Circuit Description
GB 59
L01.1E
9.
9.5
Deflection
9.5.1
Horizontal Drive
The horizontal drive signal is obtained from an internal VCO,
which is running at twice the line frequency. This frequency
is divided by two, to lock the first control loop to the incoming
signal.
When the IC is switched ‘on’, the ‘Hdrive’ signal is
suppressed until the frequency is correct.
The ‘Hdrive’ signal is available at pin 30. The ‘Hflybk’ signal
is fed to pin 31 to phase lock the horizontal oscillator, so that
TS7462 cannot switch ‘on’ during the flyback time.
The ‘EWdrive’ signal for the E/W circuit (if present) is
available on pin 15, where it drives transistor 7400 to make
linearity corrections in the horizontal drive.
When the set is switched on, the ‘+8V’ voltage goes to pin 9
of IC7200. The horizontal drive starts up in a soft start mode.
It starts with a very short T
ON
time of the horizontal output
transistor. The T
OFF
of the transistor is identical to the time in
normal operation. The starting frequency during switch on is
therefore about 2 times higher than the normal value. The
‘on’ time is slowly increased to the nominal value in 1175 ms.
When the nominal value is reached, the PLL is closed in such
a way that only very small phase corrections are necessary.
The ‘EHTinformation’ line on pin 11 is intended to be used as
a ‘X-ray’ protection. When this protection is activated (when
the voltage exceeds 6 V), the horizontal drive (pin 30) is
switched 'off' immediately. If the ‘H-drive’ is stopped, pin 11
will become low again. Now the horizontal drive is again
switched on via the slow start procedure.
The ‘EHTinformation’ line (Aquadag) is also fed back to the
UOC IC7200 pin 54, to adjust the picture level in order to
compensate for changes in the beam current.
The filament voltage is monitored for ‘no’ or ‘excessive’
voltage. This voltage is rectified by diode 6413 and fed to the
emitter of transistor 7405. If this voltage goes above 6.8 V,
transistor 7405 will conduct, making the ‘EHT0’ line ‘high’.
This will immediately switch off the horizontal drive (pin 30)
via the slow stop procedure.
The horizontal drive signal exits IC7200 at pin 30 and goes to
7401, the horizontal driver transistor. The signal is amplified
and coupled to the base circuit of 7402, the horizontal output
transistor. This will drive the line output transformer (LOT)
and associated circuit. The LOT provides the extra high
voltage (EHT), the VG2 voltage and the focus and filament
voltages for the CRT, while the line output circuit drives the
horizontal deflection coil.
9.5.2
Vertical Drive
A divider circuit performs the vertical synchronisation. The
vertical ramp generator needs an external resistor (R3245,
pin 20) and capacitor (C2244, pin 21). A differential output is
available at pins 16 and 17, which are DC-coupled with the
vertical output stage.
To avoid damage of the picture tube when the vertical
deflection fails, the ‘V_GUARD’ output is fed to the beam
current limiting input. When a failure is detected, the RGB-
outputs are blanked. When no vertical deflection output stage
is connected, this guard circuit will also blank the output
signals.
These ‘’ and ‘V_DRIVE-‘ signals are applied to the
input pins 1 and 2 of IC 7471 (full bridge vertical deflection
amplifier). These are voltage driven differential inputs. As the
driver device (IC 7200) delivers output currents, R3474 and
R3475 convert them to voltage. The differential input voltage
is compared with the voltage across measuring resistor
R3471 that provides internal feedback information. The
voltage across this measuring resistor is proportional to the
output current, which is available at pins 4 and 7 where they
drive the vertical deflection coil (connector 0222) in phase
opposition.
IC 7471 is supplied by +13 V. The vertical flyback voltage is
determined by an external supply voltage at pin 6
(50V). This voltage is almost totally available as
flyback voltage across the coil, this being possible due to the
absence of a coupling capacitor (which is not necessary, due
to the ‘bridge’ configuration).
9.5.3
Deflection Corrections
The Linearity Correction
A constant voltage on the horizontal deflection coil should
result in a sawtooth current. This however is not the case as
the resistance of the coil is not negligible. In order to
compensate for this resistance, a pre-magnetised coil L5457
is used. R3485 and C2459 ensure that L5457 does not
excite, because of its own parasite capacitance. This L5457
is called the 'linearity coil'.
The Mannheim Effect
When clear white lines are displayed, the high-voltage circuit
is heavily loaded. During the first half of the flyback, the high
voltage capacitors are considerable charged. At that point in
time, the deflection coil excites through C2465. This current
peak, through the high-voltage capacitor, distorts the flyback
pulse. This causes synchronisation errors, causing an
oscillation under the white line.
During t3 - t5, C2490//2458 is charged via R3459. At the
moment of the flyback, C2490//2458 is subjected to the
negative voltage pulses of the parabola as a result of which
D6465 and D6466 are conducting and C2490//2458 is
switched in parallel with C2456//2457. This is the moment the
high-voltage diodes are conducting. Now extra energy is
available for excitation through C2465 and the line deflection.
As a consequence, the flyback pulse is less distorted.
The S-Correction
Since the sides of the picture are further away from the point
of deflection than from the centre, a linear sawtooth current
would result in a non-linear image being scanned (the centre
would be scanned slower than the sides). For the centre-
horizontal line, the difference in relation of the distances is
larger then those for the top and bottom lines. An S-shaped
current will have to be superimposed onto the sawtooth
current. This correction is called finger-length correction or S-
correction.
C2456//2457 is relatively small, as a result of which the
sawtooth current will generate a parabolic voltage with
negative voltage peaks. Left and right, the voltage across the
deflection coil decreases, and the deflection will slow down;
in the centre, the voltage increases and deflection is faster.
The larger the picture width, the higher the deflection current
through C2456//2457. The current also results in a parabolic
voltage across C2484//2469, resulting in the finger length
correction proportionally increasing with the picture width.
The east/west drive signal will ensure the largest picture
width in the centre of the frame. Here the largest correction is
applied.
East/West Correction
In the L01, there are three types of CRTs, namely the 100º,
110º and wide screen CRTs. The 100º CRT is raster-
correction-free and does not need East/West correction.
The 110º 4:3 CRT comes with East/West correction and
East/West protection.
The wide screen TV sets have all the correction of the 110
4:3 CRT and also have additional picture format like the 4:3
format, 16:9, 14:9, 16:9 zoom, subtitle zoom and the Super-
Wide picture format
