
the integrator. Such a change is illustrated
in Fig. 3. The .002 pfd. condenser
is changed to .005 to compensate,
in part, for the elimination of the
resistor.
April. 1953
REMOVE 52K
22
(
9.2K 8.2K
TO VERTICAI.
OSC KFPOT
.002 .005 .0047 aRCDIT
CHANGE.002 '.0.005
T
Fig. 3. In a Jackson Industries' receiver
(as well as in other sets) vertical stability
was imp oved by removing the 22,000 ohm
resistor in the integrator and increasing
first .002 pfd. condenser to .005 pfd.
The vertical sync pulse amplitude
at the output of the integrator will
now be : arger. at the expense of an
impairm( nt in the discrimination of
the integrator against horizontal sync
pulses. aorizontal sync pulses will,
in other words, be able to charge up
the integrator to a greater extent
than formerly. Since the horizontal
sync pulses are weak (in fringe areas)
no impai:ment of interlace is likely to
result.
Another change that may be tried
(in place of the one just suggested) is
to increase the amplitude of the vertical
sync signal applied to the sync
separator by changing the values of
the components in the voltage divider
input circuit of the separator. In a
circuit li w the one shown in Fig. 4,
for exarr.ple, the series resistor may
be reduced from 47,000 ohms to 10,000
ohms, ani the coupling condenser increased
f mm 220 ppfd. to .001 pfd., reducing
the loss in vertical sync signal
across these components and increasing
the input across the grid resistor.
The grid resistor in this case is reduced
in value from 10 megohms to
1.5 megoltms, to prevent an excessive
horizontal sync input to the separator,
and also to keep the vertical sync input
corre2t in size. The addition of a
small condenser, approximately 47
ppfd. in size, will further aid in keeping
the horizontal sync input from being
too large in amplitude. Similar
changes nay be made in other circuits,
exa:t values of components used
depending on the results of experimental
st- bstitutions.
The changes described will alter the
frequency response characteristic of
the video amplifier. Possibly smearing
may resu t. The smear will, however,
generally be masked by the excessive
Fig. 6. Vortical slipping. Retrace lines indicate
thc:t blanking pulse amplitude has
decreaserl, probably due to increased a.g.c.
47K CHANGED
TO
FROM VIDEO AMR.
SYNC. TAKE -OFF
2201414 POINT.
CHANGED TO
.001", f
SYNC. SEP.
MEG. RESISTOR
CHANGED TO 1.5 MEG.
4Tiefd. ADDED
Fig. 4. Increasing the vertical sync input
to sync separator to improve vertical hold.
Fig. 5. (A) Inserting a noise filter (dotted box)
in series with sync separator input to reject
noise and improve vertical stability. Circuit
shown is from Admiral 21 series. The
18.000 ohm resistor was originally an 8200
ohm unit. The new value reduces the impairment
of high -frequency response in the video
amplifier. (B) Noise filter insertion in the
Motorola TS -89, TS -94. and TS -95 chassis. The
100 ppfd. and 20 ppfd. condensers and the
470.000 ohm resistor were added. See text.
noise signal present so that, from the
standpoint of over -all picture quality,
an improvement may be noticed.
In another method used to improve
(Continued on page 141)
Fig. 7. An example of the type of pattern
obtained when the picture is incorrectly
phased in the horizontal direction.
49