Specifications

CA10 Course Notes
Sharp Electronics (UK) Limited
CE Technical Support Group Page 19
Figure 18 : Line Current and Voltage Waveforms ( Idealised )
Note that as the voltage across the primary winding is proportional to the rate of change of current in it and
since the rate of change is constant during the forward scanning stroke, but fast during the flyback period,
the voltage across it ( and the scanning coils ) will be a series of pulse. This is also shown in the diagram
below.
Also note that the above is an ideal explanation of the circuit operation, the actual waveforms generated
will be somewhat different to those shown on the previous page.
Line scan correction
L605 provides for line linearity correction and R611 will damp any excessive voltage spikes across it,
these spikes may cause striations on the left hand side of the screen if left undamped.
As the tube face is virtually flat, if the scanning current was not modified, this would result in the picture
being cramped in the centre and stretched at the sides. To alleviate this it is necessary to compensate
for the rate of change of the line scan current, i.e. to slow down the current at the start and end of scan.
This can be achieved by fitting the correct value of capacitor in series with the line scanning coils.
Normally referred to the S-correction capacitor, this is C613.
East/west correction is provided by the diode modulator and is explained in more detail in the East/west
correction section of these notes.
Sand castle pulse
For correct internal processing within IC201, a reference pulse needs to be generated from the line and
frame signals. This is referred to as the sand castle pulse and its wave shape is shown in the diagrams
below.
To generate this pulse it is necessary to sample both the line and frame signals. The line pulse comes
from the line output transformer, T601, and the frame pulse from the teletext processing IC, IC106. The
line pulse generated from the line output transformer is rectified by D612, ensuring that only positive going
pulses are present on its cathode. Connected to the cathode of D612 is D608 which is used to reduce
the pulse by fifteen volts. This will ensure that the overall level of the line pulse will not exceed 5.5V when
it arrives at pin 41 of IC201.