How often have we all mistaken 'spooks' for
Barkhausen oscillations? Yeah, it's embarrassing, but we've
all done it. I can't tell you how many times as a kid I saw
the tell-tale effects on our old black and white TV and said,
"Mom, can you remind Dad to do something about those dang Barkhausen
oscillations when he gets home from the newspaper office?" If
you believe that line of bull hockey, I've got some waterfront
property in the Sahara Desert to sell you. The only thing close
to 'Barkhausen' I might have known back then was the name of
a German beer house on
Hogan's Heroes (for which
I own the entire
DVD set). Anyway, this article, written in the days
of over-the-air television broadcasts, presents a solution to
the annoying 'spook' effect caused by poor oscillator circuit
shielding.
The Spook - Another Weird Effect to Haunt TV
By M. B. Knight
Tube Department, Radio Corporation of America
A simple solution to the problem of electromagnetic radiation
from. horizontal deflection circuits of set.

Fig. 1. Hand-made r.f. chokes used for minimizing
spook problem.
A number of interesting phenomena, sometimes not anticipated
in fundamental studies, have cropped up in the electronic art.
These phenomena often are identified by colorful names which
are intriguing in themselves. The "spook," an interference effect
in television receivers, may be destined to take its place in
the language of the trade with such terms as "ghost," "barks,"
"motorboat," "birdies," "snow," and "jitter."
Description of Spook
The spook originates as electromagnetic radiation from the
horizontal deflection circuits of television receivers and is
picked up by the sensitive r.f. or i.f. circuits of the receivers.
Like any other signal in the r.f. or i.f. circuits, the spook
signal is amplified, detected, and applied to the grid or cathode
circuit of the kinescope. When seen in the picture, it appears
as a narrow vertical band very close to the left-hand edge of
the raster and resembles the more familiar interference from
Barkhausen oscillations in the horizontal-output tube. If the
television signal is weak compared to the spook signal, the
line is black and has ragged edges as shown in Fig. 2. If the
television signal is of normal strength, the line is not black
but has within its margins crawling diagonal lines which are
caused by heterodyning between the television signal and the
spook signal. The appearance of the spook when the television
signal is of normal strength is shown in Fig. 3. Despite the
similarity to Barkhausen oscillation, several distinguishing
features establish the spook as a separate effect: (1) The line
is always in the same place in the raster, very near the left-hand
edge. (2) The interference, if picked up in the r.f. circuits,
is always strongest on the lower-frequency channels, whereas
Barkhausen may be more pronounced on either the low or high-frequency
channels. (3) Experiments show that the radiation does not come
from the horizontal-output tube, nor do the usual cures for
Barkhausen oscillation, such as magnets, have any effect on
it.
Actually, the spook seldom has a serious degrading effect
upon receiver performance. Because of its location on the raster,
it is usually within the blanking period and it is almost always
off the kinescope screen because receivers are normally adjusted
to have a good margin of deflection width. The more common degrading
influence of the spook is that it upsets the receiver synchronizing
circuits. Because of the action of the detector circuit, the
spook shows up in the video circuits as a pulse in the "black"
direction, similar to the sync pulses. If it is of sufficient
amplitude, the spook pulse will pass through the sync amplifier
along with the regular sync pulses and may impair the operation
of the horizontal a.f.c. circuits.
Occasionally, two receivers are close enough together so
that one picks up the spook interference from the other. The
resulting picture disturbance is quite objectionable; if the
receivers are tuned to different stations (the line-scanning
frequencies of which may differ slightly), the spook line may
move back and forth across the picture.
Discovery of Spook
To the best of our knowledge, the spook phenomenon passed
unnoticed, or at least uncommented upon, for three or four years
of commercial receiver production. There are several reasons
for this delay. First, because of the usual practice of scanning
beyond the kinescope mask, the spook line is rarely seen. Second,
because the intensity of the spook radiation is a function of
the deflection power, it has become more evident as larger kinescopes,
having larger deflection angles and accelerating voltages, have
come into popular use. Third, the older deflection circuits
were susceptible to Barkhausen oscillations, and the spook,
even if observed, could easily be dismissed as Barkhausen.
The advent of modern high-efficiency deflection circuits,
however, called attention to the spook as a unique effect. The
writer's first encounter with the spook occurred about two years
ago during the development of the RCA- 223T1 horizontal-deflection
transformer. The narrow vertical band appeared to be due to
Barkhausen oscillation; further investigation, however, showed
that cause to be unlikely because careful measurements established
that the plate voltage of the horizontal-output tube was not
negative at any time during the scanning cycle. Other engineers
observed the effect at about the same time and established that
it was not Barkhausen. No parasitic oscillation could be found
and the mysterious nature of the effect caused it to be dubbed
the "spook." The name seemed apt and has persisted. The effect
was distressing to receiver designers, mainly because of its
elusive nature, and efforts were made by the writer to locate
the cause. After some investigation, a reasonable explanation
was found, and methods of minimizing the interference were easily
devised.
How Spook Is Generated
In the investigation of the nature and cause of the spook,
a separate television receiver was used to search for the most
prominent source of radiation. Although some radiation could
be detected from most parts of the deflection circuit, the damper
tube and its leads produced the strongest radiation.

