Horizontal Drive - Philips 20PT643F 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.