T EVIS i - AmericanRadioHistory.Com

RECEIVER CHANGES TO
IMPROVE
FRINGE
RECEPTION
By
PAUL STEVENS
Fig. 1. Undesired regeneration in test pattern. Note how sections
of the vertical wedge patterns are unduly intensified.
Part 2. Concluding article covers tube changes for greater
sensitivity and other circuit alterations for fringe areas.
N LAST month's article we considered
some of the receiver changes
I that could be made to increase set
sensitivity in fringe areas. We pointed
out that. among other changes, the
a.g.c. voltage could be reduced or eliminated
if all signals coming in were
weak. If medium -level or strong signals
are received, as well as weak ones,
more uniformly beneficial results may
be obtained by removing the a.g.c. feed
to the r.f. amplifier alone, leaving the
controlled video i.f. stages as they
were.
Such a procedure is particularly recommended
when a booster is present.
Normally, with the a.g.c. circuit left
unchanged, the addition of a booster
increases the a.g.c. voltage and lowers
the gain of the r.f. amplifier. partially
canceling the benefits provided by the
booster. With the a.g.c. voltage removed
from the r.f. amplifier, however,
maximum benefits, i.e., best reduction
of "snow" and highest signal
amplification, will be obtained from
the booster.
Another change that may be helpful
is the replacement of the r.f. amplifier
tube with a similar type having a
higher gain. For instance, if a 6AG5 is
used as the r.f. amplifier, a 6BC5 may
be substituted. Similarly, tubes in
other stages of the receiver affectinir
picture and /or sound may be replaced
with more peppy types. Consulting a
late edition of a good tube manual
should supply the technician with
ideas regarding possible tube changes.
While we are on the subject of tube
changes. we should mention that poor
48
pick up of weak signals may, in some
instances, be due to a 6AL5 video detector.
Some of these tubes, while they
may perform well in other circuits,
have too high a perveance and tend to
change the i.f. response curve when
employed as video detectors. Tubes
put out by some manufacturers are
more apt to suffer from this defect
than those produced by others. Different
-make 6AL5's should. therefore, be
tried when tube -replacement checks
are made as a test of the tube's output.
Raising the plate voltage of the r.f.
amplifier by feeding it from a more
positive tap in the low -voltage power
supply may be feasible in some cases.
Reception might be noticeably improved,
if all incoming signals were
weak.
In all cases where a change is made
in the front end, a realignment of this
section, of a kind described in the preceding
article, is recommended.
Fig. 2. (A) Reducing the plate voltage of
a sync preamplifier by inserting a decoupling
resistor of suitable value in series with
plate load resistor. (B) To find correct
value of resistance, a pot may be connected
as shown and varied experimentally.
SYNC
PRE AMP
SYNC.
PREAMP
A method advocated by one manufacturer
(Philco) to boost set sensitivity
in fringe areas involves the introduction
of regeneration into the r.f.
amplifier. A 20'/. increase in sensitivity
on the upper channels, it is
claimed, may be obtained in this way.
The method is employed when a tube
like the 6AG5, having two cathode
pins, is used as the r.f. amplifier.
The tube is removed from its socket,
and one of the cathode pins is clipped
off. It is assumed that the socket's
two cathode contacts are connected.
If they aren't, and cathode circuit
components are present at each contact.
a short jumper should he run between
them. The tuner is then realigned
on the weakest (high -hand) received
channel. The theory behind the
method is that the relatively high capacitance
present between the suppressor
and cathode elements in the
r.f. amplifier. when one cathode is
disconnected. provides a feedback path
through which the video signal is regenerated.
The realignment should he carefully
made, since it is critical. Set sensitivity
will be highest on the channel on
which the receiver has been aligned,
and those channels adjacent to it. The
method is, of course. recommended for
use in fringe areas only since regeneration
on medium- and high -level signals
will markedly impair the fidelity
of the picture, at best (see Fig. 1),
and may lead to oscillation, at worst.
In some cases. increasing the screen -
grid voltage on the video amplifier
may produce a picture with better
blacks. Increasing the plate load resistor
of the video amplifier to, say,
one and one -half or twice its original
value may also effect an over -all improvement
of the picture. That is to
say, the greater contrast and better
picture lighting gained will more than
offset the less noticeable loss in high -
frequency definition (good high -fre-
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