Fig. 2. Appearance of "spook" interference
with weak signal.

Fig. 3. "Spook" interference with signal
of normal strength.
Scrutiny of the current waveforms in the horizontal deflection
circuit showed that the spook line appears at the same instant
that the damper tube begins conduction, approximately one microsecond
after the completion of retrace. The damper tube plate current
rises from zero to its maximum value of 350 to 400 milliamperes
very rapidly. The rise time of the current was not measured
precisely, but avail-\able equipment indicated that it was 0.1
microsecond or less. At any rate, it was apparent that the electromagnetic
fields associated with such a rapid change of current and voltage
must contain many high-frequency harmonics. It could be expected,
therefore, that the high-frequency harmonics could be radiated
to the signal circuits of the receiver.
This theory was checked by exploring the radiation spectrum
with a communications receiver. The receiver was tuned from
about 300 kilocycles to 18 megacycles and a signal was detected
at every harmonic of 15,750 cycles. A more significant observation
was that no other signal was detected. The intensity of the
harmonics diminished steadily as the receiver was tuned to higher
frequencies. In addition, spook interference was found to be
most severe on television Channel 2 and was successively less
severe on higher-frequency channels. If the harmonics were being
radiated as a result of the rapid plate-current change in the
damper tube, it would be assumed that high-frequency harmonics
would be of less amplitude than low-frequency harmonics.
Small r.f. chokes were placed in the leads to the damper
tube at the socket and the radiation was reduced considerably.
The residual radiation came almost entirely from the internal
structure of the tube itself. Further tests confirmed that the
interference originated with the rapid change in the damper-tube
plate current.
Minimizing Spook Interference
The rapid rise of plate current in the damper tube is inherent
in the proper operation of deflection circuits. Practical means
for slowing down the increase in current are not at hand, and
it is not expected, therefore, that the spook can be eliminated
entirely. It is possible, however, to reduce considerably the
detrimental effects.
One approach to the problem of reducing spook interference
is to minimize the susceptibility of the r.f. and i.f. circuits
to the radiation by physically separating the r.f. and Lf. circuits
from the deflection circuits. This separation is chiefly a chassis
design problem; good chassis layout in this respect is normal
in commercial receivers. The technician is more concerned with
the installation of the receiver; he should be sure that the
antenna lead-in is dressed away from the deflection circuits.
Not much can be done along this line if an in-cabinet antenna
is used.
A second approach to this problem is to minimize the radiation
from the deflection circuits. Because the damper tube and its
leads can be thought of as a transmitting antenna which radiates
the spook, a logical approach is to make the transmitting antenna
as small as possible and to provide a shield between this "antenna"
and the receiver r.f. and i.f. circuits. It was mentioned before
that insertion of r.f. chokes in the leads to the damper tube
limited the "antenna" to the tube structure itself. The value
of the chokes is not critical, but must be large enough to be
effective in the television band without being so large that
ringing is caused in the deflection circuit. Chokes having inductance
values between 1 microhenry and 5 microhenrys are suitable and
are available commercially. Chokes for the plate and cathode
circuits can be made by winding approximately 30 turns of AWG
#28 enamel or Formex wire on a one-watt resistor. Ordinarily,
it is not important to insert chokes in the heater circuit.
If heater chokes are used, however, wire at least as large as
AWG #20 should be used to carry the heater current. The stiffness
of this size wire makes a coil form unnecessary; a coil of approximately
20 turns about 3/8 inch in diameter is adequate. Fig. 1 illustrates
typical hand-made chokes.
After chokes have been placed in the damper tube leads, it
is desirable to shield the tube from the receiver r.f. and i.f.
circuits. The high-voltage enclosures used in most receivers
provide adequate shielding. The shield should be inspected to
see that it is grounded at as many points as possible. If there
are any large holes in the enclosure, they may be covered with
ordinary copper wire screen to improve the effectiveness of
the shield. Capacitive coupling between the damper tube and
any leads which come out of the high-voltage enclosure should
be minimized by careful lead dress. A close-fitting shield around
the tube, however, is neither necessary nor desirable because
of the resultant increase in bulb temperature.
In the commercially popular auto-transformer and direct-drive
types of deflection circuits, experience has indicated that
spook interference can be greatly reduced by the addition of
only one r.f. choke. In such circuits, most of the radiation
usually comes from the "B+" lead which is connected to the plate
of the damper tube. (The cathode lead is quite well shielded
by the high-voltage enclosure.) The r.f. choke, therefore, should
be placed in the plate lead of the damper tube at the socket.
The addition of a condenser of approximately 100 μμfd.
between the chassis and the "B+" side of the choke may give
further improvement.
The techniques suggested for reducing spook interference
are not all-inclusive. Each type of deflection circuit and each
mechanical layout requires individual attention. It is expected,
however, that an understanding of the source of the interference
will be of help to the troubleshooter when spook problems appear.
Posted June 27, 2015