Horizontal Drive - Philips 7603 series Service Manual

Deflection

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
Q7462 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 IC 7200. 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 IC 7200 pin 54, to adjust the picture level in order to
compensate for changes in the beam current.
The 'filament' voltage is monitored for 'no voltage' or 'excessive voltage'. This voltage is rectified by diode 6447 and fed to
the emitter of transistor 7443. If this voltage goes above 6.8 V, transistor 7443 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 IC 7200 at pin 30 and goes to 7462, the horizontal driver transistor. The signal is amplified
and coupled to the base circuit of 7460, 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.
Vertical drive
A divider circuit performs the vertical synchronization. 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. During the insertion of RGB signals, the maximum vertical frequency is increased to 72 Hz so that the circuit
can also synchronize on signals with a higher vertical frequency like VGA.
To avoid damage of the picture tube when the vertical deflection fails, the 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.