PHILIPS 28PT4425
Instrukcja Serwisowa

Circuit Description
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 TON time of the horizontal output transistor. The TOFF 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 6447 and fed to the emitter of transistor TS7443. If this voltage goes above 6.8 V, transistor TS7443 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 TS7462, the horizontal driver transistor. The signal is amplified and coupled to the base circuit of TS7460, 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 RGBoutputs are blanked. When no vertical deflection output stage is connected, this guard circuit will also blank the output signals. These �V_DRIVE+� 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 (VlotAux+50V). This voltage is almost totally available as

L01.1E AB

9.

EN 67

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 rastercorrection-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 A line, written at the upper- or lower side of the screen, will be larger at the screen centre when a fixed deflection current is used. Therefore, the amplitude of the deflection current must be increased when the spot approaches the centre of the screen. This is called the East/West or pincushion correction. The �Ewdrive� signal from pin 15 of IC7200 takes care for the correct correction. It drives FET TS7400. It also corrects breathing of the picture, due to beam current variations (the