Philips 7629, Service Manual

Page: 72 / 374

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 7443will 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 VG2voltage 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.
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
R3475convert them to voltage. The differential input voltage is compared with the