Horizontal Deflection - Philips 29PT5458/01 Service Manual

Circuit Descriptions, List of Abbreviations, and IC Data Sheets
There is a current source inside the UOC at pin 102. This
pumps energy in the capacitor connected to this pin producing
a pure saw tooth. The vertical drive signals and the E/W
correction signal are derived.
Pin 108 is the East-West drive (or AVL), and it is a single ended
current output. The correction for "horizontal width for changed
EHT" from this pin is available by setting the HCO bit to "1".
The Phase-2 Compensation available at pin 113 gives frame
correction for high beam currents. The phase compensation
signal is used to correct the phase of the picture from the
horizontal drive signal.
Pin 63 is the SANDCASTLE output (contains all sync info) and
also HORIZONTAL FLYBACK (HFB) input.
Pin 97 is the EHT tracking/over-voltage protection pin. The
HCO bit can switch on the tracking on EW. If the voltage at pin
97 exceeds 3.9 V, the over-voltage protection will be activated
and the horizontal drive is switched "off" via a slow stop.
9.3.2

Horizontal Deflection

There are several executions (depending on the CRT):
•
Sets with no East-West correction. The principle of the
horizontal deflection is based on the quasi-diode
modulation circuit. This horizontal deflection circuit
supplies the deflection current and auxiliary voltages from
the LOT.
•
Sets with East-West correction. The principle of the
horizontal deflection is based on a diode modulator with
east-west correction. This horizontal deflection circuit
supplies the deflection current and auxiliary voltages from
the LOT.
•
Sets with dynamic East-West correction. The principle
of the horizontal deflection is based on a diode modulator
with dynamic east-west correction for picture tubes with
inner pincushion. This horizontal deflection circuit supplies
the deflection current and auxiliary voltages from the LOT.
Basic Principle
During a scan period, either the Line Transistor or diode(s)
conduct to ensure a constant voltage over the deflection coil
(that results in a linear current). During the flyback period, the
Line Transistor stops conducting, and the flyback capacitor(s)
together with the inductance of the deflection coil creates
oscillation.
First Part of Scan
Pin 62 of the UOC delivers the horizontal drive signal for the
Line Output stage. This signal is a square pulse of line
frequency. L5402 is the flyback drive transformer. This
transformer de-couples the line output stage from the UOC. It
has a direct polarization. The flyback drive circuit works with
the start-up supply taken from +6V of the Aux supply (and
subsequently taking from VlotAux+9V). When the H-drive is
high, TS7404 conducts, and transformer L5402 starts to store
energy. The base of the line transistor TS7405 is low and
therefore blocks. The current in the deflection coil returns from
diode D6404.
Second Part of Scan
When the H-drive is low, TS7404 does not conduct, and the
energy that is stored in the transformer will transfer to the
secondary, making the base of the Line Transistor high. Then
the Line Transistor starts to conduct. The current in the
deflection coil returns from the transistor in another direction.
Flyback
At the moment the H-drive becomes high, the base of the Line
Transistor becomes low. Both the Line Transistor and the
Flyback Diode will block. There is an oscillation between the
flyback capacitor C2412 and the deflection coil. Because of the
inductance of the LOT, the Line Transistor cannot stop
L04E AA
conducting immediately. After the Line Transistor is out of
conduction, the flyback pulse is created. The flyback capacitor
charges until the current in the deflection coil reduce to zero.
Then it discharges through the deflection coil and the deflection
current increases from the other direction. The flyback diode
conducts and is back to the first part of the scan.
Linearity Correction
Because the deflection coil has a certain resistance, a picture
without any linearity issues cannot be expected. L5401 is the
linearity coil to compensate for this resistance. It is a coil with a
pre-magnetized core. This correction is called linearity
correction.
Horizontal S-Correction
Because the electronic beam needs to travel a longer distance
to both sides of the screen than the center, the middle of the
screen would become narrower than both sides. To prevent
this, a parabolic voltage is applied across the deflection coil
during scan. To create this parabolic voltage, a capacitor called
S-cap (C2417/C2418) is used as a voltage source during scan.
The sawtooth current of the deflection through this capacitor
creates the required parabolic voltage. This correction is called
S-Correction.
Mannheim-Circuit
When the EHT is heavily loaded with a bright line, the flyback
time can be increased a bit in this situation. As a result, the
scan delays a bit causing a DC-shift to the right in the next line,
which would create a small spike on the S-cap. This spike
oscillates with the inductance of the deflection coil and the
primary of LOT. The result is visible in vertical lines under
horizontal white line. This is called the Mannheim-effect.
To prevent this from happening, a circuit called Mannheim-
circuit is added. This consists of C2415, R3404, R3417 and
D6406. During the scan, C2415 is charged via R3417. During
the flyback, the S-correction parabola across the S-Cap
C2417/C2418 is in its most negative, and D6406 conducts.
Thus, C2415 is switched in parallel to C2417/C2418 during
flyback. As C2415 is much larger than C2417/C2418, the
voltage across C2415 reduces the Mannheim-effect oscillation.
Class D East-West Driver
To reduce the power loss of the normal used linear East-West
amplifier, a class-D East-West circuit is used. To achieve this,
the East-West parabola waveform EW_DRIVE from the
Hercules (frame frequency) is sampled with a saw tooth (line
frequency) taken from the line aux output. Then a series of
width-modulated pulses is formed via two inverted phase
amplifiers, filtered by an inductor, which then directly drive the
diode modulated line circuit.
East-West Correction
To achieve a good geometry, dynamic S-correction is needed.
The design is such that the tube/yoke needs East-West
correction. Besides that, an inner pincushion is present after
East-West correction. The line deflection is modulated with a
parabolic voltage (frame frequency). In this way it is not so
much at top and bottom, and much more in the middle.
Upon entering the picture geometry menu in the SAM mode,
the following corrections will be displayed.
•
EWW: East West Width.
•
EWP: East West Parabola.
•
UCP: Upper Corner Parabola.
•
LCP: Lower Corner Parabola.
•
EWT: East West Trapezium.
The East-West drive circuit realizes them all. The settings can
be changed by a remote control. All changed data will be
stored into the NVM after the geometry alignment.
9. EN 75