Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates generally to television
receiving apparatus, and specifically it relates to a horizontal
deflection circuit of the apparatus in which disadvantages arising
from a disconnection in the circuit of a deflection coil are eli-
mina1:ed.
In conventional television receivers, a circuit failure
in the horizontal deflection coil and its associated circuits
could trigger a chain of events which might result in secondary
failures including an overcurrent flow through a switching tran-
sistor and a burning of the fluorescent material on the kinescopescreen. Although a mere disconnection of circuit can be remedied
by locating and reconnecting the disconnected point, failure of
circuit components necessitates replacement with a new component,
which could sometimes result in a considerable expenditure. It
is particularly important to provide a failure-free apparatus
because of the increasing repair cost.
The primary object of the present invention is therefore
to provide an improved horizontal deflection circuit of a televi-
sion receiving apparatus in which a disconnection in the deflec-
tion coil and its àssociated circuits is prevented from triggeringsecondary failures of associated circuit components.
In accordance with the present invçntion, there is pro-
vided a horizontal deflection circuit for use in a television
receiver, comprising a horizontal oscillator for generating an
oscillation in response to a received horiæontal synchronization
signal, a switching transistor having a control electrode thereof
connected to the output of said oscillator to provide switching
actions in response to the generated oscillations, a sawtooth wave
generator connected to the controlled electrodes of said transistor
to generate a sawtooth wave voltage in response to the switching
action of said transistor, and means for energizing said oscillator
to allow it to generate said oscillations and de-energizing said
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oscillator when a DC voltage developed in said sawtooth wave gene-
rator reduces to a level indicatin~ a component failure of said
horizontal deflection circuit.
The invention will be understood by way of example from
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the following description taken in conjunction with the accompa-
nyinc3 drawings, in which:
Fig. 1 is a prior art horizontal deflection circuit;
Fig. 2 is an embodiment of the present invention; and
Figs. 3A-3C show a series of waveforms appearing in the
circuit of Figs. 1 and 2.
sefore describing the embodiment of the present inven-
tion, reference is first had to Fig. 1 in which the conventional
horizontal deflection circuit of a television receiver is shown.
The circuit 10 comprises a horizontal oscillator 12 which takes
its trigger signal from a sync separator (not shown) of the tele-
vision receiver which is adapted to receive television signals
including horizontal and vertical sync pulses, as well as a video
si~nal. The horizontal oscillator 12 generates square wave pulses
in step with the input trigger pulses. The output of the oscil-
lator 12 is connected to a horizontal output circuit 14 which
includes a switching transistor T, a damper diode D with its poles -
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connected in a direction of conduction opposite to th~ direction ~'
of conduction of transistor T through its collector-to-emitter
path. The anode terminal of diode D and emitter of transistor T
are connected to ground, and their respective cathode and collec-
tor electrodes to a positive voltage supply~ B through line 16.
A capacitor Cl is connected across the line 16 and ground. In
parallel with the capacitor Cl are connected a horizontal deflec-
tion coil L and a capacitor C2 connected in series to the coil L.
A flyback transformer 18 has its primary winding connected in se-
ries with the line 16 and its secondary winding coupled to a
kinescope anode via a rectifying diode 20. The horizontal os- `
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~cillator 12 is operated from the power supply ~B through line 22.
-~ 30The function of the horizontal output circuit 14 is to
generate the current,waveform depicted in Fig. 3A. Current flow ~ ,
through the horizontal deflection coil L during the first half of
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the forward scanning interval, from the left-hand side of the cen-
ter of the screen, is produced by current iTl. The second half
of the forward-scanning interval, from the center of the screen
to the right-hand side, is produced by current iT2. Retrace or
flyback is initiated at the end of the iT2 interval, and the first
half of the retrace period is produced by current iRl; the second
half of the retrace period is produced by current iR2.
During the second half of the forward-scanning or trace
period, switching transistor T controls the current flow to the
horizontal deflection coil L. Diode D controls the current flow
in the horizontal deflection coil during the first half of the
trace period. The deflection coil L, capacitor Cl and the primary
. winding of flyback transformer form a resonant circuit which con-
trols the current flow during the retrace period.
The output of the oscillator 12 is connected to the base
; electrode of transistor T of the output circuit 14. The oscillator
output illustrated in Fig. 3B has a pulse duration which ranges
from the instant that the deflection current is zero to the ins-
tant that the deflection current is at its positive peak value.
When transistor T is gated into conduction, a positive charge on
capacitor C2 will be released into the conducting transistor T
through its collector-to-emitter path to produce current iT2 which
charges capacitor C2 in the forward direction of diode D~ The
voltage across the capacitor C2 will decrease from point b until
- point c, as illustrated in Fig. 3C. Upon turn-off of transistor
T at the end of the trace interval, a counter-electromotive force
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will be generated in coil L due to the collapsing magnetic flux.
This gives rise to oscillatory currents through capacitors Cl, C2
and coil L. During the first half cycle of the oscillation, cur-
rent iRl will flow in the same direction as current iT2 to chargecapacitor Cl which will be discharged in the second half cycle of
the oscillation to cause a reverse current flow iR2 which charges
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capacitor Cl in the opposite direction. This charge on capacitor
Cl will forwardly bias the diode D to render it conductive. At
the end of the retrace period, the charge stored on capacitor C2
will be released into the now conducting diode D causing a negative
current flow iTl during the first half of the trace period. This
negative current flow will cause the voltage across capacitor C2
to increase.
If disconnection has occurred in the circuit of deflec-
tion coil L, switching transistor T will fail because of the over-
current. If transistor T does not fail under these conditions, avertical bright trace will appear on the screen because the fly-
~ack transformer will still be operating through the switching
action of transistor T, and the portion of the screen that is bom- ;
barded by impinging electrons will soon be burned.
In order to overcome this problem, the present invention
contemplates to operate the horizontal oscillator 12 from a direct-
current voltage developed across the capacitor C2 rather than from
the power supply +B. In Fig. 2 a filter network 24 comprising a
resistor 26 and a capacitor 28 is connected between a point inter-
mediate coil L and capacitor C2 and ground. Resistor 26 and capa-
citor 28 are connected in series to ground and their midpoint
connected to the power supply input of the horizontal oscillator
12. The filter network 24 filters out the ripple components of
the unidirectional voltage on capacitor C~ and supplies the oscil-
lator 12 with a constant direct-current voltage as a power supply
therefor.
With this arrangement, if disconnection should occur
in the circuit of horizontal deflection coil L, an overcurrent ~-
will flow to the transistor T and the charge stored on capacitor
30 C2 will be discharged within a period much smaller
than the time the overcurrent takes to thermally break down the
transistor T, and the oscillator 12 will become inoperative
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until the circuit failure is corrected. During the time when the
capacitor C2 is being discharged upon the circuit disconnection,
the vertical deflection coil (not shown) is supplied with the nor-
mal vertical sweep voltage as usual which will cause the cathode
ray beam to deflect vertically across the screen producing a
bright, straight line thereon so that the phosphor material of
the beam impinging portion is bombarded with a high density elec-
tron emission. However, the phosphor material is capable of
withstanding such a high density electron emission for an inter-
~ 10 val much longer than the discharge time of capacitor C2, so thatthere is no likelihood that the cathode ray tube screen is par-
tially burnt. The secondary circuit failures described herein-
before are thus effectively prevented.
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